Winter 2014-2015 Medium-Term Discussion

[thunderbolt]

Morning thoughts...

A sense of panic set in on some sites yesterday, with pattern perceptions switching wildly like a pendulum with a change in model runs that suddenly showed a milder closing week of January. In the long-range, one should not switch wildly with each model run. The runs work toward details. What matters is the larger synoptic setup. The details will vary from day to day. Such caution is even more imperative when one is dealing with a period of time when run-to-run continuity has been poor.

Therefore, one should resist the temptation to swing wildly from one model run to the next when volatility in the guidance is so high. To illustrate the "windshield wiper" effect that unfolded on some sites, which compounded some of the negative sentiment in recent weeks at AmWx, just imagine the following scenario:

Sunday-Monday Forecasts for New York City:

Forecast time: 1/16 6 am: Mainly cloudy with a shower or two. (6z GFS forecast: 0.02")

Forecast time: 1/16 12 pm: Rainy and windy. Rainfall amounts will range from 1"-2" (12z GFS forecast: 1.53")

In contrast, if one had stayed with the ensembles and also more consistent ECMWF, both forecasts would have read something along the lines of "Rainy with the potential of 1" or more rainfall."

How would one perceive the two forecasts that varied with each model run? Would one prefer the more stable second one, even if it were changed slightly as the details grew clearer e.g. if it shifted from "the potential of 1" or more rainfall" to "1" or more rainfall is likely?"

Back to the guidance. What happened overnight?

The 0z and 6z guidance moved back toiward the colder scenarios. In fact, the new GFS showed the first fantasy range snowstorm since it supplanted the earlier version of the GFS. For posterity's sake, the relevant frames are:

Of course, that's so far out, one should not even be taking it too seriously right now (though one can always dream and the image is pleasing after what has been a grueling first half of winter with a noted absence of snowfall, even if the image is virtual).

The far more important thing is that the period of moderation (less severe cold than has been the case earlier this month), will probably give way to another very cold period as the month approaches its close courtesy of a new EPO- block. Given the pattern evolution that appears more likely than not, I remain cautiously optimistic about February.

When it comes to February KU-type snowstorms, some statistics:

- 70% commenced with an AO < 0

- 60% commenced with a PNA > 0 (75% of PNA- storms occurred after February 10)

- Most common setup: AO-/PNA+ 45% (January figure: 85%)

- Second most common setup: AO-/PNA- 25%

- Favored MJO Phases when the ENSO R3.4 anomaly was positive: Phases 7, 8, 1, 2 (80% of KU snowstorms during those phases occurred when there was a positive ENSO R3.4 anomaly.

- Most notable early February storm with an AO+: February 2-5, 1961

Don in reference to the ENSO I know the MJO reacts different when that ENSO .5 or greater at least according to Raleigh weather chart my question is that what you based your reading off of thanks Chris

[donsutherland1]

Don in reference to the ENSO I know the MJO reacts different when that ENSO .5 or greater at least according to Raleigh weather chart my question is that what you based your reading off of thanks Chris

 

Chris,

 

I checked the MJO phases and ENSO for each of the KU snowstorms. As the sample size is already small, given that such snowstorms don't occur very often, I used ENSO R3.4 > 0 and ENSO R3.4 < 0. The majority of February KU snowstorms that occurred during Phases 3-6 saw ENSO R3.4 anomalies < 0. If ENSO had no impact on the MJO, I suspect that one wouldn't find such clusters.

[thunderbolt]

Chris,

 

I checked the MJO phases and ENSO for each of the KU snowstorms. As the sample size is already small, given that such snowstorms don't occur very often, I used ENSO R3.4 > 0 and ENSO R3.4 < 0. The majority of February KU snowstorms that occurred during Phases 3-6 saw ENSO R3.4 anomalies < 0. If ENSO had no impact on the MJO, I suspect that one wouldn't find such clusters.

 thank you Don

[TalcottWx]

Don,

Any new thoughts heading forward? Seem to be entering an interesting juncture toward last week of Jan and early Feb

[mrgolf]

Don, I was wondering the same thing. I've been reading about the PV going to Siberia from its place this winter over ne Canada. Not sure if that would allow things to change somewhat for us and possible some warming in stratosphere and troposphere. Perhaps its just a mirage, but I found it kinda intriguing.

[donsutherland1]

Don, I was wondering the same thing. I've been reading about the PV going to Siberia from its place this winter over ne Canada. Not sure if that would allow things to change somewhat for us and possible some warming in stratosphere and troposphere. Perhaps its just a mirage, but I found it kinda intriguing.

There would be a risk that the coldest air would also retreat to the other side of the Hemisphere. It's a little soon to be sure about the February AO. In 54% of winters where December had a positive AO, February had a negative one (vs. climatology of 66%). However, only 28% of winters that saw a positive AO in December and January saw a positive AO in February. January will finish positive if the ensembles are reasonably accurate.

 

Nevertheless, such an outcome is not assured. If the AO fails to fall below -2 during the 1/10-2/10 timeframe, that would be a strong signal for a positive AO for February. Right now, some ensemble members hint at a brief dip to -2, but one should be cautious.It's that development that will probably offer the biggest hint about February.

[donsutherland1]

Don,

Any new thoughts heading forward? Seem to be entering an interesting juncture toward last week of Jan and early Feb

The cold toward the end of January and the start of February looks to be on course. It also appears that the subtropical jet might become quite a bit more active for that period. But the state of the AO and PNA will be important if one is looking for a significant or major snowstorm.

 

Some statistics for Boston (1/15-2/15):

 

10" or more snowfall (15 cases):

AO- 67%

PNA+ 80%

AO-/PNA+ 53%

 

12" or more snowfall (10 cases):

AO- 80%

PNA+ 100%

[donsutherland1]

Morning thoughts...

 

On this date in 1978, a major snowstorm was blanketing the Northeastern United States. Prior to the storm's onset on January 19, Boston had seen 20.3" snow and New York City had received 7.3". During the January 19-February 28 period, Boston picked up 48.7" snow and New York City saw 36.6" fall.

 

To date, much of the eastern U.S., including such major cities as Chicago, Detroit, Houston, New York, Philadelphia, and Washington, DC, has been colder than normal for January. The predominantly negative EPO has played the largest role. In the meantime, the AO has been predominantly positive. While the AO matters less for cold in the East, especially along the Eastern Seaboard, it matters greatly for snowfall.

 

Going back to the 1977-78 winter noted above, the November 1-January 18 period (before the first major snowstorm), saw 56% of the days have a positive AO. During the very snowy period to the end of February 100% of days had a negative AO. The negative AO is especially important if one is looking for big snowstorms.

 

Just for the second half of January, here are statistics for New York City:

4"-8" snowstorms: 38% occurred with an AO-

> 8" snowstorms: 100% occurred with an AO-

 

The PNA also matters greatly. During the November 1, 1977-January 18, 1978 period, 42% of days had a PNA+. The January 19-February 28, 1978 period saw 100% of days have a PNA+.

 

Again, using New York City as an example, here are statistics regarding the PNA for the second half of January:

4"-8" snowstorms: 77% occurred with a PNA+

> 8" snowstorms: 100% occurred with a PNA+

 

In February, the PNA+ becomes somewhat less important, but it still matters.

 

So, at least through the remainder of January, one should keep an eye on both the AO and PNA.

 

In terms of keeping score for the possibility of a negative December-February AO average, the following averages for the January 21-February 28 period are now required:

 

DJF Average of 0.000: -1.037

DJF Average of -0.500: -2.191

DJF Average of -1.000: -3.344

 

In addition, the period of moderation discussed in #355 has unfolded. Below are the 10-day anomalies for the January 5-14 period and the most recent 5-day figures for January 15-19:

 

 

In the longer-range, period of moderation lasting a total of 7-14 days (probably around 10) still looks on course. A sustained colder period should develop around January 25 +/- a few days. There remains potential for Arctic air to get involved. Furthermore, as the colder period unfolds, it also appears increasingly likely that the subtropical jet will come to life. Hence, if some blocking can develop, there will be a growing risk of snow in portions of the Midwest, Mid-Atlantic, and New England areas. Overall, January should end cold and February should begin cold in at least the eastern half of the CONUS, along with southern Ontario and southern Quebec.

[donsutherland1]

Afternoon thoughts...

 

1. The return of cold with the potential for some severe cold beginning around 1/25 +/- a few days remains on course.

 

2. The coming cold could coincide with a more active subtropical jet, leading to increased snowfall opportunities in the eastern U.S. The potential weekend storm for a portion of the East Coast is an example.

 

3. Although the 1/21 12z ECMWF has printed out perhaps the most extreme solution (rapid intensification and qpf figures > 1" in many of the big cities including Boston, New York, Philadelphia, and Washington, the GFS is far more restrained. The 6z GFS was very dry.

 

Historically, moderate to sometimes significant snowstorms (4"-8" in NYC and Philadelphia; 6"-10" in Boston) have coincided with a lack of blocking (AO+) during the second half of January. The major or blockbuster events (>8" in Philadelphia and New York and > 10" in Boston) have occurred with blocking (AO-). Currently, the AO is -1.589 (its lowest figure of the winter), but remains forecast to rise quickly in coming days. Hence, at this point in time, it probably makes sense to lean toward a moderate to possibly borderline significant event as being more likely than a major or blockbuster one. Future model runs will be revealing.

 

4. The new GFS has continued to exhibit some wild swings in qpf in the short range. For example, the 1/21 0z run showed 1.15" at Boston, 1.20" at Islip, and 1.59" at Providence. In contrast, the 6z run showed 0.05", 0.26", and 0.17" respectively. Therefore, one should be cautious about buying into the GFS solutions right now, especially when the model has had poor run-to-run continuity and when it is an outlier when compared to the GGEM, UKMET, and ECMWF.

[donsutherland1]

Morning thoughts...

 

1. The AO has turned up. It is now -0.741.At this time, prospects of a winter average of -1.000 have now become extremely unlikely ( 3%). The AO would also need to average -2.246 for the remaining 37 days of meteorological winter to have a December-February average of -0.5.

 

2. In terms of the forthcoming East Coast storm, what will likely be a positive AO strongly indicates that a moderate not major impact is most likely.

 

3. The arrival of Arctic air next week in the wake of an energetic clipper system that could produce a stripe of moderate snowfall in parts of the Mid-Atlantic and New England areas appears increasingly likely. The possibility of a single digit reading in such cities as Philadelphia and New York could be on the table for mid-week.

[Ground Scouring]

What was the average outcome for February PNA in years with a similar mean value to that of this year in December-January? Thanks!

[donsutherland1]

What was the average outcome for February PNA in years with a similar mean value to that of this year in December-January? Thanks!

Historic experience overwhelmingly favors a PNA+ for February. Since 1950, there have been 22 winters when the December and January PNA were positive. In 18/22 (82%) of those winters, the February PNA was positive. In the smaller subset of those winters during which the PDO was positive, 16/18 (89%) saw a positive PNA in February.

[Isotherm]

Earlier this month, I posted about the potential for an AO modality reversal in late January, particularly by the 20th and thereafter, as a more Nino-like global pattern begins to develop. I continue to believe that should be the case for a number of reasons as outlined below. The NAO state is more uncertain; however, there should be a significant improvement w/ a neutralization of the index by late January, which would certainly be a far cry from the strongly positive / low height field dominating the north Atlantic so far this winter.

 

 

[1]   Pacific-tropical forcing: A major "problem" over the past several months has been a lack of real Nino-like tropical forcing in the Pacific. SST's have gradually warmed to weak El Nino criteria, and the SSTA profile is indicative of a strongly positive PDO signal; however, tropical convection has been more reminiscent of a La Nina pattern, and this had has implications as far as the mid-latitude regime. As noted before, this will be changing as we finally see an eastward propagation of tropical convection / upward motion into the central tropical Pacific, which should initiate downstream changes in the mid latitude 500mb pattern.

 

Note the tendency for suppressed convection over the central tropical Pacific over the past several months. Forecasts for the next few weeks indicate a significant change to this tropical forcing pattern as stronger upward motion / lift translates ewd.

 

 

 

 

200mb velocity potential and OLR forecasts suggest that the above tendency persists throughout the remainder of January.

 

 

 

[2] Global budget of atmospheric angular momentum: An important indicator of whether the atmosphere is in a Nino or Nina-like state is the AAM budget of Earth, namely as AAM increasing or decreasing, particularly in the tropical regions, this has significant ramifications as far as the mid latitude ridge / trough response. Lower AAM tendency represents a more Nina-like state while increasing speeds suggest a more Nino-like state. If one notes prior Nino winters, generally one finds that a Nina-like pattern occurs more frequently in Nino's when the AAM budget is suppressed. However, the Nino's which feature increased AAM tend to resemble more closely a "typical" El Nino height configuration at 500mb. This is a relationship that exists which must be utilized in concert with all other variables as well. November-early December featured a predominately +AAM, during which time, high latitude blocking and Eastern troughiness was present. That reversed to a strongly negative AAM by mid December, at which time a +EPO / +AO regime initiated. Currently, AAM has spiked significantly, at levels higher than we've seen over the past 50-60 days. This seems to coincide with the development of more Nino-like tropical forcing.

 

 

[3] Stratospheric improvement: The minor SSW event which transpired last week significantly warmed the stratosphere and can often act to warm / perturb the tropospheric vortex, thereby reversing the modality of the northern annular mode. This process tends to take at least 2 weeks, which further points to the 20th and beyond time frame I've been referencing for an AO state shift. We may not initially see a strongly negative AO, but the AO should neutralize by the 20th, then turn negative for the final 10 days of January as heights build across the pole from both the Eurasian and Pacific sides. The NAO state will likely lag due to the maintenance of a stratospheric vortex near Greenland and recent enhanced geomagnetic activity; however, it should undergo a significant negative trend toward neutral at the very least by late January. Model guidance is now in agreement on height rises beginning first in the EPO region by the 19th, and the Arctic after the 20th. Heights should improve significantly in the NAO region, but I wouldn't expect a monster Greenland block this month.

 

[4] ENSO state / progression analogs: the tendency in warm/positive ENSO years is for a mid to late winter negative AO / NAO state. Even in warm ENSO positive winters, the NAO often shifted to a negative state for February.

 

 

In conclusion: the current cold January has clearly diverted this winter from poor/warm +ENSO year and indicates that the global state remains favorable for NPAC / EPO blocking, as has been the case since autumn. Improvements in global, tropical, and stratospheric forcing implicate an AO shift after the 20th. The NAO state should improve significantly (neutralize) but blocking in that region may remain unimpressive for the time being. Sensible weather wise: anticipate a transient thaw period of approximately 3-5 days as the global regime transitions, however, the cold January should resume for the CONUS by the last week of January, with the development of a -EPO/-AO. Since the negative AO state has not been present thus far, this will likely increase the potential for wintry precipitation in the East which has largely been absent. At this point, I expect the late January transition to continue and amplify for the month of February. We'll see how it goes.

 

 

 

 

As per my post on 1/12, the AO step-down has begun over the past week, and as a consequence of the improved pattern, this morning featured the first meaningful snowfall of the season for much of the Mid-Atlantic and Northeast. The northern hemispheric regime has become more favorable, due to a number of factors, and going forward, I still anticipate an amplification of the aforementioned changes.

 

While the AO will surge back positive for a few days, it's apparent that this will be quite transient, and a more prolonged episode of neutral to negative AO conditions seems likely for the end of January into February. I anticipate a maintenance of the +PNA / -EPO couplet, which is strongly supported by +PDO / +ENSO conditions. This should permit persistent cross-polar flow and potentially frequent arctic air intrusions for the Central / Eastern US. The NAO modality is most uncertain; however, recent changes w/ solar, stratospheric, and tropical forcing, lead me to believe that we could see our first episode of negative NAO in the beginning of February.

 

[1] Over the coming days, stratospheric vortices will be displaced from their current position into northern Eurasia / Siberia. Significant warming is progged to occur in the stratosphere over North America and Greenland. EP flux is poleward over the next several days, implicating more effective heat flux transfer.This dislodging of the stratospheric vortex should have implications for increased tropospheric perturbation of the polar vortex, which has generally been stable near the Davis Straits. Models are starting to detect the possibility of significant height rises in the NATL / Greenland and thus at least a temporary reversal of the NAO state in the medium term. The stratospheric displacement event will continue to keep the stratosphere warm and perturbed, and this substantially increases the probability of a more persistent negative northern annular mode / AO.

 

[2] Solar activity has declined significantly over the past 10 days, with lower levels of solar flux and geomagnetic activity. This in concert w/ the stratospheric progression argues for a deceleration of the jet in the north Atlantic.

 

[3] As noted in prior weeks, tropical forcing would become more conducive / Nino like by late January, and current projections indicate a maintenance of forcing near the dateline --> 170W. Recent MJO wave activity into phase 7 and the associated westerly wind burst / SOI crash should aid in the continuance of a Nino-like northern hemispheric state over the coming weeks, namely, a positive PNA / AK block and higher heights across the pole.

 

 

[4] As the AO and NAO modalities become more negative over the coming week, there should be more opportunities for wintry precipitations in the East. In fact, there will be an opportunity on Mon-Tues as the AO is still technically positive, but a ridge-bridge feature could be in place w/ higher heights across the north pole / northern Greenland. Next week, the AO and NAO states rapidly become more conducive as the NPAC regime maintains itself.

 

 

Everything remains on track for a cold and potentially active February.

 

[Note] : while early week 1/26-27 offers an opportunity for Northeastern snowfall, there may be a more significant threat around the beginning of February. The pattern should be more favorable than anything we've seen so far this winter. The first image is the GFS ensemble AO forecast, depicting the negative trend resuming in a few days. The second image is the ECMWF ensemble mean for February 1st, indicating a +PNA / -EPO / -AO / -NAO regime. I expect the -NAO could be transient, but the +PNA / -EPO and -AO should be more "stable" features for February.

 

 

 

 

 

 

[donsutherland1]

The very recent blizzard that brought widespread snowfall amounts of two feet or higher to parts of Long Island and then New England and much lesser amounts to parts of Pennsylvania and New Jersey provided an illustration of the importance of blocking. Had there been blocking, the system would more than likely have taken a track closer to the fabled 1978 blizzard than its farther east track. Had that happened cities such as Philadelphia and Newark would likely have received at least the amounts that were seen in Nassau County (generally 12"-18" .

 

That is in the past now. Yet, the expression "past is prologue" may well ring true in the weeks ahead if there a strong block is unable to establish itself over the next 7-14 days, at least when it comes to blocking. There is a growing possibility that February will not see persistent blocking, although there should be more episodes of blocking than what one had seen in January. If the Arctic Oscillation (AO) does not fall to -2.000 or below on at least one day during that period, that would provide a strong signal that the AO would average above 0 during February.

 

As January approaches its end, the Arctic Oscillation stands at +2.202. For winter 2014-15, just 24% of days have seen the AO average < 0. In contrast, 19% have seen the AO average +2.000 or above and 39% have seen it average +1.000 or above.

 

With today's positive reading, if every remaining day had a negative AO (extremely unlikely given the latest guidance), just 50% of days would have had a negative AO reading (if every day were positive, 84% of days would have a positive AO). Moreover, one would need an historic blocking episode to unfold were the winter average to come to -1.000. Even if one were looking for a winter average of -0.500, the AO would need to average -2.908 for the remainder of meteorological winter. Just to bring the winter average to 0.000 would require an average AO of -1.456 from 1/29 through 2/28.

 

What does this mean about forecasting the predominant state of the winter AO. If one looks at things objectively, this is not an encouraging outcome. With a second consecutive year where such approaches as the Snow Advance Index (SAI) and October Pattern Index (OPI) missed, it means that the relationships those approaches utilized between October developments (Eurasian snowcover changes in the case of the SAI and 500 mb pattern in the case of the OPI) and the wintertime AO are weaker than what had been thought. Increasingly, it appears that at least some of the promise seen with such tools was due to hindcasting and not sustainable in a forecasting environment. Whether the couplet of forecasting difficulties means that such tools are little better than statistical approaches e.g., regression analysis tie to past AO values, remains to be seen. What is clear is that forecasting the predominant winter AO state from the fall is more difficult with a greater risk of error than had initially appeared to be the case. This is highly unfortunate, as blocking is an important contributor in shaping winter outcomes.

 

I still remain hopeful that those approaches can be refined to provide stronger insight into the winter AO, as it still appears that one is dealing with a case of a weaker relationship, not little or no relationship. Discovery of relationships, even if they are weaker, is still an important step forward. In terms of the SAI, would a variable that considers near end-of-October changes in Eurasian snowcover (something picked up early by GaWx) in a multiple regression approach add value? Would consideration of Arctic sea ice add value? Those are among the plausible lines that could be explored following the winter.

 

The pursuit of new knowledge remains a fundamental aspect of science. The scientists who have been pushing the boundaries of knowledge deserve much credit for their efforts and both successes and failures can, over time, lead to the development of new understanding.

 

In the meantime, the AO is forecast to go negative for a brief period of time as noted in earlier discussions, even as the subtropical jet remains active. Hence, snowfall opportunities for parts of the CONUS (Midwest, Ohio Valley, Mid-Atlantic, and New England) will likely be present from now through perhaps the first 7-10 days of February. At least one Arctic intrusion also remains likely.

[Chinook]

 

The NAO modality is most uncertain; however, recent changes w/ solar, stratospheric, and tropical forcing, lead me to believe that we could see our first episode of negative NAO in the beginning of February.

 

 

 

Interestingly, the 500mb flow is so cyclonic in Europe, that Amsterdam will see 500mb heights under 500dm today. (00z). The average 500mb height for this area is around 540dm (5400m).

 

[donsutherland1]

Morning thoughts...

 

1. The return of the cold and development of an active subtropical jet around 1/25 that had been discussed is in place.

 

2. Although New England was the beneficiary of the first major storm during this period and northern New England will be the prime beneficiary of the biggest snowfall from today's clipper, the guidance suggests that another storm could bring a large area of snow to the Midwest and eastward beginning on Saturday.

 

3. One or two bouts of severe to possibly extreme cold appear likely.

 

4. The cold and stormy period will likely last through 2/10 and perhaps 2/15. Afterward, the subtropical jet could relax, but whether the cold is sustained will depend on whether the EPO-/PNA+ and/or AO- is established. The PNA+ seems like the most solid scenario right now, with the state of the other teleconnections less certain.

[donsutherland1]

To illustrate the large impact the upcoming storm will have, below are qpf figures from the 1/30 12z run of the GFS for 1/31--2/2.

 

Albany: 0.34"

Allentown: 0.84"

Belmar: 0.92"

Boston: 0.73"

Bridgeport: 0.76"

Chicago: 0.64"

Cleveland: 0.63"

Columbus: 0.90"

Des Moines: 0.65"

Detroit: 0.30"

Groton: 0.85"

Hartford: 0.68"

Harrisburg: 0.87"

Hyannis: 1.11"

Indianapolis: 0.81"

Islip: 0.81"

Morristown: 0.72"

New Haven: 0.82"

New York City:

...JFK: 0.70"

...LGA: 0.69"

...NYC: 0.67"

Newark: 0.67"

Philadelphia: 1.14"

Pittsburgh: 0.87"

Poughkeepsie: 0.56"

Providence: 0.87"

Scranton: 0.64"

St. Louis: 0.72"

Trenton: 0.91"

Westhampton: 0.86"

White Plains: 0.66"

Wilmington, DE: 1.19"

Worcester: 0.59"

 

These figures are not cast in stone and some cities may see some rain and sleet in addition to snow. The figures merely illustrate the upcoming storm's expansive impact.

[donsutherland1]

Some afternoon thoughts...

 

1. The teleconnections today are AO-/PNA-.

2. That setup infrequently produces significant or major snowstorms in such cities as Philadelphia and New York.

3. Such a setup has produced 25% of Detroit's 10" or greater snowstorms since 1950. Detroit's AO-/PNA- snowstorms are as follows:

 

February 24-25, 1965: 11.0"

December 1-2, 1974: 19.3"

January 5-7, 1994: 10.4"

[andyhb]

-QBO has strengthened even further to -26.70 for January per ESRL data (strongest on record in that month) and we now have had six straight months of -20 or stronger. Only two other years in the dataset (2005 and 2012) had that occur.

 

Source: http://www.esrl.noaa.gov/psd/data/correlation/qbo.data

[illinois]

-QBO has strengthened even further to -26.70 for January per ESRL data (strongest on record in that month) and we now have had six straight months of -20 or stronger. Only two other years in the dataset (2005 and 2012) had that occur.

Source: http://www.esrl.noaa.gov/psd/data/correlation/qbo.data

What does this mean going forward into very late winter, spring and summer. I have tried to research the effects but have not had a lot of luck. Thanks in advance.

[donsutherland1]

Some statistics from among the snowiest weeks on record and perhaps the snowiest week on record in New England:

 

1/26-2/2 snowfall:

 

Boston: 42.5"

Portland: 38.4"

Worcester: 52.1"

 

Note: Some changes are still possible when the final figures for 2/2 are in.

[donsutherland1]

The February 8-10, 2015 timeframe is likely to produce a remarkable contrast between out-of-season warmth in the West and a potential significant snowfall in parts of the East.

 

If one looks at the latest GFS ensembles, one finds an area of warmth centered over parts of Arizona, Colorado, New Mexico, and Utah with 850 mb temperature anomalies approaching or exceeding 3 standard deviations above normal. Should those anomalies verify, cities such as Albuquerque and Denver could see one or more temperatures in the upper 60s to perhaps lower 70s and Phoenix could top out in the upper 80s to perhaps lower 90s.

 

 

In the East, the forecast teleconnections and 500 mb pattern favors a period of overrunning as a trough develops over the Southeast and slowly moves east-northeastward. The forecast 500 mb pattern is similar to a composite from a set of storms (2/1-15/1950-2014) that saw moderate to significant snowfall from Philadelphia to Boston.

 

 

The most similar setup for a KU snowstorm involved the 2/2-5/1961 snowstorm. However, there are differences between what is forecast and the pattern that was in place during that storm. Nevertheless, if one considers the differences in detail in the 500 mb patterns and the outcomes of some storms that occurred during such patterns, the potential exists for Philadelphia to pick up 4" or more snow and New York City and Boston 6" or more during the 2/8-10 period. Some possibility also exists for both Boston and New York City to receive 10" or more snow.

 

Currently, the GFS and ECMWF are in reasonable agreement with the kind of outcomes that have sometimes resulted from reasonably similar past patterns.

 

Finally, in the wake of the possible storm, the guidance suggests that a brutal air mass could drop southward from Quebec toward the Mid-Atlantic region. Even as that situation is still 8-10 days away, there is enough of a signal to suggest that another bout of severe cold could affect parts of the East late next week into the weekend. Some of the models, including the ECMWF and GFS have suggested that that air mass might produce the coldest readings of the winter in parts of the affected region.

[donsutherland1]

Morning thoughts...

 

1. The exceptional shot of warmth for parts of the West remains on track with the overnight guidance.

2. MEX numbers from the 2/6 0z GFS for the highest temperatures during the 2/8-10 period are: Albuquerque: 69°, Denver 68°, and Phoenix: 86°.

3. The PNA ridge is flattening a little faster than had been forecast. That could have implications for the complex overrunning/possible coastal development in the East this weekend and early next week.

4. The guidance continues to suggest that a very cold and possibly coldest air mass this winter could move into the Northeast and Mid-Atlantic regions late next week.

[The Plowsman]

Morning thoughts...

1. The exceptional shot of warmth for parts of the West remains on track with the overnight guidance.

2. MEX numbers from the 2/6 0z GFS for the highest temperatures during the 2/8-10 period are: Albuquerque: 69°, Denver 68°, and Phoenix: 86°.

3. The PNA ridge is flattening a little faster than had been forecast. That could have implications for the complex overrunning/possible coastal development in the East this weekend and early next week.

4. The guidance continues to suggest that a very cold and possibly coldest air mass this winter could move into the Northeast and Mid-Atlantic regions late next week.

OK so HIGH in Freehold (Mon cty), NJ next Fri as per NWS is supposed to be 14 with a low of 4!

[Isotherm]

As per my post on 1/12, the AO step-down has begun over the past week, and as a consequence of the improved pattern, this morning featured the first meaningful snowfall of the season for much of the Mid-Atlantic and Northeast. The northern hemispheric regime has become more favorable, due to a number of factors, and going forward, I still anticipate an amplification of the aforementioned changes.

 

While the AO will surge back positive for a few days, it's apparent that this will be quite transient, and a more prolonged episode of neutral to negative AO conditions seems likely for the end of January into February. I anticipate a maintenance of the +PNA / -EPO couplet, which is strongly supported by +PDO / +ENSO conditions. This should permit persistent cross-polar flow and potentially frequent arctic air intrusions for the Central / Eastern US. The NAO modality is most uncertain; however, recent changes w/ solar, stratospheric, and tropical forcing, lead me to believe that we could see our first episode of negative NAO in the beginning of February.

 

[1] Over the coming days, stratospheric vortices will be displaced from their current position into northern Eurasia / Siberia. Significant warming is progged to occur in the stratosphere over North America and Greenland. EP flux is poleward over the next several days, implicating more effective heat flux transfer.This dislodging of the stratospheric vortex should have implications for increased tropospheric perturbation of the polar vortex, which has generally been stable near the Davis Straits. Models are starting to detect the possibility of significant height rises in the NATL / Greenland and thus at least a temporary reversal of the NAO state in the medium term. The stratospheric displacement event will continue to keep the stratosphere warm and perturbed, and this substantially increases the probability of a more persistent negative northern annular mode / AO.

 

[2] Solar activity has declined significantly over the past 10 days, with lower levels of solar flux and geomagnetic activity. This in concert w/ the stratospheric progression argues for a deceleration of the jet in the north Atlantic.

 

[3] As noted in prior weeks, tropical forcing would become more conducive / Nino like by late January, and current projections indicate a maintenance of forcing near the dateline --> 170W. Recent MJO wave activity into phase 7 and the associated westerly wind burst / SOI crash should aid in the continuance of a Nino-like northern hemispheric state over the coming weeks, namely, a positive PNA / AK block and higher heights across the pole.

 

 

[4] As the AO and NAO modalities become more negative over the coming week, there should be more opportunities for wintry precipitations in the East. In fact, there will be an opportunity on Mon-Tues as the AO is still technically positive, but a ridge-bridge feature could be in place w/ higher heights across the north pole / northern Greenland. Next week, the AO and NAO states rapidly become more conducive as the NPAC regime maintains itself.

 

 

Everything remains on track for a cold and potentially active February.

 

[Note] : while early week 1/26-27 offers an opportunity for Northeastern snowfall, there may be a more significant threat around the beginning of February. The pattern should be more favorable than anything we've seen so far this winter. The first image is the GFS ensemble AO forecast, depicting the negative trend resuming in a few days. The second image is the ECMWF ensemble mean for February 1st, indicating a +PNA / -EPO / -AO / -NAO regime. I expect the -NAO could be transient, but the +PNA / -EPO and -AO should be more "stable" features for February.

 

 

 

 

 

 

 

 

Update on medium to long range pattern:

 

Sensible weather over the past couple weeks has progressed as anticipated with significant cold over the Eastern CONUS and a rapid uptick in snowfall totals from approximately 40N latitude northward in the Northeast. The 1-26/27 event was a historic blizzard from parts of Long Island into Eastern New England, and the 2/2 event produced significant to major snowfall from the NYC area through New England. The two week period proved to be the snowiest in Boston’s history, and much of Southern / Central New England has reached or exceeded their seasonal normal. The NYC region has received moderate to significant snowfalls over the past two weeks, and snowfall is now 20-30” for the season, on pace for a normal to above normal winter snowfall wise. Those south of 40N haven’t been as lucky; however, due to the lack of strong –NAO/AO blocking.

 

 

The theme of the winter to date, hemispherically speaking, has been the potent –EPO / +PNA couplet. Strong Pacific/NPAC/AK blocking has promoted cross polar flow and very cold weather for the Eastern US since early January, and this shows no indications of changing. It’s been a continuation of last winter’s robust –EPO, except with more +PNA forcing this season due to the positive PDO and El Nino influence on the atmosphere.

 

 

 

The state of the AO has been stubbornly positive overall, with periodic transient bursts. The January minor SSW event and further perturbations with the recent displacement have aided in promoting temporary –AO dips, but there has not been sufficient wave driving to induce a more permanent polar vortex displacement or splitting event. And thus, the AO / NAO modalities have maintained the mean positive state. This is in stark contrast to the autumn indicators for the AO, such as the SAI and OPI, which suggested a negative AO average for the DJF period. Clearly, that did not verify, and a re-examination of methodologies utilized to forecast the long term AO state must be conducted over the coming months (in order to improve next year’s idea). I have a couple thoughts concerning why the stratospheric vortex was stronger than anticipated this winter, and have alluded to these before, but that is the subject of another discussion.

 

 

Going forward:

 

 

[1] Transient –AO dips should occur in an overall neutral to slightly positive AO modality over the coming weeks. Stratospheric vortex will be disturbed with poleward EP flux, but not enough to truly do damage in terms of splitting or significant displacement. As a result, the tropospheric reflection should maintain the low-height signal at 500mb near the Arctic.

 

 

[2] The NAO state has been more variable over the past 10 days due to improved tropical forcing and some stratospheric warming over North America / Greenland, but again, the stratospheric changes haven’t been strong enough to destroy or displace the tropospheric vortex near the Davis Straights. Eastward propagation of tropical convection in the Pacific, and as associated MJO wave into phases 7-8 should aid in producing transient east based –NAO blocks in the coming week to 10 days. We’ve already seen some east based ridging aid in the Northeast winter storms, and another such block will be present for the February 9^th Northeast winter weather threat.

 

 

Strong tropical convection now near the dateline.

 

 

 

[3] Reloading of the mean -EPO / +PNA will continue over the coming weeks, and remain the primary driver of much below normal temperatures in the Eastern US through the rest of February, and potentially into the first half of March. The January-February period will finish much colder than normal across the Northeast, and the DJF mean should reflect a -AO pattern even though one wasn't present this winter (cold East / warm West).   

 

 

In terms of upcoming winter storm threats:

 

1. Early week in the Northeast / New England for overrunning, then possible weak coastal low.

 

In the medium range, models are suggesting a very amplified +PNA will develop and eventually retrograde westward to near the West Coast, in concert w/ the Alaskan blocking. Simultaneously, there may be some east based -NAO blocking in the Feb 14th-18th period as the MJO phase 7-8 suggests. Initially the trough looks quite broad and poorly placed for significant snow, and thus, the Feb 10th-15th period might be very cold / dry in the Northeast. Trough axis could be unfavorable for cyclogenesis.

 

There could be a cut-off low stationary near the SW US in the medium range, and when / if that feature ejects eastward, could offer a more significant / widespread Eastern US winter storm threat due to the antecedent arctic air present. The time frame to watch for that cut-off ejection would probably be later in President's Day weekend, toward the 16th-19th. Once the MJO reaches phase 8, that could support the release of the cut-off. This is still a long way off, but the potential appears heightened for snow threats through the rest of this month in the Northeast, particularly north of 40N in the early to middle part of the period (Feb 8th-16th).

 

Overall, a very cold, and potentially active pattern seems to be on tap for the East.

 

A snapsot of the upcoming pattern from the GFS ensembles. Notice the +PNA/-EPO, largely +AO, and potentially east based -NAO blocking. The key to the winter storm threat in the 16th-19th period will be the ejection of the cut-off low in the SW US.

 

[donsutherland1]

Morning thoughts...

 

1. The AO is now rising briskly. This morning, it stood at +1.199. If historic experience is relevant, February will likely finish with a positive AO average.

 

As noted earlier in this thread, there was a clear signal that differentiated the negative and positive February cases for winters during which the first half had witnessed a positive AO. If the AO fell to -2.000 or below on at least one day during the January 10-February 10 timeframe, 80% of February cases had a negative AO average. If, on the other hand, the AO did not reach -2.000 or below during that same timeframe, 89% of February cases had a positive AO average and the risk of a +1.000 or above February average was more than twice the 1950-2014 baseline.

 

The December-February AO average is very likely to finish positive.

 

2. The EPO remains forecast to be negative through much of the next two weeks. So, North America will likely continue to be supplied with cold that will dump into the eastern third or half of the continent as the PNA remains predominantly positive. That outcome suggests a lower than normal probability of significant snowstorms in parts of the Mid-Atlantic region. For example, 61% of Washington DC's 6" or greater February snowstorms occurred when the AO was negative and 100% of its 10" or greater February snowstorms occurred when the AO was negative. Sample sizes were 18 and 8 respectively, so there is some greater latitude for a larger snowstorm than the probabilities show, but the overall message of a lower than normal probability likely holds.

 

3. In the nearer-term, parts of the West, especially the Southwestern portion of the U.S. will experience very warm readings for the next few days, even as New England experiences another significant snowfall. The dramatic turnaround in snowfall in parts of New England is unprecedented since records have been kept. For example, on January 20, Boston had only 5.5" snowfall for the winter. Winter 2014-15 was only the 15th such winter to have less than 6" snowfall by January 20. Until 2014-15, the snowiest winter from that pool was winter 1968-69, which saw Boston record seasonal snowfall of 53.8". Through 8 am this morning, Boston's seasonal snowfall had rebounded to 57.9" and could be well over 60" by the time the current storm departs. Later in the week, frigid air could push into the Northeast with perhaps the coldest readings of the winter or possibly in several winters. That Arctic push could also be preceded by at least some snowfall.

[SnoSki14]

So how come people are suggesting stormy conditions are possible if the AO stays positive. Wouldn't we just see cold and dry or cold to warm and wet in northern mid Atlantic.

[donsutherland1]

So how come people are suggesting stormy conditions are possible if the AO stays positive. Wouldn't we just see cold and dry or cold to warm and wet in northern mid Atlantic.

Storminess is possible, but the kind of tracks and less progressive pattern that give the Mid-Atlantic (DCA, BWI, PHL, NYC) its biggest snowstorms will be more difficult to come by. New England can still do better.

[Isotherm]

So how come people are suggesting stormy conditions are possible if the AO stays positive. Wouldn't we just see cold and dry or cold to warm and wet in northern mid Atlantic.

 

The key for those further south is getting a wave to eject from the southern stream. February 2003 had a +AO/+NAO/+PNA pattern overall. Significant snows can occur in this regime, but it's tough to accomplish with Miller B's at our longitude without downstream blocking. However, Miller A's would work given the amount of cold air available. Often times these features aren't detected until < 5-6 days, and the MJO currently in phase 8 tends to increase the probability of something ejecting, so we'll see. It's a possibility. Both significant February events in 2003 were Miller A's (Feb 7th and 17th). We haven't had much of those this winter, and that is connected to the weaker Nino and poor tropical forcing. However, the MJO now in phase 8 for the first time this season lends some credence to the idea of a potential sern stream wave.

[OHweather]

GFS MEX is giving a low of 3F for KNYC Sunday night. Will the long above 0 streak be broken? It's hard to get too excited, but the cross-polar flow modeled on the ECM ensembles for this weekend looks absolutely classic. With a deep snow pack over Upstate NY and New England upstream of NYC, and the core of the coldest air at 850mb looking to move over NY state Sunday, is getting below 0 remotely possible there? Still lots of time for it to change or modify just a few degrees I guess.