Winter 2015-2016 Medium-Term Discussion

[donsutherland1]

The latest guidance continues to forecast blocking through mid-month.

 

 

At the same time, the PNA is forecast to remain positive to strongly positive through that timeframe. As a result, western North America could experience a warming trend toward mid-month, while a more sustained period of cold develops in the Plains States and eastward near mid-month +/- a few days. There is also some support for a more active subtropical jet as mid-month approaches, so that could create snowfall opportunities for parts of the Plains States, Midwest and eventually eastward to the Middle Atlantic and New England regions. The latter areas will probably have their highest chance of wintry precipitation closer to mid-month than earlier, as the storm track may be inland prior to then as the trough begins to develop in the East.

 

Right now, a transitional pattern that favors cold in the western half of North America and alternating colder and milder weather in the East is underway.

[donsutherland1]

This morning the Arctic Oscillation was -2.299. This is the lowest figure so far this meteorological winter. To date, the AO has averaged +1.063, however with blocking forecast to deepen in coming days and remain sustained through at least mid-month, that positive anomaly should shrink.

 

Even as the current block is only in its 4th day, the AO-/PNA+ combination led to a discharge of Arctic air from northern Canada into the eastern United States. This morning's low temperatures included:

 

Albany: 2°

Boston: 8°

Burlington: 2°

Caribou: -9°

Detroit: 12°

Montreal: -6° (-21°C)

New York City: 11°

Philadelphia: 12°

Richmond: 16°

Toronto: 5° (-15°C)

Washington, DC: 16°

 

Although this cold shot will prove transient as warmer air sweeps back into the East, it was sufficient to send monthly anomalies below normal in such cities as New York for the first time since early October.

 

Should the blocking be sustained as is forecast, a more sustained period of cold will likely set in from the Plains States eastward near mid-month give or take a few days. The trimmed pool of objective analogs from the GFS ensembles suggest that odds favor at least the 15th through the 20th as being cooler than normal in the eastern third of the CONUS.

 

Toward mid-month, the PNA+ will likely result in the West's experiencing the development of a sustained period of above normal temperatures.

 

Beyond the 20th, there remains a lot of uncertainty. Some of the guidance looks to relax the blocking resulting in the development of a milder regime. However, even on the ensembles, forecasting skill that far out is fairly low, so it is not assured that blocking will relax. Should blocking persist, the cold could do so, as well.

 

It should be noted that major long wave patterns have proved fairly persistent since at least the fall. If that tendency holds up, the current AO-/PNA+ dual blocking regime could hold or any break in the blocking could be temporary. The 1966 and 1978 objective analogs that are in the mix provide some credence to that possible scenario. 

[Isotherm]

This morning the Arctic Oscillation was -2.299. This is the lowest figure so far this meteorological winter. To date, the AO has averaged +1.063, however with blocking forecast to deepen in coming days and remain sustained through at least mid-month, that positive anomaly should shrink.

 

 

Don - where did you find the to date AO value (or was it your manual calculation)? Is there a site that maintains daily AO values?

 

I'm aware of this:

 

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii

 

But it only updates the previous month at the end of the current month, and doesn't provide dailies into January.

[griteater]

Don - where did you find the to date AO value (or was it your manual calculation)? Is there a site that maintains daily AO values?

 

I'm aware of this:

 

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii

 

But it only updates the previous month at the end of the current month, and doesn't provide dailies into January.

 

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/daily.index.ascii

[donsutherland1]

Don - where did you find the to date AO value (or was it your manual calculation)? Is there a site that maintains daily AO values?

 

I'm aware of this:

 

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii

 

But it only updates the previous month at the end of the current month, and doesn't provide dailies into January.

Isotherm,

 

The data is here: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/daily.index.ascii

 

It's preliminary and small adjustments are typically made when the final figures are posted.

[donsutherland1]

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/daily.index.ascii

Thanks. You beat me to it. These figures are preliminary and are typically revised somewhat.

[Isotherm]

Thanks Don and grit!

[donsutherland1]

Extreme Blocking Forecast...

 

To date, the Arctic Oscillation (AO) has averaged +0.973 during the 2015-16 meteorological winter. 76% of days have seen the AO positive and 59% of days have seen the AO at or above +1.000. On 8% of days, the AO has been +3.000 or above. Its highest value has been +4.503, which was reached on December 22, 2015. The AO has been negative on 24% of days and at -1.000 or below on 16% of days. Its lowest value so far is -2.299, which was reached on January 5, 2016.

 

However, the latest GFS ensembles are forecast the AO to plunge. There is strong agreement among the ensemble members that the AO will fall to -5.000 or below.

 

Such extreme values are uncommon. However, such extreme values have become somewhat more common since 2000.

 

Such extreme blocking would suggest troughing in eastern North America. More importantly, should such extreme blocking develop, it would provide a strong signal of a blocky February. In short, even as the most recent December proved to be an autumn, not winter month, an extreme AO-, should it develop, could offer the strongest signal so far that there will be plenty of opportunity for winter weather from mid-January onward. North America and Europe would both see such opportunities if past extreme January blocking cases are representative.

 

 

 

[Isotherm]

Great stuff Don. We can also expand the list to include 1957-58, an el nino which featured a -4 SD AO daily value in January. February 1958 had a monthly AO of -2.2.

[donsutherland1]

A closer look at the February outcomes following January cases when the AO fell to -5.000 or below:

 

 

 

 

[donsutherland1]

Great stuff Don. We can also expand the list to include 1957-58, an el nino which featured a -4 SD AO daily value in January. February 1958 had a monthly AO of -2.2.

I agree. The development of extreme blocking in January is a good sign for February. If things work out, this may well wind up becoming a classic turnaround case following December's exceptional warmth.

[Cheeznado]

It is worth noting that the ECMWF ensembles for the AO are a lot more spread out than the GFS- they mostly stay negative but only a couple of members get down as low as -4. Just FYI.

[donsutherland1]

It is worth noting that the ECMWF ensembles for the AO are a lot more spread out than the GFS- they mostly stay negative but only a couple of members get down as low as -4. Just FYI.

This is important as it reinforces that the extreme blocking scenario is not yet assured. It will be interesting to see how things evolve in coming days.

[packbacker]

A closer look at the February outcomes following January cases when the AO fell to -5.000 or below:

 

 

 

 

 

Hey Don - can you list out the years, I can't read them since it so small.  As always, thanks for the writeup!

[donsutherland1]

Hey Don - can you list out the years, I can't read them since it so small.  As always, thanks for the writeup!

1963, 1966, 1977, 1985, and 2010.

[wxman84]

Just out of curiosity...

 

Does anyone know the record daily low for both the NAO and the AO and what those values would be.

 

Joe Lauria

[griteater]

Just out of curiosity...

 

Does anyone know the record daily low for both the NAO and the AO and what those values would be.

 

Joe Lauria

 

AO:

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.ao.index.b500101.current.ascii

-7.433 (Jan 15, 1977)

 

NAO: 

Couple different sources...

ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.nao.index.b500101.current.ascii

-3.254 (Oct 21, 2002) *For any month

-2.711 (Dec 1, 2008) *For winter months only

 

ftp://ftp.cdc.noaa.gov/Public/gbates/teleconn/pna.reanalysis.t10trunc.1948-present.txt

-5.643 (Jan 3, 2010)

[wxman84]

Thank you!

 

JL

[WEATHER53]

1963, 1966, 1977, 1985, and 2010.

s

Special years every one

[donsutherland1]

More thoughts on the January AO…

 

Yesterday, in message #158 (http://www.americanwx.com/bb/index.php/topic/47332-winter-2015-2016-medium-term-discussion/?p=3832891), I highlighted the GFS ensembles’ forecast of an extremely negative Arctic Oscillation (AO). There was widespread agreement that the AO would fall below -4.000 and perhaps -5.000 toward mid-January. The post included data concerning a recent increase in extreme cases since 2000 relative to the historic 1950-2015 period.

 

The updated GFS ensemble forecast is little changed from yesterday’s.

 

This post takes a closer look at January’s Arctic Oscillation. Since 1979, the 30-year moving average of the monthly standard deviation has increased nearly 8%. If one looks closer at the data, the January cases beginning in the early 2000s accounts for this increase in variability.

 

What this means is that the AO has recently become more variable in January. Some of that variability may be contributing to the increase in extreme daily values.

 

The variability is likely a function of stratospheric developments and sea-ice-atmosphere coupling, and related interactions. Since the 1970s, the RATPAC dataset has revealed a cooling trend at 30 mb. Stratospheric cooling should, in general, favor a stronger stratospheric polar vortex and perhaps greater stability in the AO. Some of this tendency toward higher AO values should be reflected in the AO index.

 

Looking at January, the AO was +3.000 or above on 4.6% of dates in the 1950-2015 period. However, since 2000, it has reached +3.000 or above on 6.5% of dates. On the negative side, the AO has been -3.000 or below on 10.3% of dates during the 1950-2015 period. However, since 2000, it has been at or below -3.000 on 7.1% of dates in January. The respective figures for +4.000 or above are 1.5% and 1.2%, while those for -4.000 or below are 3.4% and 1.6%. This data shows that a strongly positive AO has become more frequent in January, but that rise has not translated into an increased frequency of +4.000 or greater values. At the same time, there has been a decreased frequency of strong January blocking (especially in the frequency of AO values of -4.000 or below). These outcomes appear to be reasonably consistent with what one would expect from a cooling stratosphere.

 

But then, there’s a curveball of sorts. Since 2000, there have been no January cases on which the AO reached +5.000 or above vs. the 1.0% figure for the entire 1950-2015 period. However, the frequency of extreme negative cases (-5.000 or below) has increased from 0.8% of all cases during the 1950-2015 period to 1.0% of cases during the 2000-2015 period. Sample size (all of the 2000-2015 cases occurred in 2010) may provide the explanation, but a return of -5.000 or below values as suggested by the GFS ensembles would call into question the sample size argument.

 

Since the extreme negative values are inconsistent with stratospheric cooling, it is plausible that the sea-ice-atmospheric relationship may provide the stronger explanation for that possible outcome. There has been a decline in average January Arctic sea ice extent since the 1980s and that trend may be contributing both to increased AO volatility and the possible uptick in extreme negative cases in January.

 

January Arctic sea ice extent during the 1990s was 3% lower than that for the 1980s. During the 2000s (2000-2015), average sea ice extent was 7% lower than it was during the 1980s. The last 10 years (2006-15), saw January’s average sea ice extent come out 10% lower than the 1980s average. Furthermore, the January standard deviation in Arctic sea ice extent has also declined somewhat. In sum, January Arctic sea ice extent has declined and has become slightly more stable on a day-to-day basis at those reduced levels.

 

For now, it’s too soon to be sure what mechanisms are at play. However, it is plausible that the decline in average January Arctic sea ice extent is more than compensating for a cooling stratosphere in driving an increase in AO variability in January. The question concerning the increase in extreme negative AO cases is less clear.

 

 

[DTWXRISK]

100% GREAT POST 

 

Hey Don - can you list out the years, I can't read them since it so small.  As always, thanks for the writeup!

[DTWXRISK]

 but that is not  the case with todays run

sorry 

 

It is worth noting that the ECMWF ensembles for the AO are a lot more spread out than the GFS- they mostly stay negative but only a couple of members get down as low as -4. Just FYI.

[weatherdude]

A quick run of the daily NAO Index numbers from CPC shows a January record low value since 1950 of -2.25 on January 3rd of 2010. Seems that some of the model NAO forecasts, including those on the American Weather Models page and the NOAA/PSD, suggest the January record will fall early next week:

 

 

 

[donsutherland1]

Small changes in the Teleconnection Forecasts...

 

- There is larger agreement among the GFS ensemble members that the AO will fall to -5.000 to -4.000 toward mid-month, with somewhat fewer taking it below -5.000. The overall idea of the development of severe blocking remains unchanged. Today's AO figure was -2.758, which is the lowest it has been this winter.

 

- The NAO was slightly negative. It has been positive for 37 of the first 39 days of meteorological winter or 95% of the time. In contrast, the AO has been positive 72% of the time so far. The NAO is forecast to go strongly negative.

 

- The PNA remains positive. Consistent with a PDO+, the PNA+ has been the general rule this winter so far. The PNA has been positive for 82% of days and at or above +1.000 on 38% of the days. It is forecast to remain positive through the extended range, though one cluster of ensembles takes it to strongly positive levels and another cluster toward near neutral levels by the end of the ensemble forecast.

 

All said, given climatology, the forecast sustained AO-/PNA+ pattern should lead to an increase in snowfall opportunities in the eastern U.S., with the largest increase over the baseline being focused on the Middle Atlantic region.

[donsutherland1]

With the Arctic Oscillation's beginning its descent toward forecast values of -4.000 or below over the next 4-7 days, opportunities for measurable snowfall should increase over parts of North America, particularly the Middle Atlantic region. Since 1950, there have been 10 cases during which the AO first fell to -4.000 or below during the period from January 15 +/- 10 days. With two exceptions, Boston, Detroit, New York City, Philadelphia, and Washington, DC all saw at least some measurable snowfall from the date the AO first fell to -4.000 through the following 30 days.

 

Below is the latest AO forecast and statistics for those cases:

 

[40/70 Benchmark]

Small changes in the Teleconnection Forecasts...

 

- There is larger agreement among the GFS ensemble members that the AO will fall to -5.000 to -4.000 toward mid-month, with somewhat fewer taking it below -5.000. The overall idea of the development of severe blocking remains unchanged. Today's AO figure was -2.758, which is the lowest it has been this winter.

 

- The NAO was slightly negative. It has been positive for 37 of the first 39 days of meteorological winter or 95% of the time. In contrast, the AO has been positive 72% of the time so far. The NAO is forecast to go strongly negative.

 

- The PNA remains positive. Consistent with a PDO+, the PNA+ has been the general rule this winter so far. The PNA has been positive for 82% of days and at or above +1.000 on 38% of the days. It is forecast to remain positive through the extended range, though one cluster of ensembles takes it to strongly positive levels and another cluster toward near neutral levels by the end of the ensemble forecast.

 

All said, given climatology, the forecast sustained AO-/PNA+ pattern should lead to an increase in snowfall opportunities in the eastern U.S., with the largest increase over the baseline being focused on the Middle Atlantic region.

Absolutely agree.

[griteater]

This may help a bit with the Pacific side pattern through the rest of winter.  Chart based on http://www.met.igp.gob.pe/datos/EC.txt

 

 

 

 

-OLR Anomalies for the period Nov 15 to Jan 5 have been centered a bit west of 82-83 and 97-98

 

 

[40/70 Benchmark]

Thanks.

Where do you make that top image?

[griteater]

Thanks.

Where do you make that top image?

No problem.  It's just an Excel sheet and chart that I've been maintaining based on the data in the link provided

[griteater]

No problem.  It's just an Excel sheet and chart that I've been maintaining based on the data in the link provided

^ Oh, I copied the chart into a program called SnagIt where I added the colors, dashed lines, and other labels if that's what you're asking