December 2017 Discussions & Observations Thread

[Paragon]

14 hours ago, bluewave said:

It will come down the the post Christmas Arctic cold to tip the monthly temperature departures back to below normal. Temps through the 19th are close to normal for most of the area. Just goes to show how easy it is to get to above normal snowfall here with temps breaking even through the 19th.

NYC...-0.4...LGA...+0.1...JFK...-0.9...ISP...0.0...BDR...+0.2...EWR...-0.2

But Chris do you think the final result will be +/- 1 degree? If so, we can just say we had a near normal month.

[Paragon]

12 hours ago, donsutherland1 said:

The large-scale pattern evolution described on Monday (https://www.americanwx.com/bb/topic/50504-december-2017-discussions-observations-thread/?do=findComment&comment=4707215) has changed little since that time. In general, a powerful EPO block is expected to develop. That block will then expand into the AO region, triggering the development of a negative AO (for which support is now growing on the GEFS). The AO is currently at a preliminary December 2017 high of +2.700. The PNA is likely to remain neutral (somewhat positive or negative). Its current preliminary value is +0.301 and it has been positive through December so far. What has changed is that the guidance has moved back to the somewhat earlier passage of the cold front to on or just before the 25th. Further, the guidance is starting to become more favorable toward a renewed PNA+ in the extended range. In January an AO-/PNA+ combination is associated with a higher frequency of snowfall, including significant snowfall (6” or more) in the Middle Atlantic and southern New England areas.

Applying sensitivity analysis to the latest modeled data, the probability of a colder than normal December has increased. This is the result of the guidance moving into agreement concerning a post-25th Arctic outbreak.

Monthly Mean Temperature for NYC Through:
12/19 38.9°
12/20 39.0°-39.2° (12/18 estimate: 38.4°-39.0°)
12/25 38.5°-40.0° (12/18 estimate: 38.6°-40.2°)
12/31 35.1°-37.5° (12/18 estimate: 35.9°-38.4°)

Estimated probability of a below normal monthly anomaly: 63%

The latest data has reinforced my confidence that December has not yet seen its coldest readings in such cities as Bridgeport, New York, and Newark (lowest temperatures respectively 16°, 20°, and 17°). I also expect additional snowfall at all three locations before the month has ended. It remains possible that all three cities will wind up with monthly snowfall at or above 10”. Right now, I believe the relatively snowless 6z run of the GFS is an outlier given the forecast pattern. I believe the EPS and ECMWF have a better handle on the long-range.

Further, my case preference for this winter has been and remains 2000-01 (with a possibility of a 2010-11 outcome in terms of snowfall). Late December 2000 featured cold with a sizable snowfall in parts of the Middle Atlantic region on December 30-31. While a big snowfall rivaling that outcome isn’t necessarily on the table, the idea of at least a moderate snowfall most definitely is. Currently, the 0z ECMWF shows a potential storm that could deliver such a snowfall near the end of December followed by a fresh surge of Arctic air.

For the winter as a whole, the Middle Atlantic and southern New England regions still remain on course for above to possibly much above average snowfall.

Don, I wonder if the snowfall before New Years will be the big one in the pattern or that will be reserved for just before the pattern breaks, like what we got in Jan 1996.

[CIK62]

Next 8 days to average 38degs., or about 2degs.  AN.  This is higher than I was expecting.

The month is exactly NORMAL as of today.

[bluewave]

10 hours ago, Paragon said:

But Chris do you think the final result will be +/- 1 degree? If so, we can just say we had a near normal month.

The Arctic cold coming during the last week of December should tip us back to cold for the month. Also notice how we get these multiple 50 degree  or higher days around the solstice occur ever December recently. The last  times this didn't happen was 2009 and 2010.

 

[Paragon]

8 minutes ago, bluewave said:

The Arctic cold coming during the last week of December should tip us back to cold for the month. Also notice how these 50 degree days around the solstice occur ever December recently. The last  times this didn't happen was 2009 and 2010.

 

I was wondering about that.  I was wondering if that's a coincidence or there is something deeper going on there.  There are certain dates when historic snowstorms seem to be more common than other days too- the same dates seem to have them repeatedly.

 

 

[Brian5671]

Interesting-I was thinking the same thing- Dec 20-25th or so have been quite warm the last 5-6 years....we've snowed a few times mid Dec but it doesn't have a chance of sticking around that incoming warmth

[Paragon]

5 minutes ago, Brian5671 said:

Interesting-I was thinking the same thing- Dec 20-25th or so have been quite warm the last 5-6 years....we've snowed a few times mid Dec but it doesn't have a chance of sticking around that incoming warmth

Snow seems to occur in certain patterns, same as these temp spikes/dips. Dec 1-5 (mild) Dec 6-19 (snow)  Dec 20-25 (mild) Dec 26-31 (cold/snow)  Jan 1-10 (cold/snow) Jan 11-19 (thaw) Jan 20-31 (cold/snow)

 

[bluewave]

37 minutes ago, Paragon said:

I was wondering about that.  I was wondering if that's a coincidence or there is something deeper going on there.  There are certain dates when historic snowstorms seem to be more common than other days too- the same dates seem to have them repeatedly.

 

 

The number of 50 degree days during this period around the solstice really increased after 1995.

12/18-12/25 50 degree days NYC

1996-2017...52 days so far

1974-1995...34 days

1952-1973...27 days

1930-1951...31 days

[Paragon]

2 hours ago, bluewave said:

The number of 50 degree days during this period around the solstice really increased after 1995.

12/18-12/25 50 degree days NYC

1996-2017...52 days so far

1974-1995...34 days

1952-1973...27 days

1930-1951...31 days

hmmm we were around 30 days for a long time.

I wonder why this specific period though.

I see you used an 8 day cross section, Chris, can you look at the preceding 8 day period those years and the 8 day period immediately afterwards?

so 12/10-12/17 and 12/26-1/2

 

Thanks!

 

[forkyfork]

the eps has had a cold bias. here's a forecast from earlier this month for philly compared with actual high temps, taken from another board

[bluewave]

1 hour ago, forkyfork said:

the eps has had a cold bias. here's a forecast from earlier this month for philly compared with actual high temps, taken from another board

 

It would be nice if we could compare it to the ECMWF MOS.

MDL's ECMWF MOS

 

MDL's SMB applies the MOS technique to output from the European Centre for Medium-Range Weather Forecasting's (ECMWF) models. This statistically calibrated objective guidance includes a wide variety of weather elements and time projections and is provided within NOAA.

[bluewave]

2 hours ago, Paragon said:

hmmm we were around 30 days for a long time.

I wonder why this specific period though.

I see you used an 8 day cross section, Chris, can you look at the preceding 8 day period those years and the 8 day period immediately afterwards?

so 12/10-12/17 and 12/26-1/2

 

Thanks!

 

The 8 day period before this had a much more gradual climb instead of the big jump after 1995. Uneven warming FTW.

12/10-12/17

1996-2017....49 days

1974-1995....41 days

1952-1973....35 days

1930-1951...25 days

[PB COLTS NECK NJ]

EPS 

 

 

 

GEFS

 

 

[Paragon]

8 hours ago, bluewave said:

The 8 day period before this had a much more gradual climb instead of the big jump after 1995. Uneven warming FTW.

12/10-12/17

1996-2017....49 days

1974-1995....41 days

1952-1973....35 days

1930-1951...25 days

hm that is interesting the climb is undeniable and this is linked to high temps not mins so it's not urban heat island for the most part.

[CIK62]

Next 8 days averaging 34degs., just NORMAL or slightly BN.

[CIK62]

The sub-zero readings look possible Jan. 02-4.  BallDrop Temp. looks to be in the upper teens or worse and should do it for Dec.

Broad snow cover would aide in the sub-zero conspriacy.

[North and West]

I am thinking that this may be the first chance for NYC to see single digit or lower  temperatures in December since the 1980's. This potential would match the series of extremes that we have been experiencing during this fall and winter and the 2010's in general. The record Pacific blocking produced the first record low temperatures in NYC for November since the 1970's. This was following the record warmth experienced from mid-September into October. The first below 0 in NYC since 1994 occurred in Feb 16 following the extreme record warmth. So it's not much of a stretch to believe that this mild period around the solstice could be followed by single digits or lower in NYC near the end of the month. If the evolution is a little slower, then the coldest temps would be bumped forward a few days to the very start of January.

Central Park hasn’t seen a sub-10F temperature in December in 28 years? That’s crazy if correct. Also shows the enormous difference between CPK and places where we are (MMU, for instance, for me) for clear morning lows.


.

[doncat]

11 minutes ago, bluewave said:

These are the last years that the climate sites around NYC dropped below 10 degrees in December:

NYC...1989

LGA...1989

EWR...1989

JFK...1989

ISP...2004

BDR...2004

HPN...2005

 

Yeah I hadn't realized, it's been that long...2004 at my station.

[donsutherland1]

The synoptic pattern continues to evolve toward an EPO-/AO-/PNA+ set up. Consistent with the idea that the EPO block will expand into the AO region triggering the development of an AO-, the GEFS has finally come into agreement that the AO will go negative shortly after 12/25. This outcome would increase prospects for a moderate or greater snowstorm relative to climatology for the close of December.

One variable that has not yet changed is the SOI. The SOI had previously been forecast to go positive near 12/25. Instead, the trade winds have recently relaxed with the SOI becoming more negative. This development suggests that the arrival of Arctic air in the East could be delayed. So, rather than its arriving near or after 12/25, it would likely require a more significant event to drive it into the East. A likely candidate would be any storm near the close of the month, with the last day or two of December winding up much colder than normal in the wake of the storm.
Applying sensitivity analysis to the latest modeled data, the probability of a colder than normal December has declined just a little, but it remains the favored outcome. The small reduction in the probability of a colder than normal December likely reflects the somewhat delayed arrival of Arctic air.

Monthly Mean Temperature for NYC Through:
12/21 38.9°
12/25 38.8°-39.6° (12/20 estimate: 38.5°-40.0°)
12/31 35.4°-37.5° (12/20 estimate: 35.1°-37.5°)

Estimated probability of a below normal monthly anomaly: 59% (12/20 estimate: 63%)
At this point, I have no change in my thinking that Bridgeport, New York, and Newark have yet to see their coldest temperatures this month, as I still expect Arctic air to arrive during the last 1-3 days of December. Their lowest temperatures are respectively 16°, 20°, and 17°.

My confidence that all three locations should see additional accumulating snowfall before the month ends has increased further. It is becoming increasingly likely that all three cities will wind up with monthly snowfall at or above 10”.  The potential for a 6” or greater snowfall in the Middle Atlantic and southern New England regions has increased. Consistent with the forecast pattern, the 12/22 0z run of the ECMWF shows a storm that could bring parts of the region 10” or more of snow. Such an outcome would strongly reinforce the idea that much of the Middle Atlantic to southern New England regions (including interior sections) are likely to see above to much above normal snowfall this winter.

Earlier in the month, a storm brought 4” or more snow to Philadelphia, New York, and Boston. Total snowfall of 8” or more in all three cities during December has also been associated with above to much above average snowfall in the greater Middle Atlantic to southern New England region. However, there have been a number of cases where the above normal snowfall was suppressed south of Boston, though such suppression appeared to be somewhat more likely during El Niño winters.

Finally, any moderate or significant snowstorm near the end of this month is likely to usher in a genuine Arctic air mass. As a result, the potential for severe cold exists for the opening days of 2018.

[ForestHillWx]

I was in school in Albany from 2001-2008. Valentine's Day (2007?)was phenomenal from a snow prospective. 

 

I would have thought Albany would have been -10. Coldest I experienced was -19 at Loon Lake, Chestertown. Outstanding country up that way. 

[North and West]

The synoptic pattern continues to evolve toward an EPO-/AO-/PNA+ set up. Consistent with the idea that the EPO block will expand into the AO region triggering the development of an AO-, the GEFS has finally come into agreement that the AO will go negative shortly after 12/25. This outcome would increase prospects for a moderate or greater snowstorm relative to climatology for the close of December.
One variable that has not yet changed is the SOI. The SOI had previously been forecast to go positive near 12/25. Instead, the trade winds have recently relaxed with the SOI becoming more negative. This development suggests that the arrival of Arctic air in the East could be delayed. So, rather than its arriving near or after 12/25, it would likely require a more significant event to drive it into the East. A likely candidate would be any storm near the close of the month, with the last day or two of December winding up much colder than normal in the wake of the storm.
Applying sensitivity analysis to the latest modeled data, the probability of a colder than normal December has declined just a little, but it remains the favored outcome. The small reduction in the probability of a colder than normal December likely reflects the somewhat delayed arrival of Arctic air.
Monthly Mean Temperature for NYC Through:
12/21 38.9°
12/25 38.8°-39.6° (12/20 estimate: 38.5°-40.0°)
12/31 35.4°-37.5° (12/20 estimate: 35.1°-37.5°)
Estimated probability of a below normal monthly anomaly: 59% (12/20 estimate: 63%)
At this point, I have no change in my thinking that Bridgeport, New York, and Newark have yet to see their coldest temperatures this month, as I still expect Arctic air to arrive during the last 1-3 days of December. Their lowest temperatures are respectively 16°, 20°, and 17°.
My confidence that all three locations should see additional accumulating snowfall before the month ends has increased further. It is becoming increasingly likely that all three cities will wind up with monthly snowfall at or above 10”.  The potential for a 6” or greater snowfall in the Middle Atlantic and southern New England regions has increased. Consistent with the forecast pattern, the 12/22 0z run of the ECMWF shows a storm that could bring parts of the region 10” or more of snow. Such an outcome would strongly reinforce the idea that much of the Middle Atlantic to southern New England regions (including interior sections) are likely to see above to much above normal snowfall this winter.
Earlier in the month, a storm brought 4” or more snow to Philadelphia, New York, and Boston. Total snowfall of 8” or more in all three cities during December has also been associated with above to much above average snowfall in the greater Middle Atlantic to southern New England region. However, there have been a number of cases where the above normal snowfall was suppressed south of Boston, though such suppression appeared to be somewhat more likely during El Niño winters.
Finally, any moderate or significant snowstorm near the end of this month is likely to usher in a genuine Arctic air mass. As a result, the potential for severe cold exists for the opening days of 2018.

Always nice to learn. Thanks!


.

[snowman19]

By mid January, the new weeklies are showing a complete pattern breakdown/reversal to a very mild zonal regime. You loose the -WPO and -EPO, the PNA goes deeply negative and you have a +NAO and +AO, which is nothing new for winter anymore and classic La Niña tropical convective forcing moves to the maritime continent. Also during this time the stratospheric polar vortex strengthens considerably and consolidates over the pole and the polar stratosphere gets very cold. All this does not look like a few days of change either. Big question is does the cold pattern come back or is that all she wrote? You got your front end loaded winter? Keep in mind we have seen this happen before in January during La Nina’s, 89-90 and 10-11, both classic front end loads

[donsutherland1]

At 4 pm, the NYC temperature had reached 49°. That brings the monthly average up to 39.0°, which is 0.2° above normal through the 22nd. Given the latest guidance, the month still appears likely (`60% probability) to finish below normal.

[dmillz25]

3 hours ago, snowman19 said:

By mid January, the new weeklies are showing a complete pattern breakdown/reversal to a very mild zonal regime. You loose the -WPO and -EPO, the PNA goes deeply negative and you have a +NAO and +AO, which is nothing new for winter anymore and classic La Niña tropical convective forcing moves to the maritime continent. Also during this time the stratospheric polar vortex strengthens considerably and consolidates over the pole and the polar stratosphere gets very cold. All this does not look like a few days of change either. Big question is does the cold pattern come back or is that all she wrote? You got your front end loaded winter? Keep in mind we have seen this happen before in January during La Nina’s, 89-90 and 10-11, both classic front end loads

Remember last winter. Snowed even in a torch

[Paragon]

4 hours ago, snowman19 said:

By mid January, the new weeklies are showing a complete pattern breakdown/reversal to a very mild zonal regime. You loose the -WPO and -EPO, the PNA goes deeply negative and you have a +NAO and +AO, which is nothing new for winter anymore and classic La Niña tropical convective forcing moves to the maritime continent. Also during this time the stratospheric polar vortex strengthens considerably and consolidates over the pole and the polar stratosphere gets very cold. All this does not look like a few days of change either. Big question is does the cold pattern come back or is that all she wrote? You got your front end loaded winter? Keep in mind we have seen this happen before in January during La Nina’s, 89-90 and 10-11, both classic front end loads

Pattern changed for 3 weeks in 1995-96

Also need to look into 2000-01 since the storm next week could be very similar.

[donsutherland1]

4 hours ago, snowman19 said:

By mid January, the new weeklies are showing a complete pattern breakdown/reversal to a very mild zonal regime. You loose the -WPO and -EPO, the PNA goes deeply negative and you have a +NAO and +AO, which is nothing new for winter anymore and classic La Niña tropical convective forcing moves to the maritime continent. Also during this time the stratospheric polar vortex strengthens considerably and consolidates over the pole and the polar stratosphere gets very cold. All this does not look like a few days of change either. Big question is does the cold pattern come back or is that all she wrote? You got your front end loaded winter? Keep in mind we have seen this happen before in January during La Nina’s, 89-90 and 10-11, both classic front end loads

Using the 1981-2010 base period, there were five cases where the EPO fell to or below -3.000 sigma (12/1-1/15 period) as previously shown on the EPS. Although all cases saw the EPO reach somewhat positive values within 6-11 days after it bottomed out, it took three weeks or longer in 80% of the cases for the EPO to reach +2.000 sigma or above (60% 30 days or longer). In one case, it remained below +2.000 sigma throughout the rest of meteorological winter.

The EPO went negative on December 19. So, a return to positive values would likely occur sometime between 12/25 and 12/30. This period would likely not see any values of +2.000 sigma or above. Any such large positive departures would probably not occur until January 9-18.

More than likely, that pattern relaxation would be temporary. It would not mark a premature end of winter.

The latest EPO forecast:

[White Gorilla]

2 minutes ago, donsutherland1 said:

Using the 1981-2010 base period, there were five cases where the EPO fell to or below -3.000 sigma (12/1-1/15 period). Although all cases saw the EPO reach somewhat positive values within 6-11 days after it bottomed out, it took three weeks or longer in 80% of the cases for the EPO to reach +2.000 sigma or above (60% 30 days or longer). In one case, it remained below +2.000 sigma throughout the rest of meteorological winter.

The EPO went negative on December 19. So, a return to positive values would likely occur sometime between 12/25 and 12/30. This period would likely not see any values of +2.000 sigma or above. Any such large positive departures would probably not occur until January 9-18.

More than likely, that pattern relaxation would be temporary. It would not mark a premature end of winter.

The latest EPO forecast:

Interesting that models are showing a snowstorm when the EPO goes positive. 

[donsutherland1]

1 minute ago, White Gorilla said:

Interesting that models are showing a snowstorm when the EPO goes positive. 

The forecast AO-/PNA+ for the time of the potential snowstorm has a stronger correlation with such events than the EPO.

[White Gorilla]

9 minutes ago, donsutherland1 said:

The forecast AO-/PNA+ for the time of the potential snowstorm has a stronger correlation with such events than the EPO.

Ah, yes, thanks for reminding... 

[donsutherland1]

1 hour ago, White Gorilla said:

Ah, yes, thanks for reminding... 

Since 1950, there were 9 snowstorms that brought 6" or more snow to NYC during the 12/16-31 period. 6/9 (67%) had an AO-; 7/9 (78%) had a PNA+; and, 7/9 (78%) had an EPO+. Of the subset that saw 4" or more fall in Philadelphia, New York, and Boston, 4/5 (80%) had an AO-, PNA+, EPO+. The most common pattern, both for the larger set and the smaller subset was an AO-/PNA+ (56% of the cases in the larger set and 60% in the smaller subset).