Network News

X My Profile
View More Activity
The new Washington
Post Weather website
Jump to CWG's
Latest Full Forecast
Outside now? Radar, temps
and more: Weather Wall
Follow us on Twitter (@capitalweather) and become a fan on Facebook
Posted at 1:10 PM ET, 02/16/2011

Bad pattern for snow through end of February

By Wes Junker

But need to watch possible system early next week

Yesterday Ian wrote an article summarizing snow chances moving forward based on the climate record . Today, I'll look at the forecast pattern and how it bodes for our snow chances during the next couple of weeks.

In summary, the pattern is not one that is particularly favorable for snow. But while the pattern is not that good overall, it still could produce surprises - with a system to monitor early next week.

An illustration of the negative Pacific North American (PNA) pattern, also known as rPNA.

We are currently stuck in the reverse Pacific North American Pattern (RPNA, more information on the PNA and RPNA) as illustrated to the right. Such a pattern is composed of a ridge in the jet stream off the west coast (high pressure at the surface), a trough (a southward dip in the jet stream) and cold weather over the West and a ridge and warmth over the Southeast.

The combination of a western trough and southeastern ridge helps direct storms that form just east of the Rocky Mountains northeastward towards the Great Lakes or Ohio Valley regions. Such a track generally keeps the D.C. area on the warm side of storms as the counterclockwise flow around the storms cause winds to have a southerly component across the region.

Also, as storms approach from the west, the ridge gets suppressed and nudged back to the south as the low passes which can contribute to strong southwesterly or westerly winds developing as a squeeze play happens between the high pressure system to the south and the low to our north. That's what happened on Monday.


The observed (since mid-October) forecast (out to two weeks) PNA pattern. Source: NOAA

As shown above, the GFS model ensembles (in red) are uniform in predicting a reverse (negative) PNA pattern through the end of February. Recently these forecasts have been pretty accurate through day 10 with the skill of the forecasts decreasing rapidly by day 14. The strongly negative values being forecast suggest that the pattern in the Pacific will probably hold through the month. If so, look for more storms to track towards the Ohio Valley and for storms to reform off the New Jersey coast rather than off the Virginia Capes or Hatteras. The storm track is not a good one for producing snow storms across our area without help from a favorable phase of the Arctic or North Atlantic Oscillations (AO and NAO)

Earlier this year I discussed how the phase of the AO could modulate temperatures during a La Nina. Essentially, when the AO and NAO are positive during a La Nina, the temperatures across our region end up being above normal. However, when index drops into the negative range, our temperatures generally swing back and forth between being above and below normal and end up averaging around normal. Only when the NAO and AO are strongly negative like they were earlier this winter, do our temperatures average below normal.

This week's warmth is therefore not surprising. The AO and NAO have been positive with the rPNA pattern holding firm. Such a pattern can lead to temperatures torching like they did on Monday and might again on Friday.

So what happens moving forward?

Unfortunately, the models are much less certain in their prognostications of the AO and NAO in the longer time range. Some keep the NAO positive while some edge it back into the negative range. However, none of the negative members drop it very far from neutral. The uncertainly in the forecasts of the NAO makes 8-10 day temperature forecasts iffy and probably is the reason why CPC temperature forecasts are for us to average around normal in the 6-10 and 8-14 day ranges.

My own guess is that there will be more warmer than normal days than colder than normal ones through the remainder of the month but that's a low confidence statement. I also wouldn't be surprised by additional days that flirt with 70 degrees not including Friday's warm day. The pattern also suggests a below normal probability of snow through the remainder of the month.

While the pattern is not that good for snow overall, it still could produce surprises.


Last night's European model forecast offers hints at how. The forecast valid at 7PM on Feb. 22, shown above, is a case in point.

The model shows more blocking in the north Atlantic than the latest GFS forecast or its ensembles. It initially takes a low towards the Great Lakes but because of the blocking the low ends up getting squeezed to south and ends up as a weak wave that tracks to our south as shown above.

The freezing line is right over the DC as the dry slot moves into the region suggesting that some light precipitation during the previous 12 hours might have fallen as snow. This morning's Canadian model has the same look as the European model and would support the possibility of light snow especially north of the city.

I'm not ready yet to jump on the European and Canadian solutions as there still is uncertainty about the blocking. The bulk of last night's GFS ensemble members would argue for rain and have a warmer look than the European model for that storm. However, having two operational models hinting at a period of light snow sometime early next week makes the system worth tracking. They give snow lovers a glimmer of hope.

In summary, the pattern is not one that is particularly favorable for snow. However, there is a weather system that will be approaching next Monday or Tuesday that is worth monitoring. The pattern across the Pacific and much of the U.S. is not one that is very favorable for snow without help from a negative NAO and its associated blocking. Early next week the European and Canadian models show enough blocking to maybe supply that help.

By Wes Junker  | February 16, 2011; 1:10 PM ET
Categories:  Latest, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Classifying beliefs about climate change
Next: PM Update: Springlike air returns


The latest GFS tracks next week's system too far north for snow... but it might not be simulating a strong enough high to the north- which could force the storm track south. The latest Euro, however, has a very strong high with cold air over us and would suggest a track to the south and light snow for us.

Posted by: Jason-CapitalWeatherGang | February 16, 2011 1:43 PM | Report abuse

You all are so kind for discussing even the slightest chance for snow! Wes, as always, a great article. I have learned so much in the short time I've been reading CWG. But you've got a reader for life.

Posted by: camb67 | February 16, 2011 1:47 PM | Report abuse

The world needs a good bath. It's salty & yucky out there. Send us a proper rainstorm after nice weekend.
But not on Tuesday....we have to consider the dancers.

Posted by: FIREDRAGON47 | February 16, 2011 2:35 PM | Report abuse

The dancers and the birthday girls! No rain on Tuesday! At least until I get home from work... then it can rain and clean my car off. Free car wash is a nice birthday gift, right?

Do we have a deal? ;)

Posted by: CuseFan07 | February 16, 2011 2:47 PM | Report abuse

camb67 - thank you for your kind words. Hope you continue to enjoy reading us.

FIREDRAGON47 & CuseFan07 - a bath, car wash, and a good plant watering is needed, yes

Posted by: Camden-CapitalWeatherGang | February 16, 2011 2:52 PM | Report abuse

ok, look, we've got some opposition (bath/dance/birthday) to a mon/tue snowstorm. i'd be more than happy to give up mon/tue snow in exchange for the thu/fri/sat (24/25/26th) snow i've been eying for a few days now.

hopefully the reason wes is not talking about the 24/25/26 storm is just because it's too far in the future.

Posted by: walter-in-fallschurch | February 16, 2011 3:01 PM | Report abuse

Hmm - think the storm on the 24/25/26 will head north and wallop Boston like storms have this winter? Taking a road trip to MA on the 26th and would very much like to be driving up in pleasant weather conditions rather than a blizzard!

Posted by: CuseFan07 | February 16, 2011 3:04 PM | Report abuse

camb67 - Thanks for the nice words. I've enjoyed writing the articles.

Posted by: wjunker | February 16, 2011 3:05 PM | Report abuse

CuseFan07 - The storm is so far in the future that I hate to speculate too much. The potential storm is certainly worth monitoring but its timing could even change by a day or so. The GFs ensembles this morning continued to look too warm for snow in the dc area even the runs with a suppressed low.

Posted by: wjunker | February 16, 2011 3:16 PM | Report abuse


You have been talking about this impending storm for quite some time now.. If you keep this up, you are gonna reinstate "OPERATION SNOW HOPE"... Walter is sprinkling us snow lovers, with his magic "SNOW HOPE" dust..

Posted by: BELLASNOWQUEEN | February 16, 2011 3:18 PM | Report abuse

i'll keep tracking, if not actually reporting on, all storm (i.e. snow) chances through about mid march... but, can we call it "operation snow hope" instead? i prefer to speak softly and carry a big shovel.

Posted by: walter-in-fallschurch | February 16, 2011 3:47 PM | Report abuse

Im all in on the GFS! Bulk of precip stays north and comes in a little faster than what the EURO is saying.

HM from accuwx seems to think it will be somewhere in between what the gfs and euro is saying, basically somewhere between I 70 and I 90 could be the snow line.

Either way, theres no way we see any snow in the dc metro sun-mon or tuesday. Im no met, but I say no chance. That goes for light snow too.

Late week storm (next week) turns out to be a early morning or late night mix, at best. Id still venture to say its all rain and the temps stay warm enough throughout to keep it that way as has been the case all winter!!!!

Posted by: KRUZ | February 16, 2011 4:27 PM | Report abuse

The snow Nazi says, no more snow 4 u. Little or no chance of snow the rest of this month. Temps 2 warm & storm track 2 far N & W. Wishcasting never works. Just don't c anything over the next 12 days that will bring any decent snow. Time 2 face the facts, that this winter just wasn't going 2 b snowy.

Posted by: VaTechBob | February 16, 2011 4:35 PM | Report abuse

Wes, regardless of how this winter ends...I hope you stick around. Your articles on CWG blog have been informative & delightful. While the CWG is already awesome, you're adding and additional element of awesome-ness. Thank you!

Posted by: parksndc | February 16, 2011 4:46 PM | Report abuse

So far Doug Kammerer is the only local met showing snow chances for next tuesday. Shutt, Hill and McGrady, show rain to nothing at all. As we get deeper into spring weather the EURO is becoming less reliable imo, and it seems as though Kammerer is model hugging the EURO.

Posted by: KRUZ | February 16, 2011 6:03 PM | Report abuse

There is just enough blocking and enough cold air in east central Canada to a least wishcast for something early next week!

Probably for NYC and Boston again, but it's not completely hopeless. I believe that is what Wes is trying to say.

Posted by: frontieradjust | February 16, 2011 6:23 PM | Report abuse

So Doug Kammerer just replied to my message on facebook. I told him I liked the GFS for the next week or so and this was his reply...

"You like the GFS, no way! The models have a hard time dealing with closed llows in the Southwest and the GFS moves it out too early. The Euro, Euro Ensembles and a few other models hold that low back and allow colder air to come in from the North. Not saying we WILL see snow, but I def. like the Euro better. It has been WAY better than the other models over the last few years."

In typical meteorologist fashion he basically says it may or may not snow. I understand why mets have to do it, I just hate it soo :) No disrespect. Still, right now Doug is all by himself showing snow early next week as all other local mets show either nothing or rain.

Lets see who waffles first!!!!!

Posted by: KRUZ | February 16, 2011 6:52 PM | Report abuse




Posted by: SNOWLUVER | February 16, 2011 8:07 PM | Report abuse

I guess everyone will continue to hope for what they hope for. I'm just bummed that Saturday won't be as nice as Friday. Was hoping for a warm, pretty day for all the anime fans strolling around National Harbor...still, 50 is a lot better than below freezing =)

Posted by: angelicat | February 16, 2011 8:28 PM | Report abuse


I'd bet we dont have to worry about too many more days with below freezing temps! Maybe some nights, but no days!

Lots of average and above average daytime temps from here on out!

Posted by: KRUZ | February 16, 2011 8:33 PM | Report abuse

The 18Z gfs came quite a bit south with its low and now takes it almost over DC instead of across NY. The model would still give us rain rather than snow but the model solution is sensing some blocking. Its ensemble members continue to keep dc warm enough for rain though more are now showing a suppressed look to the surface low. The euro ensemble mean strongly supports the european model so there are basically still two camps. The euro ensemble mean has trended south from last night's run while teh gfs has trended south.

Posted by: wjunker | February 16, 2011 9:16 PM | Report abuse

parksndc - I may not be around much during summer but plan on being here next winter if the CWG still want me.

Frontieradjust/Andy - that is pretty much what I was saying but I also was thry to say how even with a pattern that isn't that good, things can change. The euro enemble mean has strong enough blocking to force the low south of us. That is even more reason why the even is worth monitoring. It could very well end up as no more than a cold rain but at this time range why lock in a forecast. Usually at these time ranges, it's not smart to try to be too definitive with a forecast.

Posted by: wjunker | February 16, 2011 9:36 PM | Report abuse

0z GFS now shows moderate snow Monday night and Tuesday. Definitely a case of models trending colder.

Posted by: Jason-CapitalWeatherGang | February 17, 2011 12:04 AM | Report abuse

The comments to this entry are closed.

RSS Feed
Subscribe to The Post

© 2012 The Washington Post Company