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 1:00 PM ET, 12/20/2010

White Christmas dream *could* come true

By Wes Junker

If you've been dreaming of a white Christmas, this might be your year. Computer models are forecasting that an area of low pressure will track across the country with plenty of cold air feeding it. The low will pass to our south and be somewhere off the coast of North Carolina by sometime early Christmas morning setting the stage for the possibility of a white Christmas.

The forecast is still much too uncertain to make specific calls on possible snowfall amounts or to address questions about travel plans; we would not advise changing plans based on a five day forecast. The main thing to take away from this discussion is that over the next several days, if you have travel plans: follow the weather closely. We will gradually be able to provide more specific information.

Next accumulating snow chance: Around Christmas day (possibly starting late Christmas Eve and ending Boxing day, 12/26)
Probability of accumulating snow (1" or more): ~55%
Probability of more than 4": 20%

Keep reading for technical discussion

TECHNICAL DISCUSSION

The crystal ball is less foggy than most at such an extended time range as the majority of high resolution models including the European model, GFS, Canadian model and United Kingdom models all suggest at least the possibility of some snow as do the majority of ensemble members from the GFS and the European models. Again there are two forecast camps, one suggesting the potential for a significant snowstorm, another suggesting we only get a glancing blow.

Why am I fairly confident about this storm producing some precipitation, most likely in the form of snow for Christmas? A strong upper level system will be coming into the West. Such a strong system should initiate the development of a well-defined low pressure system east of the Rocky Mountains. Another positive factor in favor of the storm is the strong ridge (bump in the jet stream) that develops behind the shortwave trough (dip in the jet stream, see top left panel of the figure below).

models-xmas-122010.jpg

Surface and 500 mb (18,000 feet) forecasts from last night's run of the European center
(top left panel is valid at 7 p.m. Dec 25 and top right is valid at 7 p.m. Dec 26),
and GFS (bottom left panel is valid at 7 a.m. Dec 25 and bottom right panel is
valid at 7 a.m. Dec 26).

In general, the building of a strong ridge (or bump in the jet stream - see green lines extending from eastern Oklahoma northwest into the Canadian provinces) behind a developing system amplifies the upper level trough (dip in the jet stream over the deep South) - favoring intensification of the storm. Such deepening also generally makes it easier for phasing of the northern and southern jet streams that is often associated with major winter storms. This is the reason why, despite there still being lots of impulses and lots of uncertainty about how they will mesh together, this system has the potential to be a significant storm across the DC metro area .

However, the phasing still has to occur at the right place and at the right time for a "big" storm. Many of the same phasing questions we see with this storm were also present with the storm we tracked last week, where the jet streams phased too late, and the storm went out to sea. Most snowstorms in the D.C. area are glancing blows rather than the monsters we had last winter because, more often than not, the timing for phasing is off.

A look at the European center model (top panels) and the GFS (bottom) from last night shows how the evolution and track of the storm can vary depending on whether the there is phasing or not. The European model is slower with the upper level energy that came into California allowing northern jet stream energy to catch up and phase with the surface low. Its low along the Carolina coast is 12 hours slower than that of the GFS - delaying the onset of possible snow into later Christmas day.

Once it phases the jet streams, the European produces a major snowstorm for our area and possibly the Northeast. Its low lifts rather slowly northeast just off the coast and is pretty much a classic nor'easter. By contrast, because it is faster with the initial impulse and surface low, the GFS tracks the low much farther out to sea yielding a glancing blow rather than the major storm. The Canadian model is clearly in the European camp and the UKMET model is somewhere in between the GFS and Euro solutions. The one exception is 2 a.m. run of the GFS which had a weaker, drier look than the other model runs. Still, even it is in pretty good agreement with the others for the forecast projections so far in the future.

However, a few members of the latest (12z) GFS ensembles do take the storm too far south and out to sea for snow - so we should emphasize accumulating snow is not a done deal.

models-xmas-122010-2.jpg
This morning's GFS 850 mb (5,000 feet) forecast heights (or pressure), temperatures (Celsius), and winds valid at 1 p.m. Saturday.

The latest GFS run trended a little slower with the Pacific energy so it get closer to a phase but still is only a glancing blow only giving DCA 0.25" of liquid equivalent (or a two to three inch snowstorm). One of the reasons it is so dry is that the system now in the west Atlantic is so slow to move that the flow across the Northeast remains northerly and that does not allow much Atlantic moisture to get into the system. Note on the forecast to the right how there are no easterly winds to the north of the low at the 5,000 foot level (850 mb).

If the flow to the north of the storm were to relax a little quicker, the storm might have more potential. If it does it later, then an even drier solution would be possible. That's the forecast dilemma.

The old crystal ball does think the odds of an inch of snow is rising and probably around 50 to 60 percent. However, the crystal ball also thinks of the possible solutions, the European model solution, which indicates the most snow, is the most unlikely.

UPDATE - 1:30 p.m.: The latest European model (12z) keeps the storm too far offshore (and phases it too late) for a heavy snowfall for the mid-Atlantic (but not New England). It would suggest a lighter event.

CWG's Matt Ross has noted that we get 6 inch or more snowfall events in only around 20 percent of our snow events. That doesn't preclude a big event but argues that, on long range forecasts, it is better to be cautious.

The Snow Lover's Crystal Ball appears when the potential exists for accumulating snow beyond 24 to 36 hours.

By Wes Junker  | December 20, 2010; 1:00 PM ET
Categories:  Latest, Snow Lover's Crystal Ball, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Freak pattern brings Europe record cold & snow
Next: PM Update: Chilly lunar eclipse viewing

Comments

Well if this does happen I guess its safe to say "Joey Fearless" was right more than a month out. At least %50 of the country will see a white Christmas.

Posted by: KRUZ | December 20, 2010 1:14 PM | Report abuse

Thanks for the update! So what are the odds that this storm clobbers New England (mainly Boston and areas north)?

Posted by: CuseFan07 | December 20, 2010 1:36 PM | Report abuse

KRUZ...agreed...Joe is a lucky bastard :) Wes, thank you once again for a great and detailed explanation. Your comments explain the SLCB predicitons well. Thank you for taking the time to write this.

Posted by: parksndc | December 20, 2010 1:38 PM | Report abuse

@CuseFan07

Right now the odds of a biggie in southeast New England appear greater than they do here. But this could easily change.

Posted by: Jason-CapitalWeatherGang | December 20, 2010 1:41 PM | Report abuse

I like that 20%.

Posted by: ennepe68 | December 20, 2010 1:49 PM | Report abuse

Does the 1:30 p.m. update/run preclude snow in Philly too? (I know this is Capital Weather Gang not City of Brotherly Love Weather Gang, but so many of us will be traveling near enough to still hope the snow pertains to us...)

Posted by: hils | December 20, 2010 1:51 PM | Report abuse

Doesn't Cap. Weather just confuse people by noting every model change? It seems like Mr. Junker wrote up a coherent analysis based on days of model runs of where things stand now. No sooner has he posted it, then another model comes out saying something different. So at the end of his great piece, there is now an update that would lead many to believe the storm won't happen.

However, it's just one model. By later today, who knows what they will say. This up and down forecast based on the models is what lead Cap Weather's whiplash forecast last week of maybe snow, no snow, maybe snow, all within a day or two.

I am not necessarily criticizing, but do question whether at times its better to make just one forecast a day -- based on a consensus of all the models - instead of allowing the public to think storms are big or duds based on what individual model says.

Posted by: realclear | December 20, 2010 1:52 PM | Report abuse

As this storm moves towards DC, is there any guidance on the track that it will take? I'm flying to Chicago on Thursday night, and some of the models concern me with midwest snow starting right around when I should be landing.

Will this storm be of significant enough strength to be disruptive or well within what O'Hare can handle (1-4")

Posted by: NoVAHoyaDan | December 20, 2010 1:53 PM | Report abuse

Wouldn't that 3" or 4" be pretty much perfect? Enough to qualify as a legitimate White Christmas, but not so much that it confuzzles everyone's holiday plans.

Posted by: jburksva | December 20, 2010 1:53 PM | Report abuse

I like that 20%.

Posted by: ennepe68 | December 20, 2010 1:54 PM | Report abuse

@realclear

I added the Euro info because it supports our overall idea that chance of a big storm is low. In general, we are going to just do one analysis of this kind a day... and I get your point on model whiplash.

Posted by: Jason-CapitalWeatherGang | December 20, 2010 1:58 PM | Report abuse

Very good and complete technical discussion by Wes. The trend now looks more and more like a repeat of the southern bypass of the DC area.

Posted by: ronbcust | December 20, 2010 2:02 PM | Report abuse

Hmmmm...models pushing out to sea, this is starting to sound too familiar. Like the odds of snow but sounds more like a 1-3 or 2-4 incher. Guess we'll see though, we know how these models like to do head-fakes.

Posted by: parksndc | December 20, 2010 2:02 PM | Report abuse

I'm relatively new at understanding these model maps, and this is only one of the many models, but this seems to show a decent amount of precipitation from Chicago through Kentucky and up in to Virginia and DC.

Posted by: NoVAHoyaDan | December 20, 2010 2:06 PM | Report abuse

I'm relatively new at understanding these model maps, and this is only one of the many models, but this seems to show a decent amount of precipitation from Chicago through Kentucky and up in to Virginia and DC.

http://www.meteo.psu.edu/~gadomski/AVN_18z/avnloop.html#picture

Posted by: NoVAHoyaDan | December 20, 2010 2:06 PM | Report abuse

I'm relatively new at understanding these model maps, and this is only one of the many models, but this seems to show a decent amount of precipitation from Chicago through Kentucky and up in to Virginia and DC.

(added the link this time)

http://www.meteo.psu.edu/~gadomski/AVN_18z/avnloop.html#picture

Posted by: NoVAHoyaDan | December 20, 2010 2:07 PM | Report abuse

jburksva:

There's No Such Thing As Too Much Snow. ;-)

Posted by: cassander | December 20, 2010 2:07 PM | Report abuse

Sorry for all the comment trouble. I've been having a lot of trouble using Safari browser with the CWG website.

Posted by: NoVAHoyaDan | December 20, 2010 2:08 PM | Report abuse

@hils

I think what we say for DC more or less applies to Philly. If the storm doesn't come up the coast, Philly could see less snow than DC with the storm center's nearest approach North Carolina. On the other hand, there's the possibility the storm could develop off the northern mid-Atlantic coast too late for D.C., but early enough for Philly to get into some of the snow from the ocean development (that has potential to rock southeast New England).

Posted by: Jason-CapitalWeatherGang | December 20, 2010 2:08 PM | Report abuse

If I can cut thru all of this, what it sounds like you're saying is "It might snow on Xmas Day".
As in "maybe, "possibly", "there's a chance", and "could happen".
In the meantime let's all us snowlovers lose our minds.
Go!

Posted by: FIREDRAGON47 | December 20, 2010 2:29 PM | Report abuse

@NoVAHoyaDan

That model is from yesterday afternoon so is a bit old...but your overall interpretation about what it showed is correct.

Posted by: Jason-CapitalWeatherGang | December 20, 2010 2:30 PM | Report abuse

it seems more and more like this storm could end up following the same track as last storm... go too far south and OTS and then miss us as it goes towards New England. What are the odds of that happening!?!

Posted by: KRUZ | December 20, 2010 3:25 PM | Report abuse

@KRUZ

45% :)

Posted by: Jason-CapitalWeatherGang | December 20, 2010 3:40 PM | Report abuse

Great write up Wes. For my money the more posts the better. I like hearing what new models indicate in the case of a snow storm. That's why I come to this blog. I can get the weather anywhere, but I like that you interpret the models. I understand what realclear is saying, but disagree completely. I understand it's just one model and it doesn't confuse me. Like I said it's why I come here.

Posted by: alpal3 | December 20, 2010 3:47 PM | Report abuse

I agree with alpal3,

I enjoy hearing updates as much as possible, especially about model runs and ensembles because it really lets us see how the storm is trending.

And Jason, thats a pretty high percentage (45%) and i like it :)

Im on the fence with wanting this storm. On the one hand i wont have to work and can spend the entire holiday weekend with my wife, son and family! On the other hand, theres DOUBLE OT pay out of this as well. Still id rather sepend the time with family.

Maybe I can get a little of both worlds and have a minor (1-3") storm!

Posted by: KRUZ | December 20, 2010 4:05 PM | Report abuse

I agree, the more posts the better! This site turns me into an armchair meteorologist in the winter and I love seeing how the storms come together and then fall apart...it helps me understand why so frequently winter around here sucks.

Usually I'm all about snow - want as much as possibly as often as possible! I LOVE LOVE LOVE being "stuck" in my house and spending the day in my jammies reading and watching movies while the snow is falling...ahhhh..makes me happy just thinking about it! This weekend however, I'm supposed to travel to Williamsburg to be with my family for Christmas - it would kill me to miss a good snowstorm here, they are so few and far between! Also, I'm a teacher - I'd rather not get our snow (since in a normal winter it's only a storm or 2)during winter break! How about a good storm on Jan 2 - extend winter break a bit?

Posted by: mamory1975 | December 20, 2010 4:23 PM | Report abuse

To the dude who wants 1 update a day: If you're not interested in the down and dirty of what the GFS, NAM, ECMWF, EIEIO, RUC, UKIE, RGEM, GGEM, and, occasionally, the KMA and JMA, (I made up one of those, I'll let you guess which) models say when they're run 2-4 times a day, don't visit a blog that offers in depth and up to the minute weather discussion.

I'm pleased to let you know The Washington Post and Capital Weather Gang have a place for folks like you! It's available at http://www.washingtonpost.com/weather.

Posted by: Registration1982 | December 20, 2010 4:32 PM | Report abuse

I'll take that 45%. Please send this mess out to sea where it belongs!!!!

Posted by: pbj1 | December 20, 2010 5:14 PM | Report abuse

CWG - Piggybacking off of KRUZ's comments...45% chance is very high for this to go OTS and if we've already seen one do that...and we're in a similiar pattern...my money is on OTS now. It sounds like at best we get 2-4" if it doesn't go OTS. Does that seem consistent with what you're saying?

Posted by: parksndc | December 20, 2010 5:29 PM | Report abuse

I see the 1800UTC GFS is giving us a more direct hit. Waiting for the 000 update!

Posted by: Finn1917 | December 20, 2010 5:36 PM | Report abuse

Well Bob Ryan is already on board for a big coastal, he just tweeted it looks a bit more than 50/50.

Thats not like him to do that 5 days out. But then again, technically there was a coastal this past weekend too ;)

Which my money is on XMas's coastal to be the same as this past weekends coastal AKA OTS. But we shall see.

Posted by: KRUZ | December 20, 2010 5:59 PM | Report abuse

KRUZ...agree, Bob (as I call him) doesn't usually seem that agressive with snowstorms. I'm surprised. But, I still think OTS is the right call.

Posted by: parksndc | December 20, 2010 6:01 PM | Report abuse

Henry Margusity just did a live ustream broadcast linked from his facebook fan page.

He says we get 2-4 inches this weekend.

Posted by: KRUZ | December 20, 2010 8:09 PM | Report abuse

out to sea is fine by me

Joe B and Lundberg are calling for what? 4-5 feet?

Posted by: TGT11 | December 20, 2010 8:11 PM | Report abuse

Any snow on Christmas will be great for me! I'm wondering what we did in winter before we had Wes on this blog...when of course CWG has been phenomenal all along. Thanks, Wes and CWG!

Posted by: --sg | December 20, 2010 8:56 PM | Report abuse

50/50 hybrid too far west and footprint too large, HP lacking New England to Lakes, Phasing too late. Longwave too flat.........

Favored 1-3" Garden Variety.

A light White Christmas

Happy Holidays Everyone!!!

Posted by: AugustaJim | December 20, 2010 8:58 PM | Report abuse

i love how 5 days out from a storm, everyone authoritatively posts their predictions. The guys at CWG are pros, they get paid and have been doing this for years. There is a reason they repeatedly say that they are just giving a best guess.

Posted by: samd95 | December 20, 2010 9:18 PM | Report abuse

Sorry that I've been away, I was visiting my sister in Va and just got back.

The 18Z gfs actually gave dca around .50 inches bumping up the amounts from the 12Z run. The 18Z ensemble have indeed lowered the risk of .10" over our area but right now I'd hold steady on the probabilities. I think this still is a much better shot at snow than the last event ever was but time will tell.

Posted by: wjunker | December 20, 2010 9:21 PM | Report abuse

oops, I found a type mistake in the text. It should read "Note on the forecast to the right how there are no easterly winds to the north of the low at the 5,000 foot level (850 mb). The lack of easterly winds keeps moisture from the Atlantic from being tapped.

Posted by: wjunker | December 20, 2010 10:18 PM | Report abuse

Latest GFS (0z) is a good run for DC if you like snow...

Posted by: Jason-CapitalWeatherGang | December 20, 2010 11:14 PM | Report abuse

Jason: yes, but phasing of northern S/w and southern system is too late for a big storm in DC area. from PHI and northeast, much more snow.

Posted by: ronbcust | December 20, 2010 11:25 PM | Report abuse

Let it snow! For once, I don't mind too much. This year I was granted leave for Christmas day (having tenure rocks!) and I'll spend Christmas Day with my brother and his family; his 9 year old son will be psyched if it snows, and it'll be great playing in the snow with him that day. Bring it!

Posted by: natsncats | December 20, 2010 11:28 PM | Report abuse

@ronbcust

Completely agree. But the timing of the southern stream system is improved (though not perfect) - which is important at this point.

Posted by: Jason-CapitalWeatherGang | December 20, 2010 11:34 PM | Report abuse

@jason
also agree on "improvement" , but with the track of the southern system across TN and northern NC, I think that is too far north for much more "improvement." It bombs at the coast on the 00Z GFS right at our latitude. But the odds of a white x-mas day are improved, and if this holds, we won't be trapped in our homes or have widespread power outages. That would for the best all around.

Posted by: ronbcust | December 20, 2010 11:42 PM | Report abuse

The 00Z European and Canadian models have shifted back west, The former is not quite as impressive as the euro posted in the blog but for us it still would be a nice hit. The 06Z Gfs looks similar in tersm of qpf to the 00Z version. The 06Z gfs ensembles still need some work.

Jason thanks for making the correction.

Posted by: wjunker | December 21, 2010 8:32 AM | Report abuse

The 00Z European and Canadian models have shifted back west, The former is not quite as impressive as the euro posted in the blog but for us it still would be a nice hit. The 06Z Gfs looks similar in terms of qpf to the 00Z version. The 06Z gfs ensembles still need some work.

Jason thanks for making the correction.

Posted by: wjunker | December 21, 2010 8:33 AM | Report abuse

The comments to this entry are closed.

 
 
RSS Feed
Subscribe to The Post

© 2012 The Washington Post Company