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Posted at 12:55 PM ET, 02/18/2011

Snow chance early Tuesday, but big uncertainty

By Wes Junker

Despite all of this week's warmth, and the increasing possibility President's day may be pretty warm itself, we're carefully monitoring the potential for crashing temperatures Monday night while a storm system approaches from the west and southwest. This system may bring rain that changes to snow by Tuesday morning, potentially impacting the back-to-work rush hour. This is a very complicated set up which we will be providing updates about through the holiday weekend.

Wes Junker provides an excellent technical analysis below - which is worth a read to understand why this forecast is so challenging and fraught with uncertainty. - Jason Samenow

Keep reading for Wes Junker's in-depth analysis...

Technical Discussion

Monday I concluded that the overall weather pattern was not that favorable for snow but could produce surprises (i.e. a snowstorm) if enough blocking developed (i.e. a negative Arctic Oscillation) in the North Atlantic.

At that time, the European and Canadian models were alone in one camp suggesting that an initial low would track to our north on Monday which would help pull cold air back to the south over our region prior to the approach of a second wave on the front that could produce snow. Since then, the GFS has trended toward that European model idea and has shown a low of varying intensities tracking to our south on Tuesday advertising the potential for snow. In fact some of its forecasts from yesterday were quite aggressive about the potential.

The models continue to suggest potential for a little rain on Sunday night or Monday morning as the first surface low passes by. The low passing to our north would provide the area with southerly winds putting us in the warm sector south of the front. Temperatures during the day Monday based on last night's GFS could rise well up into the 60s.

The forecast difficulty starts once that low passes far enough away to our northeast, the front sinks south, and a weaker second surface low develops on the front and approaches us from the west. How far south of our area this second low tracks will determine whether we get mostly rain, rain to snow, or get little or no precipitation.

Mean sea-level pressure and surface temperature forecast valid 1 AM Tuesday. Last night's 7 PM GFS on left and this morning's 1 AM on the right. Note the differences in the position of the low pressure between the two runs

The big problem for snow lovers is that the second low has to track far enough south for the cold air to settle southward firmly into the Washington area but not so far south that the precipitation misses us. Right now the models continue to vacillate between solutions with the Euro forecasting mostly rain but possibly ending as snow while the 1AM run of the GFS tracked the low so far south that we would get snow but only about an inch of snow because the bulk of the precipitation passes to our south. Today's GFS run has shifted the low back to the north making it wetter but also warmer at least during the first half of the storm. Any of these scenarios remain possible.

Examination of two GFS runs last night illustrates how small differences in two features (described and illustrated below) and their interaction between each other can subtly change the storm track leading to significant differences in the storm's precipitation field.


Note that on the chart above there is a vortex (the oval shaped lines near Maine) in both panels as well as a dip in the lines over the Midwest with an x or x's in the vicinity of Kentucky. This trough (dip in the lines) provides the upper support for the surface wave that is forecast to track south of the region. On the charts I've drawn an arrow pointing to where the model runs are showing confluence (where the lines start coming together as you move east to west).

Why care about confluence? The surface pressure usually rises in regions having upper level confluence. If you shift the confluence southeast from the New York-Pennsylvania border (left panel) to closer to the Mason-Dixon line (right panel) then the surface low will also be forced farther to the south as will the precipitation shield.

Notice on the two charts that line labeled 540 on the left panel extends farther to the south and west than on the right hand panel. That dip in the lines changes where the confluence is located and also produces a stronger upper level support for the surface low.

If you move the vortex near Maine a little to the north and northeast, the approaching trough probably would stay a little farther north which would delay how quickly the cold air come southward into our area in advance of the wave as simulated in last night's European solution and this morning's GFS.

In fact, this morning's GFS has jumped back northward with the storm track taking a low to West Virginia by 7PM Monday evening then tracking it to Petersburg, Virginia by 1AM Tuesday morning. The temperatures in the lowest 500 feet do not fall below freezing until the bulk of the precipitation has fallen. However, it does suggest that there could be a transition of sleet to snow across the area with the northern areas changing earlier than the southern ones. And taken literally, it would suggest the cold air and snow to coincide with Tuesday morning's rush hour with at least a light accumulation.

Plume diagram showing precipitation evolution with time based on numerous ensemble member runs of GFS model..

The run to run changes of the GFS solutions are a signal that there is tremendous uncertainty in the forecast. A plume diagram above that shows the precipitation from all the GEFS ensemble members clearly illustrates how much uncertainty there is about how much and what types of precipitation might fall. Remember that ensemble members have the same physics as the GFS but the initial conditions have been tweaked to get an idea of the predictability of the system.

The 06Z ensemble members are all over the place. Precipitation type is color coded with the color code shown on the left hand side of the figure. Most start the precipitation as rain and then change it to snow but some show only light snow at the end while some show a significant portion of the precipitation falling as snow.


So what can we say about the system? A surface low is likely to track across or to the south of the area. It is likely to produce precipitation and some of it will probably be in the form of rain and there is a chance of snow. It's too early to estimate accumulation potential but the warm surface temperatures on Monday are a big negative that will have to be overcome.

Nevertheless, we need to watch this very carefully as there is a chance of snow and falling temperatures coinciding with Tuesday morning's around rush hour after the long holiday weekend.

By Wes Junker  | February 18, 2011; 12:55 PM ET
Categories:  Latest, Winter Storms  
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Next: PM Update: Big winds follow today's record warmth


Nice update Wes,

I honestly have been enjoying watching this particular storm. Seems to me we are gonna need another one of those perfect tracks to get meaningful accumulating snow here in DC.

My thinking is either a cold rain or nothing at all. Look forward to everyone at CWG's updates!!!

Posted by: KRUZ | February 18, 2011 1:06 PM | Report abuse

Glad I just bought new windshield wipers.

Posted by: WickedRose | February 18, 2011 1:11 PM | Report abuse

Wes, i'm sure you also mean that besides "a big problem for snowlovers," the tracking could also provide relief for those who don't want to see snow...:)

Posted by: weatherdude | February 18, 2011 1:18 PM | Report abuse

Looks like the EURO moved more toward the GFS. If it were right we might hit 70 on Monday again before temps crash overnight and rain changes to snow with temps in the 20s around sunrise Tuesday.

Posted by: Ian-CapitalWeatherGang | February 18, 2011 1:33 PM | Report abuse

been waiting all morning to hear from wes. glad to see the snow scenarios are still in play. as we all suspected, it all depends on the track.

wes (and/or ian or anyone qualified to answer),
on that cool "plume diagram", are those snow amounts in qpf? like the .36" mean snow total indicates 3-4" of snow is the average result? that's pretty good! and there's one run that showed 1" qpf or 12" of snow?! (how do i "lock in" THAT scenario? - i'll pay by credit card right now.) heck, i'd be happy with the average 3-4" outcome.

also, apparently it's the track of the low#1 that brings warm air in behind it. what sort of track for low#1 do we, i anyway, want in order to minimize the introduction of warm air prior to low#2's arrival? i am fearing a scenario where it's snowing alright, but just not sticking on tuesday - because the ground is so warm.

Posted by: walter-in-fallschurch | February 18, 2011 1:43 PM | Report abuse

Wow --- can anyone here remember a time when temperatures were in the 70s during the day, but then 20s with snow during the night? Talk about weather whiplash.

Posted by: jms12 | February 18, 2011 1:44 PM | Report abuse

Wow --- can anyone here remember a time when temperatures were in the 70s during the day, but then 20s with snow during the night? Talk about weather whiplash.

Posted by: jms12 | February 18, 2011 1:45 PM | Report abuse

But even if it snows - there is no way it will stick right? Won't it just melt on contact after 70 degrees on Monday? Should be interesting none the less.

Posted by: nolagirl67 | February 18, 2011 1:45 PM | Report abuse

Walter, you're correct, they are QPF amounts. If you had a 10=1 ratio, you'd have 3.5 inches for .35"

Posted by: wjunker | February 18, 2011 1:53 PM | Report abuse


Certainly some of it would melt on contact; how much accumulates depends on how heavily it snows and for how long. Regardless, looks like the temps in the following days will be high enough to quickly melt was does stick.

Posted by: weatherdude | February 18, 2011 1:54 PM | Report abuse

wes, ian, wxperts, continued:
i asked about the influence of the track of low#1 on the temperatures before low#2 gets here. i'd also like to know if the track of low#1 influences the track of low#2. i.e., if we get the track for low#1 just right in order to minimize the between-low warmth, will it "throw off" the track of low#2 resulting in less/no snow?

Posted by: walter-in-fallschurch | February 18, 2011 1:54 PM | Report abuse

Thanks for the analysis, Wes.

My personal opinion based only on living in this area for well over 40 years is that it's simply rain.

The whole scenario seems a bit far-fetched in order to get anything more than wet snowflakes at the very end.

CWG certainly knows more than I do about this but I just don't see it happening, especially if we get near 70 on Monday. Those kind of temperature swings in a matter of hours just don't happen here that often.

I wonder if since this is still 4 days out, the long range models are showing any climatological biases here?

Posted by: ThinkSpring | February 18, 2011 2:03 PM | Report abuse

@jms12 - the March 1993 superstorm laid out like this. It was in the upper 60s/lower 70s the day before and I found it really hard to believe that this massive storm was going to hit the next day.

Posted by: nymomma | February 18, 2011 2:20 PM | Report abuse

i don't know. but wouldn't the fact that some runs are showing 70 degrees for monday argue against them showing climatological bias? a high of 70 is way more "out of whack" than a low of 20 for this time of year.

Posted by: walter-in-fallschurch | February 18, 2011 2:22 PM | Report abuse

Walter, I just saw your question on model verification scores from the morning thread. Here's a good link.

Posted by: Ian-CapitalWeatherGang | February 18, 2011 2:24 PM | Report abuse

walter, it looks to me like storm one even though it pumps in warmth in front of it helps set the stage for 2 by setting up/enforcing the cold air to our north in its wake and laying down a pathway/frontal zone for the secondary low to follow.

Posted by: Ian-CapitalWeatherGang | February 18, 2011 2:31 PM | Report abuse


You're right, and I should've been a little more clear in my post. I don't think either will happen...70 on Monday or snow Tuesday morning.

Wild temperature swings of 40-50 degrees in a matter of hours are not commonplace in this region.

But, you do make a valid point against climatological bias in this instance.

Posted by: ThinkSpring | February 18, 2011 2:45 PM | Report abuse

Come on and clobber us with another wet, fast accumulating rush hour snowstorm.
I promise the area will do MUCH better this time.

Posted by: FIREDRAGON47 | February 18, 2011 2:47 PM | Report abuse

looks like DCA tied the record and hit 76. Can't wait to witness this impressive front move through and hoping we have snow on Tuesday.

Posted by: meteorolinguist | February 18, 2011 3:01 PM | Report abuse

@jms12: "weather whiplash" I like it!!

@Firedragon47: "Come on and clobber us with another wet, fast accumulating rush hour snowstorm.
I promise the area will do MUCH better this time."

LOL. Guess I'll be back on the blog checking for the latest updates and not putting away the snowboots and snowbrum (brum = broom) just yet.

Posted by: kygurl94 | February 18, 2011 3:04 PM | Report abuse

ThinkSpring, I've got you on living here as I've been here over 60 but am too stupid to know whether it will be all rain or not. It sure will be an interesting event to unfold.

Posted by: wjunker | February 18, 2011 3:17 PM | Report abuse

"why this forecast is so challenging and fraught with uncertainty"

Why is this phrase even here? What forecast this winter has NOT been so?

Posted by: xandersun | February 18, 2011 3:23 PM | Report abuse

To my untrained, unprofessional eye, it seems like the 18Z NAM puts the rain/snow line right around the DC area:

Posted by: jms12 | February 18, 2011 3:54 PM | Report abuse

jms12, the surface temp is still around 2.7 celsius and there is a warm layer with a temp of almost 3 C at around 4500 ft. The temp at the surface is forecast to be around 36 and the warm layer temp 36 or 37. That's probably not snow yet but is a lot quicker with the cold air than the 12Z gfs. It looks like it would have a changeover fairly quickly after that. It also brings in quite a bit of rain prior to that time.

Posted by: wjunker | February 18, 2011 4:17 PM | Report abuse

Thanks, Wes. I guess that just shows how much stuff is still up in the air, no pun intended. If we can somehow get that cold air here earlier, then maybe we can pull out a decent snowfall. A snowlover can at least hope ...

Posted by: jms12 | February 18, 2011 4:21 PM | Report abuse

Wow, that "plume diagram" is fascinating! I'd love to see one of those for past events, both for huge snowstorms and big fizzles. I'm always excited to see new ways of displaying the possible outcomes of a storm system.

Posted by: ddowhan | February 18, 2011 4:23 PM | Report abuse


You raise a good point- the uncertainty is the nature of the beast...and maybe goes without saying. Having said that, we probably have some new readers with each storm who need to know there are unknowns.

Posted by: Jason-CapitalWeatherGang | February 18, 2011 4:58 PM | Report abuse

Guess I'll be hauling out my heavy coat snow or no...

Posted by: eabgarnet | February 18, 2011 4:59 PM | Report abuse

you extremist, you - thanks for the replies. ;-) i figured the storm tracks were probably linked. i'll try to make sense of that model verification page and get back to you if i have questions.

no doubt, that plume diagram is really cool. i like the "profiles" wes has broken out this year too - sort of a vertical slice of the atmosphere at a given point.

Posted by: walter-in-fallschurch | February 18, 2011 5:51 PM | Report abuse

Or it could mean that come Tuesday school systems will close in a panic that was altogether unnecessary!

Posted by: bigpaws12 | February 18, 2011 6:04 PM | Report abuse

The climitology favors up to 5 inches of snow before it is over.

Posted by: peterroach | February 18, 2011 7:40 PM | Report abuse


Where you at???

OPERATION SNOWHOPE IS IN EFFECT ******************************************************************************

Posted by: SNOWLUVER | February 18, 2011 9:19 PM | Report abuse

So the 00Z seems to keep the warm air in long enough for just a a brief shot of snow towards the end then quickly moves out. As I said early to walter, I just need this baby (2nd low) to keep on trending south and it will be a total miss! The bulk of the precip of the 2nd low is now to our south!

Then is quickly outta here:

Posted by: KRUZ | February 18, 2011 11:14 PM | Report abuse

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