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Posted at 11:30 AM ET, 01/29/2010

Slight shift north with snow accumulations

By Dan Stillman

Best chance for accumulation: late Sat. a.m. thru evening

* Very cold weekend: Full Forecast | Farewell to wind record *
* Winter Weather Advisory tomorrow for far southern suburbs (map) *


The CWG team's predicted snow accumulations Friday night through Saturday (updated @ 11:30 a.m.). Forecast confidence is Medium.

With models really having locked in on a southern track for a storm now pummeling parts of Arkansas, Tennessee, northern Mississippi and northern Alabama with snow and ice (and Louisiana and southeast Texas with intense thunderstorms), we're becoming increasingly confident that a major snowstorm is not in the cards for the immediate D.C. area.

Confidence remains not as high, however, as to how far north minor to moderate accumulations will reach. A slight shift northward in this morning's model runs was just enough to prompt a similarly slight shift to the north in our accumulation forecast (see map above to the right) compared to our earlier map. The main effect is that areas inside the Beltway now have a slightly better chance of seeing an inch or a little more rather than an inch or less. The best chance for accumulating snow should be between late morning and late evening Saturday.

Why the uncertainty in all this?

Keep reading for the answer, and for our metro area accumulation probabilities. Also, see our full forecast into next week.

Because a hard-to-pinpoint band is likely to set up somewhere near the D.C. area where accumulations will decrease (from south to north) from several inches to possibly no accumulation over a span that could be as small as around 50 miles or less.


Forecast map for midday Saturday from the Global Forecast System model shows high pressure to the north of D.C. with the storm centered to the south. Credit: NCEP.

The cause of this tight accumulation gradient is a strong high-pressure system that will be centered to our north/northwest -- the same system responsible for our very cold temperatures -- and that we expect will act as a wall stopping the northward progression of accumulating snow. (The sinking motion associated with high pressure tends to dry out the air and limit precipitation).

Considering that a very small shift in the storm's track could have a significant impact on who sees snow and how much, it's still appropriate to provide accumulation probabilities for the metro area, even this close to the storm's arrival:

45% chance: Less than 1"
30% chance: 1-3"
15% chance: 3-6"
10% chance: 6"+

Stay tuned for more updates, and see our full forecast into early next week.

By Dan Stillman  | January 29, 2010; 11:30 AM ET
Categories:  Forecasts, Updates  
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Next: PM Update: Accumulation forecast on the rise

Comments

I'm very confused. How come you guys are saying the models are trending slightly north while the 1135am NWS discussion says this... MODELS HAVE CONSISTENTLY NUDGED THE SURFACE LOW PRESSURE FURTHER
SOUTH...AND IN SOME CASES SLOWED THE SYSTEM DOWN BY A FEW HOURS.
THE GFS AND THE NAM HAVE COME INTO FAIRLY GOOD AGREEMENT WITH THE
TIMING AND LOCATION OF THE SURFACE LOW SATURDAY MORNING/EARLY
AFTERNOON. THE SNOWFALL GRADIENT WILL SET UP ACROSS THE CENTRAL
SHENANDOAH VALLEY EASTWARD THROUGH THE METRO AREAS. CONSIDERING
THIS...WILL HOLD OFF ISSUING THE ADVISORY FURTHER NORTHWARD (INTO
THE METRO AREAS). SNOWFALL WILL ALSO START LATER IN THIS AREA.

Did the NWS have a typo? BTW, I have a big event tomorrow night in Herndon and would prefer this storm to stay south. Maybe next wknd would be better for a big snow.

Posted by: formerwxman1 | January 29, 2010 11:53 AM | Report abuse

The low itself has not moved a ton, though the most recent run it is a smidge north I believe. The precip shield however has shifted north for sure. Still think 9 out of 10 times models shift north into the event -- how much so or if it keeps happening here is still up in the air.

Posted by: Ian-CapitalWeatherGang | January 29, 2010 11:59 AM | Report abuse

I think Sterling is talking about the general shift south from the past few days, not few runs of the models...we were looking like a 6-10 inch hit here in the Metro area on Tues...and now that low is clearly (to Sterling) going to be well south. What CWG is saying is that a very slight shift in the last few model runs is going to get our ground north of the Occoquan maybe juuuuust a bit whiter, but by no means are we looking at a large event.

Posted by: DullesARC | January 29, 2010 12:02 PM | Report abuse

Also, it's hard to know whether that discussion is based on the evening and overnight model runs or the ones from late morning today, or a combination there of.

Posted by: Dan-CapitalWeatherGang | January 29, 2010 12:05 PM | Report abuse

Models aside, it looks like the low is currently a little further north than predicted.

Posted by: ntrlsol | January 29, 2010 12:10 PM | Report abuse

BINGO on this analysis. Thanks CWG for the thoughtful piece. A minor tweak north looks like the call.

Posted by: curtmccormick | January 29, 2010 12:29 PM | Report abuse

The main band of snow is DEFINITELY current tracking well below the DC area...I think you guys are going to nail this one so far...very little snow.

Posted by: parksndc | January 29, 2010 12:35 PM | Report abuse

12z GFS snowfall outputs: map | text (not to be taken verbatim, but for guidance). At this range it's often fairly good... temps in the teens to around 20 -- higher than typical ratios possible.

Posted by: Ian-CapitalWeatherGang | January 29, 2010 12:56 PM | Report abuse

In my experience, when the models say it's going to snow, they're often wrong; but when the models say it isn't, they're never wrong.

Posted by: fleeciewool | January 29, 2010 12:57 PM | Report abuse

I don't like that map Ian...I feel like you're toying with me!

Posted by: parksndc | January 29, 2010 1:03 PM | Report abuse

12 hours ago it looked like there was going to be zilch or next to zilch for most of the area. Now Fredericksburg is looking at 6-8 inches and DC is looking at 2-4. Interesting.

Posted by: morris4903 | January 29, 2010 1:09 PM | Report abuse

Interesting....keep moving north, come on snow...

Sorry-I just can't help it!!!

Posted by: celestun100 | January 29, 2010 1:20 PM | Report abuse

Ian

The 12Z Snowfall output map seems to indicate that DC will get 4-6 inches. Please let me know if I'm reading that correctly. That's a lot more than previous forecasts had.
Thanks

Posted by: rginsburg | January 29, 2010 1:24 PM | Report abuse

Arriving at Dulles at 9pm on Saturday evening from out west. Thoughts on delays (not so much from snow accumulation at IAD, but from the weather south)?

Posted by: jim22033 | January 29, 2010 1:25 PM | Report abuse

Thank you for the the probably distribution. At least for me, that alone does a better job helping me formulate expectations than any of your narrative does.

Posted by: Wallenstein | January 29, 2010 1:26 PM | Report abuse

rginsburg, I think you're reading it right, though I think it might be *slightly* generous on liquid to snow ratios but in cold air maybe not. The GFS spits out .28" liquid with the 12z run, so that map is somewhere between 15:1 and 20:1 ratios.. would lean toward the low side to be cautious for now. And of course that's just one model and one model run. Most everything has moistened up today though, lots of guidance putting the area near .25" or so... still a tight gradient to nothing as you go north so it's tricky for sure.

Posted by: Ian-CapitalWeatherGang | January 29, 2010 1:36 PM | Report abuse

What the heck is NOAA doing? Look what is showing for Spotsylvania (22553) on their forecast:

Tonight: Snow, mainly after 10pm. Low around 21. Northeast wind around 6 mph. Chance of precipitation is 90%. New snow accumulation of less than one inch possible.

Saturday: Snow. High near 25. North wind between 6 and 9 mph. Chance of precipitation is 100%. Little or no snow accumulation expected.

Saturday Night: A chance of snow before midnight. Cloudy, then gradually becoming partly cloudy, with a low around 14. North wind between 6 and 9 mph. Chance of precipitation is 30%.

Posted by: panthersny | January 29, 2010 1:39 PM | Report abuse

I don't the NOAA has caught up to the latest models runs. I would ignore that for now.

Posted by: morris4903 | January 29, 2010 1:41 PM | Report abuse

That Spotsylvania forecast is DEFINITELY wrong on "little or no snow accumulation expected" tomorrow...UNLESS this storm "busts"...it has happened before!

Specifically, intense thunderstorm mesocyclones forming off Cape Hatteras COULD potentially rob moisture from all points north of the Great Dismal Swamp...

Posted by: Bombo47jea | January 29, 2010 1:52 PM | Report abuse

.28" liquid for here! that's gotta be at least 3", right?! that's a REAL snow (for these parts...)...

you guys are such teasers.

Posted by: walter-in-fallschurch | January 29, 2010 1:53 PM | Report abuse

"great dismal swamp"...great name, that.

Posted by: walter-in-fallschurch | January 29, 2010 1:55 PM | Report abuse

Any chance this storm sits in place and slowly moves up over the next 36 hours, if that high moves a bit? Kind of like a 1962 storm? Either way, the Outer Banks might have an ugly few days...

Posted by: drc231 | January 29, 2010 2:01 PM | Report abuse

@walter-in-FC

My understanding -- based on temps tomorrow-- is that should mean a little more than 4"

Posted by: skywatcher1 | January 29, 2010 2:06 PM | Report abuse

So there was just a story here about a surprise snowstorm 10 years ago. We are due for another. Keep trending north!

Posted by: MaltyCharacter | January 29, 2010 2:24 PM | Report abuse

omg!

cwg guys, please calm me down with some of your reasoned pessimism.... surely these people are teasing me. at the moment, your awesome updated snow accumulation map puts falls church at around 1.25". do you see any reason to up that again?

or is that just the crazy old Good-For-Snow model acting up?

Posted by: walter-in-fallschurch | January 29, 2010 2:30 PM | Report abuse

walter, we'll have an update soon.

Posted by: Ian-CapitalWeatherGang | January 29, 2010 2:52 PM | Report abuse

Right now the model tell us that there should be no precipitation in eastern KY and only some in western TN. The NWS needs to email the storm and tell it to start following the models.

Posted by: ntrlsol | January 29, 2010 2:57 PM | Report abuse

Please tell me this thing keeps inching north! As of right now I am barley in the 1-4inch line. If it just keeps pushing a little more north is possible we could see more than 4inches. I am not getting my hopes up though.

Posted by: justin-N-sterlingVa | January 29, 2010 3:00 PM | Report abuse

I just made dinner plans for tomorrow night, so all the snow lovers can be assured of a big storm (you're welcome!).

Posted by: LCFC | January 29, 2010 3:10 PM | Report abuse

Latest NAM has moved the storm a bit north and gives us a couple of inches!

Posted by: snowlover | January 29, 2010 3:20 PM | Report abuse

Bastardi just upped DC area to 4-8 inches.

Posted by: bbv91258 | January 29, 2010 3:24 PM | Report abuse

Latest update - it's #&*! cold. CWG's Thursday forecast for Friday "temperatures rise into the upper 20s and lower 30s for highs" - I think we're sitting at about 25 (dewpoint at +2). 24 hours ago? 54!

Posted by: manatt | January 29, 2010 3:29 PM | Report abuse

where did you see that at ??

Posted by: cpm2cv | January 29, 2010 3:29 PM | Report abuse

AAAAAHHHH COLD RAIN is EVIL even more so when walking between class i miss the snow chances :( DEFF wont be living here in the south ... enjoy the snow yall

Posted by: jmc732msstate | January 29, 2010 3:36 PM | Report abuse

UH! OH!! Accuweather's JB is throwin' a roundhouse hook, and calling for 4-8 inches here in the DC metro area. He's calling it out. The Accuweather hype-machine is running full blast! The northward progression is on!

Posted by: johnnierat | January 29, 2010 3:43 PM | Report abuse

new post up guys

Posted by: snowedin85 | January 29, 2010 3:44 PM | Report abuse

Hey, there's an advisory in DC but not for Bethesda?? I know we're north of DC, but geez...

Posted by: lilymama | January 29, 2010 6:56 PM | Report abuse

The comments to this entry are closed.

 
 
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