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Posted at 11:17 AM ET, 12/15/2010

Snow threat up, 1 or 2 inches possible tomorrow

By Wes Junker
Snowfall accumulation probabilities for Thursday, December 16. The most likely snowfall is 1-2 inches.

The Capital Weather Gang has been discussing two separate snow threats on the horizon over the past two days. The snowfall event for tomorrow is crystallizing based on the latest guidance. The storm still is expected to be a minor one. However, the probability of an accumulation of an inch or two is quite a bit higher than was expressed in Jason's blog yesterday.

Snow timing: Thursday - between about 7 a.m. and 6 p.m.
Probability of accumulating snow (1" or more): 60%
Maximum accumulation potential: Around 3"

Keep reading for technical discussion...


The NAM and GFS forecast models are now in agreement that weak surface low will be located near southwestern Kentucky Thursday morning and will track to our south across southern Virginia by evening. The upper level support (i.e. energy) for the system is relatively weak at1 8,000 feet. Note on the left hand side of the figure below there is no strong dip in the lines south and southwest of our region so there is not much of a wave in the upper levels of the atmosphere.

Most of our big storms have a significant wave. Nonetheless, both models now are showing a period of moderate to strong warm air advection (winds coming in from the south increasing moisture). Where the winds (flag shaped symbols) are blowing across the temperature lines is where warm advection and lifting is taking place (right hand side of the figure below). This warm advection is the reason we should see snow but once it moves through the snow should end late tomorrow afternoon or early in the evening.

NAM model run valid 1PM Thursday. Left: 500 mb (18,000 feet) heights and vorticity (atmospheric spin) on the left. Right: 850 mb (5,000 feet) heights (dark black lines) and temperatures (red and blue lines), the latter depict below freezing temperatures at around 5000 ft. The magenta circle shows the area of warm advection. Source: NCEP/

The GFS and NAM runs that were initialized with data ending at around 1 a.m. last night gave the D.C. area between .08" and .17" of liquid equivalent. This morning runs of the NAM, GFS both give the area at least 0.10" with the NAM being the snowier or the two. The thermal profiles indicate the ratios of snow to liquid might be a little higher than 10-1 (10 inches of snow for every inch of rain) though that might be mitigated somewhat by the time of day as the snow should fall during day time hours with the snow most likely starting after rush hour.

One way to assess the probability of getting an inch or more of snow is to look at the one of the products of the Short Range Ensemble Forecast System (SREF). This product not only takes into account the uncertainty in being able to correctly measure the initial state of the atmosphere but also takes into account the uncertainty in how the models handle the atmosphere's physical processes. Almost 50% of the members (or models) predict that the Washington area will see more than 0.10" of liquid equivalent. The SREF guidance suggests that the probability of getting an inch of snow on Thursday has risen since yesterday. One thing to note on the produce is the tight gradient in the produce suggesting that there could be a rather sharp edge to the northern edge of the accumulating snowfall.

sref_121610.gifAs of 4 a.m. this morning, SREF probabilities of getting 0.10" of liquid equivalent (or about 1" of snow) during the 12 hrs period ending at 10 pm December 16. The probability scale is shown on the right. Source: Penn State

I'm boosting the probability of getting an inch a little above the ensemble guidance shown above because the GFS, NAM and Canadian models all suggest that we will see an inch. Note that south of Washington toward Richmond 3-5" of snow or so is possible - so you won't have to go too far to get into more substantial snow.

I'll have a separate post on the weekend threat early this afternoon. Also, stay tuned later today for a snowfall accumulation map for tomorrow and SchoolCast.

By Wes Junker  | December 15, 2010; 11:17 AM ET
Categories:  Latest, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Federal leave for severe weather revamped
Next: Weekend snow threat down, not out


The trend seems to be our friend for the weekend storm. The last four GFS runs have been progressively a bit wetter.

Posted by: JTF- | December 15, 2010 11:59 AM | Report abuse

I'll definitely take 1-2 inches tomorrow. This will be a nice snow that does not cause too many problems. I could also definitely see this being a suprise 2-4 inch event especially if most of snow falls after 5 pm when it is dark.

Posted by: ajmupitt | December 15, 2010 12:00 PM | Report abuse

This is good news for the snow lover in me. Thanks Wes!

Posted by: Rcmorgan | December 15, 2010 12:01 PM | Report abuse

Posted by: dcg35 | December 15, 2010 12:44 PM | Report abuse

2.5 inches on sunday!? what!???? dont say that please. thats pitiful. snow lovers want a foot. lol

Posted by: lll1424 | December 15, 2010 12:58 PM | Report abuse

Darn it...the "sweet spot" still seems to be to our south...though now it appears to be slated for 3 to 5-inch accumulations! If only we could bring that sweet spot up to Alexandria/Arlington/Falls Church.

Yesterday seems to have been the "cricket-killer" freeze...I may need to verify that over the weekend if it's sunny on Saturday and gets above freezing.

Posted by: Bombo47jea | December 15, 2010 1:13 PM | Report abuse

I love snow as much as anyone else but the timing of this one stinks! And it doesn't matter if it is a dusting or 2 inches. People leave all rationale at home when driving in winter weather. Tomorrow evening's commute seems like a fun one...Any idea from the pros as to whether or not this will be true?

Posted by: authorofpoetry | December 15, 2010 1:19 PM | Report abuse

you mentioned the "sweet spot" missing us as of now. well, in a strange way, i'm ok w/that - especially how it keeps moving. maybe it shifts back to over us? seems like one thing's pretty likely: where the sweet spot is now is probably not where it ends up in actuality...

Posted by: walter-in-fallschurch | December 15, 2010 1:32 PM | Report abuse

well I'm down here in Cville look like we could get 3 to 5 inches of snow with this storm tomorrow and I'm liking what the models are doing for this Sunday storm

Posted by: cpm2cv | December 15, 2010 1:37 PM | Report abuse

cpm: what are the models showing for this sunday in the dc area?

Posted by: lll1424 | December 15, 2010 1:45 PM | Report abuse

cpm- I think you spoke too fast. Euro was an axe to the snow lover's heart.

Previous models today were moving it east.

Posted by: kallieh | December 15, 2010 1:50 PM | Report abuse

I mean west. Sorry. Less out to sea.

Posted by: kallieh | December 15, 2010 1:50 PM | Report abuse

seems like you all are higher then everyone else weather. gov still has less then an inch

Posted by: minerdude | December 15, 2010 2:53 PM | Report abuse

I'm a 1st grade teacher and I can ALWAYS tell when weather is brewing by the behavior of my kids (full moons, snowstorms and hurricanes make them a bit bonkers) - I'm pretty sure something is coming!

The timing of tomorrows snow is all off though - not happening early enough for delay/closure or early release - sounding like we'll all be trying to get home in the worst of it. ugh. And to pour salt in the wound it's the 1st snow event which freaks everyone out a little more! Maybe it will be enough for everyone's favorite "the 2 hour delay" on Friday! :-)

Posted by: mamory1975 | December 15, 2010 3:46 PM | Report abuse

The 18Z gfs and NAM models continue to support the amounts discussed in the blog. Walter, with such a weak system, I think the sweet spot will be narrow and is likely to be to our south.

Posted by: wjunker | December 15, 2010 6:02 PM | Report abuse

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