Rain or Snow?

6:00 AM UPDATE:

The warmth won. This is a bust. T-3 throughout the county.

10:10 UPDATE:

The GFS appeared to come in with more snow than previous for the more southern areas. However, the snow maps are not yet out.

The snow maps for the NAM is out, showing Trace-6 inches for Moco

Another interesting this is the NWS snowfall total probability chart. It is evenly spread on snow totals. Interestingly enough, we have a higher chance of getting 8-12 inches than any other category. If only the temps were lower :(

Location	Min	Likely	Max	0"	0.1-1"	1-2"	2-4"	4-8"	8-12"	12-18"	>18"
Burtonsville, MD	0	3	9	10%	13%	15%	17%	18%	24%	3%	0%
Chevy Chase, MD	0	2	9	12%	18%	20%	9%	17%	23%	1%	0%
Damascus, MD	2	5	11	1%	5%	8%	24%	25%	30%	7%	0%
Germantown, MD	1	4	10	1%	10%	13%	26%	16%	28%	6%	0%
Poolesville, MD	1	4	11	1%	10%	13%	26%	16%	28%	6%	0%
Rockville, MD	0	3	9	12%	13%	16%	15%	16%	24%	4%	0%
Silver Spring, MD	0	2	9	12%	18%	20%	9%	17%	23%	1%	0%
Takoma Park, MD	0	2	8	12%	20%	19%	9%	17%	23%	0%	0%

10:00 UPDATE:

The NAM came in farther north, and wetter. But farther north means less snow.

Temperatures are really an issue now. 50@DCA 49@IAD 46@BWI. Seems to be a wet raw mess tomorrow with rain, sleet, and snow possible. The NWS has kept it's WWA for Moco calling for 2-5 inches

I have an issue with their timing for snow though.

TIMING...RAIN WILL MIX WITH SNOW BEFORE DAWN AND EVENTUALLY
  CHANGE TO ALL SNOW BY SUNRISE MONDAY MORNING. THE HEAVIEST SNOW
  IS EXPECTED DURING MONDAY MORNING. SNOW WILL MOVE EAST OF THE AREA
  MONDAY AFTERNOON.

I think it could change significantly later but this being the official advisory, I will raise chances of having no school tomorrow to 60%. I live in SE Moco so my expectations are very low. This forecast may verify for those living farther north.

GFS will come out around 10:30 PM so I will try to have an update around then.

ORIGINAL TEXT:

Today was the warmest day of the year. Reagan Nat'l made it to 64, Dulles to 61,  and BWI to 57, all during the late afternoon, around 4 just prior to the passage of a cold front. Temps have dropped 12-13 degrees in response to a switch from a southerly flow to a northerly flow.

 A strong wave of low pressure will ride along this front tomorrow to impact us with a mix of rain and snow. As it has been many times this winter, temperatures will be a problem. The models put DC on the rain snow line, making this a snow event for points north while areas to the south get heavy rain. The NWS posted a Winter Weather Advisory for Moco with 2-5 inches of snow expected. Temperatures are forecast to be marginal for snow. Model means for DC proper shows temps above freezing until the precip has passed by. temps will be lower the father north and west one goes. The colder we drop, the farther the rain snow line move south.



There are  few ways of dropping the temperature. The most effective is to move cold air in (Cold air advection) through a large scale mechanism like a cold front. A second way is through evaporative cooling, where precip evaporates to saturate the air. The phase change from liquid/solid to gas takes heat out of

 the air, dropping temperatures. Although minor compared to CAA, this can still make a difference. the dewpoint is significantly lower than forecast, which may work in our favor in allowing more evaporative cooling to take place. However, I do not expect it to snow from the beginning of the onset. In fact, this is highly unlikely. Rain will come first. When it changes over to snow is the main question.


There is extremely high uncertainty in this forecast. The SREF, a suit of 22 model runs ,each with slightly different initial data, demonstrates this. The models run call for anywhere from nothing to a foot of snow. As usual, the lower end is more likely than the higher end because temperatures have the tendency to stay higher than modeled or forecasted in these type of the situation.


All the major global models have us on the southern fringe of the snow with a VERY tight gradient. 10 miles north/south is the difference between nothing and 6 inches of snow.
GFS:
The GFS has been consistently too warm for areas farther north but is probably decent for our area. The 18Z run came in colder and wetter, with us on the "right" side of the rain/snow line. The previous run had us on the "wrong" side, with around an inch for S Moco and 6 for north Moco.

NAM:

The NAM shifted farther north from the 12z-18z, almost exactly the opposite of the GFS. Both maps are based off of 10 inches of snow per inch of liquid. This is probably too high. This will be a wet, sticky,  heavy snow. The ratio will be lower and probably closer to 7:1. Anywhere that receives more than a couple of inches of this snow runs that chance of power outages. 

This is a classic example of a few degrees bring the difference between heavy rain and 6" of heavy wet snow. I hazard a initial forecast of 1-3 inches in SE Moco and 3-6 in northern Moco based on current temps and models trends although totals can go either way. There is low chance of delays because the heaviest snow is supposed to come during the day with rain initially in the morning. A closing is 50/50 since there is a Winter Weather Advisory up for Moco. I will try to post the 0z model runs if I stay up that late (10:30 ish).

Do the snow dance, do your hw, hope for snow, thanks for reading!

-Alex