Thanksgiving Eve Coastal Storm: Nov 24 2014 Update

More Like White Rain or some SnowTV?

The already marginal event is becoming even more marginal. Recent model runs have converged on a solution which puts us right on the rain/snow line. Every little bit matters for us now. Small shifts in temperatures, precipitation duration and intensity, low track, and cold air supply can change our snowfall outlook by a lot. That being said, this looks to be a complicated slop fest, with mostly white rain. Let's break the situation down:

Low track:
We are set with this. Almost track is just a tad west, with the low forming near Wilmington, NC, tracking NNE through the Outer Banks, and up more or less parallel to the East Coast (I-95 Corridor). This is the standard "Miller A" track storm.

 Precipitation:
We have plenty of precipitation due to the vigorous low moving north off the coast. This low will be wrapping in moist air from the Gulf of Mexico. Precipitation amounts between .6 and .9 inches are expected across the area. The precipitation maximum or around an inch is expected to be centered over the I-95 corridor.

Temperatures:
 Surface temperatures is the main (and very complicated)  issue. Our average high for November 26 is 54 degrees. This means we need temperatures to be 20-25 degrees below normal. 

The mid and upper levels of the air column are cold enough for it to snow. Not perfect, but decently cold. However, there will not be a source of very cold air like we usually see in heavy snowstorms. Our blockbuster storms typically have a strong high pressure in Southern Canada, entrenching cold air in the region. Warm air will be on the move out as the storm arrives so a slight increase in speed, as some models have been indicating, would result in the precipitation arriving before the cold air is in place. There is a weak high pressure signature to the north of the region, which means that relatively cold air will be attempting to filter into the region. However, this will likely to be too little too late. With this storm, the infiltration of cold air is not enough; the storm has to manufacture its own through a process called dynamic cooling, where intense lift associated with deepening low pressure cools the air. 

A few degrees will make the difference. Most of the area will start in the low 40s and with some upper 30s in the higher elevations. Temperatures will drop but how far it will drop will be largely elevation dependent. Areas with lower elevation will remain warmer and can expect to get less snow.

 Yes, I realize, I forgot to talk about dynamic cooling again. Will do later. :P

What the NWS says:

HOWEVER...PRECIPITATION IS EXPECTED TO OVERSPREAD THE AREA QUICKLY
EARLY WEDNESDAY MORNING AS THE COASTAL LOW TRACKS NORTHEAST
TOWARD CAPE HATTERAS. THE LOW WILL INTENSIFY AS IT TRACKS
NORTHEAST...AND THIS WILL ALLOW FOR MOISTURE FROM THE ATLANTIC AND
GULF OF MEXICO TO GET WRAPPED INTO THE STORM.
THEREFORE...PRECIPITATION WILL BE MODERATE TO LOCALLY HEAVY AT
TIMES DURING THE DAY WEDNESDAY. THE MAIN QUESTION IS HOW MUCH
COLD AIR WILL WORK ITS WAY INTO THE REGION...CAUSING THE THREAT
FOR SNOW. AS OF NOW...IT APPEARS THAT THERE WILL BE ENOUGH COOLING
FROM THE WET-BULB EFFECT AND DYNAMICAL COOLING FOR ANY RAIN OR
MIXED PRECIPITATION TO QUICKLY CHANGE TO SNOW FOR LOCATIONS NEAR
THE MASON-DIXON LINE AND ALSO NEAR AND WEST OF THE BLUE RIDGE
MOUNTAINS. THE HEAVIEST SNOW ACCUMULATION IS MOST LIKELY TO FALL
IN THE MOUNTAINS...BUT THERE IS A POTENTIAL FOR SEVERAL INCHES OF
SNOW ACROSS MOST OF THESE AREAS SO A WATCH HAS BEEN ISSUED.

FOR LOCATIONS CLOSER TO INTERSTATE 95...THERE MAY BE ENOUGH WARM
AIR FOR PRECIPITATION TO START OUT AS RAIN...BUT EVEN ACROSS THESE
AREAS PRECIPITATION WILL LIKELY CHANGE TO SNOW DUE TO THE COOLING
AFFECTS PREVIOUSLY MENTIONED. HOWEVER...SURFACE TEMPS WILL HOVER
CLOSE TO OR PERHAPS EVEN A COUPLE DEGREES ABOVE FREEZING AND THIS
MAY MAKE IT DIFFICULT FOR SNOW TO STICK. THE HIGHEST DEGREE OF
UNCERTAINTY IS IN THESE AREAS SO PLEASE STAY TUNED TO THE LATEST
FORECASTS FOR UPDATES. FOR LOCATIONS SOUTH AND EAST OF
I-95...MOSTLY RAIN IS EXPECTED...BUT THERE IS A POTENTIAL FOR
SNOW TO MIX IN WEDNESDAY AFTERNOON.

PRECIPITATION WILL END FROM SOUTHWEST TO NORTHEAST LATER WEDNESDAY
AFTERNOON INTO WEDNESDAY EVENING. EXACT TIMING IS STILL UNCERTAIN
BUT THE FORECAST LEANS A BIT TOWARD THE MORE PROGRESSIVE GUIDANCE
SINCE THIS SHOULD BE A FAST MOVING SYSTEM AND IT IS NOT EXPECTED
TO PHASE WITH THE NORTHERN BRANCH UNTIL AFTER IT IS OFF TO OUR
NORTH. TEMPS WILL DIP BELOW FREEZING WEDNESDAY NIGHT AS CONDITIONS
DRY OUT. THEREFORE...ANY WET OR SLUSHY ROADS MAY FREEZE UP.

They show the uncertainty pretty well. (They have a grammaring mistake too)
They seem pessimistic about our chances of it actually sticking much too:

LWX snow map shows a dusting for most of MoCo and up to 3 inches for the people above the fall line.

Conclusion:
The dynamics of the event have not changed much since yesterday apart from a better model consensus and some tweaks to the areas close to the rain snow line ( cough cough us). ... Ok. I guess to us it matters. The main story is that we will be sweating it out on temps. Without the arrival of heavy precipitation, we may not get anything more than white rain. Otherwise, we get SnowTV and if we're lucky, some sloppy accumulation. The potential for this storm to bust on the low (and warm) side is higher than for it to bust high (colder). While we're waiting, lets freak out. 

The Peanut Gallery:

One of the worst case scenarios: March 6th 2013, where we were forecasted to get 10 inches and we 

got like 2.

Jim Cantore standing in a snowless DC

Never Forget: The Snoquester 

If you ever wonder about my thought process: link

Link previous text: http://mdtropics.blogspot.com/2014/11/thanksgiving-eve-coastal-storm.html