Ice Tonight

9:30AM Update:
Obviously, some schools canceled today, including Moco due to the ice. Temperatures have risen above freezing across most of the area. Expect melting an improving conditions. The winter Storm warning is set to expire at 10:00 AM so that's all folks. The rest of the day will have temps rise the mid to upper 40s lower 40s and rain with clearing later.

10:20PM Update:
Light freezing rain and rain has overspread the area. Areas to the north and west have the freezing precip, but those close to or inside the beltway have just plain rain as temps are hovering just above freezing. We can expect these temps to drop below freezing eventually. The NWS has raised ice totals slightly along the eastern edge. The radar is rapidly filling in as the primary low approaches from the southwest.

8:45PM Update:
Dewpoints are quite a bit lower than forecasted, meaning evaporative cooling may lower temperatures more than expected, prolonging the cold. This also might indicate that the high is stronger than expected, once again, pushing more cold air in. Precip is approaching from the south.

SPC analysis shows the freezing line across Moco at 8:00PM. The purple line denotes below freezing temperatures and there is a 2 degree difference between the lines. The tan lines are the same except for temperatures above freezing. 

The wet bulb temperature is the temperature that the air could be cooled to by evaporative cooling if the air was completely saturated such as by evaporation of precip. Red is above freezing and blue is below freezing.

8:10 PM Update:
If you think we had it bad with the polar vortex and all this winter, take a look at Slovenia. 2+ inches of freezing rain is no joke. It was estimated that 25% of the population was without power and 40% of its forests damaged.

More pictures here.
7:00 PM Update
Surface are approaching freezing if not already below. The 18z model runs have not come in any different than previous runs so the previous forecast will remain unchanged. Temperatures will continue to drop before rising early in the morning in response to a surge of warm air from the south. Something to note is that there seems to be a weak low pressure system developing off the Carolina coast. The effects, if any, of this new secondary low is yet to be seen.

Main low is working NNE to the west, weak baby low is off the Carolina coast

Original Text:
The winter storm watch has been updated to a warning, calling for .25 inches of freezing rain and up to an inch of sleet. The sleet accumulation will be mostly confined to the far north and west. While the northern half of the county is in for an icy mess, the southern half is less certain. Models show a surface low moving from the gulf north towards us and passing us just to the west. The NAM and CMC also show a secondary low developing off the coast while the old low dissipates. This will keep a northerly component to the winds for a longer period of time, keeping it cold. The GFS, on the other hand, keeps the primary low strong, eroding out the cold air much faster than forecasted. Temperatures should rise above freezing from southeast through northwest mid tomorrow morning as the temperatures rise in response to southerly flow from the approaching low.

The NWS scaled down totals to the south and east, likely in response to warmer temperatures and higher uncertainty in the forecast. We shall watch the temperatures and depending on how far we can drop, adjust the forecast accordingly. If this forecast were to verify, we would likely have no school but due to the uncertainty of the forecast, I think that Moco will delay the decision until as late as possible, forcing us to wake up to check tomorrow at 5:00AM. 

Thanks for reading!

-Alex

Hmmm Ice...

10:30 UPDATE:
The NWS came out with their ice map. The .25 inch line, which is the warning criteria level, is in  Moco.

ORIGINAL TEXT:
Today was a forecaster's nightmare come true. The temps failed to drop like expected and as a result, rain prevailed, with only a few wet snowflakes to remind us that we are at the mercy of Mother Nature. tonight should be quite cold though, with lows in the low 20s. On Wednesday, temps will only rise into the low to mid 30s, keeping the ground cool, which is better for accumulation. At night, temps drop into the upper 20s as an low pressure system approaches from the south. High pressure to the northeast will force cold air against the mountains, keeping us below freezing as the upper levels rise above freezing. depending on how much cold air is in place, precip can range from snow, to sleet, to freezing rain ,to even plain rain. The most recent NAM model runs have come in colder and wetter and as a result, the NWS has extended the Winter storm watch to cover Moco.

THE NATIONAL WEATHER SERVICE IN BALTIMORE MD/WASHINGTON HAS
ISSUED A WINTER STORM WATCH...WHICH IS IN EFFECT FROM TUESDAY
EVENING THROUGH WEDNESDAY AFTERNOON.

* PRECIPITATION TYPE...SLEET AND FREEZING RAIN

* ACCUMULATIONS...A COATING TO ONE INCH OF SLEET ACCUMULATION.
  POTENTIAL FOR A QUARTER INCH OR MORE OF ICE ACCUMULATION FROM
  FREEZING RAIN.

* TIMING...FREEZING RAIN AND SLEET WILL BEGIN TUESDAY EVENING.
  PRECIPITATION WILL CHANGE TO ALL FREEZING RAIN LATE TUESDAY
  NIGHT AND IT WILL CONTINUE THROUGH WEDNESDAY MORNING.
  PRECIPITATION WILL END WEDNESDAY AFTERNOON.

Once again, a few degrees difference can affect the precipitation type. However, cold air damming has a tendency to be colder and stay colder than what models predict. Sleet and freezing rain, falling on roads that are devoid of salt, can turn it into a skating rink. Not something fun to drive on. School closures, if we are upgraded to a warning, are likely. Otherwise, a delay is likely and closing possible.

Since the NWS has not issued an accumulation map, I will make one based off of their totals:

Freezing rain accretion

Sleet

The sleet and ice accumulation may be a little high but the ranges are generally accurate, just don't expect the high side of the estimates. Temps will rise above freezing on wednesday morning progressing from SE to NW. When it will rise is tricky and will depend on how far the high leaves and how strong our easterly winds off the ocean are. In any case, cold air can be very stubborn and areas to the north may deal with an extended period of freezing precipitation.

Thanks for reading!

-Alex

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