Tuesday Evening February 19 2019

The weather map this evening is looking very impressive. A large band of rain and snow dominate the eastern half of the nation. It’s developing Low Pressure in east Texas that will drive the snow. That will help to push a lot of tropical moisture north of the Gulf of Mexico our way. But it’s running into dry air. We have a set up for snow to be shown on radar for a few hours in the morning, but it may take a little longer to reach the ground.

The Winter Storm Warning in place will prove itself. High Pressure in Pennsylvania is currently blocking the progress, but that will move out of the way. In place will be air just cold enough for snow to begin at the beginning of the day. But it’s a little more interesting than that. If the snow takes an extra hour or two to reach you, that will be the atmosphere actually helping to generate more cold air to hang on longer.

Baltimore Weather Conditions at BWI 10 PM

Temperature = 35ºF
Dew Point = 9ºF
Relative Humidity = 33%
Wind: NE at 3 mph

The air is very dry!

That means the initial moisture on radar will dry (sublimate) before reaching the ground. That is called virga, and it shows up extensively on the radar simulation. This will also help to drop the temperatures a few degrees.

Cleaning up the noise in this display, we can see the NAM 3 Km Model break down of what should reach the ground. That is what is shown in the sliders below. Compare that to the NWS onset graphic, and it does look to be an hour or two behind.

Winter Precipitation Simulation (Morning) —> slider

[metaslider id=73615]

This is a good set up for evaporative cooling. Since the air is so dry, as the snow finally reaches the ground, it will cool the column of air by a few degrees.

So if you wake up and see temps barely near freezing, it should drop and then hold. That ‘hold’ will be important in the potential ice storm for the afternoon.

Temperatures at 7 AM

Colder At Temperatures at 11 AM

Winter Precipitation Simulation (Afternoon) —> slider

The colder air would allow ice to linger longer near and north of Baltimore. This is what I suggested for the past few days, as models often move the cold air out too soon. Here the NAM is jumping on that idea.

[metaslider id=73628]

Winter Storm Warning (pink)

Southern PA, Cecil and Kent Co in MD have been included.
Winter Weather Advisory (purple): Southern Maryland and most of the Eastern Shore/Delmarva

Storm Notes:

Wednesday morning most of the area will see snow. This includes Southern MD and Delmarva.
A burst of moderate to heavy snow could bring 1″/Hr at times.
Wednesday Afternoon: Snow will change to sleet and freezing rain from south to north. That change over will be around lunchtime between Washington and Baltimore.
Wednesday afternoon and evening: There will be icing across central Maryland
Wednesday night: The normally colder zones will have heavy icing
Thursday morning: It is possible that leftover ice has not fully melted or fallen off of trees in some areas even with warming and rain

My Call For Snowfall

The snowfall range really depends on the burst of snow and timing before the change over. Once the ice hits, the snow will compact anyway.
Sleet and freezing rain will be an additional problem to deal with through evening and possibly at night.
The bust potential would lean towards overachieving. I am still staying lower that possible due to the sleet.
Note that the initial snow intensity can reach rates over 1 inch per hour.

Zone A:

5″ to 8″ of snow. *Bust potential for less snow if the ice arrives sooner. The white circled region can get up to 12″ if they stay all snow longer.
The highest snow may be where it begins earlier allowing for more stickage before late morning.
This Zone will have the most snow and heaviest icing Wednesday night

Zone B:

3″ to 6″ of snow. It will fall heavy at times in the morning. They higher end is based on heavy bursts… But if sleet moves in sooner, then the lower end will be more common.
Change over will be around lunchtime (between 11 AM and 3 PM).
I expanded this zone to include Kent, Queen Annes, and Caroline Countes.
Moderate icing in sleet and freeing rain
Should change to rain from south to north between 3 PM and 8 PM Wednesday.

Zone C