Friday January 17 2020

Evening Update

The break out of snow during Saturday morning is pretty much a lock. There is only a minor discrepancy of the start time among the short range high resolution models. I have prepared two simulations to compare below: The HRRR and NAM 3 Km Models.  Before we dive into them, let’s look at the set up for the storm with the morning conditions.

Saturday Morning

Arctic air will be full in place for our region a daybreak. The central of the storm shows Low Pressure in northern Illinois. But we will be getting the far edge of this system. It is that big!

Part 1: A band of snow will be associated with a mid level warm front. This push of snow will have some moderate bands. What falls will stick. Possibly a coating or so in Annapolis and Easton on the Eastern Shore. The heavier bands on the northern edge could bring 1/2″/Hour or more intensity. But it will only last a few hours.

Part 2:  The return of precipitation will be mostly a wintry mix, freezing rain, and then rain (southern areas).

There will be a break or lull for a few hours. The duration of this is the big debate shown in the models below.  The longer the lull, the better chance for temps to warm up above freezing. But a quicker return, the afternoon is what I expect, so the ice will last all day just north and west of Baltimore. The city to Annapolis and Eastern Shore will have an afternoon with just rain.

 

Morning Temperatures

Deep Freeze applies here as most of the region will start the day in the mid 20s. Again, any flakes that fall will stick. We have all night to further cool the pavement.

 

Model Simulation Comparison

The two models see include the HRRR (updates every hour), and NAM 3Km (updates every 6 hours).  Both continue to show most of our region gets the snow arriving between 7 AM and 10 AM. The HRRR comes in an hour later and suggests a lull or break between the morning snow and ice up to 6 hours long. The NAM 3Km shows the snow an hour earlier and shortens the lull. The afternoon icing will be moderate and will affect areas in north and west of I-95. Place by the Bay will be rain by then.

Here’s a Look:

HRRR 8 AM to 2 PM—> slider

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Compare this to the NAM 3 Km and see the snow a little earlier. The ice by early afternoon builds back quickly.
NAM 3Km 7 AM to 2 PM—> slider
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Temperatures at 2 PM

Notes:
Ice will remain a problems on roads through evening including:
  • northern Baltimore County
  • Carroll County
  • western Howard/Montgomery Counties
  • Frederick and Washington County
  • Southern Pennsylvania

 

NAM 3Km 3 PM to 8 PM—> slider

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Temperatures Saturday Night
Freezing temperatures remain in the colder inland counties. The counties in the Winter Weather Advisory (below) are likely to remain icy and have the time extended. I would reconsider your plans to travel north and west of the city.
Winter Weather Advisory
Most of these counties will have ‘over’ 1 inch of snow, ice in the afternoon, and likely remain below freezing through Saturday night.
My Original Forecast

My Call For Snowfall (and ice)

Notes:

  • The timing is critical! Earlier arrival means more impact for all.
  • Plan for the start between 7 AM and 10 AM
  • Snow Burst or ‘Thumping’: Arrival of snow can come with a push of moderate intensity.
  • The warmer Chesapeake Bay will play a role in stickage, but the initial push should coat the roads and impact travel during the morning.
  • The transition from snow to sleet and freezing rain in metro areas will be between 11 AM and 2 PM. Then rain to follow before evening.
  • More Snow North! The colder air will last longer (blue shading). This region will also stay frozen with ice into Saturday evening.
  • Western Maryland/Wisp: They will get ice Saturday afternoon, then more snow returns Sunday.

 

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