Tuesday Afternoon February 6 2018

We are still under a Winter Weather Advisory Wednesday morning February 8 starting at 3 AM for most of central Maryland west of the Chesapeake, and northern Virginia. Also in Pennsylvania from Lancaster and York, westward. There are a lot of similarities to the event we had this past weekend, but not an exact repeat. The main issue again will be tracking the freezing line, which even the coldest modeling had some trouble with before. Below are a few model comparisons to help with the timing, but first keep these key notes in mind:

Cold Afternoon

Even though many got above freezing, the cloud over has limited solar warming of the pavement.

Cold Ground

Temperatures have reached the mid and upper 30s today, but cloud cover has limited solar radiation warming the pavement.
Timing of the first flakes and sleet pellets should be in the dark hours before dawn Wednesday. That will act on the coldest time of the day for the ground.
Temperatures may warm slightly, then chill again with the main batch of precipitation.

Also see: What is Winter Precipitation: Snow, Sleet, and Freezing Rain

Thawing Can Be A Slow Process

Once the ground is frozen and covered with ice, it can take a few hours to thaw after thermometers are reading above 32ºF. This can vary just enough to not be widespread based on elevation and even exposure to solar energy vs shaded spots.

Valleys Hold Cold Longer

Winter events with mixed precipitation is based on warmer air mixing in. Since warm air will rise, it reaches the clouds first but also hill tops. So valleys can stay colder and icy longer than surrounding hills.

Weather Set Up

We should have the light snow and sleet arrive before sunrise. That may help schools make their call sooner.

GFS Model: Wide View

The GFS Model (which was too warm this past weekend) has caught on to the cold air… But these are 6 hour snapshots. A more detailed view in the time between is below.

By early afternoon, the modeling shows rain north of I-80 in PA. But the trough seen as a kink in the isobars is shown here. This is Cold Air Damming, a classing winter set up with cold air holding east of the mountains and west of I-95. This is very hard for models to predict thawing. That is why a wider window of time is needed to be considered here than just noon.

HRRR Model

This high resolution look shows the cold air in place at 4 AM. This is actually warmer than the NAM 3 Km model below. But all it takes is one or two degrees if you live near I-95 and the Chesapeake Bay. This happens to be where the bulk of the population is and the widest margin of error.

I need to point out that as of my time to write this post, 4 AM was the extend of the model run. It is a short term model…

I do see temperatures dropping colder as the main precipitation moves in.

The ground can be frozen at night even when the air at thermometer levels of 5 Ft are a little warmer. So I still see frozen ground for Baltimore and Washington even though is shows 33ºF here.

NAM 3 Km Model

This model has done the best job overall all winter with our marginal events…

This shows the cooler air at 7 AM after the initial precipitation arrives.

This hints at an initial snow/ice event, then a break for an hour or two before steady ice spreads in. This suggests an upper level warm front that moves the snow north and behind it the clouds warm up while the ground temps may actually get colder between 4 AM and 8 AM… and hold near freezing most of the morning.

Notice 32ºF around sunrise for Aberdeen and Baltimore. Washington on the fence and all places inland N/W colder.

Pre-Dawn Initial Snow/Ice moves north…

Late Morning

Solid Freezing Rain inland of I-95/ North and west of Baltimore. Bel Air and Ellicott City on the line. This may not be exact. We have a margin of 20 or so where the freezing line may set up.

Suggestion but not a promise of mostly rain. This was the issue last Sunday as the cold air hung around longer for parts of Baltimore/Carroll/Frederick Counties in Maryland.

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