Update on the Midweek Event (Wed-Thurs, 2/29-3/1)

Just a quick update on the situation shaping up for midweek.  A modestly potent low pressure system will track to our west over the upper midwest and southern Great Lakes region during the day on Wednesday, giving heavy snows to Minnesota and Wisconsin. While this happens, a strong 1028mb arctic high pressure will hold its ground over Quebec to our north and northeast, funneling cold air southwest along the interior and coast of the northeast US, with surface winds out of the northeast due to the clockwise flow around the high as seen on the graphic below, a process called Cold Air Damming (CAD). The counterclockwise flow associated with the low to our west will allow for an increasingly moist southwest flow at the upper levels of the atmosphere, known as warm air advection(WAA):

This WAA aloft overrunning the colder air (or CAD) at the surface results in precipitation, which can sometimes be quite heavy:

As the warm air advances up and over the colder air, snow breaks out initially, and depending how much the warm air erodes the cold air first higher up, and then eventually at the surface, sleet, and freezing rain, and even rain can follow.

This type of CAD/WAA situation is usually difficult for the models to predict. It’s been my experience they usually undo the extent and duration of the cold air resulting from the strong high to our northeast. This can potentially be a fairly snowy and icy senario for our region. Snow will likely begin the storm sometime early on Wednesday, and may transition to ice, and as the low to our west weakens and a secondary low tries to form off the coast, cold air may get pulled back south at the upper levels, possibly turning the ice back to snow. Some models have the high strengthening during the event, which would allow for more snow than ice. There can even be a lull in the action later in the day on Wednesday or Wednesday during the transfer to the coast, with precip picking up again overnight Wednesday.

As you can see, it’s a complicated situation, and we’ll have updates throughout the day on www.lehighvalleyweatherpatrol.com and on the Lehigh Valley Weather Patrol Facebook page.

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