Bolaris: Significant snow upgrades

Snow began falling about noon in the Philadelphia region and picked up steadily.

We're seeing what is called a "dynamic overrunning event" taking place right over the Delaware Valley.

In reality, we're not watching a storm in the true sense of the word, but a very significant slug of moisture that will form across the Tennessee Valley and then build as it pushes toward us.

The snow broke out as the warmer moist air collided with the Arctic air mass now in place.

If I were to totally believe the computer guidance, about 2 to 3 inches would fall across the I-95 corridor before we see a change to sleet, brief freezing rain and then rain.

However, from past experience I have seen this very dense cold now packed across the Northeast -- along with snow cover on the ground -- make it very difficult for the milder air to scour out the colder air now in place.

The result is what I call a "Big Thumping," a pre-warm frontal snow. This could greatly enhance the accumulating snow rates to 1 to 2 inches an hour before it's pushed out by the milder air.

Also, the computer models are not considering the snow pack that already exists, which could slow the changeover process from 1 to 3 hours.

With this in mind, I believe total snowfall could reach 4 to 6 inches in the city before that change-over occurs. Expect higher amounts to the west, where more liquid will fall and temps will remain a degree or two colder than models suggest.

After 7 p.m.: Snow intensity will decrease.

8 p.m. to 9 p.m.: Any remaining snow will change to sleet, then rain. Pockets of freezing rain are possible north and west of the city.

After 10 p.m.: Periods of rain and drizzle. The northwest suburbs will see freezing rain and sleet linger up until about midnight.

Sunday: We'll be drying out, as some sun returns. High is 42 degrees.

Contact John Bolaris at jweatherboy@weathersavior.com and on Twitter @weathersavior