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Wild weather last night…any more coming?

Although it was a close call at the time last night’s wild weather – due to an upper level disturbance crossing the region – stayed just to the northwest of Fredericksburg.  Heavy rain, large hail, and storm rotation was all evident on radar, and I just read through some of the flooding and large hail reported to the NWS Sterling office.  The rainfall totalled up on radar was impressive:

The darkest red shade splayed across Culpeper indicates over four inches of rain fell during the storms.  And you can at least partially blame this morning’s widespread fogginess on the moisture left behind by all the precipitation last night.

The Storm Prediction Center is predicting another small possibility of strong to severe storms today across most of Virginia as the slow-moving cold front finally edges its way eastward toward the coast.  There is a better chance of just plain rainfall after sunset, with today’s (Friday’s) high temperature climbing into the low 80′s (F) ahead of the frontal passage.  The front looks like it will go far enough to the east to provide partly sunny conditions on both Saturday and Sunday, but a couple of weak “wrinkles” will form along it and possibly provide some showers for the ‘Burg both days (with higher chances of rain further east of the city).  The highs both weekend days will rise only into the low 70′s.

Now, why has our weather been so hard to get a handle on recently? It has to do with two main factors involving the jet stream (winds above 25,000 feet):

(1) “zonal flow”, a condition in which the jet stream orients itself in a west-to-east configuration.

(2) The jet stream retreats northward into Canada.

This upper level flow of air helps to separate the cold dry air to the north from the warmer and moister air to the south, and it also “steers” surface weather systems. When zonal flow sets up across North America the clash between the two air masses (necessary to create strong surface weather systems) is inhibited and only weak fronts and low pressure systems tend to form. In addition, when the jet stream moves north into Canada these weak surface systems are left to fend for themselves without any “steering”, thus meandering somewhat randomly across the landscape and making it difficult to forecast the timing and effects on our weather.

So when zonal flow kicks in, expect the forecast to jump around more than usual!