Thursday, February 24, 2011

Update On Friday's Severe Weather Threat

A front will sweep through the area midday Friday and bring us a brief window of opportunity for severe thunderstorms. This will occur between 11 AM to 6 PM from west to east. Any storms that develop will be capable of damaging wind gusts over 70 mph.

Technical Discussion

There are three key ingredients for severe thunderstorms: instability (heat and humidity), atmospheric lift (or a trigger), and wind shear. I'll discuss all three below.

Here is a map for tomorrow at 1 PM, when a front will be moving into the Blue Ridge. This front will provide the "lift" in the atmosphere. Notice the wind arrows on the map, which are strong from the west/southwest at 25 to 35 mph with gusts up to 45 mph. This should bring in moisture at the surface, indicated by the light shades of green.

There should be *just* enough low level moisture to produce minor instability in the atmosphere. Below is an image for tomorrow at 1 PM, which shows low instability. Sunshine could add some heat to the atmosphere, which would increase our instability.

An upper-level disturbance will be passing the area during this time, which will also produce "lift" in the atmosphere. In addition, jet stream winds will be very strong in the upper atmosphere. Notice the reds and purples on the map below, which indicate winds from the southwest at around 100 to 115 mph. This is a prime example of speed shear, or winds that increase with height. Although this is a key ingredient for severe thunderstorms, the shear might be too high for storms to develop.

To Summarize:

1) We will have strong atmospheric lift (surface front, upper-level disturbance)
2) Instability is low, though increased sunshine could help
3) Wind shear is very high...almost too high

We will be watching this very closely tomorrow. Any storms that develop will be capable of powerful downbursts in excess of 70 mph.