Discussing the Latest on Wednesday Afternoon's Severe Threat

        Seems like severe weather season has awakened as Central Virginia is expected to see its first significant round of severe weather tomorrow afternoon. A very powerful storm system will sweep east tomorrow, bringing the chance for some damaging winds and a few tornadoes. The Storm Prediction Center has most of Virginia in a level (2/5) "Slight" risk area while the Tri-cities and points south are in an elevated "Enhanced" Risk area (level 3/5). Peak timing for metro RVA looks to be anytime between Noon and 2 PM. 

SPC Day 2 Categorical Risk. Most of Virginia is outlined in a "Slight" risk while southern Virginia is outlined in an "Enhanced" risk area. 

        As of this evening, a very powerful mid and upper level trough with a deepening surface low pressure is centered over the midwest, bringing strong winds and isolated tornadoes. This low pressure system will deepen some more tonight and track near the Great Lakes region by tomorrow, with a strong cold front sweeping to its south. It is along this front that we expect to see the development of a north-south orientated squall line that will race through the metro sometime around Noon-2 PM. This squall line has the potential for damaging wind gusts of 60+ mph that could bring down trees and powerlines. In addition, the very strong wind field will promote the potential for a few "spin-up" tornadoes embedded within the line. 

GFS Forecasted Precipitation, Pressure, and Height Map for Tomorrow Around 1 PM. GFS forecasts a squall line near the metro around this time.  

        At this point, confidence has increased that the Richmond metro region could see significant wind gusts and a few tornadoes. However, there are still some caveats with tomorrow that could dampen the threat. While the kinematic (wind) environment and forcing for ascent (mechanism to lift air) will be very robust tomorrow, the question remains on how much instability can build in before the cold front approaches. The latest GFS forecasted weather sounding from our region (see below) still shows some issues getting enough "oomph" to produce vigorous thunderstorms. These issues include marginal temps and moisture, a lack of very steep temperature change with height (lapse rates), an already saturated air column aloft that promotes limited buoyancy, and limited updraft Convective Available Potential Energy (CAPE). 

GFS Forecasted Weather Sounding for Wednesday at 1 PM. As discussed, this sounding shows a lack of "instability" but an extremely favorable wind profile for severe thunderstorms. 

        In a nutshell, thunderstorms need a source of "unstable" air that wants to readily rise due to density differences. Warm, moist air is less dense than colder, drier air and will rise until it cools enough that it is the same or cooler that the air around it. In essence and without getting too technical, tomorrow features a lack of a "density difference" between the air at the surface and air in the upper atmosphere to promote that moist air to rise and condense into clouds. The lack of "juice" for big thunderstorms tomorrow may limit the severe potential. However, these issues can be made up by very strong forcing for ascent and an extremely favorable kinematic environment, which tomorrow features. At the very least, we can expect strong wind gusts of 30-40 mph before the line approaches and a band of heavy rain along the front. Charge up devices tonight and tune in tomorrow with your local news for all the latest updates.