Mesoscale Convective Vortex In NC

5/15/2012 radar, severe weather No Comments

An interesting thing happened on Monday night  in central North Carolina as a cluster of storms (called a mesoscale convective system, or MCS) moved between Charlotte and Greensboro. The northern end of the squall line developed something called a bookend vortex, which is when the end of a line of storms begins to curl back and rotate. Sometimes this bookend vortex can strengthen and take on a life of its own, long outliving the storms that produced it. This resultant rotation is called a mesoscale convective vortex, or MCV. They can live for half a day or more, and can go on to serve as a focus for future storm development, and sometimes they provide the nuclei upon which tropical cyclones develop.

The storm that formed on Monday night had a pretty well defined MCV near Greensboro that lasted for about 2 hours, and maybe even longer (although it wasn't detectable by radar anymore). The animated GIF I took with GR Level 2 Analyst is a bit big, but a pretty cool animation. Click the image to enlarge it.

Update: Here's another MCV that formed in Oklahoma a few weeks back on April 30 2012. In this scenario, a strong supercell thunderstorm was embedded within a line of storms and was able to take on an incredible amount of rotation. As a result, the storm produced a strong tornado near Medford, OK that evening. This radar animation depicts wind velocities, which show wind moving towards and away from the radar. Green indicates wind moving towards the radar, and red indicates winds moving away from the radar. When you see intense green/red colors side-by-side like they are in the first few frames up near Medford, OK, that indicates strong rotation and a likely tornado. Notice that the rotation broadens as the animation advances, indicating that the rotation spreads out as an MCV over a wide area. Click to enlarge.

For the curious weather geeks among us, this is what the wind velocity radar image looked like when the tornado was on the ground near Medford:

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Severe Weather Possible Across Mid-Atlantic & Great Lakes

5/15/2012 forecast, severe weather No Comments

It turns out that today was going to be a bit more active than I thought it would be yesterday. The Storm Prediction Center has added a huge part of the Mid-Atlantic and Southeast under a slight risk for severe weather, and they've also extended the slight risk area over the western Great Lakes to include parts of Iowa, Wisconsin, Illinois, Lake Michigan, and Michigan.

SPC's severe weather outlook for Tuesday, issued Tuesday morning.

Eastern Maryland to the SC/GA Border:
A "soupy" airmass (as the NWS in Roanoke put it) is in place across much of the Southeast and Mid-Atlantic this afternoon, which will continue to grow as daytime heating takes hold and moisture continues to advect (move) northward from the Atlantic. The latest surface observation from the SPC shows a tongue of moisture extending northward into southern Pennsylvania, with uncomfortable dewpoints of 60-70+ degrees across the whole area from the Blue Ridge to the coast. Air temperatures range from the mid 60s near the mountains to the 80s near the NC/SC coastal border region.

Surface observations from the SPC's mesoanalysis, from 16z (noon eastern).

An inhibiting factor to convection for most of the afternoon will be the large expanse of cloud cover over the area, which should slowly erode throughout the day. The longer this deck of clouds stays in place, the harder it will be for sunlight to break through and help to destabilize the atmosphere.

Visible satellite image from 12:45 PM Eastern.

Any sunlight that gets through the breaks in the clouds will help contribute to the destabilization of the atmosphere. The RAP (Rapid Refresh) model that the SPC uses for severe weather forecasting shows that CAPE (Convective Available Potential Energy...essentially the fuel that storms use, the higher the better) will be more than sufficient for strong thunderstorms, clocking in between 2500 and 3000+ J/kg over much of the slight risk area.

RAP Model forecast SB CAPE for 21z (5PM Eastern). 

Given the parameters in place (and the relatively weak winds), the storms should form in clusters and move slowly towards the east-southeast once they decide to do so. The biggest threats with the storms will be damaging winds, marginally severe hail (probably between 1.00-1.50"), and the SPC has a 2% chance for tornadoes across the area, so one or two weak ones can't be ruled out.

Iowa to Michigan:
The RAP model shows a fairly weak cold front draped parts of the Upper Midwest extending from Lake Michigan down through southeastern WI into central Iowa later this afternoon. Temperatures in parts of Iowa and Illinois are expected to climb up into the upper 80s and lower 90s as the day progresses, setting the stage for some instability out ahead of the front. As the boundary approaches, it'll provide a focus for storms to develop despite the low dewpoints (40s and 50s). The SPC states that the storms will quickly dissipate once the sun sets and the instability fueling the storms quickly erodes. There is a general risk for damaging winds and large hail in the strongest storms that are able to form.

RAP model forecast temps/dewpoints/pressures for 21z (4PM Central). The  cold front is located in the sharp curving of the isobars from Lake Michigan through Iowa.

You can follow all current severe weather at the Storm Prediction Center's website, as well as using the models and maps located in the drop-down menus at the top of the website.

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