(Cross-posting this from State of the Skies because I feel it's important enough for a heads up, given the population of the areas affected.)
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.
The severe weather won't be terrible like we've seen previously. The threats today will be damaging winds from downbursts, some quarter to half-dollar sized hail, and maybe...maybe...a weak tornado or two in the Mid-Atlantic and Southeast.
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 -- with individual cells moving northeast, but the clusters moving southeast -- once they decide to do so. The biggest threats with the storms will be damaging winds (downbursts coming from the entrainment of dry air in the midlevels), 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.
12:02 PM PT: Severe T'storm Watch for eastern Carolinas until 900PM:
The SPC gives a 60% chance of a severe thunderstorm watch going up for parts of NC and the Mid-Atlantic in the next few hours:
12:17 PM PT: In addition to the severe weather, very heavy rainfall is expected in the storms that form today. The heavy rainfall will be exacerbated by the slow movement of the storms, so flash flooding is a real threat today. The counties in green are under a flash flood watch. Click to enlarge in a new window or tab.