The SPC has a slight risk of severe weather out for various isolated spots in the upper midwest with even more conditional possibilities of severe weather in the southern plains over the next few days.
So, let's look at the overall setup. Here's Sunday morning's 12Z 300mb upper air analysis:
|Fig 1 -- 300mb analysis of winds (colors) and heights (contours) for 12Z, Sunday, May 8, 2011.|
However, this isn't a typical kind of trough, at least according to the model forecasts. Here's the GFS forecast down at 500mb on Monday morning.
|Fig 2 -- GFS 18 hour forecast of 500mb heights (contours) and winds (colors) for 12Z, Monday, May 9, 2011.|
|Fig 3 -- GFS 18 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 12Z, Monday, May 9, 2011.|
|Fig 4 -- GFS 30 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 00Z, Tuesday (Monday evening), May 10, 2011.|
Of course, remember from the thermal wind arguments that I've talked about frequently in these blog posts that the upper-air winds, particularly the upper air jets, are dictated by temperature gradients below. By effectively erasing that weak cold frontal boundary, the temperature gradient there has been erased too. Without that temperature gradient, the jet aloft should tend to weaken. Here's the GFS 500mb forecast for Monday night:
|Fig 5 -- GFS 30 hour forecast of 500mb heights (contours) and winds (colors) for 00Z, Tuesday, (Monday night), May 10, 2011.|
However, on Tuesday, the surface low and the upper-air trough finally seem to be getting their act together. Cold air from eastern Montana seems to sneak in behind the surface low and re-establish a weak, but strengthening cold front.
|Fig 6 -- GFS 48 hour forecast of surface temperature (colors) mean sea-level pressure (contours) and winds (barbs) for 18Z, Tuesday, May 10, 2011.|
|Fig 7 -- GFS 48 hour forecast of 500mb heights (contours) and winds (colors) for 18Z, Tuesday, May 10, 2011.|
So what's the consequence of this slow-developing cyclone? As you can see, over much of the central part of the country there will be southerly winds and lots of warm air advection from the south at the surface. This will bring very warm air pretty far north--notice that high temperatures will probably get into the low 80s as far north as Iowa and northern Illinois. Furthermore, this southerly advection will bring in more moisture, too. Here's the dewpoint forecast at the surface for Tuesday afternoon:
|Fig 8 -- GFS 48 hour forecast of surface dewpoint temperature (colors) for 18Z, Tuesday, May 10, 2011.|
But remember--instability isn't everything. First we'd have to check if the atmosphere was capped at all. Here's a different way of looking at potential capping inversions. This is the GFS forecast for 850mb temperatures on Tuesday afternoon:
|Fig 9 -- GFS 48-hour forecast of 850mb temperatures (colors) and height (contours) for 18Z Tuesday, May 10, 2011.|
With a capping inversion in place, we're going to need some sort of forcing mechanism to get storms going. But what forcing mechanism? We've already seen that the cold front is being washed out or weakly present way out west over the high plains--far removed from our area of greatest moisture and heating. Furthermore, there's no wide-spread upper-air support. That upper-level jet at 500mb is just barely keeping the low pressure center going--with no divergence at all suggested over the area of warm temperatures and high dewpoints. As such, there's nothing really to support thunderstorms except the warm moist air at the surface.
So, the chances for severe weather will be highly conditional over the next few days. Yes, it's going to get warm and sticky over the central part of the country. And there are a few areas where there seems to be some convergence of winds at the surface. There looks to be a dryline forming in Oklahoma and Texas by mid-week. There also will probably be a warm or stationary frontal boundary stretching across the upper midwest at the leading edge of all that warm air as this low tries to organize itself. The Storm Prediction Center is focusing on any convergence along that warm/stationary front as the best chance for severe weather over the next couple of days. However, there will be a possibility for thunderstorms anywhere that there happens to be enough convergence near the surface to cause lift that can overcome the capping inversion. And with such warm, moist air to draw upon, any storms that form will have a high probability of becoming severe.
Furthermore, with how chaotically the GFS seems to be handling this low pressure center's development and the upper-air response, this is a highly uncertain forecast--bound to change as the week goes on. So...we'll have to be alert.