|Fig 1 -- KICT 0.5 degree base reflectivity from 0345Z, April 9, 2011.|
But let's get right down to it. The SPC has another slight risk of severe weather tomorrow for two regions--one in the upper midwest and the other in the Ohio Valley down through the Carolinas. The big buzz as of late, though, is for the day 3 moderate risk issued on Friday for Sunday.
|Fig 2 -- SPC day 3 convective outlook issued 07346Z, Friday, April 8, 2011.|
To get severe weather (in general) there are five basic ingredients (at least, this is how I was always taught to think about this:
So let's start looking at these particular parameters, though not in that order. The "Exhaust" parameter is supposed to refer to upper-air features that can evacuate air out at the top of a thunderstorm from fast-rising convective plumes. I prefer to think of this ingredient as "upper-level support" for a more wide-scale convective situation--i.e., I'm looking for divergence aloft. Here's the current synoptic pattern at 300 mb (the upper troposphere):
|Fig 3 -- 00Z analysis of 300 mb winds (colors) and geopotential height (contours) for April 9, 2011.|
So what's going to happen aloft? The models are in agreement that the deep trough will be slowly moving across the country over the next two days. What's remarkable about this trough right now is that it's staying very deep but is still advancing eastward fairly steadily. Often when we get really deep troughs they have a tendency to "cut-off" from the main flow and just spin around over one area for a while without really moving. But this one seems to be on the move. Here's the NAM model 300 mb wind forecast for Saturday evening:
|Fig 4 -- NAM 24 hour forecast of 300 mb winds (colors) and geopotential height (contours) for 00Z, Sunday (Saturday night), April 10, 2011.|
|Fig 5 -- NAM 24 hour forecast of surface temperature (barbs), mean sea-level pressure (contours) and winds (barbs) for 00Z, Sunday (Saturday night), April 10, 2011.|
|Fig 6 -- NAM 24 hour forecast of surface dewpoint temperature (colors) and winds (barbs) for 00Z, Sunday (Saturday night), April 10, 2011.|
The structure of this low-pressure center is also kind of unique in these plots. If you go based on the structure of the moisture field, the main low-pressure center seems to be somewhere in northeastern Nebraska at this time. Futhermore, it looks like the dryline extends all the way to the low-pressure center. The cold front that we saw on the surface temperature forecast can also be seen in the moisture field as the pronounced wind shift between northerly to southerly winds stretching through western Nebraska and into northern Colorado. But look at the location of the cold front on the dewpoint forecast map--the cold front is advancing into the dry air behind the dryline. This means that, though the cold front would provide lift, there isn't much moisture in the air it would be lifting. As such, the cold front isn't going to be much of a player on Saturday.
That upper-level trough is forecast to move slowly--by Sunday afternoon the NAM forecast has it positively tilted out over the great plains:
|Fig 7 -- NAM 45 hour forecast of 300mb winds (colors) and geopotential height (contours) for 2100Z, Sunday, April 10, 2011.|
|Fig 8 -- NAM 45-hour forecast of surface temperature (colors), mean sea-level pressure (contours) and winds (barbs) for 21Z, Sunday, April 10, 2011.|
|Fig 9 -- NAM 45-hour forecast of dewpoint temperature (colors) and winds (barbs) for 21Z, Sunday, April 10, 2011|
Let's re-evaluate our conditions for severe weather in that area with what we know so far (out of order):
- Moisture--This one seems to be very well-established. Dewpoints at the surface are forecast to be in the 60s or greater all the way up to southern Wisconsin in the forecasts. That's definitely the kind of moisture we would be looking for to get severe weather. Moisture is not an issue.
- Exhaust/Upper-level support--The 300mb winds for Sunday afternoon do have the broad left-exit region of a jet streak over the moderate risk area on Sunday afternoon. That should provide enough divergence aloft to support thunderstorm growth below.
- Wind shear--Haven't really touched on this one much yet, but we can check it quickly. Note that the surface winds in the warm, moist sector at this point are forecast to be rather strong--20 knots out of the south-southwest in the moderate risk area. The winds are somewhat lighter further north, but I don't really trust the NAM's winds in that "area of ambiguity" surrounding the low-pressure center. Let's compare these winds with the forecast 850mb winds (a little ways off the ground):
|Fig 10 -- NAM 45-hour forecast of 850 mb winds (colors) and geopotential height (contours) for 21Z, Sunday, April 10, 2011.|
Note that the winds are more southwesterly just a bit off the surface over the moderate risk region. This points to directional wind shear, at least. Winds are also somewhat faster than at the surface--30-40 knots across the moderate risk region. So there's good speed shear as well. However, remember in one of my recent blog posts I talked about how the low-level winds (but not the surface winds) tend to pick up right after sunset. Here's the forecast 850 mb winds for 6 hours later at 03Z (around 10 PM CDT).
|Fig 11 -- NAM 51-hour forecast of 850 mb winds (colors) and geopotential height (contours) for 03Z, Monday, April 10, 2011|
Look at how much the wind speeds increased as soon as night fell! Now we're looking at west-southwesterly winds at 50-60 knots in the low levels of the atmosphere right above the surface in southern Wisconsin. That's a whole lot of low-level wind shear. So, I definitely think that wind shear will be sufficient to produce severe storms in the moderate risk region.
But what about the last two--lift and instability? Some would argue that these are the most important ingredients to look for. Instability seems to be a given. Notice how warm the air was at the surface in the forecasts above. Some people in southern Wisconsin and northern Illinois are forecasting highs in the mid 80s on Sunday. Combine this with dewpoints in the 60s and it becomes very hard to consider the situation "stable". Let's check the forecast NAM soundings from Earl's Skew-T page to see what we're looking at. Here's the forecast sounding for Davenport, IA (roughly in the middle of the moderate risk region) for 21Z Sunday (Sunday afternoon):
|Fig 12 -- NAM 45 hour forecast souding for KDVN, valid 21Z, Sunday, April 10, 2011.|
|Fig 13 -- NAM 48 hour forecast souding for KDVN, valid 00Z, Monday (Sunday evening), April 10, 2011.|
Just for comparison, let's look at a point further south like Saint Louis:
|Fig 14 -- NAM 48 hour forecast souding for KSTL, valid 00Z, Monday (Sunday evening), April 10, 2011.|
So what about that lift? I believe that's the most conditional part of this setup, actually. We've seen that the model says there will be upper-level support, wind shear, moisture, and instability. All it takes is a little convergence at the surface to provide the lift necessary to start thunderstorm development. This is particularly true with a very unstable and uncapped profile like we see in the forecast for Davenport. Since some thunderstorms are expected in the region on Saturday, residual outflow boundaries and other perturbations in the wind field from the Saturday storms may linger on into Sunday and provide focal points for new convective development then. With so many factors favoring thunderstorm development, it's virtually certain that storms will form. In fact, with wide-spread vertical motion favored by the divergence aloft, we could see storms firing up all over the place. It will be interesting to see how this event is organized.
Without an organized forcing mechanism like a cold front, the storms that form will have a greater tendency to remain surface-based, drawing warm moist air directly from the surface layer. Combine this with the strong low-level wind shear and you have the ingredients for tornadic thunderstorms. This also justifies the heighted "moderate" severe risk that the SPC has put out there.
More updates will come as this event draws closer.