The shortwave trough that moved through a week or so ago that initially spawned some severe weather hype passed through without much of a fuss in terms of severe weather--big snow event, though. This shortwave moving through this week, however, has a bit more certainty to it with regards to it being a severe weather producer. So what's different about this week?
Let's look at the current 500mb map:
|Fig 1 -- RUC 18Z analysis of 500mb winds (shaded and heights (contoured) on Tue., Feb. 22, 2011. From the HOOT website.|
|Fig 2 -- GFS 42 hour forecast of 500mb winds (shaded) and heights (contoured) for 12Z, Thursday, Feb. 24, 2011. From the HOOT website.|
During much of the winter, we typically see only one major jet stream across the US--the polar jet. Remember that strong winds aloft form as a response to temperature gradients below. As such, this one polar jet lies over the "polar front"--the boundary separating frigid arctic air from warmer, moister subtropical air.
|Fig 3 -- Schematic of the single polar jet.|
So what happens when the polar jet moves further north, particularly as we start warming up during the spring and into summer? It leaves behind a somewhat cooler (but not frigid) air mass over the country. However we also still get warm, moist, subtropical air trying to intrude north. This time, however, it's moving into a less-cold air mass. Furthermore, there's still a boundary between the cool air over the continent and the frigid air up north underneath the polar jet. So we have two areas of temperature gradients--one between the frigid air up north and the cool air over the central continent and another between that same cool air over the central continent and warm, moist air coming from the Gulf of Mexico. With two temperature gradients, we see two jets:
|Fig 4 -- Typical spring time pattern with the polar jet further north and the "subtropical" jet to the south.|
So we have a jet pattern similar to this forecast for this Thursday morning. Does the surface temperature profile match this model? It certainly does.
|Fig 5 -- GFS 42 hour forecast of surface pressure (contoured), temperature (shaded) and winds (barbs) for 12Z, Thursday, Feb. 24, 2011. From the HOOT website.|
Is there moisture in that expanding warm sector across the southern plains? The GFS seems to think there will be:
|Fig 6 -- GFS 42 hour forecast of surface dewpoint temperature for 12Z, Thursday, Feb. 24, 2011. From the HOOT website.|
|Fig 6 -- GFS 60 hour forecast of surface dewpoint temperature for 06Z, Friday, Feb. 25, 2011. From the HOOT website.|
|Fig 7 -- GFS 60 hour forecast sounding for Jackson, Mississippi (KHKS) at 00Z, Friday, Feb. 25, 2011. From Earl's Skew-T Page.|
|Fig 8 -- GFS 54 hour forecast Surface-Based CAPE forecast for 00Z, Friday, Feb. 25, 2011. From the HOOT website.|
The OWL-WRF model does fire some storms out ahead of the main front, but the structure seems very unorganized:
|Fig 9 -- OWL-WRF 60 hour forecast of simulated composite reflectivity at 00Z, Friday, Feb. 25, 2011. From the HOOT website.|
|Fig 10 -- SPC Day 3 Convective outlook, issued 0824Z, Feb. 22, 2011. From the SPC website.|
Tomorrow I'll turn my attention to the snow event up in Seattle that may even be ongoing by tomorrow evening. Lots of weather going on...