Sunday, March 8, 2009

Early Season Severe Weather Setup

On Tuesday morning, showers and thunderstorms will be ongoing north of a stalled warm front near the Interstate 72 corridor in association with strong convergence as the low-level jet impinges on the 850mb warm front. I don't really see this activity being severe as soundings during this time period are totally saturated and lapse rates marginal. Soundings actually look more tropical than anything, and would expect the main threat from this early morning activity to be flooding with more heavy rain likely added to already saturated soils. On Tuesday afternoon, a shortwave trough will eject out of the Southern Plains and become negatively tilted as it passes through the upper Midwest. At the surface, an elongated area of low pressure will house an advancing warm front near the Interstate 80 corridor and an advancing cold front near the IA/IL border. Perhaps the most interesting feature during the late morning and afternoon is the presence of very dry in the mid-levels of the atmosphere. This slot of dry air will be advected over a the early morning high RH air-mass, and as this slot of dry air begins to make its way into the area (around 18Z or so), clouds should begin to thin and allow for some sunshine in these areas. If this materializes, instability in the warm sector will quickly increase as temperatures rise with the passage of the surface warm front and dewpoints rise from moisture advection and the evaporation of the morning rainfall. With the center of surface circulation near central IA at this time, surface winds in a majority of places in central Iliinois will have a southerly to southeasterly component (even south of the WF). Just 1 km above the surface winds at this time will be howling near 50 knots out of the southwest creating ideal directional shear in the lowest levels of the atmosphere. I see "mini" or "low-topped" supercells as the greatest threat with this system in central and west central IL as the thermodynamic environment will struggle to become unstable. Given the ample directional shear and dry air aloft, I see damaging winds and isolated tornadoes as the main threat. Not to mention storms will probably already have a forward moving component that is already near severe criteria. However, severe hail would certainly be possible given rotating supercell updrafts, but lapse-rates in the lowest 3 km are not all that impressive and may be one limiting factor to this entire setup. However, I plan to chase since I am on spring break, and am currently targeting near or just west of Galesburg, IL.
My "Convective Outlook" for Tuesday March 10th.

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