27 March 2008

Forecasting for tomorrow has been exceptionally difficult because model forecasts have been anything except convergent on a single solution. Part of the reason for this rests in the progressive, relatively small amplitude nature of the upper level disturbance, which is embedded in predominantly zonal flow.


















Tomorrow's winter weather

Yesterday's GFS and NAM model runs had a fairly decent snowstorm setting up over MN/WI/MI for tomorrow with 6" or more snow accumulation over a fairly narrow zonal band north of the warm front and ahead of a surface low pressure system. This morning, the model runs showed very insignificant accumulations of much precipitation of liquid or frozen phase in this region. But once again, this evening, a fairly decent swath of precipitation appears in the 00 UTC NAM and GFS, and this time the band sits over northern IL. This NAM suggests over 0.5" precipitation over the Chicago area during the day tomorrow in a region of frontogenesis, and if this were in the form of snow, accumulations of over 5" are not out of the question. However, the temperatures will be around or above freezing near the surface, which will probably give northern IL a mixture of rain and snow. Given the disappearance and reappearance of this snowstorm in various model runs, this appears to be something of a concern. Now that the weather service cancelled all winter storm watches, will we see new snow advisory or winter storm warnings posted tomorrow somewhere in a zonal band over northern IL or possibly southern WI? Currently accumulating snowfall is occurring over eastern SD and southern MN. This band is headed toward our area.

NAM's 12-h accumulated liquid-equivalent precipitation by 00 UTC tomorrow evening is posted above.

Severe Weather

Yesterday's model runs also hinted at the possibility of some marginal to locally enhanced severe weather tomorrow. This morning, the model runs were showing a substantial capping inversion over the most favorable kinematic and thermodynamic region for supercells (i.e., OK). Thus, the severe weather outlook has been as uncertain as the resolved model details surrounding this system. While tomorrow's outlook still looks somewhat unentertaining in the latest model runs, there could be some severe opportunities that crop up in the warm sector. The image below shows the NAM's forecast position of the surface low, over the TX panhandle, with a surging cold front over northern OK and MO. Warm and moist low-level flow throughout the warm sector will provide energy for thunderstorms that may form along the frontal boundary, all the way through the OH River Valley. The dryline south of the low pressure center would be the other focus for thunderstorms. Moderate to strong vertical wind shear should aid in storm organization, whether the storm mode is squall line (more probable along the cold front) or supercells, which would be likely if storms can overcome the strong EML-induced capping inversion, which will require ample insolation and most preferably forcing from surface boundaries.

This scenario is also portrayed in the latest GFS, but given the lack of temporal consistency in model runs, confidence is still low in the exact details of the surface flow and pressures tomorrow. While convection is more likely northeast of the surface low in the TX panhandle, my ideal position would sit somewhere near the Red River Valley in OK, just east of the forecasted dryline bulge (seen in the 00 Z 28 March NAM dewpoint plot):
There is high bust potential due to the stout cap, but if this cap breaks in my ideal target zone, the 2500 J/kg + MLCAPE and marginal to decent storm relative helicity (> 100 J/kg 0-1 km srh) may provide for isolated tornado development, and at least a photogenic supercell or two, that is, if the storms can form before the sun sets. The other drawbacks to this event are the SWrly flow just above the surface advecting in drier air, which could negatively affect the moisture available to storms. Other severe threats, although perhaps marginal, should include hail and and wind along the cold front, from central OK toward the northeast into the Ohio River Valley. Embedded supercells may enhance severe weather in these regions, but the threat again is conditional upon the strong cap breaking, which will be more likely as one heads toward the northeast into less favorable thermodynamic and kinematic conditions. Overall, tomorrow is still a very marginal day severe weatherwise, but the threat is there for some severe weather. Won't be surprised to see SPC post a slight risk over this region if the model runs don't flake out again on us. But this is a very risky storm chase situation, with a nice surprise if that relatively small probability supercell can overcome the potential negatives.




C.R. 11 pm