Minneapolis (MPX) — the National Weather Service forecast in plain English · Plaincast

Area Forecast Discussion · National Weather Service Minneapolis (MPX) · issued Friday, September 11 at 5:59 AM CDT.

Revised in 4 passages since the 2:51 AM discussion. See every revision.

Messages

A few thunderstorms will develop late this afternoon and tonight, primarily north of I-94. There is a level 2 out of 5 risk for severe thunderstorms.

Widespread rain arrives Monday, but forecast rain amounts have trended lower.

Turning much cooler this weekend and next week with highs in the 60s to low 70s.

Discussion

Clear skies are present across the Upper Midwest early this morning. Clouds will increase today as a disturbance over northern WY currently ejects eastward to the northern Plains and Upper Midwest by this evening. Models indicate this wave will weaken with time with the best forcing remaining across northern MN. However, a LLJ will strengthen through the day to 45-50 knots overhead/across the warm sector and a narrow plume of higher pwats (~1.5 inches) will be present just ahead of a surface cold front. That, in combination with fairly steep mid level lapse rates around 7.5 C/km, will lead to around 2000 J/kg of MLCAPE. Capping will be weakest across western and northern MN which is where storms are expected to be most numerous (especially with northern extent where height falls will be more impressive) late this afternoon and early evening. Bulk shear around 40 knots would allow for supercell development. Large hail and damaging winds are the primary threats, although the strengthened LLJ will increase SRH to the point where a few tornadoes are also possible. Storm development south of I-94 appears improbable with the exception across far western MN due to greater capping and weaker forcing. However, maintained 20 chance of rain in these areas for the potential that some of the initial activity is maintained into the late evening hours. The increasing LLJ throughout the day will also lead to gusty southerly winds as mixing depth maximizes. Some gusts exceeding 40 mph are likely across central and southern MN.

Zonal flow will follow for Saturday, and by Sunday, a short wave will rotate southeast across MN into WI. This may bring a chance of showers across southern MN into IA and SD, but dry high pressure to the north will limit the northern extent of any precipitation. The high will slide east to the Great Lakes Sunday night and warm air advection will begin across the Upper Midwest as a potent trough advances into the northern Plains. Very strong moisture advection will be brought on by a 50-55 knot LLJ Monday and pwats are expected to increase to around 2 inches. Isentropic lift will allow widespread showers to develop across the region, but the trough will swing through quickly and limit the residence time of the activity. In addition, the surface warm front will remain well to the south across southwestern IA and keep the more efficient thunderstorms anchored there. Models have trended down on expected rainfall locally from the 2-3 inches progged a couple days ago. Given the lift and moisture in place, still thinking an inch is not unreasonable on a widespread basis with heavier amounts south and lighter amounts north, but this is shaping up to be another disappointing event in the fight against the drought.

For the rest of next week, zonal flow will continue with mid level ridging remaining anchored in place across the south. A nearly stationary front will draped from the central Plains to mid Mississippi Valley and will keep the hot and humid air well to the south. It will also act as a focus for numerous rounds of showers and thunderstorms. At this time the odds of being under the influence of dry high pressure outweigh the prospects of any significant rain near the front. Following the cold front tonight, we'll remain in a definitively cooler pattern. This not to say we won't hit 80 again at some point into early/mid October, but this front appears to mark the beginning of fall weather for the Upper Midwest.

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