Steamy to stormy: Slight severe and flash flood risk tonight

Different weather systems present different forecast challenges for us as meteorologists.

Over the past 30 years, I have forecast massive Halloween mega-storms in Minnesota, fickle lake-effect fronts in Chicago, and incredibly subtle and complex thunderstorm forecasts during the North American monsoon in Arizona. Each forecast has its own atmospheric complexities.

I've always said forecasting winter storms in Minnesota is one of the toughest forecasts a meteorologist has to make. Accurately predicting the precise storm track? Nailing the rain-snow line? An Olympic degree of meteorological difficulty.

Our next low-pressure system moves into Minnesota over the next 24 hours. Summer severe weather forecasting is its own brand of challenge for forecasters. Multiply that by 10 for predicting hours in advance precisely where a flash flood will set up.

Heat and humidity peak over Minnesota in the next 36 hours. The moisture-rich atmosphere capable of thunderstorms with damaging winds and extreme rainfall spasms. Keep the weather radio handy across Minnesota later today and especially overnight tonight in the Twin Cities.

Slow-moving low

Here's the inbound low-pressure system and associated frontal zone draped across Minnesota. The lead wave of thunderstorms crossed the northern half of Minnesota overnight before fading as it approached more stable air over the Twin Cities.

The next wave of storms will develop in the eastern Dakotas and push across Minnesota late today and tonight. A second storm wave will likely fire ahead of the front Thursday afternoon from near the metro south and east.

By Friday, the cool front scours out the storms, heat and humidity as a fresh Canadian breeze blows in.

Broad brush: Slight severe risk

We've come a long way in severe storm forecasting in the past 30 years as a profession. But the reality is, we still can't pinpoint which communities will, and will not see a severe storm many hours in advance. Once the storms form we go into "nowcasting" mode, using Doppler, satellite and short term synoptic trends to refine thunderstorms arrival and intensity.

The National Oceanic and Atmospheric Administration's Storm Prediction Center lays out the broad brushstrokes today, a slight risk for severe storms for the eastern Dakotas and much of Minnesota.

Corn sweat makes it into NOAA's convective outlook?

This passage caught my eye in NOAA's convective outlook language this morning. Did evapotranspiration from "corn sweat" over the Upper Midwest just make it into the discussion as a contributing moisture source for possible thunderstorms today? A lot of weather geek speak below, I put the pertinent passage in bold.

...UPPER MIDWEST...

TSTM DEVELOPMENT IS EXPECTED THIS AFTN AND ESPECIALLY EVENING INVOF SFC TROUGH...LOW AND WARM FRONT...WITH ALL MODES OF SVR POSSIBLE.  AIR MASS E OF TROUGH AND S OF OUTFLOW/WARM FRONT WILL BE STRONGLY HEATED AND RICHLY MOIST...WITH SFC DEW POINTS COMMONLY UPPER 60S TO LOW 70S F...BENEATH 8-9 DEG C/KM 700-500-MB LAPSE RATES. THIS WILL SUPPORT LARGE BUOYANCY AND HIGH CAPE DENSITY...WITH MLCAPE 3000-4500 J/KG POSSIBLE AND -11 TO -13 LI SAMPLED THROUGH MUCH OF 500-250-MB LAYER. INCREASED MIXING AND DECREASING CAPE ARE EXPECTED WITH SSWWD

EXTENT ALONG AND 50-80 NM AHEAD OF SFC TROUGH. HOWEVER... EVAPORTRANSPIRATION AND MOIST ADVECTION SHOULD COMBINE TO KEEP RICH BOUNDARY-LAYER MOISTURE AVAILABLE OVER ERN SD AND MN.

Storm timing

To my eyes the finer resolution suggests storms firing in the eastern Dakotas again this afternoon and moving into western Minnesota this evening. The best storm chances for the metro appear to be after midnight, peaking between 3 a.m. and 9 a.m. Thursday morning.

There are still uncertainties with the precise timing and extent of storms overnight, but NOAA's NAM 4 km resolution model captures the flavor of the short-term "convective" forecast.

Heavy rainfall potential

With dew points in the 70s there is still the risk for heavy, even flooding rains today and tonight. Multi-inch rainfall totals are possible across a good chunk of Minnesota.

The finer resolution of summertime convective downpours is usually much more localized. We'll have to see precisely where storm boundaries set up later today and tonight. Some towns will be counting rain by the inch, some by the drop.

Nice weekend again

After the storms clear out Thursday night, we're setting up for another nice weekend as lazy high pressure drifts in.

Heat returns next week

The upper air pattern supports a return to steamy conditions next week, as this summer's southern heat dome nudges north one more time.

The long range 15-day meteogram cranks out a couple more 90 degree days next week, before a more significant cool-down the following week.

Stay tuned.