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As I've pointed out all week long, the parameters are coming together for what looks to be a high impact weather day. There are two factors to consider. One is heat, the other is strong thunderstorms. Many areas could see both between now and Sunday morning. I will tackle the issues separately starting with the heat which precedes any storms.


Saturday the remnants of a convective complex may linger into the early morning hours, especially over the NE third of the area. With the low level jet diminishing these should quickly fizzle. Any convective debris in the form of clouds erodes by mid-day allowing sunshine behind the passage of a strong warm front. The majority of the area resides in the warm sector through the afternoon which is expected to be capped. Robust heating is anticipated and highs are likely to soar into the low to mid 90s areawide with upper 90s possible in SE Iowa. The model blend shows this for highs Saturday.

The hi-res HRRR really goes off the rails Saturday depicting 100 degree highs over much of the region south of HWY 30. That's unlikely and I consider that the worst case scenario. Much would have to go right for that scenario to work out including lower dew points which does not seem reasonable. I'm sticking with the blend.

Assuming higher dew points in the low 70s, heat index values are likely to reach into the range of 100-105. T

That has prompted the NWS to go ahead and issue the heat advisories we all knew were coming.


The most challenging aspect of the day is the risk of strong to severe storms Saturday evening. At the time of this posting SPC has an enhanced risk of severe storms down to HWY 20 touching my northern counties. The slight risk extends all the way to the I-80 corridor.

The general set-up shows storms firing late afternoon in NC Iowa. They should grow upscale into a bowing MCS segment as they track E/SE along a strong instability gradient draped across this region. You can clearly see it below.

The potential is there for damaging wind gusts. SPC also thinks at this juncture that there will be enough low level shear for mesovortex formation along the advancing line and thus a few tornadoes are possible in northern Iowa, southern Wisconsin, and far northern Illinois. These typically are quick spin-ups that produce brief lower end strength tornadoes. Below you can see the helicity tracks the 3k NAM produces over northern Iowa, southern Wisconsin, and extreme northern Illinois Saturday evening. That indicates the tracks of supercells with wind damage capability. It will be interesting to see if later trends continue the aggressive look of the 3k. Right now the greatest threat of strong storms is near and north of HWY 20.

This animation shows how the 3K NAM forms the bowing squall line in NC Iowa that charges into my area during the early evening. If a decent cold pool develops as shown, the storms will have plenty of fuel to thrive as they dive southeast Saturday evening. Heavy rain is another threat that needs to be watched.

Of course this is just one models interpretation of how things unfold and it may evolve differently due to mesoscale influences that are currently unknown. At the time of this post I think this is a reasonable depiction of how things might end up going. That said, despite the fact we are entering the period or reckoning, I've seen many promising set-ups like this fail due to a single ingredient that did not live up to its potential. Some recent trends have been for the stronger storms to stay just north of my area. We'll have to see what impact Friday night's storms end up having on the overall environment Saturday.

Any lingering storms are likely to migrate out of the region Sunday morning as a cool front steadily progresses southeast out of the region. Humidity levels will gradually decline from north to south Sunday as lower dew points advect into the area from the north. Highs should range from the low 80s north to the upper 80s south. The GFS shows temperatures Sunday evening that are a good 10-15 degrees cooler that 24 hours earlier Saturday evening.

With high pressure back in control later Sunday, Monday and Tuesday look to be pleasant quiet days with temperatures near to a bit below normal with tolerable humidity levels. Just what the doctor ordered. Have a great weekend and roll weather...TS


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