RINSE AND REPEAT...

Thursday afternoon and evening, an MCV moved across far northern Iowa, acting as the catalyst for strong thunderstorms with very heavy rain. An "MCV" is a swirling low-pressure system that can develop within a larger cluster of thunderstorms (called a Mesoscale Convective System or MCS). These vortices can be small, typically 30-60 miles wide and 1-3 miles deep. They can persist well after the parent thunderstorms have dissipated, sometimes for up to 12 hours. MCVs often act as "seeds" for new thunderstorms later in the day (as happened locally Thursday) They can enhance existing thunderstorm activity and potentially increase the risk of severe weather, including strong winds, large hail, and even tornadoes. In the animation below, you can see two active MCV's blow up storms Thursday evening, one over eastern Iowa and the other over Nebraska.

This radar image at 7:45 pm. shows a circulation around the vorticity center near DeWitt, north of the Quad Cities. The intense rainfall rates north of I-80 have produced a number of flash flood warnings for counties in the green boxes. Up to 4 inches fell in under 2 hours time.

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RINSE AND REPEAT...

While this initial band of storms moved out Thursday evening, they served to enhance a baroclinic boundary that will move little through Friday evening. That will be acted on by additional impulses and the low level jet to produce additional rounds of storms overnight into Friday night.

One of the concerns I've had all along with this system was its ability to be a prolific rain producer. Water vapor is off the charts, near 95% of climatology at the Davenport NWS with values up to 2.60 inches on the HRRR.

With warm cloud tops like the ones associated with tropical systems, these types of set-ups in the Midwest can be exceptionally efficient rain producers, with rates easily approaching 2 inches per hour. With recent heavy rains and rivers already above mid-summer levels, the NWS has issued a flood watch for my central and northern counties through Friday night.

Additionally, WPC has a slight risk outlook for the possibility of some excessive rainfall totals.

Compelling evidence for rainfall of 3–5 inches exists on many models, especially the convective allowing ones. Here's what some are suggesting for possible rain amounts through Friday night.

The 3k NAM

The 12K NAM

Even the deterministic GFS, not a convective allowing model indicates some 6 inch potential.

The HRRR

While rains will be ongoing for some early Friday thanks to the arrival of another storm cluster, they should diminish as the morning progresses and the low level jet decreases. Assuming this happens, heating should be sufficient for more instability and additional storms to develop in the afternoon. The big question is where, a detail that will likely be determined by the outflow from rain cooled air, and its ability to enhance the instability axis somewhere in my central counties. This is where strong storms and additional heavy rain could be expected Friday afternoon and evening. The simulated radar of the HRRR has them as far north as Dubuque,

The 3k NAM shows them further south, from roughly the Quad Cities south.

That may not seem like a big deal, but if the 3k verifies, it implies the intense rains that develop later Friday may miss the area that was pounded with previous heavy storms Thursday evening and night. Since the mesocale details are still uncertain at this hour, I can't say for sure which solution is best. I did not expect to see the 3K so far south and have some doubt whether the run initialized properly. One thing is for sure, the eventual placement of the instability axis will be a critical wild card in the outcome of any potential flooding.

As for temperatures Friday, they are likely to be considerably cooler in the north, where readings around 80 are expected. In the far south, highs near 90 and a lot of humidity will make for another sultry day.

Storms will wind down from NE to SE Friday evening with the passage of a cold front. The heavy rain threat should come to an end areawide by midnight. Saturday will still see some lingering clouds and potentially some showers, mainly in the morning and across the south and east. High should range from the upper 70s to low 80s. Sunday should be a better day, with highs of 80-84 and notably less humidity.

Much cooler weather is still on track for the end of next week. Highs for several days are expected to be down in the 70s during the period Friday the 18th through Monday the 21st. Looking good then. Happy Friday and roll weather...TS