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JUST PASSING THROUGH...

Brisk southwesterly winds brought a good old fashioned July day to the region Tuesday. Highs of 86-91 were common with dew points in the low 70s. It was a little thick around the edges but actually about what you'd expect out of a July 19th day.


That was followed by a cool front Wednesday night that plowed a trail through the region. Despite the heat and humidity in place, any attempts at storm development were thwarted by warm air aloft and minimal forcing. A harmless passing. Attention now turns to high pressure which will bring northwest winds and a more pleasant air mass into the fold. Highs Wednesday should hold in the mid to upper 80s with slightly lower humidity. NW winds will be gusty in the 15-25mph+ range.


Thursday another weak disturbance approaches from the north which results in a quick return of warm air. It gets established first in the south and that's where things get plenty toasty. Most guidance indicates highs from I-80 south in the range of 90-95 with upper 80s in the north. The GFS is hotter, near 100. I doubt that.

Friday gets a bit tricky as strong warm air advection set-ups ahead of a nearly stationary front over southern Iowa. Some clouds are possible and even a shower or storm could pop. These are more likely Friday night, especially north of HWY 30. Highs are slightly lower in the upper 80s to near 90.

AN UP AND DOWN WEEKEND

Saturday looks to be a day where heat, humidity, and storms may all converge on the area. Starting with the heat, most guidance inches it across all but my northern counties by mid-afternoon. Such a scenario would make for a hot steamy day with highs areawide well into the 90s. Far southeast Iowa may push the century mark. The EURO shows this for highs.

The GFS is even worse with max temperatures soaring to 102 around Washington and Mt. Pleasant.

The NBM, National blend of Models is a few degrees cooler and I think it's the most plausible solution. No mater what happens, Saturday looks to be sizzler with heat index values exceeding 100. Heat headlines are likely, especially in the southern half of my area.

The other issue to deal with is thunderstorm potential. Some models indicate storms Friday night in the far north that could linger into early Saturday there. Models are split about 50/50 on storms developing in this area. The rest of the region should remain sufficiently capped for any thunderstorm development during the bulk of the day allowing the hottest temperatures to occur here. Such a set-up allows instability and CAPE to grow. On its northern periphery storms are likely to fire Saturday evening up north.

With the 500mb flow aimed right at northern Iowa Saturday evening, significant moisture is also expected to pool along the stationary front in the region. The EURO shows water vapor levels well over 2 inches.

As the low level jet combines with a short wave crossing the region, there is a good chance a mesoscale convective complex will form with the potential for a few strong storms. Some may produce high winds but the big threat as I've outlined in recent posts is very heavy rain. As you can see the EURO is showing a west to east oriented storm complex Saturday evening. The westerly wind vectors would allow back building and training with excessive rainfall potential.

The big question mark remains, where does the MCS set up and track? Nobody has the answer and we may not know until mesoscale details become defined Friday and Saturday.


One thing that has become clearer in the past 24 hours is that the heat dome gets knocked back to the south Sunday. That trend began to show up a day ago and seems to be well supported by all the guidance so we get off the hook with just a 2-3 day siege of hot sticky weather. Highs Sunday should be back in the mid 80s and most spots will go even lower Monday.


Beyond that (next week) there are major discrepancies on the evolution of the 500mb jet structure. The way models have been vacillating back and forth confidence is low on both temperatures and precipitation during that period. I see no sense in even trying to go down that rutted road so will end it here. Happy hump day and roll weather...TS

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