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THE MADNESS BEGINS...

The large scale weather pattern is in good agreement that the next 48 hours will at times be active ones across the central Midwest. Ridging continues over the southwest U.S. where record heat has baked the area for weeks.. This weekend we reside on the periphery of the hot air in a spot known as the ring of fire (the breeding ground for thunderstorm complexes). While the big picture is set, the intricate details of thunderstorm complexes are in continual flux. Even more maddening is the fact that each event alters the environment making it hard to pinpoint future storms until you see what's left behind from previous ones. It's a chain reaction type of effect which models are struggling with in a big way.


Just to show you what I'm dealing with in terms of inconsistency, the 3k NAM showed this for rain through Saturday in its morning run Friday.

6 hours later in the afternoon run, it went to this. Most of the northern half of my area going from 1-3 inches of rain to nothing at all.

The 3k NAM had readings in the low 80s Saturday while the GFS was about 20 degrees hotter in SE Iowa where highs of 100 were shown.

The same run time of the GFS was about 20 degrees hotter in SE Iowa where highs of 100 were shown.

All of this discrepancy is due to the models different interpretations of where thunderstorms form. Despite all the progress we've made in modeling, there are still mysteries in the atmosphere that are hard to reveal, especially when it comes to the complex composition of thunderstorms.


Having said all this I'm sure there will be storms around this weekend and some of them are going to be big rain producers. However, confidence in timing and placement is still lower than I would like to see at this distance.


Anyway, the first hurdle comes Friday morning. A thunderstorm complex known as an MCS (mesoscale convective system) is shown developing in Nebraska overnight moving into central Iowa Friday morning. As it advances towards SE Iowa it's expected to weaken and dissipate by late morning. Little if any rain is expected in my counties southwest of the Quad Cities. Friday's temperatures will be dictated by how much sunshine prevails as as rain cooled air or debris clouds could hold temperatures a bit cooler than guidance is showing. I like the NBM solution for highs which looks like this.

Friday night it's likely another MCS develops somewhere to the W/NW as the low level jet intensifies and veers in this direction. Precipitable water vapor levels go up too adding fuel to any storms. Again, guidance is on and off with little consistency from run to run or model to model. I certainly think some part of my area gets in on some storms but I can only broadbrush the potential. If storms fire heavy rain is possible. WPC (Weather Predication Center) does show a marginal risk of excessive rainfall over the entire area. Hopefully Friday's data tightens the placement.

The day with the most volatile potential for impactful weather is Saturday. Storms may be ongoing early in some part of the region but they are shown dissipating in the morning. How much outflow or debris clouds are found will dictate how hot the day turns out. I expect enough capping in the mid-levels to get the sunshine necessary to produce highs in the low 90s north to the mid to perhaps upper 90s in far SE Iowa. The NBM blend shows this for highs.

Adding the dew points into the equation heat index values could reach 100 to 108 and a heat advisory is likely for much of the area. It should be a steam pot. Here's the NBM forecasted heat index's

By late day Saturday, instability measured in CAPE is significant, especially on the 12k NAM where it maxes out at 4 to 5,000 j/kg.

That is big time energy for thunderstorms to thrive in. Most models break any cap early in the evening allowing storms to blossom in NC Iowa or SC Minnesota. These would have the potential to quickly become severe as they drive E/SE. Again, this is highly conditional on how the atmosphere recovers from any previous convection or outflow boundaries. As it stands now. SPC has an enhanced risk for severe weather down to about HWY 20. A slight risk covers the area north of I-80.

As they have been for days, models are really elevating water vapor showing it at 2 to 2.5 inches in EC Iowa Saturday night. Any storms that can roll through that environment have heavy rain potential. For now WPC indicates a marginal risk of excessive rainfall. That's a little lower than what I would deem it to be but it's early.

If storms materialize in the north and track southeast in a linear fashion, I think a front will get through the area Sunday ending the rain threat early in most areas. That also leads to a cooler day with slowly lowering humidity from north to south. The GFS indicates highs holding in the upper 70s to low 80s north of I-80.

Readings at mid-day are 15-20 degrees cooler in many areas than they were 24 hours earlier Saturday at that time.

That's where things stand late Thursday night. Obviously there are discrete details that could change the outcome in certain areas. Either way, warm to perhaps hot weather with humidity and scattered storms will be with us through Saturday night. Happy Friday and roll weather...TS

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