TO STORM OR NOT TO STORM, THAT'S THE QUESTION...

A rather complex scenario is developing for Thursday involving a cold front, increasing heat and humidity, and potential thunderstorms (some of which could be strong).


Let me start by saying convection is a fickle thing to predict. There's many moving parts and mesoscale details that models struggle with. You can have massive potential with a multitude of perfect parameters in place that point to robust development. But, if temperatures aloft are too warm, the atmosphere becomes capped and it all goes for naught. The analogy I like to use is a can of pop. You know if you shake that thing up there is a ton of potential energy just waiting to blow when you pop the top. However, if you keep the can untapped with time it disperses and that is the end of it.


For my southern counties, the capping issue may come to the forefront Thursday as very warm air at the surface and aloft surges into the region. The arrival of the warm air may generate some scattered storms Thursday morning, especially west of the Mississippi in Iowa. Some models show it, others don't. If they are around, they should dissipate by mid-morning and then temperatures will be off to the races. The EURO shows highs areawide in the low to mid 90s. 98 straight up in Cedar Rapids and 103 around Des Moines!

A heat advisory is out just to the SW and if the EURO is right, this could very well be extended east to include my SW counties. Debris clouds from morning convection could play a role in just how warm it ultimately gets.


Getting back to Thursday afternoon, with the heating that is likely and the increased humidity instability is really going to go up. We measure that in CAPE (convective available energy) The EURO shows values over 3,000 j/kg. in spots. That's pretty juicy and it means that the fuel is there for storms. (The can of pop is shaken up).

The question becomes, will the tab on the can be pulled allowing the cap to break and storms to form. One of the things we look for to achieve initiation is forcing, generally a front of some sort. In this case a cold front will come into the mix but for much of the afternoon and early evening it remains in far northern Iowa and southern Minnesota. As the front drifts closer to HWY 20 Thursday evening, temperatures will begin to cool aloft and the forcing comes into play. Throw in the nocturnal jet later on and storms should form and quickly expand across the north. There is a good chance a squall line evolves late evening that moves east southeast overnight putting much of the area north of HWY 30 in play for storms. Strong winds, some large hail, and locally heavy rain are a threat and SPC has increased the severe risk to enhanced for that potential. There is also a 5% tornado threat conditional on how the day evolves over NC and NE Iowa.

The biggest issue models are struggling with as I write this is the southern extent of Thursday night's storms. The majority of the models keep the action confined to the area north of HWY 30, the majority closer to HWY 20. That leaves the south, especially south of I-80 capped, high and dry. One thing that could alter that scenario is a cold pool that develops with the storms over northern Iowa. That outflow can act as a mini cold front allowing the storms to build further south where their is a richer environment for them to flourish. That is something to watch as I've seen such a development blow up a forecast in a hurry.


Honestly, there are just a lot of unresolved mesoscale issues that will become apparent Thursday but at least for now the best chances for rain early Thursday are in SE Iowa and Thursday night they exist north of HWY 30.


That said, here are the rainfall forecasts through Friday morning tied to all of the reasons I've described above. This includes anything that might fall Thursday morning but overall much of this comes Thursday night. Notice that most of the south does not fare as well from this initial rain threat. There will be other chances.


The EURO

The GFS

The 3k NAM

The 12K NAM

The 5k WRF

The next challenge is the position of the front Friday night. It is expected to stall near the Iowa Missouri border. More storms are possible then, especially south of HWY 34 near the boundary so this part of my area may get another chance at that time. If that does not cut it for rain, a fairly strong cold front Sunday should bring additional chances for showers and storms with needed rains. The bottom line is that it still looks to me like the pattern will be active the final two weeks of June and the potential is there to make up for some of the rain we've missed. My yard is toast. Roll weather...TS

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