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FXUS63 KDMX 242333 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 633 PM CDT Wed Apr 24 2024


Updated for the 00z Aviation Discussion

KEY MESSAGES

- Shower and thunderstorms late Thursday night into Friday.
Severe weather is possible during the day Friday.

- Additional thunderstorm chances expected on Saturday, with severe weather possible. Some uncertainty remains on location and timing.

- Final wave of thunderstorms possible through Sunday. High uncertainty at this time, but severe weather may again be possible.

DISCUSSION
Issued at 335 PM CDT Wed Apr 24 2024

>> Breezy with Increasing Cloud Cover Thursday

After a comfortable and quiet day today, expect a flip to a more active weather pattern starting late tomorrow and lasting through most of the weekend. Surface low pressure will begin to build in to our west as the surface high overhead drifts east.
This will result in a tightening pressure gradient ahead of the low pressure, increasing southeasterly winds through the night tonight and into Thursday, with the strongest winds on Thursday being to the west. As the low pressure system to our west deepens and slowly make it’s way east, we will also see increasing cloud cover through the day with the potential for a few sporadic showers in the south. However, with low level dry air still in place, any showers that form during the day Thursday will be fairly light with minimal accumulations.

>> Showers and Thunderstorms Thursday night into Friday

Starting Thursday night, we will see a succession of intermittent shower and thunderstorm chances through the weekend. Due to the timing of these systems being so close to each other, there is a good deal of uncertainty on how things will play out through the weekend. The evolution of the first system will greatly dictate how the rest of the weekend plays out and where the greatest impacts will take place.

This progression will start with a surface low tracking northeast out of the lower plains region through western Iowa and into southern Minnesota on Friday, which will put much of our CWA in the warm sector. Friday’s parameter space looks a bit messy and many similarities can be drawn between this and the storm system we saw last week. There will be a good amount of broad scale lift with the theta-e advection ahead of the low, which will favor widespread precipitation through the morning.
This will likely keep the low- levels fairly saturated and somewhat inhibit the ability to destabilize the boundary layer.
Therefore, initially on Friday morning, storms will likely be elevated and fed by the 40 to 50+ kt LLJ aloft. Through the day, warmer air and higher dewpoints will continue to advect north and the environment will attempt to destabilize near the surface. As of right now, this looks most likely over southern Iowa, while areas further north will be fighting a bit more convective inhibition. That being said, should any storms become surface based on Friday, the high amount of low level shear and SRH would suggest that a few tornadoes are possible.
Hail and wind would be secondary threats, but marginal instability (~1000 to 1500 J/kg) and mostly saturated low levels would inhibit these two hazards.

>> Second Round of Thunderstorms Possible on Saturday

As the Friday system departs north overnight into Saturday, a front will remain draped behind it and down to the second wave approaching from the southwest. This front will be the source for showers and thunderstorm chances again on Saturday but the location of this front is more uncertain and will partially depend on what the Friday system does. Currently, guidance has the front from roughly southwest to northeast Iowa, crossing through north central Iowa and putting the southeast two thirds of the CWA in the unstable air.
Regardless of where the front sets up, the environment on the warm side of the boundary looks conducive for strong storms, and seems to follow a more “traditional” severe environment when compared to Friday. A warm nose aloft will erode through the day, with convection becoming completely surface based in the afternoon.
Surface based CAPE will be high (~2000 to 3000 J/kg) with mostly unidirectional speed shear resulting in straight hodographs. This environment would favor rotating storms but with large hail and winds being the main threat. The lack of higher low level shear on Saturday would inhibit tornado chances some, but certainly cant rule it out. Another important factor with Saturday would be the potential for heavy rainfall along the boundary as the LLJ orients parallel to it in the evening hours and overnight. Conditions are still fairly dry, so main impacts from heavy rainfall would be confined to urban areas with poorer drainage. But again, will want to see some higher resolution guidance before getting too committed to any specific timing, locations or impacts.

>> Final Wave Lifting Through The Area Sunday

Finally, on Saturday night into Sunday, another surface low pressure system will lift up into the state, roughly following where the surface boundary was in place on Saturday. This will put portions of our area in the warm sector and bring more shower and thunderstorm chances Sunday. This system has the highest amount of uncertainty with it, but the parameter space would again be favorable for a few severe storms. Beyond that, not going to speculate too much on how Sunday will play out at this time. The important takeaway will be that multiple rounds of showers and thunderstorms are expected through this weekend, with the potential for severe weather on all three days at least somewhere in the state. If you have plans outdoors, make sure you keep up to date on the forecast for your area, especially as details become clearer in the coming days.

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/
Issued at 633 PM CDT Wed Apr 24 2024

VFR conditions are forecast through the TAF period, with increasing high clouds around or above FL100 on Thursday.

DMX WATCHES/WARNINGS/ADVISORIES
None.

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