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FXUS63 KDMX 081904 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 204 PM CDT Wed May 8 2024

KEY MESSAGES

- Showers and storms likely by later this afternoon into the evening. Some stronger to severe storms possible with gusty winds the primary threat.

- Scattered to widespread rain showers linger overnight into Thursday. Total rainfall amounts of 1-2"+ possible over the northern half of the area, lesser amounts south.

- Quiet and dry to end the week. Shower and storm chances return early next week.

DISCUSSION
Issued at 200 PM CDT Wed May 8 2024

Primary forecast challenge remains with convective and hydro related trends over the next 12 to 24 hours. 19z GOES water vapor imagery shows a large upper low centered over the western Dakotas along with numerous embedded shortwave disturbances. At the surface a weak low pressure center was noted moving into northwest Iowa, and a deeper surface flow analyzed into western Missouri. Radar has shown returns over portions of western and northern Iowa for much of the day, but much of this has failed to reach the ground due to a substantial sub- cloud dry layer and surface T-Td spreads of 20-30F.

High confidence in precip chances ramping up by mid to late afternoon through this evening as forcing associated with a mid-level shortwave rotates around the parent upper low. Hi-res guidance has focused on an area of convection developing over west central into northwest Iowa, and another corridor of loosely organized convection over southwest into central Iowa by 22-00z. The storms then steadily track northeast through the evening hours. Overall the severe threat will be limited by marginal instability as the primary axis of deep instability remains well to our south. Sounding interrogation suggests downburst wind gusts will be the primary threat with any of this acitivty given the aforementioned dry layer. Even with rather limited surface based instability dcape is progged between 750-1000 J/kg. Cool mid levels and rapid flow aloft may support a few cores with small to marginally severe hail. CAPE in the lowest 3km may provide sufficient stretching for a few funnels, but the dry air/high LCLs negates a meaningful tornado threat.

Instability wanes overnight as shower activity festers over northern Iowa. Steering flow becomes sluggish as the upper low drifts overhead. The slow progression will allow for some locations to steadily pick up more rainfall on top of any amounts from this evening. Thursday looks cloudy and cool with additional chances for light rain and the upper low meanders. This will hold highs close to 60, possibly not getting out of the 50s in some locations. HRRR PMM suggests rainfall totals over the next 24 hours of 0.5" or more generally along and north of I-80, with higher amounts possibly exceeding 1-2" toward northern Iowa.

The decaying upper low is kicked out by Thursday night as northwesterly flow is established aloft. Forecast is majority dry to end the week with highs in the 70s. The only exception is token lingering low PoPs in our far north/east associated with a shortwave crossing the Great Lakes region. Most model/ensemble guidance keeps precip off to the north and east, closer to the deeper forcing tied to the upper disturbance. Temps warm a bit early next week coincident with subtle ridging aloft.
Shower/storm chances return as a closed low slowly moves east out of the Four Corners region. Flow aloft is very weak and primary forcing mechanisms ill-defined, so it is difficult to pinpoint a particular period for higher PoPs or glean potential precip amounts. The broad PoPs provided by the NBM will have to suffice at this juncture.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/
Issued at 1238 PM CDT Wed May 8 2024

VFR conditions currently prevail across the area. Scattered to widespread showers and storms develop by mid to late afternoon and into this evening. The highest coverage is expected over northern into central Iowa. Brief strong wind gusts and visby reductions from heavy rain are possible with any tsra activity through this evening.
These may need to be handled with more tempo groups as confidence increases in timing and impacts for select terminals. More general shra will continue into Thursday morning, mainly for northern terminals. Cigs gradually begin to lower after 00z, eventually dropping into MVFR range tonight and lasting through the remainder of the TAF period.

DMX WATCHES/WARNINGS/ADVISORIES
None.

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