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FXUS63 KDMX 030455 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 1155 PM CDT Thu May 2 2024


Updated for the 06z Aviation Discussion

KEY MESSAGES

- Southeastern Iowa showers and storms ending tonight - Scattered storms roll into Iowa late Friday night/Saturday morning - Thunderstorms arriving later Monday into Monday night with severe risk possible

DISCUSSION
Issued at 323 PM CDT Thu May 2 2024

The main upper low over southern Saskatchewan will spin slowly eastward and then northeastward toward Hudson Bay from today through the beginning of the weekend. This upper low has swung a shortwave through its base with a surface low over central Iowa and a cold front near I-35 in southern Iowa. Rain and storms have prevailed over parts of our southern and more so our southeastern forecast area this morning into early this afternoon. Automated personal rain gauges have recorded 1 to 3 inches with isolated amounts just now topping 3 inches so far today from parts of Wayne to Monroe to Mahaska Counties, which lines up well with MRMS QPE. Calls to emergency managers after lunch time reveals numerous road closures - mainly gravel, but even at least one paved road - with a few vehicles driving into flooded areas. Thus, issued a flash flood warning for those areas until early this evening. Rainfall let up briefly over the warned area, but additional rainfall is transitioning up into our southeastern Iowa forecast area and this will not allow flooded areas to see appreciable improvement until the rain ends later this evening. While rain rates are not expected to be extreme, light to moderate rainfall falling on already saturated soil may lead to low end water issues. Opted for the flash flood watch over the (areal) flood watch given our already issue flash flood warning. Additional totals of 1 to 2 inches are possible in localized areas before the front pushes all the rain out later this evening.

Tonight, as winds relax, the sky clears, and with the recent rainfall, patchy fog may develop in at least favored low lying locations. Any fog will dissipate early in the morning and it will be a fine Friday with highs around 70 degrees in central Iowa.
Friday night will see another lobe of vorticity round the base of the upper low. MUCAPEs look to be under 500 J/kg, but mid-level lapse rates and deep layer shear could be favorable for a few elevated stronger storms with hail and perhaps a few strong wind gusts being the main threats late Friday night into Saturday morning. After this departs, a weak ridge will pass over Saturday night into Sunday and a southern stream shortwave trough and its associated rain will pass largely south of the state with some token chances in southern Iowa.

Attention then turns to a stronger shortwave trough and surface low moving over the central and northern Rockies, respectively, early next week. This low will occlude as it moves into the western Dakotas, but will draw moisture northward back over the state with dewpoints likely topping 60 degrees in at least southern Iowa.
Stronger kinematics will yield higher deep layer shear and instability over 1000 J/kg and steep mid-level lapse rates will also pool ahead of a cold front moving eastward into the state. This will likely yield some strong to perhaps severe storms and while the SPC day 5 outlook is just southwest of our area, the 00z runs of the Colorado State's machine learning random forest outlook and the CIPS Experimental Analog-Based Severe Probability Guidance both highlight a large sections of Iowa with 5-10% probabilities of severe with a 15% over the southwestern third to half of the state. These storms will push across the state overnight into early Tuesday, though some models have a slower timing of the front, which could lead to a lingering strong storm risk over parts of our eastern area Tuesday afternoon. Thereafter, the shortwave trough will meander and shear out while our flow stays westerly or southwesterly aloft. This could bring some shower potential later in the week as this occurs and the shearing shortwave sinks closer to Iowa, but stronger storms do not look favored as the instability and higher dewpoints will be shunted to the south. Temperatures, however, will remain above normal ranging from the upper 60s to low 70s north to the middle and upper 70s south.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/
Issued at 1153 PM CDT Thu May 2 2024

IFR/LIFR fog to impact terminals tonight. Lower confidence at KDSM so kept vsbys higher. VFR to resume after 13z with light winds becoming easterly to southeasterly through the daytime hours. Showers and thunderstorms to reach KFOD near the end of the TAF period.

DMX WATCHES/WARNINGS/ADVISORIES
Flood Watch until midnight CDT tonight for IAZ086-096-097.

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