Sunrise 6:10 AM   Sunset 8:13 PM Moonrise 12:31 AM   Moonset 9:01 AM

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FXUS63 KDMX 291101 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 601 AM CDT Mon Apr 29 2024


Updated for the 12z Aviation Discussion

KEY MESSAGES

- Thunderstorm chances return Tuesday afternoon into Tuesday night. Strong to severe storms possible with large hail and damaging winds the primary threats.

- Additional showers and storms likely Wednesday evening through Thursday. Severe threat uncertain. Much of area may see 1-2" of rain.

DISCUSSION
Issued at 228 AM CDT Mon Apr 29 2024

Precip is all but over for now as an upper low is pushing away toward the Canadian border. Deep moisture transport has been deflected well east and synoptic subsidence will further take hold over the state into the today, leading to a lull after an active stretch of days. Moisture wrapping around the backside of the upper low will keep a low stratus deck in place to start the day before starting a northward retreat this afternoon.
Clearing skies and light winds as a surface ridge axis moves through tonight will lead to ideal radiational cooling conditions. Portions of northern Iowa and other favorable river valleys and low lying areas will see lows dip into the 30s along with the potential for some areas of fog.

Pattern activity picks right back up Tuesday as a potent shortwave and attendant jet streak emerge over the central Plains. Southerly winds usher moisture back northward ahead of an approaching cold front. An increasingly unstable boundary layer ahead of the frontal boundary pushes MLCAPE to near 1500 J/kg by mid afternoon when storms fire over eastern Nebraska and move into western Iowa by late afternoon or evening. Effective shear of 40-50 kts oriented near normal to the frontal axis will support discrete storm modes initially, potentially supercellular and capable of producing large hail and damaging winds. The tornado threat is more questionable due to moisture return uncertainties as some models only increase dewpoints into the mid 50s before storms roll through. Model soundings show an inverted-V attribute below 800mb, which may support a localized threat for significant wind gusts especially as cold pools begin to merge with upscale growth into the evening. Eventually the severe threat wanes as instability diminishes by late evening and early overnight.

Cold front stalls out near the MO border on Wednesday, then slowly begins to retreat back northward as a warm front late in the day as another upper low approaches from the west. Models initiate convection along the frontal boundary by mid to late afternoon, potentially strong to severe given a favorable shear and instability parameter space along and south of the front. A further south frontal placement would limit the severe threat into southern Iowa. Shower and storm coverage expands northward Wed evening and night as the LLJ/northward moisture transport intensifies and phases with background kinematic forcing downstream of the approaching upper low. High precip chances continue into Thursday. Any severe threat appears low at this time unless there is a break in showers/cloud cover and more instability is realized. Main upper jet stream is also lagging west and displaces the most favorable shear environment. 50th percentile QPF from both the GEFS/ECENS has 1-2" of rain for much of the forecast area through the middle of week. Higher amounts are possible in areas that see repetitive rounds of deeper convection.

Models are in much better agreement late week into weekend with a more progressive pattern evolution. Upper low sweeps north and east by Friday and drier air pushes into the state. NBM is latching on to this trend with lowered PoPs in the latest run. Scattered precip chances return for the weekend as a weak shortwaves interact with a surface boundary waffling over the region. Predictability of timing and spatial coverage of potential precip is low at this time.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/
Issued at 601 AM CDT Mon Apr 29 2024

A blanket of MVFR to IFR stratus will continue to expand of much of northern into central Iowa this morning. The lowest cigs will generally be in far northern Iowa, including KMCW where IFR cigs are forecast. The stratus deck erodes from south to north beginning early this afternoon and through the evening, gradually giving way to prevailing VFR conditions at all terminals by tonight. Pockets of fog may develop toward early Monday morning, however due to low forecast confidence opted not to include any mention in the TAFs at this time.

DMX WATCHES/WARNINGS/ADVISORIES
None.

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