Sunrise 6:11 AM   Sunset 8:07 PM Moonrise 11:32 PM   Moonset 7:06 AM

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FXUS63 KDMX 271117 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 617 AM CDT Sat Apr 27 2024


Updated for the 12z Aviation Discussion

KEY MESSAGES

- Another round of severe weather is possible over southern into central Iowa later this afternoon and evening. All severe weather hazards are possible, but large hail and damaging winds are the primary threats.

- Showers and storms are likely tonight into Sunday. Additional severe weather is possible Sunday afternoon and evening, but the extent of the threat is uncertain at this time.

- Weather pattern remains active into next week with more shower and storm chances Tuesday/Tuesday night and again late in the week.

DISCUSSION
Issued at 300 AM CDT Sat Apr 27 2024

We have transitioned into a quieter period overnight as the upper low responsible for yesterday's severe weather outbreak has pushed north and east of the area. Subsidence on the backside of the departing low has cleared out most cloud cover and precipitation, although the lull will be short lived as another potent upper wave is already churning through the four corners region and ready to take aim on the Plains states later today.
A warm frontal boundary draped over southwestern and through central Iowa serves as the focus for additional convective development. Hires models percolate isolated showers and storms early in the day as instability builds along and south of the front, but this will likely have a tough time overcoming mean deep layer subsidence. Much more robust convective initiation holds off until 20z-22z as vigorous deep layer ascent overspreads the increasingly unstable warm sector. Most of the 00z model suite has honed in on nw MO into sw IA for initial development. These storms may quickly become severe as they work within a favorable environment for large hail and damaging wind gusts. Sounding analysis suggests a few tornadoes cannot be ruled out with the initial development into southern Iowa given 0-1km SRH ~100-200 m2/s2, looping hodographs, plentiful CAPE within the lowest 3km.

Convective evolution becomes increasingly uncertain into the evening hours. A few hires members (HRRR/ARW) quickly grow upscale and race the storms east, while others (NAM/FV3) take a more northerly trajectory and eventually push the storms north of the warm frontal boundary and into a less favorable environment for severe weather over northern Iowa. Redevelopment through the night is also poorly handled as some models favor the mesoscale storm outflow over MO into SE Iowa, while others favor the synoptic warm front bisecting the CWA In either case, strong background upper level divergence should support at least scattered shower and storm activity over the forecast area tonight into Sunday as the upper low pushes toward the Dakotas. Another window for a severe weather risk including hail and a few tornadoes may materialize Sunday afternoon and evening as the surface low moves and attendant dryline move into western Iowa. The extent of the threat will hinge on precip/clouds clearing out and allowing the boundary layer to destabilize.

Pattern remains active into next week as models show multiple waves traversing through the region. After another break in storm chances on Monday, the atmosphere quickly reloads with return moisture ahead of an approaching upper wave and surface frontal boundary.
Showers and storms are looking increasingly likely as the front passes through Tuesday evening and night. Latest NBM PoPs are likely too aggressive into Wednesday and may need to be trimmed back if current trends continue. The next system is quick on its heels as another shortwave digs into the central Rockies, leading to additional shower/storm chances later in the week.
Models begin to diverge with pattern evolution by this point, and ensemble data is noisy and does not provide many clues for honing in on a particular time period for higher PoPs.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/
Issued at 616 AM CDT Sat Apr 27 2024

A bank of MVFR stratus has steadily built back in over the northern half of the forecast area early this morning. This has progressed a bit further south than model guidance previously suggested which lends some uncertainty to the cig forecast at KDSM over the next several hours. Opted to leave at VFR for now but adjustments may be necessary as trends warrant. Expecting the MVFR cigs to persist through the day over much of northern Iowa, possibly dipping into IFR at times.

TSRA develops after 20z in southern Iowa and expands north and eastward through the evening. Tempo categorical impacts are likely at terminals, especially for KDSM and KOTM. Scattered to widespread shra/tsra continues into the overnight hours along with more widespread MVFR to IFR cigs.

DMX WATCHES/WARNINGS/ADVISORIES
None.

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