Sunrise 6:08 AM   Sunset 8:14 PM Moonrise 1:19 AM   Moonset 10:12 AM

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FXUS63 KDMX 301118 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 618 AM CDT Tue Apr 30 2024


Updated for the 12z Aviation Discussion

KEY MESSAGES

- Enhanced Risk This Afternoon/Evening for Very Large Hail and Damaging Thunderstorm Winds - Warm and breezy today || Next System arrives Wed Night - Thursday small risk of severe and moderate/locally heavy rainfall possible

DISCUSSION
Issued at 307 AM CDT Tue Apr 30 2024

Short Term /Today through Wednesday/

Confidence Short Term: Medium to High

Quick response in pattern to an approaching significant H500/H300 jet max now entering the Rockies, but extending back to the Pacific Northwest to a large scale H500 low over British Columbia. Early in the day we will see a gradual increase in south southwest winds as warm air advection begins to rapidly increase to our west. Lee side troughing at H850 is already beginning with a low over eastern Colorado and another more northern stream low over southeast Montana. Though the northern stream low will be dominant in this event, a remnant weak surface low is expected to maintain some integrity along the approaching cool front as the system swings east this afternoon and evening. 0-6km BWD will already be around 45kt ahead of the system by 18z, and increase thereafter to 55 to 65kt around 00z from southeast back to west central Iowa. Soundings continue to show strong turning in the 0-3km layers with significant westerly winds from H500 and above. Though a warm front is expected to lift northeast today allowing for the area to warm into the 70s to lower 80s, a second trough/warm frontal boundary will lift northeast to southern Iowa by early evening and make it just over the IA/MO border between 23-01z. The HRRR and HRRR are painting 2 favorable areas for stronger supercells/meso/helicity tracks: one nearer the US30 corridor and the other near to south of I80. Looking at the timing of the peak of the event; many of the strongest storms will reach the west between 4-5pm; nearer the I35 corridor by 5-6pm and in the south between 6-7pm; farther southeast between 7pm and 8pm. We continue to see a strong signature for very large hail greater than 2 to 3" in diameter along with the potential for higher end damaging thunderstorm wind gusts of 60 to +70 mph. Soundings continue to show appreciable drying in the lowest layers with DCAPE 800 to 1000 j/kg as well. The combination of both may result in wind driven hail in some of the stronger storms. At this time, the LCLs are not all that favorable for tornadoes; most of the soundings around 1200 to 1300m near the peak. The one concern is in the southeast where the 0-1km SRH increases as the weak southern stream low approaches around 23-00z. We could see a few tornadoes early in the event over the west, but this area will also have to be monitored as the event unfolds. The eventual line of storms/embedded supercells will move east rapidly and exit the area around 03-04z. Rainfall amounts this afternoon and tonight with the storms should bring up to 1 to 1.5 inches in the stronger cells. Lows tonight behind the system will cool slightly; with mins northwest in the mid 40s to the lower 50s southeast. Wednesday will begin rather calm, but already another H500/H300 speed max will be moving across the Rockies. This will develop another lee side trough with increasing H850/H700 flow already by 18z Wednesday. This means a rapid return of moisture is expected over the area by Wednesday night into Thursday. Highs Wednesday ahead of the next system will recover to the mid 60s northwest to the lower 70s southeast. Some showers and thunderstorms may enter the far west/southwest by late afternoon as the warm air advection aloft/moisture begin to arrive and lift isentropically into the region.

Long Term /Wednesday Night through Monday/

Confidence: Medium

As we move along to Wednesday night into Thursday, the main wave and upper level jet max will line up a bit farther west/southwest than our event today. This will mitigate the risk of any higher end severe weather over our area. The main concern right now would be an MCS with some minor severe threat overnight Wednesday night into Thursday morning. A broad H850/H700 LLJ of 40-50kts will move quickly northeast into Iowa by 12z. The stronger H500/H300 winds lags the lower level features and this is more conducive to locally heavy rainfall. With the strongest push of warm air northeast of Iowa by 21-00z Thursday, we will be mainly influenced by the wave lifting northeast, just to our west. A line of showers and storms will move across the state Thursday. With the winds generally parallel to the boundary, locally heavy rainfall and a chance of a few stronger storms will remain possible. With a few rounds of storms later Wednesday night and Thursday, much of the state will see additional moderate rainfall. The potential MCS Wednesday night will have 1.25 to 1.5 PWATs available for rainfall. Generally, a factor of 2 can be used in faster moving situations; so expecting about 2 to 3 inches in the heavier storms; lesser amounts elsewhere.
Thursday 1.5 inch PWATs line up along the boundary over the southeast/central during the afternoon. Given the progressive nature of the boundary, a 1 to 2 inch rainfall max amount will be expected in some of the heavier cells. Though widespread flooding is not anticipated, localized urban challenges may arise due to rainfall rates or repeated rainfall in the same areas and mainly in the form of ponding or rises in streams/creeks or urban travel issues. Lows Wednesday night will remain in the mid to upper 50s south with 40s far north. Thursday will be warm and humid, with highs in the mid 60s far northwest to the mid 70s southeast. The remainder of the extended may see a chance of showers or thunderstorms later Friday night and early Saturday (30%) and possibly again Monday. Mild weather is expected with highs generally in the 60s/70s and lows in the 40s/50s.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/
Issued at 618 AM CDT Tue Apr 30 2024

Main concerns today will be convection aft 21z; spreading from west to east/southeast by 04z. Mainly VFR through 20z with sct storms developing aft 21z. Timing key, but once storms roll through there will be a potential for large hail, wind gusts to 45-50kt and 1/2sm to 2sm visby. Next package will address timing and conditions with more detail. Aft 04z, most areas back to VFR with some LLWS developing from northwest. /rev



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