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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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141 FXUS63 KDMX 150545 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1245 AM CDT Thu Aug 15 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Strong to some severe convection this evening into the overnight, biggest threat southwest forecast area. - Locally heavy rainfall also possible overnight. - Additional storm chances Thursday afternoon into evening. - Decent weekend weather with relatively dry conditions. && .DISCUSSION... Issued at 227 PM CDT Wed Aug 14 2024 MCV currently moving into southern Minnesota from northwest Iowa this afternoon with extensive area of showers across northern into eastern Iowa. Meanwhile, some clearing has been underway across the southwest third or so of the forecast area in the wake of the morning activity. Instability has been slowly building in this area with more robust destabilization farther southwest into Nebraska and Kansas where insolation is a bit more robust. CAMs models continue to focus development early this evening in eastern Nebraska near the nose of surface instability axis as low level flow converges in that area. The 0-3km portion of the hodographs remain rather robust in southeast Nebraska into southwest Iowa with a large curving sweep and SRH values of 300-400. This is also where heating appears to be sufficient for convection to become surfaced based and the best tornado threat. However, soundings heading east into the forecast area have less robust near surface thermal profiles with near surface parcel acceleration becoming more subdued and anticipate the tornado threat to drop off quickly heading into the DMX forecast area. This doesn`t mean there isnt a threat, just that the it will become much more unlikely. Some hail threat will exist given the rather large positive areas in our far west. Otherwise, the convection appears to consolidate and move eastward into the forecast area mid to late evening. Some wind threat does exist but the somewhat unfavorable low level profiles may inhibit the overall wind potential. Once again, the best threat remains in the far west where storms are likely to be near surface based with the activity becoming more elevated with eastward travel. Finally, ample moisture exists for heavy rainfall during this event with some models indicating localized 3+ inch amounts. A hydro discussion below will address this potential. The storms are likely to pass east overnight with some lull in the wake of the activity into Thursday. Some insolation during the day leads to increasing instability once again as upper low passes north of the state. While the deeper convection and severe potential are likely to exist primarily to the east of the forecast area, some scattered showers and storms are expected during peak heating as another shortwave heads east southeast towards the state. Some of these may have some gusty winds along with marginally severe hail given the decent instability which arises by mid to late afternoon. This activity lingers into the early evening but should die off quickly with the approach of sunset. Thereafter, a higher amplitude eastern trof develops with Iowa on the backside. This suppresses hotter airmasses to the southwest of the state with temperatures remaining at or below normal into the weekend with generally limited precipitation chances. Warm advection does increase by late in the forecast period with temperatures climbing into the middle of next week with some threat of storms by this time as well with increasing theta-e advection. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1235 AM CDT Thu Aug 15 2024 Showers and storms are expected to continue pushing across much of central Iowa through the overnight hours with much of the storm activity lifting into northern Iowa and mainly showers with occasional lightning/thunder across southern Iowa. This shower/storm activity is accompanied by some drops in visibility and low ceilings, which will allow for MVFR/IFR, or even LIFR at times, conditions to persist across much of the state, at least into the morning hours, before gradually improving as conditions dry out. However, there is the potential for additional showers and storms into Thursday, especially in the afternoon to evening southeast, though uncertainty is on the higher end so have left out at this time. Breezy winds are expected to remain as the system passes through, gradually shifting southwest/west. && .HYDROLOGY... Issued at 227 PM CDT Wed Aug 14 2024 Seasonally moderate to heavy rainfall is expected through Thursday. At this time the primary concern will be ponding of water and potential flooding in urban areas. Outside of urban areas, there may be some isolated issues but we do not expect any widespread issues. SAC-SMA soil moisture values are over 50 percent across the far southwestern CWA (mainly Taylor and portions of Ringgold counties). Anytime those soil moisture values exceed 50 percent the risk of flash flooding increases significantly. The elevated values there are mainly from the rain last night into early this morning. Thus all other things being equal, we will have to monitor those counties more closely than normal with any future rainfall for the flash flood threat. Longer term, the river flood threat appears to be minimal at least on a large scale. The QPF ensemble hydrographs and HEFS output suggest several sites may reach or exceed action stage if precip comes on the heavier side vs. expected. Plus, some sites may even see minor flooding--but again if precip comes in on the heavy side. National Water Model (NWM) 10-day annual exceedance probability (AEP) guidance suggests the greatest rises would be on streams to the southeast through south and southwest of Des Moines where 50 percent chance AEP values (roughly bankfull) and spot 20 percent chance AEP values (flood stage or higher) are indicated. The most likely scenario across our region would be minor to moderate within- bank rises with perhaps some more significant rises in isolated areas or on smaller streams. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Cogil AVIATION...Bury/KCM HYDROLOGY...Zogg