Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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968 FXUS63 KDMX 251944 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 244 PM CDT Sat May 25 2024 .KEY MESSAGES... - Severe weather possible again later tonight into early Sunday central & south. Damaging wind the primary threat, hail, tornadoes, and heavy rain to lesser extents. - Additional chances for non-severe showers and storms Monday, then dry until late next week - Seasonal temperatures continue && .DISCUSSION... Issued at 243 PM CDT Sat May 25 2024 A more tranquil pattern is expected for next week, but in the mean time one last MCS is anticipated overnight, which will include some severe weather potential. 1-3km warm/theta-e advection continues to advance through the MO Valley this afternoon, with some mid level cloudiness and mainly precip aloft through the Siouxland and areas north. Little has reached the ground as of yet however with plenty of dry air below 3km. It will take a while, but lower level moisture is expected to return through the MO Valley later this evening behind the departing Great Lakes surface high pressure. The advancing Rockies long wave trough will continue to induce a surface wave response and subsequent warm front development into the night. The warm front is expected to remain just to our south, however 1-2K J/kg MLCAPEs may still reach the border with 2-3K J/kg MUCAPEs farther north into the state above the 500-1000m inversion. Although CINH will likely suppress tornadic development for the most part, effective shear to 40+ kts at times and ESRH 300+ m2/s2 will lead to organization with damaging wind and large hail potential, even well north of the warm front. 12Z HREF output suggests wind may be the most likely hazard, with less of a signal in updraft helicity tracks versus previous runs. Recent deterministic solutions with 750-1000+ DCAPEs also support this scenario. The only caveat is that if cells can somehow become rooted near surface based, tornadoes are not out of the question toward the border considering some model sounding depictions of 500m SRH 200+ m2/s2 and 0.025+/s streamwise vorticity ingest. Model guidance is fairly consistent with isolated-scattered 1-2" rainfall amounts tonight, which is consistent with only slightly seasonally elevated moisture content and warm cloud depths. More on expected stream response can be found in the hydro section below. Any significant convection should have exited to the south and east by daybreak Sunday, however weaker precip may linger into the day, especially the morning, until the passage of the H85/H7 trough. A pattern transition will then occur early next week with a northern Plains/Upper MS Valley short wave bringing some chances for showers and a few storms Monday, and starting more of a northern stream/northwest flow influence. The Ontario/Great Lakes long wave trough will stay in place for several days, and be followed by upper level ridging Thursday, which will result in less active weather next week and a welcome respite from convection. The next chances for precipitation appear to return late Thursday as the upper ridge departs, lasting into the weekend in response to an approaching Rockies long wave trough. Confidence in details is not great however with varied model solutions with regard to timing and short wave depth. However there does not appear to be much signal of appreciable instability return by this time, keeping any severe weather potential low, and the lack of any severe outlook in the GEFS based CSU machine learning probabilities seems to corroborate that thinking. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 100 PM CDT Sat May 25 2024 VFR conditions are anticipated for the time being with nothing beyond mid cloudiness and potentially some virga northwest. Another thunderstorm complex is expected later tonight however, likely affecting TAF sites 07-13z with lingering stratus following behind into Sunday morning. Some storms may be severe with damaging wind. Conditions are expected to transition to at least MVFR during the precipitation, and then MVFR/IFR following in more widespread stratus by morning. && .HYDROLOGY... Issued at 243 PM CDT Sat May 25 2024 The latest river forecasts utilizing QPF over the next 24 hours (7am Sat-7am Sun) mainly suggest either steady or falling river levels. Even higher end QPF simulations based on recent HEFS 30% exceedance probabilities yield similar results as sites either have 1) sufficient storage capacity, or 2) are already at such elevate flows that more appreciable precip would be needed for additional rises. The only exception to this would be locations below Lake Red Rock where the combination of runoff and Corps of Engineer releases may bring more significant within bank rises, or potentially minor flooding. Ottumwa may reach minor Flood Stage Sunday night, but confidence is medium at best and is dependent on rainfall and future reservoir release changes, so a River Flood Watch has been issued for Ottumwa for the time being until confidence in flooding increases. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...Small AVIATION...Small HYDROLOGY...Small