Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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714 FXUS63 KDMX 221922 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 222 PM CDT Sat Jun 22 2024 .KEY MESSAGES... - Showers and thunderstorms are expected this evening with the potential for strong winds, heavy rainfall, and a few tornadoes. Isolated severe hail may also be possible. - Mostly dry on Sunday with highs in the 80s - Hotter with additional shower and thunderstorm chances Monday and Tuesday. && .DISCUSSION... Issued at 219 PM CDT Sat Jun 22 2024 >> Severe Weather Threat Today The system that brought heavy rainfall across northern and northwestern Iowa last night has begun to make its way east over the region this afternoon and will depart the area this evening. However, as it passes through, showers and thunderstorms will develop initially across northern Iowa ahead of the surface low early this afternoon, then also fill in along a trailing cool front oriented roughly from southwest to northeast later this afternoon. This will bring a threat for severe weather through the afternoon and into the early evening hours, with all hazards possible mainly across the eastern and northeastern portions of the area. SPC mesoanalysis data indicates favorable instability values of 2000 to 3000 J/kg, and shear values of 40 to potentially 50 kts of effective shear. This will support organized convection and potentially a few supercells, given the 20 to 30 kts of low level shear. Hodographs also show a few hours of the traditional sickle shaped profile with near 100 m2/s2 of SRH, although the streamwise ingest does still seem to be lacking some. Combine this low level shear with LCLs around 500 to 1000 m, steep low level lapse rates, and 0-3km CAPE values of 150+, J/kg could certainly see a tornado this evening. Strong flow aloft and marginal DCAPE values of 500 to 1000 J/kg will also make damaging wind gusts a consideration with any organized storms. This is especially true if discrete convection congeals and grows upscale, although this will be a greater concern further east of our forecast area. Regardless, the chance for strong winds and a tornado are the primary concerns, but also cant rule out some severe hail with any supercells that do develop. The large hail threat does look somewhat limited by warm and moist profiles, but an organized, rotating updraft would still be able to support large hailstones, which are less susceptible to melting than smaller hailstones found in less organized storms. With moisture pooling in the warm sector ahead of the approaching system, PWATs will again be in the 2+ range, which will promote efficient rainfall rates with any storm that develops. There also looks to be a brief period this afternoon where storms may even train along the boundary in northeastern Iowa, which could cause a few areas to pick up 2-3+ inches of rainfall, especially in areas further east along the boundary. Fortunately, the overall synoptic system will be quite progressive and storm motions will pick up to around 30 to 40 kts, so extremely high rainfall amounts are not anticipated. Regardless, given the higher rainfall rates in proximity to areas that already received heavy rainfall last night, have decided to extend the flood watch through 7 PM this evening. Another extension for some of the eastern counties may also be needed later this evening, but these areas received less rainfall last night so will want to evaluate how convection plays out before continuing the flood watch into the night. >> Hot Temperatures With More Storm Chances Next Week After the low pressure departs today, central Iowa gets a brief break from showers and storms through tomorrow, aside from an isolated shower or two in the afternoon. Unfortunately, this break is relatively short lived with more shower and thunderstorm chances expected both Monday and Tuesday as weak waves pass through the northwest to zonal flow overhead. These will be accompanied by high instability values as temperatures increase into the 90s both days. Shear values at this point dont look great, at least on Monday, which currently has a marginal risk for severe weather in the current SPC day 3 outlook. Tuesday has slightly higher forecasted shear values and also has some greater severe weather probabilities from the CSU machine learning algorithm, but at this point not going to dig too much into the minutia. In addition to the precipitation chances, highs climbing into the low 90s over southern Iowa and increasing dew points may lead to some uncomfortable apparent temperatures both Monday and Tuesday, so will be closely monitoring the heat risk in future forecast packages. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1222 PM CDT Sat Jun 22 2024 Scattered to broken cumulus at roughly 2k to 3k ft have developed over the western half of the state this afternoon. These precede a system bringing shower and thunderstorm chances through the afternoon and evening, potentially impacting all TAF sites. However, storms will be scattered and therefore confidence in impacts vary for each site. Therefore, have included mention of thunder during the most likely periods for each location and will refine as needed. Overnight, an MVFR to potentially IFR cloud deck looks to develop over much of the area, again impacting all sites. These ceilings then lift in the morning hours, and are followed by a quieter and drier day on Sunday. && .HYDROLOGY... Issued at 317 AM CDT Sat Jun 22 2024 We are looking at a high-end river flood event especially across our far northwestern CWA. The river basins of most concern include the West Fork Des Moines River, East Fork Des Moines River, mainstem Des Moines River above Saylorville Lake and the Cedar/Winnebago Rivers. This higher end river flooding is due to the heavy QPF axis shifting south and now affecting many of our river basins. The West Fork Des Moines River basin is of most concern. At some locations the expected river flooding will approach the flood of record and will exceed the levels seen in 1993 (which is the benchmark major flood for many locations in Iowa). The river is rising rapidly and is above flood stage already at some locations. As for the East Fork Des Moines River, significant flooding is expected there too. The river flooding will be in the top three crests of all time at some locations. River forecasts presently include QPF through Sunday morning due to the widespread, heavy nature of this rainfall event. The river forecasts are based on our best forecast rainfall amounts and locations. That being said, location is everything in terms of rainfall and river response. If the QPF axis shifts even farther south than expected then the river responses may be even more significant and include more rivers/locations. If the QPF ends up being higher then the same thing may happen. Saylorville Lake on the mainstem Des Moines River above Des Moines is expected to rise significantly in response to this runoff. Presently the lake elevation is just under 840 ft. The latest U.S. Army Corps of Engineers forecast has it rising to between 865 and 870 ft in early July, or a rise of between 25 and 30 ft. As a point of comparison the lake reached an elevation of 860.1 ft in late May in response to the heavy rainfall from April into May. Although the forecast rise with this event will be significant it will still be well below the spillway elevation of 884 ft. Of course like the river forecasts the Saylorville Lake forecast depends heavily on the forecast rainfall. If the rainfall ends up being heavier or further south then expected then the response may be more significant than presently expected. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch until 7 PM CDT this evening for IAZ004>007-015>017- 023>028-035>039. && $$ DISCUSSION...Dodson AVIATION...Dodson HYDROLOGY...Zogg