Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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288 FXUS63 KDMX 260820 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 320 AM CDT Wed Jun 26 2024 .KEY MESSAGES... - Drying out today with cooler and lower humidity air moving in - Storms return late Thursday into Friday with locally heavy rainfall. Severe storm risk may develop later Friday. - Hydrologic discussion below main discussion details more on ongoing river flooding and future locally heavy rainfall potential - Drying out this weekend with more seasonal temperature and humidity - Intermittent storm chances return late Sunday night into early next week && .DISCUSSION... Issued at 320 AM CDT Wed Jun 26 2024 An active evening with severe storms across a good portion of central and southern Iowa, but now activity has largely shifted into Missouri with the storms in Nebraska diving along the downdraft instability gradient into northeastern Kansas and northwest Missouri. Lingering showers and elevated thunderstorms do not pose any more severe risk as the cold front slowly pushes through the state through this morning. There is a secondary cold front up in Minnesota that will drop through the state today that will help to flush out the higher dewpoints/humidity air through the day with cooler conditions as well. This secondary cold front could be the focus for a few showers over the north this morning as well. Otherwise, a period of dry weather is forecast as a high pressure zone transits the area through Thursday. The rain and storm chances do return late Thursday and more so Thursday night into Friday morning as strong theta-e advection develops into the state as the 850mb low level jet ramps up. Instability is insignificant Thursday night so not seeing much for severe risk in this time frame, but widespread showers and storms are likely in this period. By Friday afternoon, new storm development is possible ahead of a cold front as MLCAPEs top 2000 J/kg and 35 knots of deep layer shear point to some severe risk. Forecast soundings show a stout cap, though this is weaker in southern Iowa. Colorado State University`s machine learning random forest outlook does paint low severe probabilities over the state and SPC`s day 3 has a slight (2 out of 5) risk over southern into portions of central Iowa. On the heavy rainfall side, precipitable water (PW) values will be on the increase Thursday night pushing above 1.5 inches with warm cloud depths (WCD) of 4000m favorable for efficient rainfall processes across much of the forecast area. Mean flow is 25 knots and should lead to the storms being progressive with rainfall amounts looking between 1 and 2 inches. PW and WCD values increase topping 2 inches and 4500m respectively on the day Friday. An isolated flash flood risk may evolve depending on a) rainfall Thursday night and Friday morning and b) orientation of surface front with the mean flow. For now, agree with the slight risk (2 out of 4) of excessive rainfall over southern Iowa from WPC. More details in the hydrologic section below. As this cold front pushes through Friday night into Saturday morning, we`re looking at a nice weekend setting up weatherwise. It will feature cooler and lower humidity air with no rain forecast as a big zone of high pressure passes through the upper Midwest. The high`s departure late in the weekend will mean the return of storm chances as soon as later Sunday night as theta-e advection pushes back into the state. A more amplified trough largely staying north of our state may provide more widespread chances of storms Monday night into Tuesday morning along with the possible return of 90 degree temperatures in portions of the state. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1220 AM CDT Wed Jun 26 2024 Main concern is lingering convection at DSM/OTM with potential for some modest wind gusts and brief reductions of vsby. Aft 12z, all of area will be dry with light northwest winds. Likely the remainder of the period will remain VFR aft 15z where some lingering MVFR will be present near OTM. /rev && .HYDROLOGY... Issued at 320 AM CDT Wed Jun 26 2024 Major to record flooding is ongoing or continues to be forecast on the West Fork of the Des Moines River early this morning. The upper portions of the West Fork and the East Fork are either leveling off or in recession from their crest. Farther downstream on the West Fork, rises are still forecast. On the Cedar River, Janesville has crested and Cedar Falls and Waterloo are nearing their crests later today. The Shell Rock and Winnebago are also in recession. Rainfall yesterday/Tuesday fell largely south of Highway 20 and away from the basins mentioned above. There were pockets of rainfall of 2 to 3 inches along and south of I-80. With about 36 to 48 hours before new rain falls over these areas, there should be some recovery of infiltration capacity. Not surprising, NASA SPoRT relative soil moisture (RSM) has changed little since this time yesterday showing more limited capacity/saturation over northern Iowa with some limited capacity of 50 to 60% in the 0-40cm layer. Rivers and streams did respond with the rain that fell yesterday with half foot to foot rises generally at worst (e.g. NRWI4, DESI4, DMOI4, DEMI4, PRMI4, CHTI4, BDFI4) so capacity remains in many central and southern Iowa streams after this rainfall. The next chance for rainfall will be later Thursday through Friday night. The flash flood risk remains uncertain in this period with WPC excessive rainfall outlooks painting broad slight risks (2 out of 4) Thursday night into Friday. Depending on where the rain falls with the expected more widespread storms Thursday night into Friday morning and if the storms later Friday fall over the same area, then there could be an isolated flash flood risk if things line up just right, or wrong depending on your perspective. On the river flooding, QPF only goes out 24 hours so this late week rainfall is not reflected in forecasts and won`t be until Thursday morning`s updates. For areas that are not currently experiencing or forecast to go above flood stage, namely central and southern Iowa, the experimental Hydrologic Ensemble Forecasting Service (HEFS) based on GEFS QPF continues to paint within bank rises at the 30% chance exceedance level. The 10th exceedance level would bring a few locations on the Iowa, Raccoon, and Des Moines into flood stage, but that would need the higher end rainfall. For northern Iowa and rivers with above flood stage conditions or forecasts (Upper Des Moines, Cedar, Shell Rock, and Winnebago), the rainfall Thursday night into Friday may prolonged the high flows or slow the recession. Make sure to stay up to date on the latest forecasts and river information, avoid flooded areas and rivers, and never drive around barricades or through flooded areas. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Ansorge AVIATION...REV HYDROLOGY...Ansorge