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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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623 FXUS63 KDMX 281918 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 218 PM CDT Fri Jun 28 2024 .KEY MESSAGES... - Showers and storms likely through the remainder of the day into the early overnight hours, including the potential for strong to severe storms and locally heavy rainfall. - Terrific weekend with seasonably comfortable temps and humidity, but weather pattern becomes active again early next week with increasing humidity and chances for storms, including the potential for strong to severe storms and heavy rainfall. - River flooding continues within the Des Moines and Cedar River basins in northern Iowa. && .DISCUSSION... Issued at 212 PM CDT Fri Jun 28 2024 19z GOES water vapor imagery shows a compact upper shortwave pushing off the northern Rockies into the high Plains. At the surface an area of low pressure was noted over far northeastern Nebraska, with an attendant cold front draped to the south/southwest and a warm front positioned southeast through Iowa. Morning shower/storm activity within a broad elevated warm air advection regime is finally exiting east, although widespread low cloud cover remains in it`s wake as highly moist low levels are trapped beneath a very warm mid-level environment with 700mb temps pushing 14C. Elevated convection has managed to develop within a zone of mid-level convergence over northwest Iowa. This activity will generally ride the northern edge of the warmer mid-level air across northern Iowa this afternoon. Some model guidance suggests a few robust core may materialize and support a threat of large hail. The most favorable window for severe weather likely holds off until the stout mid-level cap erodes toward early to mid-evening, allowing for CI along the cold front as it moves into western Iowa. Hires models initially show discrete storm modes as updrafts work within effective shear values of 30-40 kts. All severe hazards could be on the table. Model soundings show steep mid-level lapse rates and ECAPE near 3000 J/kg which would be supportive of large hail. Low level winds gradually veer with height, and may be enough to squeeze out a tornado or two especially as the LLJ increases and enhances the low level shear profiles. Storms will eventually congeal into one or more convective complexes as cold pools merge and support a threat for damaging wind gusts before the severe weather risk begins to wane due to decreasing instability with time and eastward extent. The environment is primed for heavy rainfall rates given warm cloud depths pushing 4000m+ and PW 1.75-2"+. HREF LPMM product advertises pockets of 2-3"+ rainfall amounts. The good news is the convection should be progressive, limiting the extent and magnitude of the heavier rainfall amounts. The threat should also mostly remain south of the heavily saturated portions of northern Iowa. Cold front and any associated storm chances clear the CWA tonight. Model guidance suggests some patchy fog potential into the early morning hours within a zone of light winds behind the front. Beyond that it is shaping up to be a fantastic weekend as a cooler and dry Canadian airmass settles into the state with highs in the 70s and 80s and comfortable humidity levels. Pattern activity picks right back up as a broad upper level trough with numerous embedded lower amplitude waves is re-established over the northern Rockies into the northern Plains. A broad warm air advection wing should spur elevated shower/storm activity Sunday night into early Monday. 12z model guidance is a bit slower with pattern progression beyond that point as a surface low deepens over the Dakotas and a deep but narrow plume of moisture is pulled northward up the Plains. This may hold additional convection to our west until Monday night into Tuesday as a cold frontal boundary slowly pushes through the state, including the potential for strong to severe storms and heavy rainfall. Models are in fair agreement with stalling out the front just south of the state and serves as the primary focus for additional storm activity through mid-week. Should the boundary hang up a bit further north, then we would possibly face a threat of additional episodes of heavy rainfall and impacts to our ongoing river flood concerns. See the hydro section for more details. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1225 PM CDT Fri Jun 28 2024 Messy aviation forecast over the next 12 to 18 hours as the area deals with variable cig restrictions, rounds of shra/tsra, then some patchy fog tonight into early Saturday morning. As of 1730z, cigs over much of the area are MVFR with pockets of IFR. Radar imagery shows a complex of showers and storms moving into southeast Iowa. This will continue to push east, with additional development already noted over northwest Iowa. Expecting further development by later this afternoon and evening, impacting terminals from west to east into the evening hours. Timing of tsra and extent of categorical impacts to individual terminals remains uncertain at this time, although sporadic strong wind gusts and heavy rain reducing visby below 2sm is possible at times. Conditions should improve as storms push east by late evening into the early overnight hours. Model guidance suggests patchy fog may develop east of I-35, and have included mention of MVFR visby at KALO and KOTM. VFR conditions prevail area-wide by mid-morning Saturday and through the rest of the TAF period. && .HYDROLOGY... Issued at 200 PM CDT Fri Jun 28 2024 The observed rainfall from Thursday night into Friday morning ended up being higher than expected in some areas which led to some adjustments to the river forecasts. The most drastic examples are the Shell Rock River where renewed flooding is now expected at Shell Rock with the river going back above flood stage this evening. In addition, renewed flooding is expected on the Winnebago River with the Mason City area going back above flood stage on Saturday. In other locations the adjustments primarily involved either keeping the stages steady for longer or slowing the fall. Concern in the short term then turns to later today into tonight. We may see some locally high QPF amounts however we are not expecting widespread heavy QPF amounts. Our river forecast updates this morning take into account the QPF through Saturday morning. In the longer term, concern is about QPF next week and its potential impact on rivers. Again, with our current river forecasts taking into account 24 hrs of QPF they do not include the QPF for next week. The last few meteorological model runs have been trending higher in QPF especially across the southeast half of the state. Our latest QPF ensemble hydrographs and Hydrologic Ensemble Forecast System (HEFS) runs are latching on to this QPF. Unlike the rainfall a week ago, the QPF next week will affect more of the state so additional river basins may come in to play. The Cedar/Iowa River basins look to be the most affected however our guidance is showing possible rises on tributaries to those rivers as well as portions of the Des Moines and Skunk River basins. At this point the most likely scenario will be slowing the rate of fall of our area rivers, however some ensemble members are suggesting the possibility of additional or new crests with additional minor flooding. Some of the lower probabilities are suggesting moderate to major flooding at some locations. We cannot totally rule out these possibilities. Ultimately the location, amount and timing of the rain next week will be the biggest factor. In addition to the river flooding, we will also need to monitor for flash flood potential next week given the favorable heavy rainfall parameters Monday night into Tuesday. 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...Martin AVIATION...Martin HYDROLOGY...Zogg