Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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218 FXUS63 KDMX 240142 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 842 PM CDT Thu May 23 2024 .KEY MESSAGES... - A potentially large MCS is expected to move through Iowa tonight. - Severe weather is possible, with the primary risks being damaging winds and large hail, but tornadoes are possible as well, especially in the southern end of the QLCS/squall line. - The Storm Prediction Center has a Level 3 Enhanced Risk western counties, and a level 2 slight risk through central Iowa, lowering to a marginal level 1 risk for eastern counties. - Heavy rainfall could occur tonight, and with that, flash flooding could be a concern in isolated areas. && .DISCUSSION... Issued at 218 PM CDT Thu May 23 2024 /Tonight through Saturday/ A beautiful day is found across Iowa today, with seasonal temperatures in the mid to upper 70s, low dewpoints in the upper 40s to lower 50s, and winds of 10 to 20 mph. This pleasant air is found to the north of a developing warm front, as there is widespread 60 - 70 degree dewpoints found throughout locations to the south in Kansas through Missouri and the lower Ohio Valley. In a typical MCS set up, the advection of this warm unstable air will force strong to severe storms near and to the north of the developing warm front tonight. The upper wave triggering storms in along this boundary is found well to the west, but should help initiate storms in eastern Nebraska by mid to late evening. A deeply sheared atmosphere will be found over eastern NE through Iowa tonight, allowing for organized storms. These will most likely form into a QLCS over eastern Nebraska and race eastward, generally north of Interstate 80, as there is a notable EML/capping inversion found to the south. That southern edge of the convection will be important overnight, as this both could keep the event from occurring in southern Iowa, and also allow the southern end of the line of storms to retain a favorable profile for supercells, including tornadoes well into the night, possibly through daybreak. Thus, the chance for storms is somewhat lower south of Highway 30 late tonight, but the overall chance for severe weather occurring with them is higher than to the north where the depth of a stable boundary layer approaches 4000 ft. In a QLCS mode, +30kts of 0-1km shear will allow for possible spin up tornadoes along the leading edge, generally in areas that have dewpoint values over 55 degrees, keeping the boundary layer somewhat neutrally buoyant, vs confidently stable (highway 20). QPF tonight will be generally under 1 inch in most of the CWA, due to the fast moving storms, with low training potential. There certainly could be a warm advection wing of storms once a mature QLCS develops, but those leading cells should lift northeast rather quickly without significant QPF. This progressive scenario suggests that no flood watch be issued, despite areas that are sensitive to heavy rainfall, especially west and east CWA counties. We`ll watch those areas carefully tonight for any localized flooding threats. Friday, a cold front will sweep through during the morning hours, with any new storm development possibly only affecting eastern counties towards early afternoon, followed by a dry night Friday night and Saturday. A "windows open" night is expected Friday night, with lows in the mid to upper 40s. Saturday, highs in the 70s are expected before another strong wave approaches the region Saturday night and Sunday. Following section issued at 335 AM... .Long Term /Saturday Night through Wednesday/... Confidence: Medium As we approach Saturday night, another area of low pressure will track into the Central Plains. The bulk of the energy is still forecast by the medium range models to be just southeast of Iowa, though the Euro does bring some slightly more unstable air into the southeast/south by 12z Sunday. Though severe storms are not expected, there may be a few stronger storms in far southeast and southern Iowa. Rainfall may be the bigger issue, with another half inch to inch across the south half of the state. Certainly not needed at this time. Lows Saturday night will be in the 50s to around 60 with highs Sunday in the 60s to lower 70s. As we round out the weekend, there is lower confidence heading into Monday as operational model runs diverge. The current EPS is more supportive of the operation GFS with the light rain lingering Sunday night into Monday over the region. This will likely keep the region cooler and somewhat cloudy/wet with highs in the 60s to lower 70s again. From Tuesday through Wednesday night, a break from the wet weather is expected. Highs should gradually warm back to the mid to upper 70s. By Thursday some scattered showers and storms are possible with cooler highs in the lower to mid 70s. We may need to make some temperature adjustments at that time. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 624 PM CDT Thu May 23 2024 VFR conditions with breezy south to southeasterly sfc winds through late-evening. A dramatic change to occur after midnight/early Friday morning as a line of strong-severe storms race eastward across the state of Iowa. This linear convection may include some bowing structures with damaging/high winds as it propagates eastward across the TAF network of Central Iowa through 13-15z. Most sites will see a 1-2 hour window of these strong storms with higher gusts, mostly in the 30-40 kt range, but with potential gusts as high as 60 kts. Along with the high wind potential with these storms, there exists a potential for large hail, and even a tornado or two. Also a chance 10-30% coverage of higher based/ elevated showers and storms developing ahead of the main line this evening/overnight, but these will be hard to pin-point for TAF occurrence or VCNTY wording. CIGS generally VFR, but with likely dip to MVFR or even brief IFR levels with the storm line passage. Also, much guidance continues to indicate higher coverage MVFR stratocu in the wake of the main line Friday morning. VSBY also drops during the passing storms. Clearing is expected from west to east during the afternoon hours on Friday. Winds to remain elevated behind the main line of storms as they become more westerly during the day on Friday. && .HYDROLOGY... Issued at 218 PM CDT Thu May 23 2024 Both short-term (flash flooding) and longer-term (river flooding) hydro concerns will continue over the next few days due to the combination of current hydrologic conditions and QPF. In terms of flash flooding concerns, we may see some isolated issues tonight into Friday morning with the thunderstorm activity. With the storms expected to be rather progressive the rainfall is not expected to be too heavy. Thus based on present information a flood watch should not be needed. That being said, SAC-SMA soil saturation values remain in the 40 to 60% range across the eastern CWA roughly bounded by I-80, US 218, US 20 and I-35. In the western CWA they are also elevated in the Audubon-Guthrie-Jefferson County area. When soil saturation values exceed 50% the flash flood concern increases markedly. Thus, those areas will be the most likely areas to watch tonight into Friday morning. Fortunately in those areas the smaller streams have begun receding or have receded already. River flooding is more of a concern especially regarding how the forecast rainfall will affect already high river levels. River flooding continues or is forecast across portions of the Cedar, Iowa, Skunk, Des Moines and Raccoon River basins as well as their tributaries. Moderate flooding is occurring along portions of the Iowa River as well as Black Hawk Creek in northeast. The latest river forecasts updated this morning take into account 24 hrs of QPF ending Friday morning. Additional QPF is in the forecast beyond Friday morning. Looking beyond 24 hrs using our QPF ensemble hydrographs and Hydrologic Ensemble Forecast System (HEFS), the most likely scenario in most places is that the rain over the next several days will slow the fall of area rivers and possibly result in secondary crests. Those secondary crests would be lower than the levels we are presently seeing except for portions of the Iowa River and the South Skunk River across the eastern CWA where a long, broad crest is expected. That is due mainly to the timing of crests working their way down both rivers combined with runoff from future rainfall. Along both rivers the broad crests may extend well into the Memorial Day weekend before the river levels begin falling. In the Des Moines area as well as the Eddyville/Ottumwa areas, river levels on the Des Moines River are affected heavily by the Saylorville and Lake Red Rock reservoirs operated by the U.S. Army Corps of Engineers (USACE). Coordination with the USACE indicates they plan to adjust (minimize) outflows from the reservoirs to try to minimize flooding issues downstream. Having said that, minimizing outflows will result in building pool elevations more quickly on the lakes. As a result, we will see decent rises on both Saylorville and Lake Red Rock again. The rises on Saylorville will likely be similar to what we saw in the last event but they will be higher on Lake Red Rock. In the last event Lake Red Rock crested around 747 ft but with this event it is expected to go to around 754-755 ft. Those forecasts may change going forward due to the actual rainfall amounts. If the rainfall comes in higher than forecast then we would see long, broad crests at more locations in our CWA and higher secondary crests. We will continue monitoring. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...REV AVIATION...Percha HYDROLOGY...Zogg