Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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216 FXUS63 KDMX 232338 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 638 PM CDT Thu May 23 2024 ...Updated for the 00z Aviation Discussion... .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. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...REV AVIATION...Percha