Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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586 FXUS63 KDMX 212347 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 647 PM CDT Sat Sep 21 2024 ...Updated for the 00z Aviation Discussion... .KEY MESSAGES... - A few strong storms with large hail and damaging wind possible into mid evening. - Much cooler Sunday. Chances for showers and non-severe storms over the southeast half. && .DISCUSSION... Issued at 239 PM CDT Sat Sep 21 2024 The well advertised cold front has reached northwest Iowa this afternoon and it will move slowly southeast through the remainder of the afternoon. Temperatures and dewpoints ahead of the boundary have exceeded forecast values. Dew points in the low 70s have reached into southwest Iowa while temperatures are in the 90s in some areas of central Iowa. Waterloo was at 92 degrees at 2 pm. Southern Iowa was still cooler where more clouds and some weak elevated showers and thunderstorms have persisted. Coincidentally, the higher dew points have been in that part of the state and that has also been an area where the mid-level moisture has been highest. In fact the 700-500 mb relative humidity values has a minimum in vicinity of the boundary which has been a limiting factor in elevated convective initiation currently. The elevated mixed layer has been capping surface based convection this afternoon but that warm layer will continue to erode the remainder of the afternoon for a narrow corridor of mixed layer instability for surface based convection to develop some time between 21-23z. There remains a chance for a few strong to severe storms this afternoon and early evening. Mixed-layer CAPE values near 2000 J/kg with ECAPE values near 1500 J/kg. Shear values are relatively low in the below 500 mb with speed shear increasing above that level, which overall will just result in anvil spreading and shadows leading to the potential cooling surface temperatures which actually could lead to some stabilizing. Additional instability fuel will come from modest kinematic forcing and the additional convection overnight will likely become even less organized and mushy. The severe weather chances will be confined to large hail and damaging wind gusts, especially with the initial storms.The potential for efficient rainfall remains though with the PWAT values of 1.5 to 2 inches over central and southern Iowa and good warm cloud depths. Pockets with rainfall amounts of 2 to 3 inches may occur through 06z. The shallow surface boundary will be undercutting the moisture overnight as well so temperatures will cool and stratus should begin to form. This will lead to a much cooler Sunday with highs only in the 60s expected. Precipitation chances will continue over the southeast half of the area to begin the day then gradually settling southeast during the day. The more fall like weather will prevail through the week though highs in the mid to upper 70s will return by next Wed. Precipitation chances will be minimal with a few low chances over the southeast Monday and again late week. && .AVIATION /00Z TAFS THROUGH 00Z MONDAY/... Issued at 644 PM CDT Sat Sep 21 2024 VFR conditions to largely prevail with the exception of some localized vsby restrictions from rain tonight. A line of thunderstorms will approach KDSM and KALO over the next 2-4 hours and may pose some gusty variable winds for an hour. A front will pass over and shift winds to northwesterly. Rain will gradually fill in from the south after sunset and bring steady rain and a few non-severe storms overnight. Stratus may form behind, but enough dry air present to keep VFR. Gusty winds expected in the morning hours, lessening as the front moves farther away after 18z. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Donavon AVIATION...Jimenez