Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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991 FXUS63 KDMX 240404 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1104 PM CDT Mon Sep 23 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Chances for light rain/showers southeast tonight and far west late tonight into Tue morning. - Localized fog may occur tonight. - Next rain chances arrive late this week and into the weekend. && .DISCUSSION... Issued at 250 PM CDT Mon Sep 23 2024 Weak ridge of high pressure is over Iowa this afternoon but despite the high pressure, some moisture has been moving into southeast and even parts of central Iowa. The combination of a slow moving upper level PV anomaly directly over central Iowa and an initial wave of theta-e advection into central Iowa has resulted in some elevated light showers to develop between Ames and Waterloo. There is a very dry layer below these showers so a fair amount of evaporation is occurring yet a brief period of sprinkles or very light rain may reach the surface. The PV anomaly will continue to move slowly off to the northeast into this evening. A more robust upper level PV anomaly/short wave is moving into the western Dakotas this afternoon with good darkening showing up on the latest water vapor imagery. This system will descend swiftly southeast tonight and will arrive over northwest Iowa late. This system will bring a few showers to western and perhaps north central Iowa late tonight into Tue morning though any rain accumulations will be minor. Additional shower chances will be over far southeast Iowa as more theta-e advection rotates through that area tonight. There may be a few pockets of fog to form overnight and it is possible it could be locally dense in clearing areas near fringes of cloud cover. Rain chances will be low on Tuesday outside any residual showers lingering in the morning. The northern system will drag the thermal trough over Iowa on Tuesday. That system may keep an abundance of cloud cover around as well during the day, pending some clearing at some point in the afternoon. High temperatures on Tuesday are currently forecast in the upper 60s to low 70s but that is probably on the optimistic side and should clouds prevail, these highs may be 3-5 degrees too high. The northern short wave will evolve into a large closed upper low pressure system over the lower Mississippi River Valley by Wednesday and will remain there into the early portion of the weekend, effectively clogging up the upper level flow. The area will be mostly quiet into Friday before some Gulf moisture takes a non-typical route towards Iowa from the Florida panhandle and wrap northwest around the upper low. That moisture may be additional fueled if the currently Potential Tropical Cyclone Nine is ingested into this flow. At some point, rain chances will be on the increase this weekend as some short wave energy is ingested into the flow as well. Overall this is a low confidence period due to all of the moving parts but the potential for rain across at least southeast Iowa, is increasing. Temperatures will generally be in the 70s throughout with overnight lows in the 40s and 50s. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1104 PM CDT Mon Sep 23 2024 Expectations haven`t changed much since 00z. Conditions approaching 06z vary from mostly clear/VFR northwest to overcast VFR/MVFR conditions southeast with spotty IFR already in place as well. Confidence in how these trends evolve is low, but generally conditions should worsen along the northwestern edge of the current VFR/MVFR ceilings during the overnight hours with fog and/or stratus development. Current expectations are for MVFR/IFR at KDSM/KOTM/KMCW, but lower LIFR conditions are possible, with restrictions at other sites possible as well. Confidence increases that VFR conditions will return later in the morning and through the day with varied ceilings and cloud heights. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...Donavon AVIATION...Small