Area Forecast Discussion
Issued by NWS Des Moines, IA

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082 FXUS63 KDMX 221130 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 630 AM CDT Sun Sep 22 2024 ...Updated for the 12z Aviation Discussion... .KEY MESSAGES... - Lingering categorical showers/iso storms southeast half today and showers returning south Monday (25 to 40% chance) - Much Cooler as Fall Begins today - Tuesday rain chance? - Lower confidence and possible complex late week pattern && .DISCUSSION... Issued at 305 AM CDT Sun Sep 22 2024 .Short Term /Today through Monday/... Confidence: Medium to High Forecast pretty much on track with relatively stable model solutions and decent consensus the past few runs. Tonights subjective sfc analysis at 04z shows low pressure over east central Iowa with a warm front extending east into northern Illinois; a trailing cool front from the low into southwest Iowa and northeast Kansas along with a secondary, stronger cold front extending nearly due west from the low near Marshalltown back to Carroll and then northwest into southeast South Dakota. Aloft at H850, Iowa and much of the Central Plains remains east of the main H850 trough; resulting in continued moisture transport into our area overnight through most of today (Sunday). A large plume of +10C dew points is still being forced northeast into our area at this time. High pressure over the Southeast combined with the elongated trough from southern Canada southwest to New Mexico is assisting to focus the moisture along the boundary. Over the past few hours, the trailing cold front has become more active with a line of showers and thunderstorms from Des Moines to Omaha back west to Kearney. Though we are already drying out in the north at this time, the southeast half will continue to see showers and some isolated storms today as the upper level trough becomes more parallel to the mid level flow and begins to slow its southeast progression through the day today. The secondary cold front now edging southeast is bringing with it a rather cool airmass which will be present for the day today. H850 temperatures are expected to drop to 8C in the north to the lower teens in the southeast by afternoon. Clouds are expected to hold on in the southeast half and will also fill in farther north today as an H500 system associated with the H850 southwest trough begins to move east this afternoon/evening. There remains some uncertainty as to how quickly the systems will track far enough south to no longer impact the region with clouds/precipitation. The HIRES models show a consensus of keeping more clouds and even some precip in the south through Monday as the H500 trough pulls east northeast. Ensemble guidance supports this scenario as well; however the deterministic GFS and EC are shading the main moisture channel far enough south to keep most, if not all of the precipitation later this evening and Monday in Missouri. Given the NAMs north bias, it also generally keeps most, if not all of the additional rain showers south in Missouri. With the signal being consistent the past few days in some of the data, will lean toward the NBM which does bring back some showers later Monday morning and afternoon in the south and southeast (chances at 25 to 40%) with light amounts of up to a tenth of an inch of rainfall possible. Clouds will linger over the southeast as well on Monday as the southern stream wave brushes Iowa with some precipitation. Highs today will be 20 degrees cooler than yesterday as afternoon readings reach the upper 60s northeast to the lower 70s in the north central areas. With lingering showers south, mid 60s will be the general trend there. Tonight will be rather cool as lows dip to the lower to mid 40s with lower 50s in the south. Monday will see a similar spread of slightly warmer temperatures north where more sun is anticipated; highs ranging from the lower 70s north to the upper 60s in the south/southeast. .Long Term /Monday Night through Saturday/... Confidence: Low to Medium Confidence in the extended has dropped considerably over the past 24 hours due to a number of factors. First, though it appeared yesterday that Tuesday would be dry, the deterministic synoptic models are now picking up on a wave currently located near Lake Winnipeg, dropping it south southeast into Iowa by Tuesday with a few showers and perhaps an isolated storm. This originally was expected to drop south and remain slightly west of Iowa, but the exiting upper level system may slow down enough to now allow the two systems a better chance to phase. To varying degrees, all the 00z NAM, EC and GFS solutions show an increase in clouds and a slight chance of showers Tuesday afternoon. The current blended approach in our forecast shows no increase in rain chances and we may need to revisit that again later today and tonight as more data arrives. Beyond that, confidence erodes further as the medium range models are struggling with an H500 low and the apparent tropical storm/potential hurricane reaching the eastern Gulf states by Thursday. For the first few days of the extended, temperatures will recover to the 70s and last through Thursday. Secondly, as we move through the middle of the week, several possible scenarios are now being introduced by the medium range models with significant differences in the late week/early weekend forecast. The deterministic GFS is the most progressive of the model suites with a large ridge at H500 building across the west and extending to the Northern Plains with H850 temperatures warming back up to 18 to 22C by Friday over Iowa. The EC, on the other hand ends up phasing the two H500 waves (tonights with the Tuesday Canadian ripple) into a cut off H500 low over southern Missouri by Thursday. With the approaching apparent TS/hurricane in the Gulf by late week; the deterministic GFS pushes the system up the eastern seaboard while the EC merges the H500 closed low and the soon to be extratropical cyclone over the mid Mississippi River Valley, bringing an abundance of moisture back into the Central Plains and Iowa by Friday and Saturday. The difference between solutions GFS/EC is about 4 inches of rainfall over MO, with the EC depositing nearly 7 to 8 inches of rain across southern MO and 1 to 2 inches into the southeast half of Iowa. For grins, the deterministic GEM is similar to the EC, but phases the H500 and tropical system in the deep south, rather than closer to Iowa. Backing out of these model suites, the ensemble EC and GEFS are both suggesting some component of moisture being drawn back into Iowa by Friday and lasting into the weekend with highs tempered back to the lower to mid 70s for the period. From a climatological standpoint, it seems out of favor to get a cut off H500 low this time of year in the mid Mississippi River Valley. But with that being said, we do have quite the blocking high over the eastern CONUS right now and the mean H500 ridge may maintain its position into midweek stretching from New England to the southeast states. The difference between the GFS and EC might mainly be due to the generally faster, more progressive nature (bias) of the GFS compared to the EC. For now will need to maintain the PoPs in the extended period Thursday through Saturday due to the odd possibility that the cutoff EC solution, supported by the ensemble guidance, comes to fruition. If it does, next weekend looks rather wet for much of the area. Details will begin to be ironed out already by Tuesday or Wednesday. By then the trend toward a cutoff H500 or progressive pattern will be readily apparent in the data. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
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Issued at 630 AM CDT Sun Sep 22 2024 Showers and increasing north winds the main focus through the period. Break in showers aft 15 to 18z with winds relaxing aft 23z. VFR conditions expected with only a small chance of possible brief MVFR at OTM through 18z. System to pull out this evening with some mid to high level clouds lingering.
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&& .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...REV AVIATION...REV