Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 16 2023 00:37:36 FOUS30 KWBC 160037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 836 PM EDT Sat Apr 15 2023 Day 1 Valid 01Z Sun Apr 16 2023 - 12Z Sun Apr 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND LOWER-MID MISSISSIPPI VALLEY... ....Lower-Mid Mississippi Valley... 0100 UTC Update -- Based on the latest observational and mesoanalysis trends, have removed the Slight Risk area across the Central Gulf Coast, while trimming quite a bit of the Marginal Risk. Based on the latest guidance trends (including most recent CAMs, esp. HRRR runs), still anticipate a localized flash flood threat with additional convective formation/upscale growth ahead of the cold front, despite the limited threat for cell training (given the relative swift W-E frontal progression) and gradual decreasing MUCAPE trends this evening and overnight. Hurley Day 2 Valid 12Z Sun Apr 16 2023 - 12Z Mon Apr 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND SOUTHEAST FLORIDA... ....20Z Update... No changes were made to either Marginal Risk area for this afternoon's update. In the upper Midwest, a strong low will be moving across the region on Sunday. It will pull down lots of cold air from Canada, which will turn the western end of the precipitation shield over to snow, with Winter Storm Watches already issued for portions of Wisconsin and the U.P. of Michigan. The key will be where that rain-snow line sets up. Since FFGs are very low in this region, even if a short period of the first part of the day Sunday remains all rain, that rainfall when combined with the rain expected today falls, isolated flash flooding remains probable. Meanwhile further east into the eastern U.P. of Michigan and northern L.P., there is no concern about changing over to snow, but rather rain falling on very saturated soils with low FFGs. The areas highlighted by the inherited Marginal Risk remain in very good agreement with the latest guidance and WPC QPF forecast as to where the heaviest rainfall totals are expected. Unlike today, by Sunday there will be very little if any MUCAPE, so convective elements to the rain will be limited. With that said however, the strengthening low will itself help encourage lift, so brief periods of heavier rain can't be ruled out. For South Florida, there's considerably less confidence in the flash flooding potential, primarily due to faster recovery of the FFGs in the Ft. Lauderdale-Miami areas. It remains likely that afternoon convection will occur Sunday afternoon, but its ability to cause flash flooding will be entirely dependent on those storms moving over the most sensitive areas. Thus, the Marginal Risk was maintained for this update, but it's a low end Marginal threat at this point. Wegman ....Previous Discussion... ....Upper Midwest and Michigan... Low pressure will continue to drift north/northeastward through the region as the mid/upper level energy begins to close off by Sunday evening. Along/downstream of the 700 mb low, an axis of heavier precipitation is expected to set up across portions of Wisconsin, initially on the warm side of the system, before much colder air crashes eastward, leading to a changeover to rain/snow or all snow. The impressive dynamics/lift will help support a narrow band of intense precipitation, likely over areas that are somewhat more susceptible to flooding given the snowmelt concerns and previous day rainfall. The latest guidance suggests potential for another 1-2" areal averages across portions of central/eastern Wisconsin, the U.P. of Michigan, and northern L.P. of Michigan which may lead to isolated instances of flash flooding. ....South Florida... Deeper convection is expected to initiate along existing sea breeze convergence ahead of an approaching cold front slated to arrive later in the period. The latest guidance shows anomalously high moisture pooling in the region, upwards of 1.75", which when combined with the daytime heating/sea breeze interactions, several clusters of strong thunderstorms capable of producing intense rain rates will be possible. Some of the area is still susceptible/vulnerable due to recent rainfall so this additional threat of heavy rainfall may lead to flash flooding over the urbanized corridor of southeast Florida. Taylor Day 3 Valid 12Z Mon Apr 17 2023 - 12Z Tue Apr 18 2023 ....20Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-En-ROTbIqErakQXE2ES5nc1K2komM2BBWRECd3HXe1m= JVNeE7nOSD6r5zOAyhkWneU3JKpnEielY3Ochvdjh4I9MRU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-En-ROTbIqErakQXE2ES5nc1K2komM2BBWRECd3HXe1m= JVNeE7nOSD6r5zOAyhkWneU3JKpnEielY3OchvdjeGJiBPo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-En-ROTbIqErakQXE2ES5nc1K2komM2BBWRECd3HXe1m= JVNeE7nOSD6r5zOAyhkWneU3JKpnEielY3OchvdjwKGOMaY$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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