Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 05 2023 00:58:42 FOUS30 KWBC 050058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Fri Aug 04 2023 Day 1 Valid 01Z Sat Aug 05 2023 - 12Z Sat Aug 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR AREAS IN AND NEAR SOUTH DAKOTA... ....Northern High Plains... Heavy rainfall continues to fall across portions of the Dakotas in and near the center of a deep layer cyclone, with totals over the past 24 hours in the 2-3" range for western portions of those states, with local amounts to 5" per radar estimates. Pockets of 1500-2500 J/kg of ML CAPE remain across the area along with effective bulk shear of 25-40 kts and precipitable water values of 1.75-2". As the rainfall pattern is forecast to constrict with time, so have the risk areas, which has shrank the areal extent of the Moderate Risk somewhat from the previous issuance. With flash flood guidance lowering with time across the western Dakotas, this will leave the region more susceptible to additional heavy rainfall. The 18z HREF paints a ~50% of an additional 3" for portions of northwest and south-central SD, with non-zero probabilities of 5"+, which could bring overall areal averages/totals closer to 5" and local amounts closer to 10" by 12z. This is a highly unusual amount of rain and continues to be considered on the high end of a Moderate Risk -- High Risk impacts on a very localized basis cannot be ruled out, but at the moment do not appear to be a broad enough threat or at an exact enough precision to depict, given the neighborhood probabilities. ....Northern Rockies... As convection is expected to fade over the next several hours, lowered the excessive rainfall risk to Slight. Hourly rain totals up to 2" are possible wherever cells can manage to backbuild, train, or merge in the meantime. ....Mid-Mississippi Valley... The Slight Risk across northern and eastern MO is mostly in place for the developing small MCS. Precipitable water values remain in the 2-2.25" range, which along with ML CAPE of 2000-4500 J/kg, could foster hourly rain totals to 3", which may try to hug the WNW-ESE instability gradient, with some chance of backbuilding, as the related mesocyclone forward propagates eastward. The 18z HREF has high chances for 3", with a ~50% chance of 5", and a non-zero chance for 8" across northern MO. Where the rotating thunderstorm complex has been moving, local amounts of 3-5" have fallen within 1-2 hours. If some training band can form in the wake of the system or enough backbuilding convection can form to its south or southwest, local 5-8" amounts and associated Moderate Risk impacts cannot be ruled out. Since there's enough uncertainty about the higher end amounts in this scenario, and after coordination with the LSX/St Louis MO and ILX/Lincoln IL offices, maintain the idea of a general higher end Slight Risk for the region, with the greatest impacts expected across north central into central MO where local amounts to 9" have been seen over the past 3-4 days. ....Northeast... The Slight Risk has been moved to southern New England for short term potential impacts as described in Mesoscale Precipitation Discussion #864, valid through 0330z. Training storms are possible across portions of MA, CT, and RI over the next few hours. ....Southeast... Anomalous moisture (PWs > 2") combined with favorable forcing for ascent associated with shortwave energy moving through the region will keep an active/unsettled pattern in place and the threat of heavy rainfall and flash flooding into Saturday morning. Flash flood guidance is very high over sandy soils which should help mitigate any concerns. Roth Day 2 Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS INTO THE MID-MISSISSIPPI VALLEY, MUCH OF ILLINOIS AND INDIANA, AND PORTIONS OF THE GULF COAST FROM THE WESTERN FLORIDA PANHANDLE TO MISSISSIPPI... ....2030Z Update... ....Northern Plains... A Slight Risk remains across much of the Northern Plains as a compact but vigorous vertically stacked low imports highly anomalous moisture into the low, and wraps that moisture around the north and west sides of the low. For those locations generally over ND, a prolonged period of lighter rain, but with some embedded convective elements included is expected over areas that don't normally see prolonged periods of precipitation, especially on the order of multiple inches. Meanwhile a bit further south across northern SD, closer to the instability and moisture plume, shorter bursts of heavy rain associated with stronger convection are likely, especially east of the low center. HREF probabilities are very impressive across this area, with over 90% chance of 3 inches of rain in the neighborhood probabilities, and over 20% chance of 8 inches of rain, which would approach both Dakota's state records. Soils in the eastern Dakotas have been dry, which may allow for more efficient absorption of the heavy rain. However, the potential for 1-2 inch per hour rates could overwhelm local streams and creeks resulting in flash flooding, instances of which are expected to be widely scattered. Warm rain processes in a very moist environment with record PWAT noted in this morning's 12Z Bismarck, ND sounding are also points of concern for localized very heavy rain and subsequent flooding. Inflow into the low across SD will also likely cause heavy rains along the MO river along the NE/IA border, to as far south as Kansas City, where the Slight was extended. ....Northern IL and northwest IN... A separate compact low that will be moving across MO/IA tonight will slowly push eastward as well into IL, where the potential also exists for several inches of rain. A Slight was introduced in coordination with LOT/Chicago, IL forecast office. These high amounts of rain are a rather new development in much of the 12Z guidance, so just a single upgrade was coordinated until there's some run-to-run consistency. Otherwise expect very efficient warm rain processes with PWATs around 1.75 inches to continue through the day along the MS River in IA/IL and push eastward towards the Chicagoland area by the end of the day. This will be a very slow-moving system, which in turn means there will be lots of time for heavy rain to fall over any one area over northern IL to potentially cause flash flooding. ....Coastal S. Alabama and the western FL Panhandle... Very slow moving convection along the sea breeze an old frontal boundary will develop Saturday afternoon, moving southeastward along the boundary. Expect training storms to be possible in this localized area, which may include Mobile and/or Pensacola. These areas are at the highest concern for localized flash flooding, though potential for 3-5 inches of rain could mean localized flash flooding even outside those cities. Wegman Day 3 Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY... ....2030Z Update... A major shift in the guidance for Sunday occurred with the 12Z runs. The NAM/GFS/UKMET each shifted west a good 100-200 miles with the heaviest precipitation, indicating the guidance is really struggling with the speed of this system. Thus, much of the inherited Slight Risk centered over Lake Michigan was now rain-free in the latest guidance, so the Slight was adjusted accordingly, but the confidence in exactly where the Slight should be is very low. The vertically-stacked low over the Dakotas continues to strengthen as a deep shortwave trough, likely enhanced by the low itself, supports the low. The shortwave energy will remain rather slow moving, as it will be somewhat cutoff from the broader flow between the 2 jet streams, so the surface low is also expected to continue a slow-movement, going from near Sioux City at the start of the day to near Chicago by Monday morning. PWATs will remain above 1.75 inches, so efficient warm rain processes being forced by very favorable upper level support will produce the potential for heavy rains. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5aHxAX8vaoOA0JCu6ZOtnWQv8TwS3s5b6gXrBiEGJaHu= pbZO2dy8Sxx_DVeR87PGz-sPv61tvv-bth1RZLIH4nvt8HU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5aHxAX8vaoOA0JCu6ZOtnWQv8TwS3s5b6gXrBiEGJaHu= pbZO2dy8Sxx_DVeR87PGz-sPv61tvv-bth1RZLIHo9PRogQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5aHxAX8vaoOA0JCu6ZOtnWQv8TwS3s5b6gXrBiEGJaHu= pbZO2dy8Sxx_DVeR87PGz-sPv61tvv-bth1RZLIHzPe5zhA$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .