Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 04 2023 20:26:10 FOUS30 KWBC 042026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Fri Aug 04 2023 Day 1 Valid 1929Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR THE WESTERN DAKOTAS... ....Northern High Plains... Indicators for a potentially very significant rainfall continue to sound across northwestern SD primarily, but also into southwestern ND. 100 year ARI values (which represent total rainfall between 4 and 5 inches of rain) have spiked over 60% for a small portion of northwestern SD, with a 50% area into far southern ND. This is now considered a high-end Moderate Risk area. The broader Moderate Risk has been expanded north and east to include more of the steady but still stratiform rainfall ongoing across west-central ND, with the Slight Risk brought nearly to the Canadian border, as the steady rain is likely to amount to 1-3 inches, but will be falling over a longer period of time. This will still be a highly unusual amount of rain, but for northwestern ND may fall just slow enough that the river systems can mostly handle it. Such is not expected to be the case further south, where instability both from daytime heating, but far more significantly from the advection of high theta-e air continuously into the vertically stacked and intensifying low over SD will likely make for much higher rainfall rates in thunderstorms, which are expected to largely overwhelm the low FFG values between 1 and 1.5 inches in an hour over much of the Moderate Risk area as storms capable of producing 2 inch/hour rainfall rates are possible this afternoon and evening. ....Intermountain West... The Slight and Marginal Risks were expanded to include more of OR and ID, where there's good signal for 1-3 inches of rain on persistent easterly upslope flow, and the Marginal into OR where light to moderate rain has been ongoing all morning and convection is expected to continue developing this afternoon. ....Mid-Mississippi Valley... The Slight Risk across northern and eastern MO is mostly in place for an expected MCS that will develop this afternoon -- with the initial thunderstorms appearing to form currently -- and evening and move over the area, with a leading squall line followed by training convection. This area has been quite hard hit in recent days, and more rainfall is occurring presently, though fortunately is largely moving at a fast pace. This area will be monitored into the evening for a potential Moderate Risk upgrade as well. ....Northeast... The Slight Risk over the area matches messaging with the ongoing Flood Watches across portions of northeast NY and all of VT, NH, and southern ME. Training storms are beginning to develop with an MCV moving into ME and new convection forming presently across NY, which will move across the region this afternoon. After heavier than expected flooding in VT last night, the Slight Risk looks good for this region with no significant changes planned. ....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. The notable difference however from yesterday's Day 2 ERO was an uptick in the rainfall along the NC/VA coastline as a weak shortwave approaches from the west later today. Extended the Marginal risk area to cover the potential that some flooding or run-off problems occur. On the other hand...flash flood guidance is very high which should help mitigate any concerns. ....Southeast FL... No changes were made to the Marginal Risk as slow-moving convection with heavy rainfall, which has mainly left the area this afternoon, may return during the overnight hours which may result in additional areas of localized flash flooding and ponding on roads. 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!7ByNTRuIFhtZtx1N57swcwSHsAs4XRVzqVEdcV4FQsDn= _p_FLcgLCaJ-NzS2lRyITWygLKjmL72GKoVl60bUu1vxl58$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ByNTRuIFhtZtx1N57swcwSHsAs4XRVzqVEdcV4FQsDn= _p_FLcgLCaJ-NzS2lRyITWygLKjmL72GKoVl60bUgUFXm00$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ByNTRuIFhtZtx1N57swcwSHsAs4XRVzqVEdcV4FQsDn= _p_FLcgLCaJ-NzS2lRyITWygLKjmL72GKoVl60bUOpEESQw$=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) .