Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 14 2024 20:10:50 FOUS30 KWBC 142010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Wed Aug 14 2024 Day 1 Valid 16Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH DAKOTA AND MIDDLE MISSISSIPPI VALLEY... 16Z update... A swath of 3 to 5 inches of rain will likely setup as moisture flux encounters a strong convergence zone over western portions of the North Dakota. These storms will be capable of producing 1/1.5 inches/hour rain rates which elevates the threat for localized flash flooding. For additional details refer to WPC's Mesoscale Precipitation Discussion #882. As such the western/northwestern bounds of the Slight Risk area was expanded. The convection across Missouri is expected to continue tracking to the southeast while producing heavy rain along its path. Further south, the storms crossing into northern Arkansas are starting to weaken so the the risk for localized problem as the afternoon and evening progresses will lower in response. Campbell A bit greater uncertainty than normal with how convection evolves this morning across portions of IA/MO/AR, although things are becoming a bit clearer as of 08z. The convection moving across NE (as of 08z) will likely be over portions of western IA and northwest MO by 12z this morning. It will likely align itself more northwest to southeast by this time, allowing for a greater training potential. However the longevity of this training is a bit uncertain as the low level jet gradually weakens. Suspect that an isolated to scattered flash flood risk will exist, but not confident in anything more significant than that at this time. Another axis of training convection is likely this morning across portions of southern MO into northeast AR. As of 08z starting to see the beginning of this scenario starting to evolve, and expect to see an uptick in organization and training between now and 12z. NAM Nest runs have indicated localized 5-10" rainfall with this event, and while not impossible, it does seem probable that amounts will stay below that level. However still thinking several inches of rainfall is likely, locally higher, and thus do expect a corridor of isolated to scattered flash flood risk to develop. This activity will likely continue into the morning hours before dissipating by later in the morning By this afternoon into evening expect convection to develop across far southeast NE into western IA and northwest MO, with the focus expected to be near a warm front slowly lifting north across the region. Stronger mid/upper forcing will move into the region, which combined with strengthening low level moisture transport, should allow for organized convective development. Suspect that the convection this morning across NE/IA may help keep the warm frontal position a bit further south today. Given the heavier rainfall rates should generally be near and just south of the warm front, we were able to trim some of the northern areas (MN and northern IA) out of the Slight risk with this update...and this is generally supported by 00z HREF QPF probabilities and FFG exceedance probabilities. Area of heavy rainfall are still likely over northern IA into MN, but rates should stay low enough to keep any flash flood risk more localized in nature. Even over northern MO into central IA this late day and overnight convection will probably be rather quick moving, limiting rainfall total potential. However we could see some brief training near the warm front, and some of these areas will have also seen heavy rain from the morning round of convection. Thus do think at least some flash flood risk will exist with this later convection as well. Overall the broad Slight risk stretches from central IA to northeast AR, and covers the risk from both the convection this morning and also later today/tonight. The southern part of this Slight risk is primarily for this mornings activity, with the northern portion of the risk potentially getting both rounds of storms. A secondary rainfall maximum should occur over portions of ND this morning into early afternoon along another slow moving convergence axis. Hard to ignore the impressive 00z HREF signal over this area, with 3" neighborhood probabilities in the 50-80" range, and 6hr FFG exceedance probabilities over 40%. There was a localized training cell that resulted in a flash flood warning last night over ND, and the pattern does appear favorable for that potential this morning as well, except probably a bit more convective coverage today. Thus think adding a Slight risk is the way to go, with isolated to scattered flash flooding a possibility. Chenard Day 2 Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF MISSOURI, ILLINOIS, INDIANA AND KENTUCKY... 21Z update... The latest guidance continues to have better instability and moisture influx aligned over the Mid-Mississippi=20 Valley/Midwest with consensus of 1 to 3 inches as the areal average. The Slight Risk area that was inherited highlighted where there will be increase threat for flash flooding from training storms. Minor adjustments to the Slight were made with this update to include a W/NW expansion across central Illinois. There was also a trend for portions of central/eastern Iowa to be a little drier while the heavier QPF focused to the east over central Illinois than the previous cycle so a small trimming of the back edge of the Marginal was made. Campbell The heavy rainfall threat shifts east into the Great Lakes and OH Valley by Thursday. A bit more uncertainty with the details by this time, but the overall ingredients remain in place for areas of excessive rainfall. Expect one round of convection to be moving across the region Thursday morning, but currently anticipating this activity to be on a downward trend by this time and thus should not be posing much of a flash flood risk. However with deeper troughing still upstream at this point, would expect to see additional convective development by the afternoon hours. This convection is likely from portions of eastern MN and WI southward into portions of MO/IL/IN/KY. Convection across the northern half of this area will probably stay pretty quick moving off to the east, likely limiting the extent of the flash flood risk, keeping the threat more at Marginal risk levels. The greater instability and moisture axis will likely end up further south from southeast MO into portions of IL/IN and KY. Even here cell motions should be rather quick, however with this area further displaced from the forcing to the north and mean flow more parallel to the front, do expect that we could at least see some brief training of convection. Given the favorable thermodynamic environment for heavy rainfall rates, any training would likely result in at least some flash flood potential. This appears most likely along this corridor, and thus will carry a Slight risk here. Chenard Day 3 Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST INTO THE GREAT LAKES... 21Z update... There were no significant changes to the WPC QPF or the environmental setup for this period. There are a few signals for very isolated areas of accumulations of 1 to 2 inches but the exact location remains uncertain. No changes were made to the Marginal Risk area that is in effect from northern Alabama/Georgia to Michigan/western New York. Campbell On Friday the likelihood of convection shifts eastward stretching from portions of the Southeast up into the Great Lakes region. The probability of flash flooding by this time appears to decrease, but at least some risk likely persists. Stronger deep layer flow supports quicker cell motions and not really seeing much of a low level focus for training. However, the troughing moving overhead is quite broad in nature, likely meaning the duration of lift will be extended, potentially supporting a few rounds of convection. Also deep layer flow, while quick, is fairly unidirectional, which can support some brief backbuilding of convection at times. Thus while the overall setup for flash flooding and model QPF output are not all that impressive, there are enough favorable ingredients in place to suggest at least an isolated flash flood risk will exist. This warrants the continuation of the Marginal risk. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!77RA43z_akFlcb1GT6aEdygpei6xSytGT01O7P4PqX4R= kC80NXQbBF6UtCp74M4NzqNXD7AUYJyP5NT-KSniEPa3zZ8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!77RA43z_akFlcb1GT6aEdygpei6xSytGT01O7P4PqX4R= kC80NXQbBF6UtCp74M4NzqNXD7AUYJyP5NT-KSniXgBOwFI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!77RA43z_akFlcb1GT6aEdygpei6xSytGT01O7P4PqX4R= kC80NXQbBF6UtCp74M4NzqNXD7AUYJyP5NT-KSniev4G_jE$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .