Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 26 2024 20:26:57 FOUS30 KWBC 262026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EST Mon Feb 26 2024 Day 1 Valid 16Z Mon Feb 26 2024 - 12Z Tue Feb 27 2024 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO AND MID-MISSISSIPPI VALLEYS AND THE CENTRAL APPALACHIANS... ....2030Z Update... Very few changes were made to the inherited Marginal Risk area. The pattern remains one quite favorable for severe weather, but nowhere near as much so for flash flooding. Despite PWAT values 2-3 sigma above normal, a 60 kt LLJ, and 500-1000 J/kg of instability noted in the latest guidance (with locally higher maxima) these ingredients favor wind-related impacts more than flash flooding impacts. The primary factor working against flash flooding is the incredibly fast motion of any storms that develop in this region. This will be supported by the 60 kt LLJ and a supporting 150 kt upper level jet. Much of the CAMs guidance that covers the Tuesday afternoon and overnight period suggests anywhere from 1 to 3 rounds of storms...separated by a few hours...will move across the Ohio Valley. These storms will be capable of heavy rainfall rates, with embedded hail with the stronger storms. Hail in this area will cut down on overall precipitation totals. The very rapid movement of the storms suggests any one area may see an hour or less of rainfall with any one storm, so it will likely take 2 or 3 rounds of rain for flash flooding to develop. The storms are likely to grow upscale into MCS's and lines rather quickly, so that will favor a bit more widespread rain, though not everyone will see heavy rain due to more widely scattered storms and hail. Secondly, atmospheric moisture. As noted, PWAT values are 2-3 sigma above normal. However as this is the dry time of year, that only equates to PWATS of 1 to 1.25 inches. This will therefore limit overall precipitation totals as well as more moisture goes into maintaining the storms with less resulting in rainfall. Finally we have very dry soil conditions, with NASA Sport suggesting soil moisture over the Ohio Valley near 50% saturation, which in some areas is less than 10% of normal. The empty river basins will be easily able to handle any and all precipitation. Flash flooding would most likely be in any local creeks, streams, and poor-drainage urban areas. Taking all of this together, think it will take multiple rounds of heavy rain (or mostly rain) moving over a poor drainage area to result in flash flooding. Thus, instances of flooding should be very isolated. The Marginal was considered for cancellation, but the prevalence of urban areas, somewhat low FFGs for Ohio and points east (1-1.5 inches per hour FFG values), and then terrain considerations in the east may be enough to support an isolated occurrence or two, as well as keeping with consistency. For western areas (IL/IN) where FFGs are a little higher, the storms are likely to be a bit stronger as they will move through closer to peak heating Tuesday afternoon and evening. Wegman ....Previous Discussion... A robust mid-level shortwave will propagate southeast out of the Pacific Northwest on Tuesday with increasing difluent flow ahead of the mean trough that will help initiate a round of convection within the confines of the Mid-Mississippi and Ohio Valley's. Focused meridional flow ahead of the mean longwave trough will advect deeper moisture and associated theta-E's poleward with a tongue of elevated instability building within the areal theta-E advection regime. Getting more into the hi-res time frame, we are seeing some forecast MUCAPE values approaching 500-1000 J/kg with locally higher lying along a projected warm front situated to the east of a developing surface reflection in the central Midwest. PWAT indices soar to 2-3 standard deviations above normal within a large zone spanning from the Mid-Mississippi Valley up into the Great Lakes by Tuesday afternoon as the enhanced low and mid-level flow usher Gulf moisture into the outlined area. By late-Tuesday afternoon, convective development is likely thanks to the approach a mid-level vorticity maxima moving out ahead of the shortwave trough as it migrates eastward. Models are keying on a band of heavy rainfall within the proximity of a warm front, denoted fairly well within the theta-E fields on guidance. The question that still needs to be solved is where that boundary will lie. Current consensus is north of I-70 with some guidance as far north as southern MI which leads this to be the focal point of expected convective development. Recent HREF probability fields for at least 1"/hr have risen to 25-30% within the confines of the IN/MI/OH border which is where most of the CAMs currently have the warm front situated when examining the surface moisture/temp fields. Further south will have the best deep layer moisture and elevated PWATs, but the lack of a focused boundary like the warm front in question will lead to more isolated flash flooding concerns, mainly within urbanized areas, and terrain as you head further east. Any convective development will continue into the overnight hours as we approach Wednesday with upscale growth of convection likely after 06z as the shortwave trough pivots east of the Mississippi and begins to take on a more neutral tilt by the end of the period. Large scale forcing will be at its maximum by the end of the forecast cycle with a line of heavy rain likely extending from eastern OH, south into the Tennessee Valley. This will continue to advance eastward and the focus shifts into the Appalachian front and terrain-centric areas in the Mid-Atlantic and Ohio Valley as we roll into D3. The Marginal Risk from the previous forecast was maintained, but adjusted to reflect the latest QPF trends within the convective-allowing environment being forecast. A Slight Risk upgrade is non-zero, but current forecasted rates within any convection keep this capped for the time-being. Kleebauer=20 Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5mKlbXJ10GsymyQt5QRJ2igGgacsMsA61TWEO9Koz-9y= e9wS3j6CoWM4pqYM4EWaZtDKsHWnVWA8khQtNFresCV2GIY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5mKlbXJ10GsymyQt5QRJ2igGgacsMsA61TWEO9Koz-9y= e9wS3j6CoWM4pqYM4EWaZtDKsHWnVWA8khQtNFrefazuvW8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5mKlbXJ10GsymyQt5QRJ2igGgacsMsA61TWEO9Koz-9y= e9wS3j6CoWM4pqYM4EWaZtDKsHWnVWA8khQtNFreL2Q8Smg$=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) .