Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 28 2022 19:20:00 FOUS30 KWBC 281919 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 219 PM EST Mon Nov 28 2022 Day 1 Valid 16Z Mon Nov 28 2022 - 12Z Tue Nov 29 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Tue Nov 29 2022 - 12Z Wed Nov 30 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS AND FOR MUCH OF ALABAMA... ....1930Z Update... Few changes needed with this update. A severe weather outbreak remains likely over most of the northern and western portions of the risk areas. Any repeating supercell thunderstorms will easily have the capability of locally producing 3 inches/hour rainfall rates, albeit for a short time in any one location due to storm movement. According to most of the CAMs, these thunderstorms will persist mainly across Mississippi well into the overnight hours. By around midnight or so, the storms will likely congeal into 1 or 2 squall lines. They will still be strong enough to cause local flash flooding, especially given the moist soils from recent rains over far southern MS & AL but forward speed will be a mitigating factor. That said, there may be some stalling of that squall line over far southern MS & AL, but a bit inland from the coast. If that happens the flash flooding threat locally could be substantially higher. Wegman ....Previous Discussion... A strong shortwave trough ejecting out into the Plains Tuesday will induce deepening cyclogenesis over the Upper Midwest with an associated cold front progressing across portions of the lower Mississippi Valley. The presence of a strong upper-level jet, especially positioned in the favored right entrance region over the Southern Plains, will help bring widespread forcing for ascent across the region. Scattered to widespread thunderstorms is expected to fire up across the Lower Mississippi Valley, the Deep South and into portions of the Tennessee Valley as this is where PW values upwards of 1.5 inches (~2+ std dev) will be streaming northward and encountering an axis of sufficient instability lifting northward with MUCAPE of 1000-2000 J/kg possible. A surge in the low level winds/moisture over portions of eastern Louisiana, Mississippi and Alabama will help support and maintain multiple hours of enhanced rainfall rates of 1 to 2+ inches/hour. For Tennessee and areas north into the Ohio River Valley, dry antecedent conditions, less moisture and weaker storms should keep any training convection to just isolated instances of flash flooding. However, the Lower Mississippi Valley and the Gulf states will be more sensitive to flash flooding and ponding, especially in urban areas. These storms will be capable of producing intense rain rates of 1-2"+/hr and there is some training signal given the southwesterly/southerly mean flow orienting somewhat parallel to expected storm motions. The latest guidance is showing some of the higher QPF spreading further into northwest Georgia and southern/southeast Tennessee. There is the potential for a broad areal average of 1-2" with some signal for isolated 3-4"+ totals during the period. Soil moisture varies from drier than normal across the Tennessee and Ohio Valleys while further south near the Gulf Coast, above normal precipitation over the last few days has brought the top 40 cm soil layer saturation to above 80-90 percent in places. This lead to the eastward expansion of the Slight Risk area that was already in effect. Campbell/Taylor Day 3 Valid 12Z Wed Nov 30 2022 - 12Z Thu Dec 01 2022 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND ALONG THE COAST OF OREGON AND NORTHERN CALIFORNIA... ....1930Z Update... ....West Coast... No changes made in coordination with the EKA/Eureka, CA forecast office. Up to 4 inches of rain are expected, particularly in southwest OR/northwest CA, especially Wednesday afternoon and evening. The front causing the rain will be fairly fast moving, and northern portions of OR will see rapidly falling snow levels Wednesday night. However, the primary area of rainfall will stay ahead of any falling snow levels. Thus, all rain is expected both at the coast and the adjacent mountains. Isolated flash flooding is possible as a result. ....Southeast... A squall line moving across central AL/GA will be ongoing at the start of the Day 3 period Wednesday morning. The strong thunderstorms associated therewith should be fairly fast moving by this point, using up the last of the instability from Tuesday. that said, southern areas of the Marginal Risk already have saturated soils that could be prone to isolated flash flooding from the storms as they drop as much as 2 inches of rain locally as they pass through Wednesday morning. The forcing and instability diminish through the day, so the Marginal Risk is largely for remaining thunderstorms Wednesday morning. ....Northeast... Largely stratiform rainfall is expected across the Northeast on Wednesday, as any significant instability gets shunted to the Southeast. The rainfall will also be rapidly progressing as cold air rushes in behind the cold front. Despite low FFGs for portions of the Catskills and Southern Tier of NY this morning, the fact the rain doesn't begin until Wednesday should allow the, FFGs time to recover such that the 1.5" or less of rain over most of the area will not fall quickly enough to cause flash flooding. Thus, the previous Marginal Risk was removed. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4ZPlM9Mg_wR_QKPQPstTMvUW7oSKYzizjrweFLOEIEY8= 4gAg_FGrOnHsefKXGY3qSa-ctuEPWslLHtdp1vfCCVoQx9g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ZPlM9Mg_wR_QKPQPstTMvUW7oSKYzizjrweFLOEIEY8= 4gAg_FGrOnHsefKXGY3qSa-ctuEPWslLHtdp1vfCkHZY_gA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ZPlM9Mg_wR_QKPQPstTMvUW7oSKYzizjrweFLOEIEY8= 4gAg_FGrOnHsefKXGY3qSa-ctuEPWslLHtdp1vfCTwogaXk$=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) .