Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 28 2022 15:57:56 FOUS30 KWBC 281557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1056 AM 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 VALLEY INTO NORTHERN ALABAMA AND SOUTHERN TENNESSEE... 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 THE SOUTHEAST, NORTHEAST AND ALONG THE COAST OF OREGON AND NORTHERN CALIFORNIA... ....West Coast... Onshore flow and shower coverage will increase across the Pacific Northwest as a low pressure system approaches the West Coast. Moderate to heavy rainfall is expected along the coast from northwest Oregon southward to northern California. With the flow nearly perpendicular to the terrain there will be some local enhancement, especially for the Coastline. A Marginal Risk area spans from northwest Oregon to northwest California given the increased risk for rapid run off, flash flooding and possible debris flow near sensitive/prone locations. ....Southeast... The system will continue to advance eastward while the trailing front makes its was across the Southeast. Showers and thunderstorms will continue to fire up and train in proximity to the frontal boundary. Parts of the Southeast will already have had wetting rains to raise soil saturation levels and reduce local FFG. Given that the potential for rainfall rates over 1 inch/hour will be possible and the antecedent conditions expected, there will be an elevated risk for local run off and flash flooding concerns. A Marginal Risk area cover most of the Southeast and into the southern Mid-Atlantic. ....Northeast... The leading low of this Plains system will continue to lift north and east during period while a reinforcing shot of cold air plunges in Great Lakes region in its wake. Abundant moisture from the Gulf and from the Atlantic will be drawn into the system spreading up 1 to 2 inches across the Northeast. Local FFG across much of this part of the country is as low as 1 inch/3hr especially near areas with higher elevations. Given the degree instability and lift that will still be present there will be the potential to reach or exceed FFG. A Marginal Risk is in effect for a large portion of the Northeast. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5dWRYchmBOVPuPrUT0sn_EaXsplbSsV65SQAJdhc-nuC= lYSjtzbtgD_pLvMKTi09sQGDq-qCtWeJjPThDAT1KsmAG_c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5dWRYchmBOVPuPrUT0sn_EaXsplbSsV65SQAJdhc-nuC= lYSjtzbtgD_pLvMKTi09sQGDq-qCtWeJjPThDAT1Bsi-3mY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5dWRYchmBOVPuPrUT0sn_EaXsplbSsV65SQAJdhc-nuC= lYSjtzbtgD_pLvMKTi09sQGDq-qCtWeJjPThDAT1-SlbggM$=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. 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