Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 27 2022 20:27:38 FOUS30 KWBC 272027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Sun Nov 27 2022 Day 1 Valid 16Z Sun Nov 27 2022 - 12Z Mon Nov 28 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Mon Nov 28 2022 - 12Z Tue Nov 29 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 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... ....2030Z Update... Few changes of note with this update, as the broad scale features described in the previous discussion below remain valid. The primary change was to expand the Marginal Risk area north and west to better align with the Storm Prediction Center's (SPC) Slight Risk area over extreme eastern TX through to southern IL. Expected supercell development early in the day on Tuesday have the potential to train over these areas. AR and TX have both picked up several inches of rain in recent days, so soils in this area are quite saturated, unlike areas further north and east. Any supercells that develop, especially as part of multiple rounds, will be capable of producing isolated instances of flash flooding. The rationale behind the Slight Risk remains unchanged. Much of the rainfall in this area is expected Wednesday night, as a 50 kt LLJ kicks into high gear with the loss of daytime heating. This will also advect plentiful instability of 1000-1500 J/kg (an issue that caused problems with verifying in the previous event) into this region. For TN north into the OH River Valley, dry antecedent conditions, less moisture and weaker storms should keep any training convection to just isolated instances of flash flooding. Finally, the Marginal Risk area over southeastern AL/FL Panhandle was trimmed towards the northwest a bit, with the expectation any flooding rains over this area will happen after the end of the Day 3 period on Wednesday morning. 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. Favorable right entrance region jet dynamics of a 120 kt jet positioned over the southern Plains will help bring widespread forcing for ascent across the region. Southerly flow ahead of the approaching system should surge northward anomalously high moisture, characterized by PWs approaching 1.5"+ which is between 2-3 standard deviations above normal. Model CAPE fields show an axis of sufficient instability lifting northward with MUCAPE of 1000-1500 J/kg possible. This overlap of deeper moisture and instability combined with the approaching front and large scale lift should support scattered to numerous showers and thunderstorms Tuesday afternoon across the Lower Mississippi Valley then further eastward into Tuesday night across portions of MS and AL. 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 model guidance shows potential for a broad area of 1-2" with some signal for isolated 3-4"+ totals during the period. However, there remains some spatial uncertainty, particularly the northern extent of the deeper convection and heavy rainfall potential. Soil moisture varies from drier than normal across the TN and OH 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. The Slight Risk inherited was largely unchanged and adjusted based on the latest ensemble probabilities and trends in deterministic guidance, and highlights the greatest risk area for isolated to scattered instances of flash flooding. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-TZrUoLqBEq7cDODErFypaPo6c2_uSjplpYe0XOycrSm= R316cxPTKJNTsMFRw6OttKxgHkTmJLtyd8GuSmhf-9keZN0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-TZrUoLqBEq7cDODErFypaPo6c2_uSjplpYe0XOycrSm= R316cxPTKJNTsMFRw6OttKxgHkTmJLtyd8GuSmhflwnFxPc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-TZrUoLqBEq7cDODErFypaPo6c2_uSjplpYe0XOycrSm= R316cxPTKJNTsMFRw6OttKxgHkTmJLtyd8GuSmhfeVAJSMc$=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) .