Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 29 2023 07:45:27 FOUS30 KWBC 290745 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 244 AM EST Sun Jan 29 2023 Day 1 Valid 12Z Sun Jan 29 2023 - 12Z Mon Jan 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND CENTRAL GULF COAST... Very few notable changes with this morning's forecast update, based on the latest guidance QPF trends (deterministic and probabilistic). A slowly strengthening upper level jet streak will put much of the Slight Risk area in the right entrance region of the jet streak. Small disturbances riding along the jet may locally trigger convection across the Slight Risk area as they race northeastward with the jet. Much of the dynamics for this rainfall event will be in the lower levels of the atmosphere as broad southerly flow covering the whole of the Gulf peaks in intensity as far as the LLJ is concerned over southern LA midday Sunday. The LLJ will reintensify as the convection moves east across the FL Panhandle and southern GA Sunday night, including values as strong as 60 kt over the FL Panhandle, far SE AL and far SW GA. Thus, expect convection to be quite efficient at producing heavy rainfall as it moves eastward Sunday and Sunday night. To be sure, the greatest threat within the Slight Risk area will be across western areas from TX through MS, with slightly lower threats for flash flooding further east. Peak storm total rainfall may exceed 3 inches in portions of southern LA through southern MS-AL-GA, within a more favorable thermodynamic environment with PWs close to 1.75" along with better deep-layer instability (MUCAPEs 500-1000 J/kg). The 12Z HREF in fact indicates clusters of 40-60+ percent probs of >3"/24hrs within the Slight Risk, with the highest probs (60-80%) just north of the Upper TX Coast through much of southwest LA. There continues to be good model consensus however that the axis of heaviest rain will remain north of New Orleans. Changes with this update were all small tweaks, including dropping a bit more of the Mississippi River Delta south of New Orleans down to a Marginal. There remains a bit of uncertainty as to how far north the convection can get into MS/AL/GA, but with a general very small southward shift, did trim the northern edge of the Slight Risk area through those states a half a county (on average) southward. Wegman/Hurley Day 2 Valid 12Z Sun Jan 29 2023 - 12Z Mon Jan 30 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Mon Jan 30 2023 - 12Z Tue Jan 31 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The previous Marginal Risk which extended from the Big Bend area of southwest TX northeastward to about Memphis, TN was eliminated with this update. There were a few reasons listed below for this change: 1. Guidance continues to struggle with when a large and potent cutoff low over the Southwest ejects eastward into the southern Plains. There is excellent agreement that it holds off from doing so until Wednesday, and so therefore there is little upper level forcing other than a broad right entrance region of a jet streak across this region. The much weaker forcing left over for Tuesday means much lower probability of even isolated instances of heavy rain/resultant flash flooding. 2. Surface flow across the southern Plains into the Tennessee Valley will remain northerly, with dew points dropping into the 20s to 40s range (20s to the north and 40s to the south), even taking the higher values, this low atmospheric moisture content continuous with the southward-moving Arctic air mass over the northern Plains will not support there being enough atmospheric moisture for flooding rains. 3. Areas along and north of the line from the Big Bend of TX through Memphis are likely to experience icing concerns as the aforementioned shallow, dry, and cold air mass is overridden by increasing Gulf moisture in the mid-levels. While this mid-level return flow will be important for priming the atmosphere for future flooding concerns on Days 4 and beyond, on Tuesday and Tuesday night the cold, dry air will not only limit atmospheric moisture content, but what rain that falls will likely freeze on contact with the ground (i.e. freezing rain) over portions of north TX, southern OK, and AR. Thus, the primary hazard will be icing, not flooding. For those areas that remain all rain, amounts will be meager with no instability and low level dry air advection. An early wave of rain is expected Tuesday morning with a second round Tuesday night. Thus, any rain that falls early will have a chance to drain before the second round, also a scenario not favorable for flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-rfkHwDMhPt_ZCXNgJC5h_zjOo3qlGreM8KNukvfsRbS= TusuYzBLto-ZBGpR9ejPabIWNSNHMK1fGkEfyPTXeg8_dug$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-rfkHwDMhPt_ZCXNgJC5h_zjOo3qlGreM8KNukvfsRbS= TusuYzBLto-ZBGpR9ejPabIWNSNHMK1fGkEfyPTX14a-ka4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-rfkHwDMhPt_ZCXNgJC5h_zjOo3qlGreM8KNukvfsRbS= TusuYzBLto-ZBGpR9ejPabIWNSNHMK1fGkEfyPTXrFJWIfc$=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) .