Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 28 2023 08:16:37 FOUS30 KWBC 280816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 AM EST Sat Jan 28 2023 Day 1 Valid 12Z Sat Jan 28 2023 - 12Z Sun Jan 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS & WESTERN LOUISIANA... A digging longwave positively-tilted trough will cause a jet streak to slowly strengthen from the MS River Valley into the Northeast Saturday night. Meanwhile, deep Gulf moisture will be directed northward from the Bay of Campeche into eastern TX and southwest LA, as a front pushes south and east out of the Plains and into northeast TX through AR. As is typical of most January nights, instability will be the primary limiting factor. MUCAPE will increase to between 100 and 400 J/kg late Saturday night. Only a small northward nudge of the inherited Marginal was made to account for the ARW's depiction of potentially heavy convection across northern LA, though this is far from certain. Antecedent conditions remain favorable in the Marginal Risk area with nearly saturated soils, though that may be mitigated somewhat by the dry conditions expected today. PWATs will increase Saturday to between 1.5 and 1.75 inches along the TX Gulf Coast, which is as much as 2.5 sigma above normal for this time of year. The LLJ will increase to 40-50kt across northern LA/southern AR, while further south in the Marginal Risk area, expect the LLJ to be closer to 30 kt. Nonetheless this will advect plentiful Gulf moisture into the area which will support training convection. While the forcing will be greater further north, again the lack of instability should disallow short-term heavy rainfall rates, even if there are training cells into southern AR. Wegman Day 2 Valid 12Z Sat Jan 28 2023 - 12Z Sun Jan 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND CENTRAL GULF COAST... Very few notable changes with the latest forecast update. 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, though there is some consensus that the axis of heaviest rain will remain north of New Orleans. Changes with this forecast update were primarily on the eastern end of the inherited area. In coordination with the CHS/Charleston, SC forecast office, trimmed the Slight out of SC and far eastern GA entirely, as the greatest threat and the highest forecast QPF are well to the west of that area. While main-stem rivers and soil moisture across all of SC remain high, with a few of the main-stems in flood stage, the flashier creeks and smaller tributaries are quite dry, and are likely to handle the 1 to 2 inches of rain expected during the overnight hours Sunday night in this area. Model guidance is in excellent agreement on the swath of heaviest rain falling in the new Slight Risk area. Additional trimming of the Slight Risk was done on the western end with good agreement that metro Houston may largely miss out on the heaviest rain, and also that the heaviest rain will miss the Mississippi River Delta to the north. Confidence wasn't quite there to remove Houston from the Slight given some variability, but extreme southeastern LA was downgraded to a Marginal with this update. Wegman Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9QVKA0aMB8lb6m9PDBBH4ywv-0K1qn1VDzScYmE7AdC3= rCVB8LupfftS7dYyegOw5K2kwXQyRbyofZBHR5pxba1W6Tc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9QVKA0aMB8lb6m9PDBBH4ywv-0K1qn1VDzScYmE7AdC3= rCVB8LupfftS7dYyegOw5K2kwXQyRbyofZBHR5px_why_Io$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9QVKA0aMB8lb6m9PDBBH4ywv-0K1qn1VDzScYmE7AdC3= rCVB8LupfftS7dYyegOw5K2kwXQyRbyofZBHR5px9nx9CAU$=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) .