Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 07 2023 00:48:34 FOUS30 KWBC 070048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 847 PM EDT Thu Apr 06 2023 Day 1 Valid 2203Z Thu Apr 06 2023 - 12Z Fri Apr 07 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHEAST TEXAS TO WESTERN LOUISIANA... A stalled frontal boundary is now draped along the Texas and Louisiana Gulf Coast into portions of central Mississippi and Alabama this evening. Fairly anomalous moisture has pooled along/ahead of that front from eastern Texas into southern Louisiana with the most recent blended TPW products showing upwards of 1.8" PW while upwards of 1000 J/kg of MUCAPE reaches further inland. Ongoing convection across the area will remain fairly stationary or slowly drift over the next several hours as another weak/embedded shortwave trough lifts through within the large scale southwesterly flow. This should support a continuation of convection from the Texas Gulf Coast through southern/south-central Louisiana overnight into early Friday morning. The ingredients in place and the 18Z HREF neighborhood probabilities all point toward 1-2" hourly totals possible with localized instances of 2-3" hourly totals, particularly through 06Z across southwest Louisiana and far eastern Texas where recent convection has anchored south of Lake Charles and is also slowly expanding westward. 12-hr HREF neighborhood probabilities through 12Z for 2-3" are high (above 70%) from far eastern TX into southwest LA with some signal for localized higher amounts exceeding 5-7" possible as suggested by some of the hi-res guidance including the last few runs of the HRRR. Given the frontal boundary placement and the expectation that the best instability will likely be along the coast, these higher amounts are most likely along the TX/LA border along the Gulf Coast. Elsewhere, earlier afternoon/evening convection across portions of the Mid-Atlantic continues though will begin to wind down over the next few hours. Some isolated instances of flash flooding over mainly urban areas will remain possible, particularly over the NC Piedmont where the latest HRRR suggests additional convection to develop overnight. But it's likely to be widely scattered and the lack of deeper instability should keep rain rates in check such that any issues would be localized. Finally, steady moderate rainfall continues across the Pacific Northwest where some isolated to scattered 0.25-0.5" hourly totals remain possible through the night. The warmer air mass giving way to mostly rain onto of a deeper snow pack may contribute to additional snow melt and when combined with the ongoing precipitation, may lead to some flooding issues. Taylor Day 2 Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM CENTRAL TO SOUTHERN LOUISIANA AND SOUTHERN MISSISSIPPI... ....2030Z Update... The focus Friday will remain on an axis of heavy rainfall producing storms over the Southeast, to northwest of a frontal boundary draped over the Southeast into the western Gulf of Mexico, with additional storms likely to develop to the south of the front from southeast Louisiana into southern Mississippi, Alabama, and Georgia. The hi-res guidance paints a bit of an uncertain picture on where exactly this axis of slowly progressing storms, and thus risk for heavier rainfall totals, will set up. This is likely tied to the forecast position of the front which will continue to meander a bit with southwestward extent given the lack of appreciable upper-level support to help progress it along. However, the Slight Risk continues to cover the region most likely to see heavier rainfall totals both southeast of the boundary over southeastern Louisiana as well as to the northwest over central Louisiana and southern Mississippi. Hi-res guidance suggests a shortwave aloft will help to kick off an initial round of convection south of the front over southeastern Louisiana while helping to maintain storm coverage north of the front into central Louisiana and southern Mississippi. Storms will likely linger a bit to the west over eastern Texas/western Louisiana but expect that the heavy rainfall threat will move offshore as the front pushes a bit eastward over the Gulf, with the northern extent of these storms also pushing eastward into the Slight Risk area over southern Louisiana. Additional post-frontal storms are expected to the northeast into the Southern Appalachians and Carolinas. However, as was noted overnight, the cold front is forecast to push southward much quicker over this region with little expected lingering instability aloft to support heavier rain rates. Uncertainty over storm coverage grows significantly further east into Alabama and Georgia as well, with the noted progressive nature of the front limiting areal coverage of the warm sector with northward extent rather quickly Friday afternoon/evening. Putnam ....Previous Discussion... The Mid-South and as far south and west as east Texas remains under the influence of the diffluent right-entrance region of a roaring 160 knot jet streak over the Northeast. In addition, the mean 250-500mb trough over northern Mexico is also producing its own diffluent flow aloft over southeast Texas. The frontal boundary that has acted as the trigger for rounds of heavy rainfall will continue to stick around off the coast while southerly 850mb flow out of the Gulf of Mexico continues to deliver anomalous moisture to the region. The atmosphere looks to remain quite saturated (1000-500mb RH values >90% for most of the Slight Risk and Marginal Risk areas) and soils in these areas will have continued to have grown more sensitive from multiple days worth of rainfall. One limiting factor is that MUCAPE remains below 1,000 J/kg and mean 850-300mb mean winds remain around 20 knots, allowing for storms to remain on the progressive side. That said, the added soil sensitivity, anomalous moisture content, and just enough instability aloft to allow some storms to produce rainfall rates up to 2"/hr is enough to keep the Slight Risk in place over the Lower MS River Valley. The Lower MS Valley will reside directly in front of an approaching 500mb vort max, while the region also features better odds for seeing more modest instability levels. The Marginal Risk extends as far south and west as the Houston metro area to as far east as the southern Appalachians. The latter could see MUCAPE range between 500-750 J/kg Friday afternoon and PWs as high as 1.5". Revisited the Marginal Risk potential all the way to the Carolinas, but the cold front sinking further south into GA made it seem less likely sufficient instability and high enough PWs would be present to promote a more sufficient flash flood setup. Should there be a trend for more instability and moisture to become available in future forecast cycles, an extension of the Marginal Risk into portions of the central Carolinas may be necessary. Mullinax Day 3 Valid 12Z Sat Apr 08 2023 - 12Z Sun Apr 09 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE TO THE CENTRAL CAROLINAS... ....2030Z Update... Only very minimal adjustments were made to the current Marginal Risk area based on the updated global guidance. As is the case for Friday, there is a bit of disagreement on the axis storm coverage and thus heavy rainfall threat through the Southeast as the meandering frontal boundary finally begins to push offshore over the Atlantic. However, southeasterly low-level flow setting up over the Atlantic and over the boundary will help to continue to support the threat for areal average rainfall totals of 1-2", with locally higher amounts prompting an isolated threat for flash flooding. Putnam ....Previous Discussion... Saturday features another favorable setup for a very wet day for much of the Southeast with a pesky frontal boundary still positioned from the Gulf Coast to the Carolina coast. A strong dome of high pressure over the Northeast will work with lower pressure near the Bahamas to produce strong onshore winds, providing the region with some Atlantic moisture. Aloft, the Southeast remains beneath the diffluent right-entrance region of a 250mb jet streak located over the Northeast. There is also a 250-500mb trough traversing the Mid-South that will enhance vertical motion atop the atmosphere too. This trough will tap into a stream of 850mb moisture flux and direct it at the front, providing additional moisture into the region and aid in the development of numerous showers and thunderstorms. PWs will generally hover between 1.25-1.50" while MUCAPE generally struggles to eclipse 500 J/kg. These values don't necessarily scream flash flood potential, but 1000-500mb RH values will be >90% for much of the area and soils will have grown more saturated from rainfall that transpired on Friday. Given these factors, the inherited Marginal Risk still appears to be on track, but did take out portions of the Carolina beaches where, barring a bigger increase in instability and moisture content, have the soil type to be able to better manage the steady periods of rain throughout the day. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-dV3gFI-QZ5saViPe8RYtysjiwWL4JyCKenBrb9k4IY7= YCWU5ughX533j9E5EGvCs0aHMg6Ddf8qqq4kkAnJGIWO29c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-dV3gFI-QZ5saViPe8RYtysjiwWL4JyCKenBrb9k4IY7= YCWU5ughX533j9E5EGvCs0aHMg6Ddf8qqq4kkAnJSebaOsI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-dV3gFI-QZ5saViPe8RYtysjiwWL4JyCKenBrb9k4IY7= YCWU5ughX533j9E5EGvCs0aHMg6Ddf8qqq4kkAnJBdeDaoM$=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) .