Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 05 2024 08:22:36 FOUS30 KWBC 050822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 AM EST Tue Mar 05 2024 Day 1 Valid 12Z Tue Mar 05 2024 - 12Z Wed Mar 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... CAMS guidance continues to struggle this morning with the convective outlook for the first few hours of the Day 1/Tuesday time period. The latest guidance picking up on current radar trends suggests that much of the heaviest rainfall expected this morning along the Gulf Coast will remain offshore. Further, the portion of the Mississippi Delta region of southeast Louisiana now is likely to be on the western/back edge of the area of heavy rain extending east towards the northern Florida Peninsula. Thus, the threat for flooding rains in the previous Slight Risk area has diminished, despite ongoing rains. The rain is almost universally below a half inch per hour, with only isolated embedded convection potentially producing heavier rainfall rates. Further, there remains considerable uncertainty as to whether a squall line will move through around midday/early afternoon into southeast Louisiana. The 06Z HRRR suggests that the answer to that is no, which would mean the ongoing rain is the last of it that the area will see today. Should older runs verify and a line of storms move through, it will be so fast-moving that any heavy rain that would be capable of producing flash flooding will be through and done in minutes. For all these reasons and in coordination with LIX/New Orleans forecast office, the inherited Slight Risk has been downgraded. Northerly flow behind the front will advect much drier air into the central Gulf Coast and Lower Mississippi Valley today. This will effectively end any flash flooding threat across the area. As mentioned above, any storms that form will accelerate with time, which in turn will progressively diminish their flash flooding potential. Thus, the Marginal Risk area was trimmed out of all of Mississippi and western Alabama for today. For the rest of the Marginal Risk area, there are few changes. The convection largely remaining off the coast across the northern Gulf today will eventually be drawn northeastward into the Florida Panhandle, eastern Alabama and Georgia tonight. A developing low in between two jet streaks and associated with a strong shortwave trough will increase the southerly flow ahead of the low track, which will draw the convection northward. The rain associated therewith tonight is largely expected to fall over areas with high FFGs and are therefore well-prepared to handle the rainfall. Tonight the greatest flash flooding threat will be around Atlanta as the eastward progression of the area of rain will slow as the low forms. Nonetheless, the rain will largely leave the instability that has fueled the much heavier-rain producing thunderstorms behind across the Mississippi River Valley and points west, increasingly favoring a warm-conveyor-belt scenario. While rainfall will be efficient, the lack of instability will greatly restrict maximum possible rainfall rates across Georgia. Thus, the flash flooding threat will be largely tied to where the rain is most persistent for the longest time. Little has changed in this regard from previous forecasts, as much of western Georgia remains in the area of highest threat. Wegman Day 2 Valid 12Z Wed Mar 06 2024 - 12Z Thu Mar 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF LONG ISLAND AND NEW ENGLAND... ....New England and the Coastal Mid-Atlantic... A surface low that starts the day near the southern Appalachians will track northeastward to off the Jersey Shore by Thursday morning. Along this track a warm conveyor belt of deep moisture will track east and northeast of the low center from the Outer Banks northward to coastal Maine on Wednesday. Little has changed in the overall forecast for this area. The warm conveyor belt will lead to very persistent moderate rain into these areas as individual cells and any embedded convection race northward. There is considerable uncertainty as to where the westernmost edge of the heaviest rain sets up respective to New York City. Most of the guidance remains very close, but remaining east of the city across Long Island and southeastern New England. As such the Slight Risk area remains in place for these areas with up to 3 inches of storm total rain possible. A tight gradient in rainfall is expected on the western edge of the warm conveyor belt, which will be close to both Philadelphia and New York. Given this uncertainty, the Marginal Risk remains in place for those 2 metros, with higher confidence that Philly will avoid the heaviest rain. Meanwhile much of Long Island, Connecticut, Rhode Island, eastern Massachusetts, and potentially coastal New Hampshire are likely to get the brunt of the heaviest and most persistent rainfall Wednesday and Wednesday night. While hourly rates are not expected to be noteworthy due to a distinct lack of instability, the persistence of the rain, deep moisture that may approach 4 sigma above normal for this time of year Wednesday night, and very wet antecedent conditions in southern New England which is still experiencing river flooding from previous events easily justifies the Slight Risk for this area. Further, forecast rainfall in these areas has increased yet again from previous forecasts. Changes with this forecast update include trimming out the Jersey Shore and Cape Cod/Martha's Vineyard from the Slight due to high FFGs and uncertainty in the western extent of the heaviest rain in New Jersey. The Slight is unchanged across Long Island, bisecting Nassau County. For the surrounding Marginal, the risk area was trimmed from almost all areas west of the Chesapeake Bay, as a lack of instability and much less forcing should hold rainfall amounts to between 1/2 and 3/4 inch broadly, which over the day will not be enough rainfall to cause flash flooding concerns, even in the urban areas of DC and Baltimore. Hampton Roads was kept in the Marginal owing to the heavy rain that occurred there over the past few hours. A bit more of interior New England was added since soils are saturated in this area, and even lesser rainfall amounts around an inch should all convert to runoff and worsen any ongoing flooding. ....Southern California... A fast-moving but somewhat potent low will move into the southern California coast from north to south on Wednesday. The low will have some modest instability to the tune of a couple hundred J/kg of most-unstable CAPE. This should allow there to be embedded convective elements to the rainfall as it moves into the coast, which is well agreed upon in the CAMs guidance. Given the sensitivity of soils in this region to more rainfall, albeit modest, the Marginal Risk remains largely unchanged with this update. South/west facing slopes of the Transverse Ranges are likely to see the most rain from this low, but should remain an inch or less. Meanwhile L.A. and San Diego should pick up a half inch of rain or less from this quick moving rainfall event. 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!-sIuZaWI1cveRQTQuetQXf773oCoyB81wOZzBAVhQ5sx= 5_-ltWNGh2azH2htX9jXfb-Axl7c1d6vmDlxu75dmK3Ccz0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-sIuZaWI1cveRQTQuetQXf773oCoyB81wOZzBAVhQ5sx= 5_-ltWNGh2azH2htX9jXfb-Axl7c1d6vmDlxu75dq3wUmzY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-sIuZaWI1cveRQTQuetQXf773oCoyB81wOZzBAVhQ5sx= 5_-ltWNGh2azH2htX9jXfb-Axl7c1d6vmDlxu75dxQBqOSI$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .