Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 05 2024 16:13:40 FOUS30 KWBC 051613 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1112 AM EST Tue Mar 05 2024 Day 1 Valid 16Z Tue Mar 05 2024 - 12Z Wed Mar 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST TO OHIO VALLEY... Broad southwesterly mean flow (southerly closer to the surface in the Southeast) is in place across the areas that could see flash flooding concerns today, providing ample moisture. In the near term, an MCV is present in the Lower Mississippi Valley. CAMs continue to struggle with this ongoing convection, though the HRRR seems to have a comparatively decent handle on it. Some higher rain rates are present with this feature and this could continue as it tracks east. Thus the Marginal Risk was expanded into much of Mississippi due to this convection hanging on with the MCV. Meanwhile farther south, broad rainfall is present across the north-central Gulf of Mexico. The bulk of this rainfall looks to stay south of the Gulf Coast since the instability is pooling offshore, and rain sneaking onshore into northern Florida for example is pretty stratiform in nature with rainfall rates that do not look to exceed the high FFGs near the coast. Per coordination with the Metwatch forecaster, removed the Marginal from the Gulf Coast areas that can handle a lot of rain. Most hi-res guidance in the near term looks to be overdoing the potential totals based on what is falling now. Later today into tonight, convection looks to increase across Alabama and Georgia as a surface low develops and tracks northeastward, while convergent flow in the Southeast with a couple of likely surface trough axes and the warm conveyor belt contribute. Particularly into Georgia, most unstable CAPE looks to be around to below 500 J/kg and certainly lower than 1000 J/kg, which would preclude terribly heavy rates from forming and limit the flash flood risk. But the persistence of multiple hours of moderate rainfall rates along/near the low track could be an issue, especially around the Atlanta metro given lower FFG in the urban areas. Farther north in the Ohio Valley, a cold front is slowly moving west to east, with rain ahead of the front as moisture pools (likely around/above the 90th percentile for PWs). Instability is currently somewhat meager but there does look to be some clearing on satellite for solar insolation that should increase instability. CAMs vary but most have over 1000 J/kg of CAPE this afternoon near the Ohio River. This would increase rain rates, and HREF probabilities show the potential for FFG to be exceeded, so the Marginal Risk was expanded northward. This is also supported by an earlier Flash Flood Warning upstream in central Illinois this morning. Tate 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 Valid 12Z Thu Mar 07 2024 - 12Z Fri Mar 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS THROUGH THE MID-MISSISSIPPI VALLEY... ....Southern and Central Plains through the Mid-Mississippi Valley... A developing surface low over Oklahoma Thursday will draw increasing Gulf moisture northward into the Marginal Risk area as a 160 kt southwesterly jet moves overhead. Further, clashing air masses are expected over northern portions of the Marginal Risk area as a polar air mass pushes south across the Dakotas, intersecting the warm, moist air mass across the southern Plains. The result of all of these various ingredients coming together will be widespread thunderstorm development over the Marginal Risk area from northeast Texas north across all of Arkansas and Missouri through Thursday night. The storms will have plenty of instability and moisture to be capable of heavy rainfall rates locally. Generally dry soil conditions will diminish any flash flooding threat broadly, but portions of northern Arkansas and southeast Missouri saw locally heavy rains overnight last night, which could help support flash flooding in these isolated areas by Thursday. Guidance has sped up just a bit from previous forecasts, so the Marginal has been shifted east a few counties to include more of the Mississippi River, and less of the central High Plains. With several urban areas in the Marginal Risk, the local flash flooding threat will be higher in these areas should storms become persistent. Should high-resolution guidance come into agreement on storm structure and location with future forecasts, a local Slight may be needed somewhere in the current Marginal Risk area. Without the benefit of that guidance at that time, confidence is low on where that may be, and as such the Marginal Risk remains for now. ....Coastal New England... With much uncertainty noted...the guidance for rainfall across coastal New England Thursday morning has shifted a bit to the east and sped up from previous forecasts. As such, the heaviest rainfall amounts expected after 12Z Thursday will focus across Cape Cod and the eastern tip of Maine. Both of these areas have high FFGs and are unlikely to flood or worsen ongoing flooding. Despite this, it is important to note that with the vast majority of rain from Day 2/Wednesday having fallen by the start of the period, it's very likely there will be ongoing flooding in New England at the start of the Day 3/Thursday period. However, confidence is increasing that almost all of the rain will be over. Thus, the Marginal Risk area inherited across coastal New England was removed with this forecast update. It's probable that should trends reverse and shift back to the west that a Marginal may need to be reintroduced, but rainfall amounts for all but Cape Cod and the eastern tip of Maine should be a half inch or less after 12Z Thursday, which should preclude any new flooding or significantly worsening any ongoing flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9vHE8ctMt9ySuYkrVo40lGaOjGAIwxVJih6wYpDPtIrI= ujlVkxtcB7yuRB8NKfvZCKt94tNOykPCxNLDLKovqp7U0ls$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9vHE8ctMt9ySuYkrVo40lGaOjGAIwxVJih6wYpDPtIrI= ujlVkxtcB7yuRB8NKfvZCKt94tNOykPCxNLDLKovFE-Ce54$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9vHE8ctMt9ySuYkrVo40lGaOjGAIwxVJih6wYpDPtIrI= ujlVkxtcB7yuRB8NKfvZCKt94tNOykPCxNLDLKovN1cza1Y$=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) .