Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 06 2024 08:29:32 FOUS30 KWBC 060829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Wed Mar 06 2024 Day 1 Valid 12Z Wed Mar 06 2024 - 12Z Thu Mar 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE OUTER BANKS OF NORTH CAROLINA AS WELL AS PORTIONS OF NEW JERSEY TO NEW ENGLAND... ....Outer Banks... A nearly stationary boundary currently over the Outer Banks is drawing plentiful Atlantic moisture from the nearby Gulf Stream. While rates associated therewith are not overly impressive...generally under 1/2 inch per hour, this boundary is not forecast to move much of anywhere for as long as the next 12 hours based on the latest CAMs data. Embedded convection may move through in the broader rainfall area, which will locally enhance rates for a short time. As a low develops over the southeast and tracks roughly parallel to the coast and the stationary area of rain, it will hold the rainfall band in place. The result is a forecast of 3 to 5 inches of rain with locally higher amounts possible. Further, the area recently picked up 3 to 5 inches of rain, so despite the sandy soils, the development on the Outer Banks and the likelihood for ponding since the sandy soils are fully saturated make ponding on area roadways and potentially basements increasingly likely through the day. Ponding has been known to block sections of the one road on portions of the Outer Banks, NC-12, which may effectively cut off entire communities, resulting in outsized impacts. Considering all of the above and with extensive coordination with MHX/Morehead City, NC forecast office, the Outer Banks were double upgraded to a Slight with this forecast update. The 06Z HRRR suggests it may take until late afternoon/evening, when the parent low tracks near the Outer Banks to finally move the area of rain off to the north and east, resulting in a rapid shut-off of the rain. Thus, it's probable that this Slight will be canceled before the end of the period 12Z Thursday. ....New Jersey into New England... No significant changes were made to the Slight Risk area. The same band of rain that will impact the Outer Banks will continue northward ahead of the developing low, resulting in another long-duration steady moderate rain for the Slight Risk area. Rainfall amounts may not be quite so high into New England as the Outer Banks, due to lesser instability and slightly less moisture since sea surface temperatures supporting the moisture plume cool with latitude. Nevertheless, several urban areas including NYC and Boston will be under the plume of rain for an extended period of time today. Forecast rainfall amounts of 2 to locally as high as 4 inches of rain remain, and despite the lack of instability keeping rates down, the constant rain for an extended period of time over urban areas is likely to cause widely scattered flash flooding. Further, soils remain near saturated across the entire Slight Risk area and into New England, so almost all of today's rainfall is expected to convert into runoff. Thus, while there were no significant changes to the Slight Risk area, the surrounding Marginal was expanded to the Canadian border to include all of VT and NH. Rainfall amounts will gradually diminish towards the north and west, but low FFGs and already high river levels across New England are likely to make up for that. ....East Coast of Florida... The tail end of the cold front associated with the same low yet again will impact the Florida Peninsula today. When added to local forcings, the ongoing heavy rain over Florida will only very slowly creep east to the Atlantic coast, where it will effectively stall out. With daytime heating and local forcings such as sea breezes, additional showers and thunderstorms are likely in the Marginal Risk area this afternoon with plentiful moisture in place and weak steering flow keeping any storms that form moving very slowly or remaining stationary. Lower FFGs in urban areas will support isolated flash flooding with the most persistent storms. ....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 2 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... Very few changes were needed once again to the broad Marginal Risk area across the center of the country. The guidance remains remarkably consistent on the convective evolution across the area. With the benefit of CAMs guidance, the overall structure will be for greater instability and storms on the southern side of the precipitation shield associated with the low, while most areas get a long-duration steady rain with embedded convection. Otherwise the pattern remains largely unchanged. There remains some potential for a Slight Risk being needed, but with remarkable consistency and a rather underwhelming convective signature across the area, it appears the most likely flash flooding will occur with highly localized areas of very persistent convection, and are highly unlikely to impact larger areas. Thus, it appears the potential for a future Slight upgrade has decreased. Certainly should any persistent convection occur over an urbanized area (KC the most likely city to be impacted), then impacts will exceed Marginal criteria. 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... There is no change to the forecast reasoning this morning. The very last of the rain will end after 12Z Thursday for Cape Cod and the eastern tip of Maine, but the rain is not expected to amount to much after 12Z and will not cause additional flooding. Thus, the area remains with no flash flooding risk 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!8iGefp3_104CM1URL0GhulZVKvt-nhZBI6tA77JMgdx2= QO-UVqON0i82Lsq1KTdB3df8PQBpVVtpYVpwm5ZflJQRsE0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8iGefp3_104CM1URL0GhulZVKvt-nhZBI6tA77JMgdx2= QO-UVqON0i82Lsq1KTdB3df8PQBpVVtpYVpwm5ZfyK042fo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8iGefp3_104CM1URL0GhulZVKvt-nhZBI6tA77JMgdx2= QO-UVqON0i82Lsq1KTdB3df8PQBpVVtpYVpwm5Zf_TZiU7g$=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) .