Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 06 2024 08:27:04 FOUS30 KWBC 060826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 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 The Day 2 outlook will be updated by 0830Z. 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!_3fRLWFEeakTcOSR8Q9uY-vsJEd1oimJo3fNzRcp6q2L= jEhILkGrTiNw6sZXcKlHF80jypYPwOUytAugZ-xfnO1dTAg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_3fRLWFEeakTcOSR8Q9uY-vsJEd1oimJo3fNzRcp6q2L= jEhILkGrTiNw6sZXcKlHF80jypYPwOUytAugZ-xfKcORWH0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_3fRLWFEeakTcOSR8Q9uY-vsJEd1oimJo3fNzRcp6q2L= jEhILkGrTiNw6sZXcKlHF80jypYPwOUytAugZ-xfU5qlZ6A$=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) .