Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 06 2024 16:00:06 FOUS30 KWBC 061600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Wed Mar 06 2024 Day 1 Valid 16Z 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 THE DELMARVA TO NEW ENGLAND... ....16Z Update... No major changes seemed necessary to the Day 1 ERO at this time, just some relatively minor tweaks. These include a bit of a northward expansion for the California Marginal where there are some low-end HREF probabilities of exceeding FFG. Expanded the Slight Risk into the Delmarva Peninsula--though the area is not necessarily sensitive, high-res models do show 2-3 inches of rain there, which is along the order of the totals expected farther north. Considered eliminating the Slight Risk from eastern North Carolina, as rain rates thus far have stayed below thresholds for notable flooding, but removing it may be a bit premature because of additional moderate to heavy rain that is likely to come in this afternoon after a possible break. The Flood Watch by the Morehead City WFO goes through the evening so keeping the Slight is consistent with that too. Tate ....Previous Discussion... ....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 Valid 12Z Fri Mar 08 2024 - 12Z Sat Mar 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY INTO THE SOUTHEAST... ....Southeast... A potent positively tilted longwave trough will eject out of the Rockies and across the Plains into the Mississippi Valley by Saturday morning. A vigorous 150 kt southwesterly jet will lead ahead of the trough as a shortwave at its base drags the entire trough eastward. A low will develop near the Arklatex Friday afternoon, then track northeastward to the Tennessee Valley by Saturday morning. The low will support the development of a strong low level jet emanating from the Gulf and into the Southeast Friday night. The LLJ will intensify both in intensity and depth of moisture as it tracks eastward across the Southeast. The mid-level low is expected to remain south of the surface low, which will support the development of a mid-level front somewhere over central Alabama and eventually extending into Georgia. This front will become the primary forcing mechanism which will force the warm, moisture-laden air mass associated with the LLJ upward, becoming the boundary along which numerous storms are expected to develop and then train towards the northeast, with continuous backbuilding expected. The upper southwesterly jet streak and mid-level low will gradually shift the axis of this front from east-west to a more southwest to northeast orientation. Thus, the streak of heaviest rainfall is likely to focus in a southwest to northeast oriented band through the Slight Risk area from southwest Mississippi to northwest Georgia. The LLJ is expected to be strongest and most persistent across Alabama, but the boundary will be the focus for training thunderstorms across all three states. While these kinds of events aren't all that uncommon, unfortunately the mesoscale nature of the boundary makes it notoriously difficult for the guidance to resolve. There's very good agreement that given this Day 3 forecast that there will be an area of very heavy rainfall that persists for an extended period of time, there is enough uncertainty as to whether the heaviest and most persistent rains impact an urban area, which would greatly enhance the flash flooding risk and resultant impacts. While even rural areas would be greatly impacted by as much as 6 inches of rain (locally higher) in 12 hours in any area, the high FFGs and relatively dry conditions across the Slight Risk area should require those heavy rains to occur in order for impactful flash flooding to result. Thus, the corridor from east-central Mississippi, central Alabama, and northwest Georgia is considered under a higher-end Slight, and it's probable a Moderate Risk may be needed in this area with future updates with CAMs guidance and better agreement where the heaviest rains will be. Coordination with BMX/Birmingham, AL and FFC/Peachtree City, GA forecast offices resulted in agreement that the area remains in a Slight, but all acknowledge the ingredients are coming together for a potential Moderate Risk with future updates. Areas that pick up 6 inches or more of rain in 12 hours (or less) could see numerous instances of flash flooding with considerable flash flooding likely. The area will continue to be closely monitored. ....Mid-Mississippi and Lower Ohio Valleys... On the cold frontal side of the low, interaction with the stronger forcing associated with the baroclinic interaction will result in a local maximum of heavier rains from south-central Missouri through east-central Illinois. Instability will be greatly lacking in this area, so any isolated flash flooding will occur as a result of a long-duration heavy rain before much colder air moves in and ends any flash flooding threat. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!86iCnoscgO47F-cclAL-dvcr_qNKSfhfqMAZxwlH5C2A= t0KYwkE0SHR0yc_ec1RfxHonn8fzD6vIp3p_Wlw1RRPkgTE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!86iCnoscgO47F-cclAL-dvcr_qNKSfhfqMAZxwlH5C2A= t0KYwkE0SHR0yc_ec1RfxHonn8fzD6vIp3p_Wlw1DRCeXwY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!86iCnoscgO47F-cclAL-dvcr_qNKSfhfqMAZxwlH5C2A= t0KYwkE0SHR0yc_ec1RfxHonn8fzD6vIp3p_Wlw19nQjlng$=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) .