Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 17 2024 19:37:20 FOUS30 KWBC 171937 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 237 PM EST Sun Nov 17 2024 Day 1 Valid 16Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 1600Z Update... The 12Z HREF guidance along with 06Z RRFS/REFS output continues to support a locally heavy rainfall event across areas of the southern Plains by this evening and extending into the overnight period. There has been some shift farther west with some of the swaths of heavy rainfall potential across west TX as some of the individual CAMs are supporting the heavier rainfall/convection along the immediate cold front and in close proximity to the deepening surface low that will be evolving across the TX High Plains tonight. However, a complication to the rainfall forecast is the downstream evolution of convection along the front ahead of the surface low involving areas of northwest TX through southwest to central OK. This is where strong moisture convergence and forcing may foster a threat for some training areas of convection that become aligned in a southwest to northeast fashion. Overall, still expecting some swaths of 2 to 4 inch rainfall totals, with isolated potential for heavier amounts and especially across northwest TX into southwest OK where the better convective training threat will exist. Given the latest CAM guidance, the Slight Risk area has been expanded somewhat to account for the varying modes of convective evolution that are expected this evening and overnight. Elsewhere, no changes have been made to the previous D1 outlook including the Marginal Risk area across western OR where there continues to be some lingering, but modest atmospheric river activity. Orrison Previous Discussion... ....Southern Plains... A southern stream cutoff low is expected to eject into the Plains today with very impressive IVT and accompanying anomalous PWs and forcing as it moves across Texas and Oklahoma by this evening into the overnight hours. This looks to be enough to overcome the limited forecast instability and result in deep convection and heavy rainfall rates across portions of the Southern Rockies and the adjacent Southern/Central Plains. Guidance continues to suggest 2-4" of rain (per latest HREF and WPC QPF), but would expect isolated swaths as high as 4-6" within this setup (indicated by some of the heaviest CAMs). The overall progressive nature of the system and limited instability will cap the upper bound of rainfall totals, but this system is dynamic enough with plentiful moisture to suggest at least localized flash flooding is likely. A Marginal Risk area remains in effect for central/eastern New Mexico, northern Texas, most of Oklahoma and southern Kansas. The inherited Slight Risk area saw only minor adjustments to reflect the latest trends and WPC QPF and spans from northern Texas/panhandle to western Oklahoma. ....Coastal Oregon and northwest California... A fairly progressive, weak/moderate atmospheric river will be ongoing across the region. Total rain of 2-4" (through 12z Mon) and hourly rainfall rates possibly approaching 0.5" within a narrow southward shifting axis suggest that some minor flooding issues could arise along coastal areas/mountains. A Marginal Risk area remains in effect along the coastline. Churchill/Campbell Day 2 Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND THE PLAINS/UPPER MIDWEST... 20z Update: A Slight risk was added across portions of southeast Louisiana into coastal Mississippi including New Orleans and=20 Biloxi. The higher QPF with this event is still forecast to fall on day 3 over areas further east. However, while heavy QPF coverage=20 may not be as high on day 2, the ingredients will still be there=20 for intense rainfall rates over southeast LA. The 12z HREF is not=20 overly aggressive, but given the setup and ingredients would not be surprised to see an upward trend with the 00z guidance. Overall,=20 the environment in place is conducive for a conditional flash flood threat over urban locations within the Slight risk. A slower=20 evolution (as indicated by the ECMWF and AIFS) would bring Baton=20 Rouge more into play, but will let the Marginal risk cover that for now and keep the Slight focused over the corridor from New Orleans to Biloxi where the probability of some training convection is higher Monday night. Chenard ....Previous Discussion... ....Central U.S. ... The precipitation shield will shift eastward as the cold front advances further into the Plains. Moderate to locally heavy rainfall is expected from northern Texas to southern Minnesota with areal average of 1 to 3 inches. The higher amounts will likely concentrate over eastern Kansas, eastern Nebraska and western Iowa. A Marginal Risk area was maintained once again with only minor adjustments for this issuance (not enough confidence for 2"+ totals to introduce a Slight Risk). ....Gulf Coast... A plume of enhanced tropical moisture will likely advect northward through the Gulf of Mexico well in advance of tropical cyclone Sara; which will increase the potential for heavy rain and localized flooding concerns along portions of the Gulf Coast. A Marginal Risk area is in effect from southeast Texas to southern Mississippi (changed very little from inherited with 3" localized totals possible). Churchill/Campbell Day 3 Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND CENTRAL GULF COAST... 20z Update: Only minor changes to the inherited risk areas, as the forecast generally looks on track. Expecting a backbuilding=20 convective complex to push eastward across the central Gulf Coast=20 Tuesday. Anticipate rainfall rates with this complex will be=20 intense enough to overcome the high FFG on an isolated to scattered basis. 3 to 5" totals remain most likely, although localized=20 swaths exceeding that are probable. The progressive nature of the=20 system will put a cap on the upper bound of rainfall=20 magnitudes...although still think locally significant flooding is a possibility given the intense rates and potential for backbuilding into the strong southerly low level inflow. Chenard ....Previous Discussion... ....Central Gulf Coast and Southeast... By Tuesday significantly more tropical moisture will likely advect northward through the Gulf of Mexico in association with the remnants of Tropical Storm Sara; which will increase the potential for heavy rain and localized flooding concerns along portions of the Gulf Coast. An inherited Slight Risk area was maintained for southern AL and surrounding portions of FL/GA/MS/LA, given the potential for localized 3-5" totals (per ensemble guidance and WPC QPF). ....Coastal Washington, Oregon, and northwest California... Another weak/moderate atmospheric river is expected for region for Day 3. Total rain of 2-4" (similar to Day 1) are expected with hourly rainfall rates possibly approaching 0.5" once again suggest that some minor flooding issues could arise along coastal areas/mountains. An inherited Marginal Risk area remains in effect along the coastline. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5O-DO2dOxLckVMvkwFsmgNTHTy7eBGuxpxxuevCQfcBw= 5myiNJ-hneHl08sn4LQYANgg9I2uGcxtJ7DrbpJTYmDMswE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5O-DO2dOxLckVMvkwFsmgNTHTy7eBGuxpxxuevCQfcBw= 5myiNJ-hneHl08sn4LQYANgg9I2uGcxtJ7DrbpJT42E0bZg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5O-DO2dOxLckVMvkwFsmgNTHTy7eBGuxpxxuevCQfcBw= 5myiNJ-hneHl08sn4LQYANgg9I2uGcxtJ7DrbpJTXK2gSIs$=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) .