Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 16 2024 19:31:45 FOUS30 KWBC 161931 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 231 PM EST Sat Nov 16 2024 Day 1 Valid 16Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF THE PACIFIC NORTHWEST... Conventional GOES satellite imagery along with ALPW data shows an offshore atmospheric river beginning to gradually impinge on the Pacific Northwest. An offshore warm front will gradually begin to overspread the region by this evening which will coincide with strong warm air advection and moisture transport. The latest 12Z HREF guidance suggests a 40 to 50+ kt low-level jet arriving with an orientation gradually becoming more orthognal to the coastal terrain of western WA including the Olympic Peninsula and extending southward down into northwest OR. A cold front will then begin to approach and gradually cross western WA by 12Z/Sunday. This atmospheric river event will be progressive, but there will be a corridor of IVT values locally exceeding 500 kg/m/s tonight across these coastal ranges based off the 06Z GFS which seems reasonable considering the elevated CIRA-LVT parameters offshore. Some rainfall rates associated with the combination of strong warm air advection and orographic forcing should reach into the 0.25" to 0.50"/hour range between 00Z and 12Z, and this will help facilitate some rainfall totals of 2 to 3 inches by 12Z for the windward slopes of the coastal ranges. Given the expected rates and totals by early Sunday morning, and thus earlier arrival of this event, a Marginal Risk of excessive rainfall has introduced for this period. Orrison Day 2 Valid 12Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 20z Update: Main change with this update was a minor east and=20 southeast expansion of the Slight risk over north central TX into=20 central OK. Model agreement is pretty good on the large scale, but as is typically the case, smaller scale convective processes will=20 likely drive where the heaviest rainfall axis ends up. This is=20 less predictable and warrants a slightly larger Slight risk area.=20 If there is a model error with convective QPF placement, more=20 often than not the heavier QPF is south and east of the forecast.=20 This is why we were a bit more generous on the east and southeast=20 side with the Slight area...with the 12z HREF guidance also=20 supporting the southward expansion. Chenard ....Previous Discussion... A southern stream cutoff low is expected to eject into the Plains and be accompanied with very impressive IVT, PWs and forcing as it moves across Texas and Oklahoma by Sunday evening into the overnight hours. This should 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, but would expect isolated swaths as high as 4-6" within this setup. 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 a minor expansion 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. Campbell Day 3 Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND THE PLAINS/UPPER MIDWEST... 20z Update: Only minor changes to the inherited Marginal risk areas. We tightened up the Plains Marginal to try to keep it focused along/ahead of the low track where weak instability may=20 briefly enhance rainfall rates. There is some chance we'll need a=20 Slight risk over portions of OK into south central KS, as a flash=20 flood threat may be ongoing at 12z Monday (a continuation of the=20 day 1 Slight risk). However for now will maintain the Marginal and=20 allow for a look at the overnight HREF guidance. There is also a chance we'll need a Slight risk over portions of the central Gulf Coast for Monday night. Convection should develop and grow upscale during this time, although the stronger model=20 signal is after 12z Tuesday (and we do have a day 4 Slight risk=20 covering that threat). Certainly plenty of moisture moving in with a deep connection to tropical moisture from the remnants of=20 Sara...just a question with regards to how quickly we see a more=20 robust uptick in forcing and convective organization. This is more=20 likely by Tuesday (day 4), thus will keep this day 3 risk at=20 Marginal for now and continue to monitor. 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 with minor adjustments for this issuance. ....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. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V= 4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35fAkIlOA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V= 4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35eIYGMAw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ObbgveIoPJkmw1DtI_DwVoqJnBtuGlSCBXGhNYGeq9V= 4IGmEwSeswDP0XOS4Qbz7QEVkWD2t8Uw8v602G35BbmZwEc$=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) .