Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 27 2024 01:17:37 FOUS30 KWBC 270117 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 817 PM EST Thu Dec 26 2024 Day 1 Valid 01Z Fri Dec 27 2024 - 12Z Fri Dec 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST=20 TEXAS INTO THE LOWER MISSISSIPPI VALLEY... Southeastern Texas into Lower Mississippi Valley...=20 An axis of thunderstorms, located ahead of an advancing frontal=20 system over TX, will steadily translate east tonight with strongly=20 diffluent/divergent flow supporting enhancements to lift,=20 especially from northern LA into AR, just ahead of a potent mid-=20 level vorticity max. Instability will be a limiting factor for=20 northern locations with only a small region of 500-1000 J/kg=20 expected from central LA into AR. However, strong forcing combined=20 with potential for training may allow for areas of training with=20 1-2 in/hr rainfall rates. To the south, the Slight Risk across TX/LA was trimmed on the=20 western side to account for where precipitation has ended. The=20 flash flood threat looks to be the greatest across far southeastern=20 TX into western/southwestern LA where a combination of lingering=20 warm sector instability (500-1500 J/kg MLCAPE) and slower eastward=20 progression of an ongoing axis of thunderstorms will result in=20 areas of training where line orientation matches the mean steering=20 flow from the southwest. Sufficient shear for organized cells will=20 continue into the night with supercell structures supporting=20 enhanced rainfall rates of 1-2 in/hr and localized totals of 2-4=20 inches through 12Z.=20 ....Pacific Northwest through Northern California... Another atmospheric river/surge of heavy rain will bring a renewed threat for heavy rainfall and localized flooding to the coastal=20 ranges of western WA to roughly the San Francisco Bay region. As=20 an occluded frontal system approaches the West Coast tonight, an=20 initial warm-advection driven swath of precipitation will move=20 across the West Coast with rainfall rates increasing toward 06Z as=20 higher IVT values arrive ahead of the frontal system. The latest=20 forecast from the RAP supports IVT values increasing into the=20 500-600 kg/m/s range for the central and southern coast of OR=20 tonight and as high as 800 kg/m/s for the central CA coast near San Francisco, though winds will become less favorable with veering toward 12Z for the San Francisco region. 850 mb winds of 50 to 60+ kt are expected to impact a broad region of the OR and=20 northern/central CA coast through 12Z along with impressive PWAT=20 values peaking in the 1.4 to 1.5 inch range, at least for the=20 central CA coast near San Francisco. Peak rainfall rates of 0.5 to=20 0.8 in/hr will be possible but have a relatively limited window of=20 ~3-5 hours with these peak rates. The southwestern OR coast into=20 the northern and central CA coast will have the best potential for=20 higher/impactful rainfall rates with 12 hour totals of 1-3 inches.=20 Higher rainfall will also fall downstream into the lower elevations of the Sierra Nevada and for northern locations across the central WA coast down to the northern OR coast, though rates are not expected to be as high. Localized flooding concerns will exist=20 from the expected surge in rainfall. Otto Day 2 Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND PORTIONS OF MISSISSIPPI, ALABAMA AND TENNESSEE... ....1930 UTC Update... Minor tweaks made to both Marginal Risk areas, based on the latest guidance/trends (now within the high-res CAM windows). Still expecting the risk areas to remain sub-Slight, based on the degree and duration of favorable moisture transport, also supported by the latest CSU ERO first guess fields. Hurley ....Previous Discussion... ....Southeast... A Marginal risk was maintained across portions of MS, AL and TN with this update. Convection will likely be ongoing at 12z Friday along this corridor as a front pushes eastward. The front slows and starts to wash out during the day Friday as large scale forcing weakens. This weakening of the front and forcing decreases confidence in convective evolution, as these factors could result in decreased convective coverage/organization and little to no flash flood risk. However a corridor of modest convergence will likely still exist near the fading front, and some chance outflow from earlier convection could locally enhance this convergence. There will be a supply of instability upstream, and so if convection is able to persist then there is a potential for a few training segments to evolve within the Marginal risk area. Given the dry antecedent soil conditions and streamflows, and only a conditional threat of a few training segments, this is generally considered a lower end Marginal risk. However given this risk was only recently introduced, and the continued non-zero risk of localized training resulting in 3"+ amounts, we will hold on to the Marginal for now and continue to monitor trends. The risk area was shifted a bit off to the southeast to better match where the latest models depict the best chance of these localized higher rainfall amounts. ....Northwest... After what should be a relative lull in rainfall to start the period, another round of stronger IVT should move into the Pacific Northwest Friday night. 24 hour rainfall totals through 12z Saturday will generally not be too significant. Most areas are looking at 1- 2", with peak values around 3" possible from far northwest CA into far southwest OR. These amounts themselves would typically not pose much of a flood concern. However it has been a wet recent stretch over this area, and thus soul saturation and streamflows are running well above average. Given this increased sensitivity, the uptick in rainfall coverage and intensity Friday night may continue to pose some flood risk. Chenard Day 3 Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA AND SOUTHWEST OREGON... ....1930 UTC Update... Made adjustments to the Marginal Risk area across the TN Valley and Deep South by extending it to the Gulf Coast region. Guidance (model QPFs) have come up a bit along the coast, most notably the ECMWF and UKMet. Despite being a bit farther south of the best right-entrance region upper jet forcing (and thus low-level FGEN), these areas closer to/along the coast will have a much better chance of seeing more intense short-term rainfall rates with mixed-layer CAPEs of at least 1000-1500 J/Kg within the warm sector. For now the models continue to show a fairly progressive upper trough and surface front, with sufficient low-mid level shear (i.e. forward or downwind propagating Corfidi vectors) that would limit cell training over any given area for a lengthy duration (several hours). Given this, along with the continued spread in the guidance with respect to the QPF maxima, have maintained no higher than a Marginal Risk in the D3 ERO over the TN Valley, Deep South, and adjacent Gulf Coast area. Hurley ....Previous Discussion... ....Northwest... A stalling front and IVT plume will allow for a prolonged period of moderate to locally heavy rain Saturday into Saturday night across portions of northwest CA into southwest OR. The magnitude of the event should peak Saturday night as IVT values likely exceed 750 and large scale forcing increases ahead of an approaching mid level trough. Not expecting there to be any instability to work with, but the increasing IVT and forcing should allow for hourly rainfall around 0.5", and the stalling of the front/IVT axis will allow these higher rates to persist. Thus the Slight risk in the ERO still looks in good shape, with the WPC 24 hour rainfall currently peaking at 3- 5" in the terrain of far northwest CA into far southwest OR. There are some model solutions that even suggest 5-7" of rain is a possibility. Even though this area is used to seeing rainfall of this magnitude without significant impacts, in this case this rain will be falling after what has already been a prolonged wet period. So saturated soil conditions and elevated stream and river levels will likely increase the susceptibility to flooding. For this reason do think we will see a notable increase in the flood risk over this area by Saturday night, justifying the Slight risk. The inland extent of the heavier rain could also end up greater than normal, with 1-3" possible even in inland area of southwest OR. This should be enough to result in at least localized flooding warranting the Marginal risk over these areas. ....Tennessee Valley into the Southeast... A swath of heavy rain is likely across portions of the Southeast into the TN valley on Saturday into Saturday night. A strong shortwave at the base o the longwave trough will eject eastward into the area, accompanied by a strong upper jet. These two features support robust divergence aloft and support a widespread convective threat. The system as a whole should remain progressive, however as low level moisture transport increases there does appear to be a window for some training/backbuilding of convection. Overall this looks to be a widespread 1-2" event, however localized swaths of heavier amounts are likely. Given the magnitude of mid/upper forcing in place, combined with PWs increasing over the 90th percentile for late December, and sufficient upstream instability forecast...it seems plausible that we could see localized swaths of 3-4". While the progressive nature of the system will likely cap the magnitude and coverage of flood impacts...do think the event has enough going for it to support embedded Slight risk level impacts. However exactly where this higher threat materializes remains a bit unclear. Model guidance has been wavering a bit with the depth of the longwave trough and shortwave strength, which ends up playing a large role in the placement of heaviest rain. Our forecast aligns closer to the somewhat consistent GFS and ECMWF solutions...however it should be noted that the 00z GEM and UKMET are quicker with the system and focus the heaviest rainfall quite a bit southeast of the WPC forecast. While these solutions are considered lower probability outcomes, they can not be ruled out. The inherited Marginal risk was expanded a bit south and east...the eastward shift to account for the aforementioned further east solutions (and a modest eastward trend in the 00z ECMWF as well)...and the southward shift to account for the better upstream instability pool. Given the antecedent dry conditions over this region and the progressive nature of the system, this event is probably going to mostly result in a localized flash flood risk, supporting the Marginal risk. Embedded within this there could very well be a smaller scale area where impacts are a bit greater due to short duration training of higher rates. Best guess location at the moment seemingly centers around central/northern AL, but confidence remains low. Thus will not introduce any Slight risk area at this time and will continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89= j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQxkhB-wk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89= j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQTH8zadU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-JPGZY7hWxrldnz8kWbLTh7Er5QcOxFszKsw5fAaGX89= j_RcVvOmRLC7bFbFhcU2MBroECVEJ7xOKphX1SxQCMlNLGg$=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) .