Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 28 2024 01:01:03 FOUS30 KWBC 280100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Fri Dec 27 2024 Day 1 Valid 01Z Sat Dec 28 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... ....Mississippi, Alabama and far western Georgia... ....Northwest... GOES West water vapor imagery showed a shortwave rounding the top of a ridge axis 400-500 miles west of the WA coast with an elongated W-E axis of mid-upper level moisture to its south, moving across OR. A longwave upper trough axis was noted back to the west near 160W with Blended TPW imagery showing the next round of moisture poised to reach the West Coast later tonight, containing PWAT values of 1.0 to 1.4 inches a couple hundred miles west of OR. As the shortwave and longwave troughs track eastward/downstream, a surface cyclone and accompanying warm front will allow for a surge in IVT values from northern CA into southern OR (peaking near 800 kg/m/s through ~05Z). With this initial surge, rainfall rates are likely to peak near 0.5 in/hr from the central/southern OR coast into the northern CA coast. Beyond 05Z, IVT values are forecast by the latest model consensus to come down a bit but remain moderate in intensity (~500 kg/m/s) as the moisture transport axis shifts southward, primarily into northern CA. Low level flow oriented nearly orthogonal to the coastline will still maintain the threat for higher rain rates near 0.5 in/hr. Peak 12 hour rainfall=20 totals, ending at 12Z Saturday, are expected to range from 2 to 4 inches (locally higher) from the favored west-facing terrain of southern OR into northern CA. Additional rainfall atop wet=20 antecedent conditions and high water levels in area streams/rivers=20 may result in localized flooding. ....South...=20 As a negatively tilted shortwave trough axis over the=20 MS Valley continues to lift north and upper level jet forcing=20 begins to weaken, the threat for flash flooding across the central=20 Gulf Coast into inland areas is expected to decrease in coverage=20 and become more localized, likely focused near the greatest=20 instability (500 to 1000+ J/kg MLCAPE) near the coast. PWATs near=20 or just over 1.5 inches and potential for SW to NE training will=20 continue to support the potential for 1-3 in/hr rainfall rates=20 across southern MS/AL into far western FL. Farther north into=20 interior portions of AL and far western GA, while MLCAPE reduces to near zero, weak elevated instability will maintain a lower end=20 flash flood threat through the night. Otto Day 2 Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA... 20Z Update... Forecast remains largely unchanged from overnight. Over the Southeast, 12Z models still show enough spatial spread to preclude a focused Slight Risk outline, and the higher QPF may lie over areas with higher FFG (even after today's rain). Fracasso ....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 J/kg/m^2 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 rainfall rates around a half inch per hour, and the stalling of the front/IVT axis will allow these higher rates to persist. 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 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 areas of southwest and central Oregon. Given that setup, in coordination with MFR/Medford, OR forecast office, the inherited Slight Risk was expanded northeastward to include much of Coos, Douglas, and Jackson Counties in Oregon. An internal higher end Slight was introduced to include Coos & Curry Counties in OR and Del Norte County in CA. The lion's share of the rainfall expected in this Day 2 period will be from the evening through much of the overnight. Since the AR will remain relatively stationary through this period, expect widespread 4-6 inch rainfall amounts. This will be added to the rainfall expected today into tonight and well-above-average river levels already established in this area. ....Lower Mississippi 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 of 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". As yesterday, it appears with some instability and potential for training convection that there remains a reasonable likelihood that somewhere in the Marginal Risk area that Slight Risk impacts will materialize. Unfortunately, guidance remains all over the map as to where that potential will be realized, from the mountains of the western Carolinas to central Louisiana. Given last evening's rainfall and several Flash Flood Warnings, it seemed prudent to expand the inherited Marginal south and west to cover much of Louisiana. While the heaviest totals remain most likely over northern MS/AL and TN, the dry antecedent soil conditions should preclude more than isolated Slight Risk impacts...so the large Marginal Risk remains for this update. Wegman/Chenard Day 3 Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA... 20Z Update... Very little change to the forecast from overnight. Rain rates into southwestern OR may still exceed 0.5/hr across 12Z thus supporting the Slight Risk. Trimmed a bit from the northeastern side of the Marginal outline into the Northeast and expanded a bit on the southwest side to account for just a tad slower motion of the system in the east, but otherwise left a somewhat broader Marginal in place that may focus more to the northwest of I-95 per current trends. Fracasso ....Northwest... The A.R. moving into the Northwest will weaken through Sunday morning. Thus, rainfall rates should diminish with time. Since the bulk of the Day 2/Saturday A.R. will occur overnight Saturday night, much of the impacts of the A.R. will be felt with the diminishing rainfall on Sunday morning. Thus, for consistency both in messaging and the timing of the worst impacts, a small Slight Risk area was introduced with this update for the southwest corner of Oregon and the northwest corner of California. This area will be the hardest hit area through Saturday night, so the continued rainfall into Sunday morning will have the greatest impacts in this area. Otherwise, expect continued rainfall all up and down the Pacific Northwest, with high elevation snow. ....Mid-Atlantic... A mature low over the Midwest will track north up the Mitten of Michigan and into Ontario by Monday morning. Ahead of the low's cold front, a plume of subtropical moisture will stretch up the entire Eastern Seaboard. The combination of the moisture and much warmer temperatures will support widespread light to moderate rain over much of the Mid-Atlantic. While much of the Mid-Atlantic has been dry, especially south of the Mason Dixon line, some upslope may help wring out the moisture along the I-81 corridor through New York and the I-95 corridor from Richmond through NYC. Any localized training may cause widely scattered instances of flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!76mZebh4wIyC6vu-YVEMxqKWwf0InXEA0XuKxtZmMfEy= c0qOiqX57RpIRcEEKCkjApcmrcGOJNTDJHEQpAGNQchkHt0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!76mZebh4wIyC6vu-YVEMxqKWwf0InXEA0XuKxtZmMfEy= c0qOiqX57RpIRcEEKCkjApcmrcGOJNTDJHEQpAGNJxBLBn8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!76mZebh4wIyC6vu-YVEMxqKWwf0InXEA0XuKxtZmMfEy= c0qOiqX57RpIRcEEKCkjApcmrcGOJNTDJHEQpAGNMKVfMU8$=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) .