Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 27 2024 19:53:52 FOUS30 KWBC 271953 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 253 PM EST Fri Dec 27 2024 Day 1 Valid 16Z 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 THE MID-SOUTH... ....Northwest... 16z update...No changes were required given new guidance, timing and placement continue to support current Marginal Risk level of concern. ---Prior Discussion--- After what should be a relative lull in rainfall for most of the day, there will be another round of stronger IVT/atmospheric river activity arriving this evening and continuing overnight across the Pacific Northwest with an emphasis on southwest OR and northwest CA. This will be driven by the arrival of another Pacific shortwave trough and attendant surface low that will be yielding an eastward advance of strong low to mid-level flow. The latest GFS/ECMWF solutions suggest IVT values here surging upwards of 750+ kg/m/s in the 00Z to 12Z time frame tonight across especially southwest OR and far northwest CA. A combination of strong warm air advection, enhanced moisture transport and orographic ascent over the coastal ranges will support 0.25" to 0.50"/hour rainfall rates with these rates likely exceeding a 0.50"/hour at least occasionally across areas of Coos, Curry and Del Norte Counties. More broadly across the region, the additional rainfall amounts should reach 1 to 2 inches by early Saturday morning, but the latest HREF guidance suggests some of the favored windward slopes seeing as much as 2 to 4 inches where the rainfall rates will be highest in conjunction with the stronger IVT values. Given the continuation of the very wet pattern across the region and with streamflows already running high, these additional rainfall totals may pose concerns for increasing runoff problems and flooding. Thus, a Marginal Risk for excessive rainfall remains in place for this period. Orrison ....Mid-South... 16z update... As main polar forcing/jet support continues to lift northward though the Ohio Valley/Great Lakes, the trailing cold front and deep layer warm conveyor belt will align with depth and continue a very slow eastward progression through Middle TN into northern AL. Prolonged slight to moderate deformation zone showers will lead in increased totals from Chattanooga to Birmingham and with naturally lower FFG, opted to maintain a narrow Marginal Risk but with less than 100 J/kg of CAPE, sufficient rates may be hard to come by and area is on the lower edge of the threshold. Further south (Louisiana)...the cold front is starting to stall in eastward progression across S MS and central LA and has fully across the northern Gulf of Mexico where it lays more orthogonal to the mean flow and is producing a convective complex that is likely to maintain for a few hours and inhibits further moisture transport across/along the front though some showers may affect Marsh Island into the Atchafalaya Delta, it does not rise to Marginal status. Further east (S MS/AL/W FL)...the confluence band along the western edge of the subtropical high, continues to be a corridor for enhanced moisture flux and instability advection across the Central Gulf Coast Plain of S MS/S AL. Active convection has developed in the convergence zone across SE LA into S MS ahead of the main cold front. CAPEs into the 1000-2000 J/kg range will be expected along the coast and advect up toward the cold front. Scattered thunderstorms are expected with some solid rates of 1.75-2.25"/hr possible given 20-30kts of flux with 1.5-1.75" Total PWats (mainly focused below 700mb). While the area has naturally higher FFG, Hi- Res CAMs have been trending toward greater convective coverage from the frontal zone in SE MS/SW AL toward this return flow axis across the coast; with the FV3 CAM even suggesting localized totals in excess of 5". While this is not completely unfounded; it does appear to be a worst case evolution for the current environmental setup. More reliable guidance suggest spots of 2-4" and more scattered in nature more in line with a Marginal Risk of Excessive Rainfall. As such, have made a southward and eastward expansion of the risk area to include Mobile Bay and urban locations in the far western FL Panhandle. Gallina 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=20 areas with higher FFG (even after today's rain).=20 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.=20 Rain rates into southwestern OR may still exceed 0.5/hr across 12Z=20 thus supporting the Slight Risk. Trimmed a bit from the=20 northeastern side of the Marginal outline into the Northeast and=20 expanded a bit on the southwest side to account for just a tad=20 slower motion of the system in the east, but otherwise left a=20 somewhat broader Marginal in place that may focus more to the=20 northwest of I-95 per current trends.=20 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!4aOhDumIYgDkVn_07TiVxy3_LRLILpAEpd3cOQYvPecE= 9eoncBW8uFGjlXzwfPNRXjSuK0U_aYLKz2FOg0fiyKHE3XA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4aOhDumIYgDkVn_07TiVxy3_LRLILpAEpd3cOQYvPecE= 9eoncBW8uFGjlXzwfPNRXjSuK0U_aYLKz2FOg0fi-GmUHJI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4aOhDumIYgDkVn_07TiVxy3_LRLILpAEpd3cOQYvPecE= 9eoncBW8uFGjlXzwfPNRXjSuK0U_aYLKz2FOg0fiTUoq2jA$=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) .