Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 25 2024 00:58:37 FOUS30 KWBC 250058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 758 PM EST Tue Dec 24 2024 Day 1 Valid 01Z Wed Dec 25 2024 - 12Z Wed Dec 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EAST=20 TEXAS INTO NORTHWESTERN LOUISIANA AND FAR SOUTHWESTERN ARKANSAS... Texas into Louisiana... The base of a mid to upper-level trough continues to amplify across central Texas with scattered thunderstorms ongoing as of 0030Z from just south of the DFW metroplex to near a trough axis east of I-35 and with more isolated development near a weak boundary near=20 Galveston Bay. Continued amplification and advancement of the upper trough should allow for thunderstorms to maintain across=20 central/eastern Texas through the overnight, feeding off of MLCAPE=20 that is estimated to be 1000-2000 J/kg across east-central TX per=20 SPC mesoanalysis data (1750 J/kg at CRP/00Z sounding). Low level=20 southwesterly flow, increasing slightly into the 15-25 kt range,=20 coupled with mean southwesterly steering flow will likely allow for some repeating/training and possible backbuilding of cells=20 overnight. High rainfall rates of 2-3 in/hr seem likely where cell training/stalling occurs.=20 The previous Marginal Risk was removed over Oklahoma into central Arkansas where decreasing instability and forcing has allowed for a reduction in rainfall intensity. The Marginal Risk was expanded slightly to include the TX coast where some isolated runoff could be possible later in the night, although the latest CAM guidance suggests an overall increase in forward propagation toward 12Z. Sierra Nevada Foothills... The previous Marginal Risk was removed due to a reduction in moisture flux and rainfall intensity as an upper trough axis has=20 moved inland across California. Otto Day 2 Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS OF WASHINGTON, OREGON, AND NORTHWEST CALIFORNIA... ....20Z Update... The only noteworthy change to the Day 2 ERO was to expand the Marginal Risk down the Pacific coast to include Eureka and Cape Mendocino. This was due in part to the recent heavy rainfall (high relative soil moisture with the top meter soil moisture percentiles averaging above 75%). The other factor was the uptick in QPF, with isolated orographically-enhanced additional totals between 2-3" per the majority of high-res CAMs. Late in the period (Wednesday night), elevated CAPEs get close to 250+ J/Kg, which will enhance the rainfall rates. The 12Z HREF in fact shows probabilities between 40-60+ percent of 0.50+/hr rates between 06-12Z Thu, which will impact several of the 2021-23 burn scars. Hurley ....Previous Discussion... There has been little change in the forecasted evolution of the next atmospheric river (AR) surging off the Pacific and into the PAC NW by Wednesday afternoon. Model guidance is keen on a surge of moisture represented by fairly elevated IVT signatures on the order of 600-800 kg/ms within the current global ensemble blend and 12z NAEFS. A steady stream of moisture will advect onshore of the Pacific Northwest with the current progression indicating a wave of heavier precipitation entering coastal WA/OR by Wednesday afternoon and continuing through Thursday morning. The latest NBM mean QPF is pretty substantial for the areas between the central OR coast up into the Olympic Peninsula with the latter being the beneficiary of the expected QPF max thanks to added orographic enhancement from the Olympic terrain just inland of the coastal plain. Totals of 2-4" along the coast will be common over the span of 12-18 hrs with 3-5" likely occurring across the Olympic Mountains and adjacent Olympic National Forest where soil moisture anomalies are hovering ~80% leading into the event. The forecasted precip is causing some forecast rises in rivers within the local watersheds of the areas anticipating the heaviest rainfall, especially within the Skokomish basin south of the Olympics. The elevated stream flows and heavy rain will enhance local flash flood prospects for anywhere in proxy to those watersheds, so the threat is a bit higher than normal compared to what is usually forecast for an AR of this strength. The coastal OR area will be more prone to urban flooding, especially for places like Astoria that are within the secondary maxima of the ensemble QPF for the incoming event. The previous forecast was generally maintained, but did trim a small area across the Olympics that will likely maintain wintry precipitation during the entire period leading to a mitigated flash flood threat and more of a snow load/heavy snow concern. Kleebauer Day 3 Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST AND ACROSS EAST TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY... ....20Z Update... Based on the latest guidance (trends), have shifted the Marginal Risk area a little farther east across eastern TX-OK and across portions of the Lower-Mid MS Valley. Moisture (TPW) and moisture flux anomalies (850-700 mb) peak around +2 standard deviations early in the period, then drop after 00Z Fri as the upper shortwave flattens and moves quickly up the western side of the Eastern Seaboard ridge. Transient nature of this system will be the main inhibitor for flash flooding, as will the limited deep-layer instability (particularly over northern portions of the outlook area). Closer to the Gulf Coast, MUCAPEs will likely reach at least 500-1000+ J/Kg, however FFG values are also higher across these areas (particularly the 3hr FFGs, which are between 3-4 inches). Farther north, current 3-hour FFGs are considerably lower in some areas (1.5-2 inches), however so is the available instability and short-term QPF potential. Global guidance current indicates the likelihood of at least isolated areas of 3-4+ inches of rain during Day 3 across the Lower MS Valley to the Gulf Coast; however, there remains quite a bit of areal spread with these higher totals. Therefore, for now at least, will hold off on including a low-end Slight Risk within the current Marginal Risk area. Hurley ....Previous Discussion... ....Pacific Northwest... Secondary IVT pulse will occur on Thursday during the prolonged AR event creating a flash flood potential extending into the D3 time frame. The multi-day precipitation forecast is upwards of 3-6" with locally higher for many areas extending from the Olympics down through coastal OR with the highest centered over northwest WA and some of the coastal ranges in OR. The second pulse will actually protrude inland further creating a risk for flash flooding inland compared to the previous period, so the MRGL risk is more pronounced in areal coverage compared to D2. The AR will eventually slide southward with more emphasis placed on southwest OR into northern CA leading to more heavy precip over likely saturated soils. This lead to the MRGL risk extending further south, encompassing areas that will be at risk for back-to-back events within the last 72 hours. ...East Texas and Lower Mississippi Valley.. A repeat of the D1 setup is progged for east TX extending into the Lower Mississippi Valley as a shortwave disturbance will make headway into the Southern Plains with favorable ascent aloft coupled with surface cyclogenesis and an increasingly destabilizing environment ahead of the trough. This setup does have a bit more vigor within the 500mb evolution as the ensemble output has a better defined closed height field with neutral to negative tilt within the mean trough. This will likely lead to another period of convection with heavy precipitation cores in-of a tongue of enhanced theta_E's that will position over eastern TX into the Arklatex. As of this time, the QPF output via ensembles is generally within the 1-1.5" range, but is lacking the data input from regional hi-res guidance and relevant CAMs. If the current 500mb evolution being forecast goes as planned, expectation is for a general crescendo of QPF in future model output. Considering the threat expected in the D1, it is very plausible that there could be a higher risk area (SLGT) embedded over a broader MRGL risk forecast. As of now, the forecast inherited was only changed to capture the areal QPF average >0.75" which extended the western flank of the risk a bit further west compared to previous issuance. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9= 7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_DVVX7e4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9= 7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_QPZ_7ME$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_7LBziUy-zIxyCMNm-BXRGdjoBeqa_wW-mhW1CbPjps9= 7lW5IHZOMRLKXjGXcejXORzwGgRsJTj4UDcYWDO_MuDH95U$=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) .