Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 26 2024 15:42:59 FOUS30 KWBC 261542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1042 AM EST Thu Dec 26 2024 Day 1 Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EAST TEXAS THROUGH THE ARKLATEX AND ADJACENT AREAS OF THE LOWER MISSISSIPPI VALLEY... ....16z update... Recent observational trends, 12z Hi-Res CAMs and HREF probabilities further support initial Day 1 reasoning and placement of Slight Risk area across E TX into the Arklatex and Lower MS Valley later today. As such, only slight westward adjustments were needed with this update, to account for slightly earlier convective initiation observed in the observational trends.=20=20 Additionally, southward adjustment with Marginal Risk in central=20 CA also appears on solid track with the arrival of the next AR wave in the later 6-12 hours of the Day 1 period.=20 Gallina ~~~Prior Discussion~~~ ...East Texas...ArkLaTex...Lower Mississippi Valley.. GOES satellite imagery shows a strong shortwave trough digging southeastward across the southern Rockies which will be ejecting and amplifying out across the southern Plains this afternoon and the Lower MS Valley tonight. The mid to upper-level trough is expected to become negatively tilted this evening as it crosses through the ArkLaTex and moves downstream, and there will be a substantial amount of upper-level jet energy/forcing in place via the arrival of a 90+ kt jet streak and associated jet couplet which the guidance has been quite consistent in adverting. Coinciding with this will be surface cyclogenesis and the northward advance of a warm front into the Lower MS Valley which will foster the northward advance of moisture and instability. The 00Z HREF consensus favors a convergent low-level jet increasing to the order of 50 to 70 kts this evening across areas of far eastern TX through western and northern LA and much of southern AR and western MS. Strong convection will be developing over areas of central and especially eastern TX this afternoon and impacting much of the broader ArkLaTex region and the Lower MS Valley this evening and overnight. Multicell and supercell thunderstorm activity is likely given the available moisture and instability transport and the enhanced shear profiles. The convection should tend to become more progressive in time with a QLCS evolution expected tonight, but concerns with cell-merger activity and some periodic cell-training will support rainfall rates reaching 1 to 2 inches/hour with some 2 to 4+ inch storm totals possible. Scattered areas of flash flooding, especially around the more sensitive urban locations, will be possible. This will also be aided by the fact that at least locally there has been recent rainfall that has moistened the soil conditions and increased the streamflows. Therefore, the Slight Risk area has been maintained and locally expanded to account for the heavy rainfall threat and antecedent conditions. ....Pacific Northwest through Northern California... The arrival of a new Pacific shortwave trough and associated atmospheric river surge will bring a new round of heavier rainfall to the coastal ranges of the Pacific Northwest on down through northern CA later today through Friday morning. The shortwave energy will be quite vigorous and will be amplifying which will drive strong cyclogenesis just offshore of western WA/OR tonight. Enhanced IVT magnitudes around the southern flank of the low center are expected to impact especially southwest OR and northwest CA with values increasing to as much as 750 to 1000 kg/m/s. This strong moisture transport and related warm air advection will facilitate rainfall rates increasing to as much as 0.50" to 0.75"/hour at least briefly as the core of the stronger IVT arrives and with aid from orographic ascent over the coastal ranges. Additional rainfall amounts by Friday morning with this next atmospheric river surge are expected to be on the order of 3 to 6 inches, with the heaviest totals likely focused over southwest OR and northwest CA. Given the level of additional rains and wet/saturated soil conditions, there will be increasing concerns for more flooding impacts. The Marginal Risk area will be maintained across the region, except it has been expanded somewhat farther south in northern CA to include the San Francisco Bay metropolitan area given expectations for heavy rain arriving here by early Friday morning along with some urban flooding concerns. Orrison 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... ....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... ....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!7-hJNO3AXdMril-hpaDxIRLcuzd_D0CLQwyDYyc-nO7_= M1AIGDVZLbmDLk4fR1O5f6hhhY4rf1SCGeIuQG3dANl84TQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7-hJNO3AXdMril-hpaDxIRLcuzd_D0CLQwyDYyc-nO7_= M1AIGDVZLbmDLk4fR1O5f6hhhY4rf1SCGeIuQG3dyBQF2Ik$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7-hJNO3AXdMril-hpaDxIRLcuzd_D0CLQwyDYyc-nO7_= M1AIGDVZLbmDLk4fR1O5f6hhhY4rf1SCGeIuQG3d3H5h3DA$=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) .