Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 03 2025 08:16:19 FOUS30 KWBC 030816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 316 AM EST Mon Feb 3 2025 Day 1 Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN=20 CALIFORNIA... The atmospheric river event across much of northern California will=20 continue through the entire Day 1/Monday period. A narrow (100-200=20 mile) wide plume of rainfall rounding the northwestern periphery of=20 a broad subtropical high is also being steered around a positively=20 tilted upper level trough centered over British Columbia near the=20 northern tip of Vancouver Island. The jet stream is rounding the=20 upper level trough from the northeastern side of the trough around=20 to its west and south then ejecting around the southeastern side of=20 the trough. The trough will retrograde southwestward, allowing a=20 cutoff upper level low to form as the jet stream eventually cuts off=20 from the upper level low and maintains a trough over much of eastern=20 Canada. The upper low will drift southwestward through 12Z Tuesday, which=20 will work to reorient the atmospheric river from its current west-to- east to more of a southwest-to-northeast orientation. This will have=20 several ramifications: First, it will maintain the plume of rain=20 into northern California as the reorienting atmospheric river pivots=20 over this area. Rates are expected to remain largely steady based on=20 the latest HiRes guidance. Second, the "pivot" will be associated=20 with a subtle surface low tracking along the atmospheric river. This=20 will locally increase rainfall rates from midday through this=20 afternoon area-wide...though localized convergence in the northern=20 Sacramento Valley will keep rainfall rates elevated there throughout=20 the period. Third, the southwesterly flow may support the ongoing=20 low level jet in the Sacramento Valley better since the broader flow=20 will align better with the low-level jet. The result will be=20 continued heavy rain for the northern and eastern Sacramento Valley=20 and the foothills of the northern Sierras with a widespread 3-5=20 inches of rain expected. The southwesterly reorienting will then begin to push the=20 atmospheric river southward from this afternoon through tonight,=20 reaching the Bay Area this evening. Upslope flow into the Sierras=20 will keep heavy rain ongoing into the foothills of the Sierras all=20 the way up the Sacramento Valley, however, the rain will finally=20 taper off and give the northern coastal ranges a much needed break=20 by tonight as the core of the AR moves south of the Bay Area and=20 weakens with time. The faster southerly movement of the AR from late=20 this afternoon through the overnight should significantly lower the=20 flooding threat with decreasing latitude, so the southern end of the=20 Slight risk is largely the same, stopping just north of the Bay=20 Area, as amounts from the Bay Area south should only necessitate a=20 Marginal Risk since they've been relatively dry in recent days. With this expected evolution, a few minor changes were made to the=20 Day 1 ERO from continuity, namely to begin expanding the Marginal=20 Risk southward to include the Santa Cruz mountains through Monterey=20 Bay. Otherwise, only minor tweaks were made to better match both=20 Marginal and Slight risk areas with the local higher elevations that=20 will see mostly snow, while including some of the valleys in between. Wegman Day 2 Valid 12Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... The atmospheric river event will continue into Tuesday across central and northern CA. Model solutions continue to indicate the=20 approach of a mid level shortwave and a stronger upper jet after=20 18z Tuesday. The approach of these features should help slow/stall and increase the magnitude of the IVT axis during the day. The=20 increase in forcing/IVT and a strengthening of the low level=20 convergence along the front, will both support an uptick in=20 rainfall rates from near San Francisco into the Sacramento Valley=20 and foothills of the Sierra. Hourly rainfall could approach and=20 locally exceed 0.5" along this front, which should drive an=20 increase in the flood threat. The good news is that most of these=20 areas in line for the heaviest rainfall Tuesday have been south of=20 the heaviest rain the past few days. This combined with the fact=20 that the front/IVT axis (while briefly stalling), will start to=20 make quicker southward progress by 00z, should limit the threat of=20 more widespread significant flood impacts. Nonetheless, do=20 anticipate some flooding and landslide potential to accompany this=20 rainfall Tuesday. The Slight risk extends south into coastal Monterey county, where stronger orographic effects will help drive heaver rainfall totals in the 4-7" range. As the front drops into this area there will=20 also be an uptick in hourly rainfall, with high probabilities of=20 0.5" in an hour and localized totals approaching 0.75" in an hour=20 probable. Heavy rainfall is common along this corridor, and=20 antecedent conditions this far south are quite dry. Both these=20 factors should help put a cap on the magnitude of flood impacts=20 from this rain, however would still expect to see at least some=20 flood and landslide potential. The Marginal risk extends further down the coast into Santa Barbara, Ventura and far western Los Angeles counties. The front=20 should be quick moving by this time, and the IVT axis should be weakening. The GFS is the weakest model, but most other models have come around to at least a short period of locally heavy rainfall=20 getting into the favored upslope terrain areas. Even these wetter=20 models (and the WPC forecast) only get rainfall totals into the=20 1-2" range given the fast frontal motion. So generally not=20 expecting widespread or significant impacts at this time, although=20 rainfall rates could be briefly high enough to result in some=20 localized minor flood concerns. Chenard Day 3 Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY... Strong southwesterly flow will advect abundant moisture northward into the OH Valley on Wed into Wed night, with PWs forecast to increase over the 90th percentile for early February. Increasing WAA, weak mid level shortwaves moving east within the quick zonal=20 flow, and upper level divergence in the right entrance region of a jet to the north and left exit region of a strong jet over the MS=20 Valley, will broadly support an expansion of showers across the=20 region through the period. Model guidance also indicates decent=20 instability near and south of a northward moving warm front, thus=20 expect we will see embedded thunderstorms as well. Given the=20 forcing, moisture and instability forecast, do anticipate we will=20 see some locally heavy rainfall rates materialize, especially by=20 Wed evening into the overnight near the lifting warm front. Still some uncertainty on the location of this front, with the=20 ECMWF the furthest north solution and the GFS on the southern side of the model spread. Precipitation north of the front will likely=20 be either sleet or freezing rain, and thus the position of this feature will be important to delineate the northern edge of the=20 flood risk area. At this time generally favor something in the=20 middle, which would favor an axis from KY into southern IN/OH and=20 into WV for the greatest rain rate potential, and the Marginal risk encompasses these areas. However, if a warmer/further north=20 solution becomes more likely, then the Marginal may need to be=20 expanded further north, with the opposite being true if a=20 colder/south scenario comes to fruition. Portions of=20 central/eastern KY into WV have above average soil saturation and=20 streamflows, and so this area is likely most susceptible to some=20 flooding impacts if higher rainfall rates do materialize. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7MEWS3ioW44KEYAdH_jDGwwhI80VCMOrHDQhhrhEiOVd= SEv90Efj7x0FhRpYHHFTBlmrHiVc2ejlw3BQrBj194xEGG8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7MEWS3ioW44KEYAdH_jDGwwhI80VCMOrHDQhhrhEiOVd= SEv90Efj7x0FhRpYHHFTBlmrHiVc2ejlw3BQrBj1uyfyeE4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7MEWS3ioW44KEYAdH_jDGwwhI80VCMOrHDQhhrhEiOVd= SEv90Efj7x0FhRpYHHFTBlmrHiVc2ejlw3BQrBj1YutczJ0$=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) .