Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 03 2025 00:48:03 FOUS30 KWBC 030047 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 747 PM EST Sun Feb 2 2025 Day 1 Valid 01Z Mon Feb 03 2025 - 12Z Mon Feb 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA... The only change with this update was to nudge the northern portion of the Marginal risk area northward a bit in deference to radar=20 trends as of late aternoon...but few (if any changes) needed=20 elsewhere. Given instability forecasts from the high resolution=20 guidance...earlier thinking that rates should gradually wane with=20 rates settling under 0.5 in/hr and less impactful still seems to be on track. A surge of heavy rain has persisted across portions of northern California from this morning into the afternoon with several=20 reports of hourly rainfall in excess of 0.5 inches across the=20 northern Sierra Nevada. Peak IVT values are forecast by the 12Z and 18Z GFS to lower from the earlier morning values of 700-800 kg/m/s into the 400-600 kg/m/s range for the remainder of the period=20 (through 12Z Monday). This steady influx of moisture will continue=20 to carry the potential for hourly rainfall in excess of 0.5 inches=20 but the coverage of these higher rates should shrink. Nonetheless,=20 in advance of the next mid- level shortwave impulse from the west,=20 rainfall rates will increase slightly tonight with steady rain=20 continuing through 12Z Monday.=20 Bann Day 2 Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... 19Z Update... No significant changes were made to the previous outlook areas. Previous Discussion... Not much change to the IVT axis is forecast into Monday, meaning the prolonged moderate to heavy rainfall event will continue across northern CA. Snow levels should begin to drop by this time resulting in less impacts over the higher terrain of the Sierra, but a continued uptick in flood impacts over coastal areas into the foothills where conditions will continue to saturate. Model guidance continues to show a strengthening of the frontal convergence by this time over northern CA to go along with an uptick in IVT values moving into the coast. This should result in an increase in rainfall rates by late morning into the early afternoon, with this axis of higher rates shifting south through the afternoon and evening. There is some potential for higher end Slight risk, or maybe even MDT risk, probabilities along coastal areas given the expected uptick in rainfall rates over what should be fairly saturated ground conditions by this point. However a lack of stronger dynamics, and lack of instability interacting with the IVT plume, should put a cap on upward vertical motion and rainfall rates. Thus while some flooding impacts appear likely, more widespread significant impacts may be harder to come by, and thus not planning on any upgrade at this time. Pereira/Chenard Day 3 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... 19Z Update... No significant changes were made to the previous outlook areas. Previous Discussion... The atmospheric river will continue dropping southward on Tuesday, bringing another day of moderate to heavy rainfall and some flooding risk. Still some uncertainty with the latitude of the AR by this time, although models do appear to be converging on a solution covered by the Slight risk area. Some trimming on the northern end of the Slight risk may eventually be warranted, but with some lingering uncertainty and the saturated conditions over that area, we were a bit conservative with the northern extent of the risk area. Model solutions indicate the approach of a mid level shortwave and a stronger upper jet after 18z Tuesday. These features will help drive an increase in IVT, and may also help briefly stall the southward progression of the front. Thus it appears likely that we will see an uptick in rainfall rates after 18z Tuesday from approximately San Francisco south into Monterey county. Still some uncertainty on the exact location of this enhanced rainfall rate activity, as well as exactly how intense the rain will be...but current model solutions support upwards of 1.5" in a 3 hour period, and 2-4" in total. This rainfall will likely drive an increase in the flood threat over these areas. Luckily most of the heavier rainfall on the preceding days will have been north of this corridor, so not expecting conditions to be too saturated leading into the uptick in rain. This fact, combined with the overall progressiveness of the system, keeps the flood risk at a Slight level. Although will need to continue to monitor, as if rainfall rates trend higher can not rule out a localized corridor of MDT risk level impacts. The Marginal risk extends further down the coast into Santa Barbara, Ventura and Los Angeles counties. The front should be quick moving by the time it makes it this far south, but a quick shot of heavy rainfall is possible. The 00z GFS and NAM do not bring the better moisture transport this far south, however the 00z ECMWF, UKMET and GEM do. Even these latter models only get rainfall totals into the 1-2" range given the fast frontal motion. So generally not expecting widespread or significant impacts at this time, although rainfall rates could be briefly high enough to result in some localized minor flood concerns. We will keep a close eye on the sensitive burn scars over Los Angeles county, but at this time even the more aggressive ECMWF would suggest rainfall rates will be on the decline by the time they get that far south. Will need to continue to monitor, but think a Marginal risk will suffice for now. Pereira/Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_T2bDionanPE0oyGyjoV0vbQRxmxQQV-ag-KOjcPsGMd= cGkz5f3zIx3uZM7donWQwNTIgYVMlBBVKzA4V6ou04HAdYc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_T2bDionanPE0oyGyjoV0vbQRxmxQQV-ag-KOjcPsGMd= cGkz5f3zIx3uZM7donWQwNTIgYVMlBBVKzA4V6ouAedW-bo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_T2bDionanPE0oyGyjoV0vbQRxmxQQV-ag-KOjcPsGMd= cGkz5f3zIx3uZM7donWQwNTIgYVMlBBVKzA4V6ouiLUlePA$=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) .