Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 02 2025 19:36:33 FOUS30 KWBC 021936 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 236 PM EST Sun Feb 2 2025 Day 1 Valid 16Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA... ....16Z update... The only change with this update was to trim back the southern portion of the Slight Risk over the northern California Coastal Ranges. As the ongoing surge of heavier rainfall wanes into the afternoon, additional rainfall rates through the rest of the day into tonight look to be less than 0.5 in/hr and less impactful. As mentioned in the previous discussion, a surge of heavy rain has been ongoing this morning with several reports of hourly rainfall in excess of 0.5 inches across the northern Sierra Nevada. Peak IVT values are forecast by the 06Z 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 (through 12Z Monday). This steady influx of moisture will continue to carry the potential for hourly rainfall in excess of 0.5 inches but the coverage of these higher rates should shrink. Nonetheless, in advance of the next mid- level shortwave impulse from the west, rainfall rates will increase slightly tonight with steady rain continuing through 12Z Monday. Peak additional rainfall totals through 12Z Monday for the northern California Coastal Ranges are expected to be 3-5 inches and 5-10 inches for the northern Sierra Nevada. Otto ....previous discussion follows... A prolonged atmospheric river (AR) event is underway across almost all of northern California this morning. A large area of high pressure off southern California is teaming up with a pair of lows...one well north of Hawaii and the second near the northern tip of Vancouver Island, BC. These features will interact to form a long corridor where abundant tropical moisture can continue riding northeastward from the tropics and into northern California. Meanwhile, the lows are limiting the northward extent of the moisture, especially the one off Canada, which is supplying colder air into Oregon and Washington State. In the upper levels, a stationary upper level low will guide upper level disturbances around its southern periphery and into northern California throughout this period, perhaps even retrograding a bit west by tonight which will only further lengthen the fetch over the Pacific. The strong high in place near southern California will form a tight gradient between the two features, resulting in a hyperactive zonal jet which will support continuous forcing for lift supporting the rainfall within the AR. NASA Sport Soil Moisture imagery shows much of the soils of=20 northern California have become nearly saturated as a result of the 1-4 inches of rain that fell yesterday through the time of this=20 writing. Thus, expect much of the rainfall going forward associated with this AR to largely convert to runoff. A pair of upper level disturbances are moving along the atmospheric river. The first will clear California and move into the interior this morning. Heavy rain ongoing now over far northern California will gradually drift south through the day in response to this first disturbance. This will give coastal regions a brief break in the rainfall behind the disturbance from the start of this period through late morning/midday. Then the second disturbance will renew the heavy rainfall beginning midday/early afternoon and continue right through tonight. The rainfall plume with this second round of rain will be very slow to move, only crawling northward with time. Most areas of far northern California will see several hours of continuous, steady rain. Rates don't look to be quite as high with this second disturbance, likely averaging between 1/4 and 1/2 inch per hour. Thus, the primary hazards with it will be associated with the long-duration of the rain. Potential impacts from this AR today and tonight include possible mudslides and rock slides in the foothills and mountains below snow level (~6,500 ft), as well as the potential for localized flooding in flood sensitive and low-lying areas with poor drainage. Wegman 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...=20 Not much change to the IVT axis is forecast into Monday, meaning the prolonged moderate to heavy rainfall=20 event will continue across northern CA. Snow levels should begin to drop by this time resulting in less impacts over the higher=20 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=20 frontal convergence by this time over northern CA to go along with=20 an uptick in IVT values moving into the coast. This should result=20 in an increase in rainfall rates by late morning into the early=20 afternoon, with this axis of higher rates shifting south through=20 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!6Pf3C6wLAtAbMu7hSpJJdArwzWn4S8AVG8HYT-DFBn16= bsSTAB8itgad5X7olt5hbuBeEBA73-wUV_Z-iy39xZ5SPMQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Pf3C6wLAtAbMu7hSpJJdArwzWn4S8AVG8HYT-DFBn16= bsSTAB8itgad5X7olt5hbuBeEBA73-wUV_Z-iy39__3KhZU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Pf3C6wLAtAbMu7hSpJJdArwzWn4S8AVG8HYT-DFBn16= bsSTAB8itgad5X7olt5hbuBeEBA73-wUV_Z-iy391y-Zbpk$=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) .