####018009061#### AGUS76 KRSA 052057 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 800 AM PST Mon Feb 5 2024 ...HEAVY PRECIP ACROSS SRN CA THROUGH TUES AM WITH SCATTERED SHOWERS ACROSS NRN CA AND ISOLATED TSTORMS OVER CNTRL CA... ...LINGERING PRECIP, MAINLY OVER SRN CA TUES WITH CONTINUED CHANCES FOR SHOWERS THE REST OF THE WORK WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: MON AM - THU AM)... Overnight, heavy precip was mainly concentrated across the Transverse Range where observations report 1.50" to nearly 4" just over the past 6 hours. Heavy rain was also focused over the LA metro area where observations report about 1 to 2.5" since the last update. Additionally, a couple lines set up one from the Sacramento Area to the foothills along I-80 and the other from the Monterey Bay through the Santa Clara Valley training showers across the areas for most of the night. Obs show precip totals from 0.75" to over 2" along the line near Sac with almost nothing in surrounding areas. Parts of the Santa Cruz Mountains have seen about 0.25-0.75" spilling over into the Santa Clara Valley north of Gilroy while just to the south obs only report a few hundredths of an inch. The north coast has benefited from its position near the core of the surface low as precip rotated onshore most of the night as well reporting 0.10-0.50" and locally over 1" around the Kings Range. Widespread gusty winds occurred overnight as well at roughly 40-70 mph across the state. An ob at Mammoth Mountain reported 125 mph gusts late last night. Local storm reports showed numerous flood reports and downed trees across the state as well as swift water rescues in the LA region. Taking a look at current conditions...The surface low still resides just offshore of the north coast this morning with an upper low stretched across the west coast from BC and out to sea west of soCal at around 130W. The tropical moisture plume remains focused over southern CA mainly around LA County and southward. Lightning detection capturing multiple flashes and strikes over the coastal waters as radar imagery shows convective showers across a corridor from the Monterey Bay through to the northern Sierra. The rest of today, expect the moisture plume to remain focused across southern CA bringing continued heavy precip to the San Gabriel/San Bernardino Mountains and surrounding coastal areas. Convective showers and thunderstorms remain possible across central CA the rest of today while the north coast through to Shasta will continue to receive showers spun up by the core of the surface low. The southern edge of the upper low will inch closer to the CA coast throughout the day as the surface low continues to rotate offshore of the north coast and the moisture plume slowly creeps towards San Diego. By this evening, the moisture plume is expected to mostly shift south of LA County though still bringing moderate/heavy precip into the San Gabriel/San Bernardino Mountains. There is still some disagreement on the speed with which the moisture moves across soCal tonight. The ECMWF remains the quicker model while all the others take a slower progression. The official forecast continues to lean on the slower side, but there is uncertainty still regarding how slow it will be and how much precip will be spread northward across the rest of the soCal coast overnight. Tuesday morning, the moisture plume will exit to the south into Baja while the surface low will have weakened offshore and the upper low begins to slowly make landfall throughout the day. The weakening of the surface low will allow precip to dissipate across northern CA with scattered showers persisting across central/southern CA thanks to lingering moisture and the upper trough. Highest amounts Tuesday am still over the eastern Transverse Range. Chances of thunderstorms persist for Tuesday, though this time mostly for southern CA. Uncertainty remains regarding the QPF for the next 24 hours. QPF clusters ending 00z Weds are divided into 3 approximately equal groups with 24 hr QPF across the western Transverse Mountains ranging from 0.75" to nearly 3". Much of this will depend on how quickly the moisture plume moves the rest of today and overnight and where any additional thunderstorms develop. For the moment, 24 hour QPF ending 12z Tues is as follows: 3-8+" San Gabriel/San Bernardino Mountains, 1.5-4" coastal soCal LA to San Diego County, 1-4" Shasta, 0.75-2" north coast (highest Kings Range/Cape Mendocino), 0.50-1.50" Sierra, and 0.10-0.75" across the greater Bay Area/down the valley with locally higher amounts in any developing convective cells. Freezing levels 4.5-6.5 kft north of Point Conception and 6.5-9 kft to the south most of today, lowering overnight to 3.5-5.5 and 5.5- 6.5 kft respectively by Tuesday morning. The upper low will continue to slowly move through the region the rest of Tuesday and overnight into Wednesday. Most of the precip for Tuesday will occur in the morning and across coastal soCal still due to lingering moisture and the presence of the upper trough. Looking at another 0.50-1" along the coast from LA to San Diego and 1.5- 2.5" in the mountains. Another system then slides in behind the current one off the coast of the PacNW. Models disagree on where exactly this system will end up. The GFS takes the trough just west of the coast stretching across WA through to soCal by Weds afternoon while the ECMWF shifts this same system eastward right along the coast. The GFS also shows a larger overall trough encompassing the entire western U.S. by Thursday morning while the ECMWF separates the system into two troughs one over WA/OR/ID and into northern NV with the other over soCal. Either way, it does not look like this system will produce much precip with most of the ensembles in agreement on that point. Precip will focus for Weds across the Sierra and over far southern CA (San Diego County area) thanks to some energy at the southern edge of the trough both models have passing into soCal. QPF 12z Weds to 12z Thurs has 0.25-0.75" along coastal soCal, 0.25-0.50" over the Sierra, and generally a few hundredths to a quarter of an inch across the rest of the region. Freezing levels remaining lower through the rest of the period after Tuesday evening down to 2.5-5 kft across CA/NV. .METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)... Troughing is forecast to remain over the western US through much of the extended range with general model agreement on the overall large- scale pattern. This should keep the forecast area active with lower elevation showers and mountain snow, though accumulations will be lower when compared to the short range. While model guidance is in general agreement on the overall flow regime, differences mainly arise in the timing and track of s/wv activity embedded in the upstream portion of the larger scale trough and how much moisture aggregates in the warm sector of the surface lows trekking southward over the region. Several model solutions suggest the development of an upper level cutoff low somewhere over California sometime on Thursday, though the timing, location, and magnitude this cutoff differs depending on the model and the run. A more westward formation of the low will allow for the system to tap into more Pacific moisture and result in stronger onshore moisture flux whereas a more inland track may produce a drier system and spread precipitation further inland. For Thursday into Friday, the EC seems to suggest a more northerly track, focusing heaviest precipitation accumulations over the North Coast whereas the GFS favors a southerly tack that brings the precipitation into Southern California and the Southern Sierra. Ridging is forecast to begin to build through the weekend as the broader trough axis shifts inland, diminishing the chances of precipitation for the forecast region by Sunday. Precipitation forecasts generally followed the guidance of the NBM and WPC spreading light precipitation, up to 0.5 inches across the crest of the northern Sierra, and higher amounts across Southern California (up to 1.0 over higher elevations) from Thursday afternoon through Saturday morning. QPF will continue to be adjusted as guidance converges towards better agreement. Freezing levels will start the extended forecast window at around 2,000 feet in the north and 4,000 feet in the south before rising under the building ridge to over 8,000 feet across California and 1 to 6,000 feet across Nevada on Sunday morning. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS/CH $$