####018006014#### AGUS76 KRSA 271501 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 800 AM PDT Wed Mar 27 2024 ...A COLD FRONT WILL BRING PRECIPITATION TO NRN/CNTRL CA TODAY/THURS ALONG WITH ISOLATED THUNDERSTORMS... ...A SURFACE/UPPER LOW TO THEN TRAVEL ALONG THE CA COAST FRI/SAT SPREADING PRECIP ACROSS THE REGION BEFORE MOVING INLAND LATER THIS WEEKEND... .METEOROLOGICAL CONDITIONS (SHORT TERM: WED AM - SAT AM)... A surface/upper low continue to approach the PacNW from the Gulf of Alaska sending a cold front towards northern CA and the west coast. TPW satellite imagery shows a stream of 0.75-1" PW of moisture surrounding the front stretching from WA to the Bay Area and back to HI. The result has been precip across the north coast since late last night now inland towards I-5 along the CA/OR border as evident on current radar. Observations report about 0.10-0.50" for those areas overnight. The rest of the region remains under an upper ridge that will continue its eastward journey the rest of the day as the front sweeps through CA and the low continues its approach. Precip will spread across the rest of northern CA this morning and then into central CA in the afternoon as troughing from the low begins to move in behind the ridge. This may also result in some isolated thunderstorms this afternoon/evening over northern CA and into the nrn/cntrl Sierra. Thursday morning, the front will have passed the Bay Area bringing precip into the central coast as the upper trough takes over and the ridge moves into the central U.S. The front will then pass Point Conception Thursday afternoon, but by then the majority of the moisture will have already been depleted producing only maybe a few scattered showers across soCal. Meanwhile another surface low and upper shortwave will spin up off the back side of the main low that sits offshore of WA/BC later Thursday. This system will head east towards CA reaching the coast mid/late Friday morning providing another surge of precipitation. The upper shortwave will also join back with its parent low settling just above the surface system offshore of nrn CA in the afternoon. The low will then slowly move southward along the CA coast overnight drawing some additional moisture from the sw into soCal. There is some uncertainty on Friday precip along the central coast/Bay Area as ensembles disagree on how close the low will actually get to the coast. The closer it comes the farther more moderate precip will spread. 24 hr QPF clusters ending Friday afternoon are slit into 3 groups (52%, 25%, 23%). Cluster 1 shows about 0.10-0.50" across the central coast while cluster 2 is more confined to the Bay Area northward, and cluster 3 is wetter at 0.50-1.50". The official forecast was a blend of WPC guidance and the latest NBM. The majority of precipitation Friday into Saturday will be focused along the central and soCal coasts as well as inland across the Sierra where lower freezing levels may bring additional mountain snow. To summarize, a surface and upper low will send a cold front through CA today/tomorrow before the main system travels southward along the CA coast the rest of the period. Highest precip amounts are forecast along the nrn/cntrl CA coast, Shasta, and across the Sierra. QPF through 12z Saturday: 1-4" Sierra, 1-3.50" Shasta, 1.50-3" CA coast from Marin County northward (3-5" Smith Basin), 0.75-1.50" CA coast SF to Ventura, 0.30-1.50" down the valleys, and a few hundredths to 0.75" rest of coastal soCal. Freezing levels 5.5-7.5 kft this morning north of I-80 lowering to 3.5-6.5 kft this evening and 6-12 kft to the south. Freezing levels will gradually lower from n to s the rest of the week with lower levels spreading across CA/NV. Friday morning expecting 3.5-4.5 kft north of I-80 and 4.5-9.5 kft to the south. By the end of the short term, forecasting 3.5-5.5 kft for most of the region north of Point Conception and 4.5-9 kft to the south (nw to se). .METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)... Low pressure will reside off the central CA coast...while a moisture plume ahead of the cold front with PW values around 1.00-inch intersecting the coast will be moving across coastal southern CA from the LA Basin down to the CA/MX border to start the period. Total amounts between Los Angeles and San Diego will range from 1.00- to 4.00-inches...greatest over south and west facing slopes. A secondary area of low pressure forming across west-central NV will also be a focal point of precip...bringing widespread moderate amounts to the state...including a deformation zone on the northern portion of the feature. Could see anywhere from 0.25- to as much as 1.00-inch for much of NV. Through Saturday into Sunday...the offshore area of low pressure will drop southeast past Point Conception before weakening and eventually turning toward the coast near the CA/MX border. At the same time...the "Tonopah Low" will make its way northeast across the state and weaken before crossing the UT/NV border close to Wendover. Eventually the overall positively tilted upr trof will slide inland and an offshore upr ridge will begin to nose across the Pacific Northwest and northern CA to end the period with dry north to northeast flow on Monday with precip tapering off early in the Day 6 period. Freezing levels on Saturday into Sunday will generally range from 4000- to 7000-feet from north to southeast across the area...then significantly rebounding across north and west areas on Monday as the high pressure begins to build toward the area...up to 9000- to 11000-feet. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS/Kozlowski $$