####018006616#### AGUS76 KRSA 291455 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 800 AM PDT Fri Mar 29 2024 ...A LARGE SURFACE/UPPER LOW WILL TRAVEL ALONG THE COAST TODAY INTO THE WEEKEND BRINGING WIDESPREAD PRECIP AND ISOLATED TSTORMS WITH LINGERING SHOWERS PERSISTING INTO MON AM... ...HIGH PRESSURE WILL DRY CONDITIONS OUT EARLY NEXT WEEK BEFORE ANOTHER SYSTEM RETURNS PRECIP CHANCES MID WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: FRI AM - MON AM)... A surface low is approaching the CA coast from the west as its parent continues to merge with the upper low offshore of the PacNW. The result is showers beginning to move into the central coast this morning with more on the way. Additionally, these systems are dragging moisture towards the area from the southwest set to reach coastal srn CA tonight. The two upper lows are expected to merge later this afternoon just sw of Cape Mendocino above the surface low. The vertically stacked system will then gradually progress to the south just off the CA coast spreading precip across central CA to the Sierra and then into soCal overnight. Isolated thunderstorms are also anticipated mainly for the central coast/Bay Area today. HREF exceedance probabilities show 50-70+% chances of at least 0.50"/hr rain rates this afternoon into this evening for the Big Sur coastal mountains. The combination of the low and moisture plume is expected to produce moderate to locally heavy precipitation particularly along the western Transverse range and over Santa Barbara/Ventura counties earlier in the evening spreading to the San Gabriel/San Bernadino mountains overnight. Models also show a 120- 130 kt jet streak into the same area overnight. HREF exceedance probabilities show a 30-80% chance of 0.50"/hr rain rates overnight for these locations. Saturday morning, models only have the low progressing as far south as the SF Bay Area while the moisture plume continues to travel along coastal soCal. The presence of the moisture plume will concentrate heavier precip across coastal srn CA Saturday morning with scattered showers persisting over much of the rest of the region thanks to the large sfc/upper low. The moisture plume should begin to exit to the south and weaken in the afternoon as the low reaches the central coast. At this time precip will begin to diminish as well, though with showers and isolated thunderstorms continuing. Chances for thunderstorms for Saturday will be more widespread as the low expands to engulf more of the region. By Saturday evening, a secondary low is expected to form over NV while the core of the main system sits just offshore of Point Conception. This will result in two areas of precip for Sunday, one across coastal soCal as the low continues its southerly offshore journey, and the other over NV as the second upper low heads into UT. By late Sunday afternoon the system will look more along the lines of a large positively tilted upper trough stretched from Baja to MT/WY. The trough will continue to generate showers across central/srn CA and NV as well as additional thunderstorm chances for soCal and NV. The same sources of uncertainty in the QPF remain as they did yesterday. Ensembles still in disagreement on how much precip will fall across coastal soCal through Saturday afternoon. QPF clusters ending 00z Sunday remain split into 3 groups (46%, 29%, 25%) this time divided more along the lines of their parent models. The spread is not extreme as it has been in other systems this winter, but the clusters are showing values spread across the coast between 0.75" and over 1.50". In spite of this, the models are in relatively good agreement on the path and timing of the system. Still, as is always the case with these offshore lows any slight shift toward or away from the coast will make a difference. The forecast was a combination of the morning WPC guidance and the latest NBM. Highest precip amounts along the Big Sur coastal mountains, the Transverse Range, central/soCal coasts, and across the Sierra. QPF through 12z Mon: 2.50-5.50" Big Sur Coast/Transverse Range, 1-3" Sierra, 1.50-2.50" coastal soCal, 0.75-2" Bay Area/central coast, 1-2" NV mountains, 0.25-1" down the valleys lower elev areas of NV (higher Sierra foothills), and 0.10-0.50" for ne/se CA. Freezing levels generally 3.5-6.5 kft north of Point Conception rest of today becoming 4.5-6.5 kft over the weekend. Levels will begin to rise nw to se into Monday as high pressure moves in. Freezing levels along nrn cntrl coastal CA 6.5-10 kft lowering inland to 4.5-5.5 kft across NV and interior CA. .METEOROLOGICAL CONDITIONS (LONG TERM: MON AM - THU AM)... The positively tilted upr trof extends all the way from Hudson Bay back southwest toward the lower Colorado River basin to start the period. There may be a few scattered showers near the base of the system on early Monday across portions of southern CA. At the same time...an offshore upr ridge is expected to nudge across the Pacific Northwest and northern CA...developing drier north to northeast flow over much of the west coast for the beginning of the work week. This upr ridge will not be long-lived as a s/wv trof moves across the northeast Pacific and pushes the upr ridge downstream of the area. This will return the west coast to a more onshore southwest to west flow with a moisture plume that shows a slight uptick in available PW (just over 0.50-inch) moving across northern CA and northern NV on Wednesday with some scattered light precip mainly focused over areas near the CA/OR border...far northern NV (I-80 northward)...and across the crest of the northern/central Sierra. Freezing levels will start relatively low on Monday before high pressure begins to build...from approx 4500- to 6000-feet. Then as the upr ridge moves across the area...these freezing levels will peak later Tuesday from 10000- to just over 12000-feet. Finally...as the troffing moves toward the Pacific Northwest and northern CA...cooler air will usher in across the area with 1500- to 4500-feet across northern CA and northwest NV...with less of a dip for areas south of I-80 from about 5500- to 11000-feet north to south. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS/Kozlowski $$