####018005314#### AGUS76 KRSA 061515 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 715 AM PST Wed Mar 6 2024 ...SCATTERED PRECIP FOR NORTHERN AREAS WITH INCREASING AMOUNTS FOR COASTAL SOCAL... ...CONTINUING CHANCES OF PRECIP THIS WEEKEND FOR AREAS N OF I-80... .METEOROLOGICAL CONDITIONS (SHORT TERM: WED AM - SAT AM)... With broad upr troffing over the Pacific Northwest over toward the northern Plains states and a s/wv trof nearing Point Conception this morning...an associated frontal boundary meanders from northern NV back across northern CA from the northeast CA Plateau back toward the Eel River basin before arching southward just off the SF Bay Area and central CA coast. Radar imagery shows echoes focused primarily along this boundary with the best precip this morning falling across portions of the Santa Cruz mountains up toward the SF Bay Area...and near the Shasta Lake drainage. As the s/wv trof approaches Point Conception later this morning expect precip to be on the increase along the central CA coast down toward the Santa Ynez mountains. Then as the disturbance makes its way over the Los Angeles area by this evening before shifting eastward across the southeast CA deserts...precip will spread east across the rest of the transverse mountains and down toward coastal areas near the CA/MX border. Freezing levels today will range from about 4000- to 4500-feet along the CA/OR border and far northern NV to 5500-feet along I-80 and then 6000-feet or so across central CA and central NV with 6500- to 8000-feet for southern CA and southern NV. As the overall troffiness slides to the east of the area...a positively tilted upr ridge will begin to nose across the Pacific Northwest and northern CA on Thursday before transitioning inland. This will bring drier northerly flow on Thursday into early Friday...waiting for the next approaching s/wv trof to reach the eastern Pacific late in the period. Light precip with this system will start to spread along the immediate north coast early on Saturday with the majority expected beyond the forecast period. .METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)... An active pattern is expected for the long term as a few systems bring rounds of precipitation to northern CA. The first system takes the form of an upper trough reaching the coast of the PacNW and nrn CA early Saturday morning. The trough will then travel inland in a northeast direction bringing showers to nrn CA and srn OR. Precip will diminish later in the afternoon as the trough lifts into ID. Not long afterwards, a surface/upper low will approach from the northwest with the core of the system headed towards BC. The system's front will reach the PacNW/nrn CA sometime Sunday morning. There are still differences between the GFS/ECMWF on the arrival timing of precip. The ECMWF remains the quicker model bring precip into nw CA early Sunday morning while the GFS is delayed by about 6 or so hrs. The GFS is also showing a deeper slower moving system compared to the ECMWF. The ECMWF has a shallower trough mostly over the PacNW moving inland Sunday afternoon. The GFS has a much deeper trough that stretches all the way across CA and progresses through the west coast more slowly spreading precip further to the south across the state. By Monday morning, the trough in the ECMWF has lifted out of the area into ID/MT while another low approaches. The GFS at that time still has the Sunday trough over WA/OR and into NV/soCal. The quicker pattern progression in the ECMWF det run is resulting in the next system bringing moderate precip into the nrn CA coast late Monday afternoon into the early evening while the GFS is delayed until Tuesday morning. These differences mean the ECMWF is predicting 1-2" of QPF for Monday along the north coast while the GFS has 0.10-0.30". There is still a variety of timing and precip amounts in the model ensembles as well. Looking at the 24 hr QPF clusters ending Monday afternoon, amounts along the north coast range from 0.25-1" in cluster 1 to 1.50-3+" in cluster 2. It is no surprise that the majority of the GFS ensemble members (73%) are in the drier cluster 1 while the ECMWF and CMC are split between the three. All this to say that model differences in the timing and overall pattern make for uncertainty in the QPF in the extended, particularly Sunday onward. For the QPF, went with a blend of the morning WPC guidance and the latest NBM. Highest precip amounts expected along the north coast. QPF 12z Sat-12z Tues: 1.50-3.50" north coast (2-4.50" Smith Basin/Kings Range), 1-2" Shasta, 0.75-1.50" srn OR Cascades, 0.50-1" northern Sierra, 0.10-0.50" central Sierra, and a few hundredths to 0.50" the rest of nrn CA. Freezing levels 4-7.5 kft north of I-80 early Saturday lowering to 3.5-6 kft in the afternoon and remaining around there through most of Monday. Expecting levels to rise into Tuesday by about 0.50-1 kft. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php Kozlowski/AS $$