####018005110#### AGUS76 KRSA 071507 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 705 AM PST Thu Mar 7 2024 ...SYSTEM EXITING REGION TODAY WITH LIGHT PRECIP SOUTH/EAST... ...SEVERAL DISTURBANCES MOVE THROUGH THIS WEEKEND INTO EARLY NEXT WEEK MAINLY NORTHERN AREAS... .METEOROLOGICAL CONDITIONS (SHORT TERM: THU AM - SUN AM)... The general upr troffing across the west coast is beginning to shift off toward the east...while a s/wv trof at the base of the overall flow is crossing the lower Colorado River basin this morning. Best concentration of echos according to radar imagery is now affecting areas near the CA/NV/AZ triple point along with more scattered precip downstream across the Desert Southwest. Behind the upr trof axis...drier northerly flow is starting to develop across the region. There may be some scattered light precip this afternoon with some daytime heating across the higher terrain of central/southern CA into portions of southern/eastern NV...but chances for precip should wrap up overnight as an offshore positively tilted upr ridge initially moves over the Pacific Northwest and then slides across the rest of the west coast on Friday. Freezing levels this afternoon should range from 5500- to 6500-feet across areas south of I-80. As the upr ridge gets pushed downstream of the area...the west coast will be open to weaker progressing systems moving across the northern Pacific toward the Pacific Northwest and northern CA. The first will arrive early on Saturday...spreading precip across areas north of I-80 with the best amounts along coastal areas near Cape Mendocino northward (0.25- to 0.75-inch) and lesser totals inland over the crest of the southern OR Cascades down toward the Shasta Lake drainage (0.10- to 0.33-inch). Freezing levels will generally range from 3000- to 5000-feet. The next disturbance after this one will be just off the Pacific Northwest and northern CA coast on early Sunday with light precip beginning to make its way along the immediate coast from Cape Mendocino northward. .METEOROLOGICAL CONDITIONS (LONG TERM: SUN AM - WED AM)... A pair of systems will bring periods of precipitation to the region late this weekend through mid next week. The first as a surface/upper low moving in from the Gulf of Alaska early Sunday before opening up into a broad trough and heading inland into Monday. There are still some timing issues and other differences with this system between the GFS/ECMWF. The ECMWF continues to bring precip into the CA coast sooner on Sunday than the GFS (early morning vs afternoon). The GFS also has the trough deeper Sunday afternoon onwards into Monday while moving it through CA/NV slower than the ECMWF. This would impact how long showers persist over the region Monday and on freezing levels. Both models do show the trough exiting into the Four Corners sometime in the afternoon with another system then approaching. A surface/upper low will move into the Gulf of Alaska Monday afternoon sending a front towards the PacNW and nrn CA. Precip is expected to move into the nrn CA coast Monday afternoon/evening before spreading inland across nrn CA and into central CA overnight as the troughing sets up along the west coast. The trough is then progged to head inland the rest of Tuesday beginning to exit to the east into Wednesday. The pattern differences between the GFS/ECMWF are more subtle than the Sunday system, but the result is the ECMWF predicting higher QPF than the GFS. 24 hr totals ending 12z Weds from the GFS show 0.10-0.30" or so across the Sierra while the ECMWF has 0.50-1.25". Ensembles are divided as well. Looking at the 50th percentile 24hr QPF clusters ending 00z Weds, ensemble members are split into 3 clusters at 45%, 30%, and 23%. No one grouping is dominated by a majority of members from any one particular model. QPF totals along the north coast range from 0.50-1" to 1.50-3" while totals across the northern Sierra range from 0.10-0.25" to 0.75- 2.50". Bottom line is there is still some uncertainty in the QPF for the extended. The official forecast was a combination of the latest NBM and the morning WPC guidance. Highest precip amounts for days 4-6 expected along the north coast, over Shasta, and across the northern Sierra. QPF 12z Sun-12z Weds: 1.75-3" north coast (2-4.50" Smith Basin/Kings Range), 1.50-3.25" Shasta, 1-2.50" srn OR Cascades/northern Sierra, 0.50-1" central Sierra, and a few hundredths to 0.50" for much of the rest of nrn CA. Freezing levels 4-6.5 kft north of I-80 Sunday morning lowering into Monday down to 3-4.5 kft before rising again in the afternoon to 4.5- 6 kft. Some fluctuation early next week but generally 3.5-6.5 kft north of I-80 and 5.5-9.5 kft to the south. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php Kozlowski/AS $$