Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 750 AM PDT Tue Apr 29 2025 ...WEAK SYSTEM LATER TODAY THRU THURS WITH LIGHT PRECIP AND ISOLATED T-STORMS, MAINLY OVER THE SIERRA AND EASTWARD... ...ANOTHER LARGE COLD CORE SYSTEM EXPECTED FRI THROUGH THE WEEKEND... .METEOROLOGICAL CONDITIONS (TUE AM - MON AM)... A shortwave approaches the PacNW this morning as ridging sits over nrn CA/NV with an upper low over the Four Corners. The ridge overhead will keep conditions dry for much of today along with near to slightly above normal afternoon high temperatures. The approaching shortwave will reaching nrn CA late this afternoon into the evening generating showers over parts of nrn CA/NV. This system will continue to dig further to the south across CA through mid-week deepening as it does so. This will continue to produce scattered showers for Wednesday, mainly over the Sierra and eastward into NV. Additionally, models persist in showing instability as the trough deepens particularly for Wednesday afternoon/evening when thunderstorms are most likely. Best chances will be across the Sierra and eastward into NV. As this trough travels through CA, an upper ridge over the eastern Pacific will push inland behind it into the PacNW and nw CA raising temperatures to about +5 to +10 deg F above normal for Wednesday. Some time Thursday morning, the trough is expected close off into an upper low over srn CA. This will shift shower and thunderstorm chances a bit south over the central/srn Sierra and eastward. Some showers may stretch as far west as the srn CA coast, but accumulations should be minimal. QPF for the Tues- Thurs system is forecast at 0.10-0.75" across the Sierra and 0.10- 0.25" or so over the NV mountains. Lower elevation areas can expect generally less than 0.10". Locally higher amounts in any developing thunderstorms. Above normal temperatures again for Thursday over nrn CA as the ridge drifts inland over WA/OR. A larger system is still set to approach heading into the weekend. Models have an upper low over the Gulf of Alaska with an elongated trough and front moving towards the west coast. Showers expected on Friday between lingering precip from the previous system and initial showers ahead of the next. There are differences between the GFS/ECMWF on the structural and timing aspects of the next system, but it looks like the southern end of the front will reach the north coast by the evening with it and the eastern edge of the trough moving inland overnight. Precip should then spread into the Bay Area and across the Sierra Saturday morning as the core of the system approaches the coast. By Saturday afternoon, the trough should've closed off into an upper low somewhere along the nrn CA coast before traveling through the Bay Area while hugging the coastline overnight. Both the GFS and ECMWF have the core of the low somewhere near Point Conception by early Sunday (though the ECMWF is a bit further south). Things diverge a bit from there with the ECMWF taking the low inland across srn CA Sunday morning and beginning to exit se CA into AZ in the afternoon. The GFS keeps the low along the coast continuing to head south across coastal soCal through most of Sunday afternoon before taking its inland shift in the late afternoon/early evening. Either way, the core of the low looks to move out of the area by the end of the period with the back edge of the system still encompassing the entire region. Expect this system to generate scattered showers across CA/NV for most of this time period. Highest precip amounts are expected across the Sierra and eastward into nrn/cntrl NV with lesser amounts across CA. QPF 12z Fri-12z Mon: 0.75-1.50" across the Sierra/nrn NV mountains, 1-2.25" central NV mountains, 0.30-1" nrn/cntrl NV, 0.10-0.30" rest of NV, 0.25- 0.75" Shasta/ne CA (locally up to 1" mountains ne CA), 0.10-0.50" rest of nrn CA/srn CA mountains east of San Diego, and generally 0.10" or less for the rest of CA. Again locally higher amounts for any thunderstorms. This will be a colder system as well dropping both afternoon high temperatures and freezing levels across the region. A few thunderstorms may be possible as well. By Saturday, most areas will see anomalies of -5 to -15 deg F with some locations reaching -20 deg F. Sunday will be the coolest with anomalies at -10 to -20 deg F. Freezing levels will go from 9-11.5 kft today through Thursday down to 4-8.5 kft north of I-80 Saturday morning spreading to Point Conception by the late afternoon. Freezing levels will reach a minimum early Sunday down to 4-7.5 kft for most areas outside of far se CA and eastern NV before rebounding from nw to se into Monday as the system exits. Areas north of I-80 by then should be back up to 8- 9.5 kft over the Sierra and up to 11 kft along the north coast. |