AGUS76 KRSA 091429 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 730 AM PDT Thu May 9 2024 ...WARMING THROUGHOUT THE WEEK AND OVER THE WEEKEND AS HIGH PRESSURE BUILDS OFFSHORE... ...GENERALLY DRY CONDITIONS ASIDE FROM ISOLATED SHOWERS/T-STORMS OVER NV THURS AND THE SIERRA/NV FRI/SUN/MON AFTNS/EVES... .METEOROLOGICAL CONDITIONS (THU AM - WED AM)... An elongated low is oriented ne to sw from NE all the way through southern CA this morning as an upper ridge resides over the eastern Pacific just offshore. The core of the upper low will continue to progress back west towards the region the rest of today pushing the ridge further offshore of CA with the northern end wrapping around the low into the PacNW. The result will be both warming temperatures and dry conditions for most of CA with continued below normal temps for NV and se CA along with chances of isolated showers and thunderstorms (mainly for E NV) in the afternoon/evening. This setup will remain in place for Friday as well with the offshore ridge bringing temperatures across nrn and cntrl CA 5-15 deg F above seasonal normals while most of NV remains below normal due to the upper low. The presence of the low will also generate additional chances of showers and thunderstorms for the cntrl/srn Sierra and much of NV. On Saturday, the low will shift its trajectory and begin to head eastward taking the core of the system into the Four Corners in the morning. This will replace the more moist mid level air with dryer flow from the offshore ridge as it is allowed to build into CA and nrn NV. The intrusion of high pressure will allow for higher temperatures to spread further across the region with most locations above normal. While the ridge moves over land, models have an upper low moving into the Gulf of Alaska sending a trough towards the PacNW while a weaker low develops under the ridge just west of CA. The PacNW trough will move through to the north throughout the day Sunday sending some mid-level moisture and instability across the CA/NV borders and over the Sierra. This may result in some isolated showers and thunderstorms in the afternoon/evening. The trough will exit the PacNW into ID/MT Monday morning. There is some uncertainty after that as ensembles disagree on the 500 mb height pattern Monday afternoon. The det GFS/ECMWF and some of the ensembles have an upper low just sw of Point Conception with an upper ridge to the northwest over the eastern Pacific. About 60% of the GFS/ECMWF/CMC ensembles do not show the low offshore or have it weak enough to not make much of a difference beyond cooling temperatures along the coast. The GFS/ECMWF have this low along with the exiting system to our north generating some mid-level moisture/instability over the Sierra as well as parts of NV/nrn CA. This does creep into the NBM which generates some precip over the Sierra Monday afternoon/evening, isolated thunderstorms also possible. Should the low develop, it looks to move to the south offshore of the srn CA coast into Tuesday allowing the upper ridge to sneak back in closer to the west coast. This will keep cooler conditions for central/srn coastal CA while above normal temperatures by at least 5-15 deg F remain for the rest of the region. QPF over the next 6 days is fairly minimal the result of some afternoon/evening convective showers/isolated thunderstorms today/tomorrow and then again Sunday/Monday. Totals are forecast at generally a tenth of an inch or less across the Sierra and parts of NV with locally higher amounts possible should thunderstorms develop. Freezing levels will continue to rise from west to east the rest of the week up to 9.5-12 kft this afternoon with most of the region above 10 kft by tomorrow afternoon. Levels then generally 10- 12.5 kft over the weekend rising to 11-13.5 kft early/mid next week. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS $$