####018004669#### AGUS76 KRSA 211437 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 735 AM PDT Sun Apr 21 2024 ...GENERALLY DRY CONDITIONS WITH ABOVE NORMAL TEMPERATURES THRU MONDAY BEFORE GRADUAL COOLING BEGINS TUESDAY... ...PRECIP CHANCES RETURN MID TO LATE WEEK, BEST CHANCES OF MORE WIDESPREAD PRECIP THURS/FRI... .METEOROLOGICAL CONDITIONS (SUN AM - SAT AM)... Dry conditions and above normal temperatures will continue through Monday as high pressure builds over the eastern Pacific. Afternoon high temperatures are expected to reach 5-15+ deg F above seasonal normals across much of the region. Gradual cooling will then take place starting Tuesday as an upper low approaches CA from the west. Temperatures will ease from west to east throughout the week starting at the coast Tuesday. Additionally, a weak surface low setting up off the nrn CA coast may result in some scattered showers over nrn CA/NV throughout the day with chances of thunderstorms in the afternoon/evening. Regarding the mid-week low, models have come into better agreement on the timing of this system. The GFS has slowed down the arrival of the low which is much closer to the ECMWF than it has been the past several days. Both models have the low shifting to the south into Wednesday approaching coastal soCal, but the GFS is still slightly faster on the core of the system reaching land compared to the ECMWF (Weds aftn vs Weds eve). Either way, expecting the low to bring some showers to parts of the central/southern Sierra and eastward through central/ne NV Weds/Weds night. Some light showers may also develop near the core of the low as it passes through soCal. By early Thursday, the low is expected to begin exiting CA into western AZ. Precip amounts through 12z Thurs are not expected to be significant, with the highest amounts about 0.25-0.50" on Wednesday from the central/southern Sierra into ne NV. Only forecasting about 0.10" or less for the rest of nrn CA/nw NV in the near term. Later Thursday, another system will drop in from the Gulf of Alaska as a surface/upper low. There are differences between the models on this system as well. The GFS has the surface low further to the north mid Thursday morning just offshore of BC while the ECMWF has it just west of the WA/OR border. This is resulting in the ECMWF predicting showers further across CA into the Bay Area and central Sierra while the GFS confines precip mostly to northern CA/NV. This also means the positions of the upper level system are different in similar respects. The ECMWF has an upper trough stretched across the PacNW and through much of CA/NV Thursday evening while the GFS shows a shallower trough. The GFS is also further westward with the trough axis on Friday with the core of the system still offshore while the ECMWF has the system fully enveloping the region. Models come closer together by Friday evening both showing an upper low and elongated trough encompassing the entire western U.S. spreading precip to much of the region. To illustrate what these differences mean in terms of QPF, the ECMWF is predicting 1-1.50" along the north coast 12z Thurs- 12z Fri and 0.75-1.25" over the northern Sierra while the GFS has 0.25-0.50" and <0.10" respectively. To summarize, dry conditions with above normal temperatures through Monday before gradual cooling and a transition to a wetter pattern begin the rest of the week. Still some uncertainty on the details due to model differences. The majority of the QPF for this week is expected on Thursday/Friday with that second and larger system. Highest precip amounts over nw CA, the srn OR Cascades, and over the nrn/cntrl Sierra. QPF 12z Thurs-12z Sat: 0.50-1" north coast (up to 1.50" Smith Basin), 0.25-0.80" srn OR Cascades/nrn/cntrl Sierra, 1- 2" eastern NV mountains, 0.10-0.50" rest of nrn CA/NV and a few hundredths to 0.30" down the valleys and around the Bay Area. Freezing levels 10-13 kft from N to S the rest of the weekend and early next week. Freezing levels to then lower mid week down to 8- 10.5 kft then 6.5-10.5 kft by Thursday reaching 4.5-6.5 kft north of I-80 and 6.5-11.5 kft to the south Friday morning. Lower freezing levels will continue to spread across the region into the weekend down to 4.5-6.5 kft north of Point Conception across CA and 6-7.5 kft generally elsewhere. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS $$