Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 750 AM PDT Thu Apr 24 2025 ...COOLER AND WETTER CONDITIONS TODAY TODAY THRU SUN WITH T-STORMS POSSIBLE IN THE AFTN/EVES... ...GENERALLY WARMER/DRIER CONDITIONS EARLY NEXT WEEK AS HIGH PRESSURE RETURNS... .METEOROLOGICAL CONDITIONS (THU AM - WED AM)... Dry conditions across the region this morning as an elongated trough off of a low in the Gulf of Alaska heads our way. The southern portion of this system will break off and form its own closed low later today as it continues to approach the west coast. This will generate some showers over the Trinity mountains and up into the Cascades this afternoon/evening. Models show some instability over these same areas as well making for at least some chance of thunderstorms. Showers will spread along the north coast and across interior northern CA Friday morning as troughing from the low pushes inland. The core of the system will shift to the south offshore of the CA coast reaching the Bay Area in the afternoon. More widespread instability across the nrn CA coastal mountains, the OR border, over the Sierra, and nrn NV are predicted for Friday afternoon/evening resulting in better chances of thunderstorms in addition to showers. The low will continue to move south while inching closer to the coast the rest of Friday with the core of the low centered around the Big Sur coast Saturday morning. This will bring additional shower activity to northern CA while spreading it into central/southern coastal CA as well. By the afternoon, the low will cease its southerly heading and shift inland across central CA generating more widespread scattered showers across the region along with isolated thunderstorms. The low will cross into NV overnight heading northeastward into UT throughout Sunday morning/afternoon. Shower activity will pull away from the coast as the low travels inland becoming confined to the Sierra and eastward into NV by Sunday afternoon. Troughing from the low will hang over the region into Monday keeping shower chances over ne NV while an upper ridge builds in over the eastern Pacific behind the exiting system. A secondary trough will also break off from the back side of the exiting low over srn CA. This may generate a few additional showers on Monday across the srn Sierra. In addition to showers/thunderstorms, the low will bring cooler temperatures to the region through the weekend spreading from west to east starting today. Afternoon highs will drop to below normal with anomalies peaking on Saturday at -10 to -20 deg F. Temperatures will start to rebound on Sunday, though still below normal, rising from west to east early next week back to near/above normal. Models then disagree regarding another potential weaker system Tuesday into Wednesday. The ECMWF carries a small closed low into the PacNW on Tuesday while the GFS has a barely discernible shortwave around the same time. Both models take their respective systems into nrn CA overnight Tuesday dropping a few showers. The ECMWF solution would produce more QPF with the CMC on the wetter side of both. The CMC 24 hr QPF around SHasta ranges from 0.50-1" ending 12z Weds while the GFS has 0.10" or less and the ECMWF has about 0.10-0.50". The current forecast is on the drier side in line with WPC guidance and the 13z NBM. Overall, the QPF was a blend of WPC guidance and the 13z NBM. Blended in a bit of the det ECMWF/GFS for Saturday evening period over ne CA where models were in agreement on potential for locally higher amounts. The majority of the QPF for the period will fall over CA before the end of the weekend and over NV early next week. Highest amounts across the Sierra and higher terrain areas of nrn CA. Days 1-6 QPF: 0.50-1.50" Sierra/higher terrain of nrn CA, 0.25- 1" nrn/cntrl NV, 0.25-0.50" central/soCal coast from Big Sur to Ventura county, 0.10-0.30" for much of the rest of nrn/cntrl CA, and less than a tenth around the Bay Area up the I-80 corridor into Sacramento and over the rest of coastal srn CA. Freezing levels 7.5-11.5 kft today lowering tomorrow down to 5.5-9 kft from the coast to the eastern Sierra mid Friday morning. Lower freezing levels will spread inland on Saturday down to 4.5-7 kft for most of CA outside of far se CA. By Sunday morning, most of the region will be down to 5-7.5 kft. Freezing levels will then quickly rebound heading into next week up to 7-10 kft by Sunday night and 9- 11.5 kft by Monday afternoon. |