Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 740 AM PDT Thu May 8 2025 ...MOSTLY DRY CONDITIONS AND ABOVE NORMAL TEMPERATURES THROUGH SATURDAY AS HIGH PRESSURE BUILDS OVERHEAD... ...A LARGE COLDER SYSTEM IS EXPECTED TO BRING PRECIP TO THE REGION LATE SUN THRU EARLY NEXT WEEK... .METEOROLOGICAL CONDITIONS (THU AM - WED AM)... Dry conditions across the region this morning as an upper ridge builds overhead. Behind this ridge, a large upper low is traveling across the Gulf of Alaska. This low will eventually make its way to CA to bring the next round of precip. High pressure will continue our warming trend today with above normal temperatures in the forecast by about 5 to 15 deg F. TPW imagery shows some lingering moisture over parts of CA (0.50-0.75") as a front continues to traverse the area. This may generate a shower/thunderstorm or two over the Sierra later today, but nothing significant is expected. The ridge will continue to build in overhead the rest of the week keeping dry conditions in place and allowing temperature anomalies to rise to +10 to +20 deg F for Friday and Saturday. A trough will develop off the south side of the Gulf low on Saturday headed towards the west coast. This will allow some initial cooling over the north coast for Saturday while as mentioned before the rest of CA/NV will see above normal temperatures. The core of the low will then shift to the south throughout Sunday sending showers into CA. The timing of precip has slowed down since yesterday, and models continue to disagree on the details of this system. Ensembles of 500 mb height patterns are divided as well on the timing and positioning of the low into early next week. For now, the majority of the precip for Sunday is forecast to stay north of Cape Mendocino aside from lighter showers reaching Sonoma County. Both the GFS and ECMWF show the low moving inland into the PacNW on Monday while troughing digs into nrn CA. Precip to spread into central CA throughout the day. Troughing will dig deeper into the region into Tuesday with showers possible over parts of srn CA. The trough should begin to exit into the Four Corners by the afternoon with showers lingering into early Wednesday (mainly over the Sierra and eastward). As the low/trough moves overhead, thunderstorms will also be possible. Ensemble spread regarding the QPF remains similar to yesterday with 24 hr QPF values at Arcata anywhere from 0" to nearly 2" between the GFS/ECMWF. There is decent agreement on a system impacting the area late this weekend into early next week, but low confidence on the timing and amounts of precip. All the QPF in the forecast, aside from a couple hundredths over the Sierra, is for the extended with the best day for precip likely to be Monday. Highest amounts over the northern/central Sierra and the Smith Basin. QPF 12z Sun-12z Weds: 0.50-1" north coast/nrn Sierra, 0.25-0.75" Shasta, 0.10-0.75" central Sierra, a few hundredths to 0.25" southern Sierra and the rest of nrn CA, and less than a tenth for coastal srn CA. In addition to precipitation, the system will lower temperatures from nw to se starting Sunday when much of the CA coast will drop to below normal. This trend will continue into Monday with widespread anomalies of -5 to -15 deg F. Cooler temperatures will persist for Tuesday as well. Freezing levels will drop from 10-13.5 kft or so this week to 5.5-10 kft north of I-80 Sunday afternoon. Lower freezing levels will spread across the region early next week down to 4.5-8 kft north of I-80 mid Monday morning, these lower levels will push as far south as Monterey County by the evening. Freezing levels to bottom out early Tuesday at 4.5-7.5 kft north of Point Conception before rebounding from sw to ne as the system exits into Wednesday. Please Note: This product may NOT be routinely updated this summer. Please refer to the following product issued by the CNRFC www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of weather and hydrologic conditions. |