November 15, 2017 – Pacific storm brings rain, wind and mountain snow, heavy at times, today into Thursday night. Dry Friday and Saturday, then wet weather returns Sunday into early next week.
Discussion
Cold low off VRISL with associated baroclinic zone and upper troughing digging slowly along the West Coast. Meanwhile moisture plume to the south with embedded PW core around 1.5 inches and weak subtropical wave are approaching CA. Mosaic 88D showing some light rain falling over portions of the Bay Area attm and beginning to spread into the Central Valley.
Precip will increase over the CWA during the morning hours as AMS moistens. Moderate to strong mid level southwesterly flow is already occurring, and combined with deepening moisture will also initiate precip over the Sierra Nevada and Western Plumas mountains during the morning hours. Precip then increases this afternoon as PWS continue to trend up and deepening surface wave approaches the NW CA coast. Snow levels will rise above pass levels by this afternoon into evening as WAA increases over Interior NorCal.
Surface low weakens and moves across far NW CA this evening. Baroclinic zone with colder system to the north then begins to invade Interior NorCal as associated polar jet merges with subtropical jet. Models showing precip ramping up tonight, especially over the Sierra and Western Plumas mountains with periods of heavy precip likely.
Snow levels will rise above 8000 feet overnight resulting in heavy rain for most mountain areas. Will need to monitor rain rates over burn scar areas for potential debris flows.
Periods of heavy precip and gusty wind continue Thursday along and ahead of the front as it drops south through the CWA. Post frontal showers will be possible with a threat of isolated afternoon thunderstorms in the Central Valley, from about Sacramento northward, and along the eastern foothills. Precip in the Sierra winds down Thursday evening with lingering showers possible into early Friday morning. Storm total QPF attm showing around 1 to 2+ inches for the Central Valley, and 1 to 7 inches for the foothills and mountains. Several feet of snow possible at the highest elevations by Friday morning.
Drier weather on tap Friday into Saturday as upper ridging builds over Interior NorCal. Some patchy valley fog possible Friday morning becoming more widespread and locally dense Saturday morning.
Extended discussion (Sunday through Wednesday)
Models are coming into better agreement regarding the potential for another storm system late Sunday into early next week. However, there are still model differences in terms of timing and precipitation amounts, especially with the GFS showing lower precipitation amounts and the storm pushing eastward fairly quick. Therefore, confidence in terms of storm impacts remains low. Similarly to previous days, storm arrival has slowed down so have adjusted PoPs accordingly.
After brief drying Monday afternoon into early Tuesday, the pattern could remain active as guidance indicates another system approaching the area by mid-week. At this point, the ECMWF is the fastest and most aggressive solution compared to the GFS, which is slower and keeps the area generally dry. For now, have followed the ECMWF approach but things will still need to be monitored this far out.