May 18, 2020 – Rain showers, mountain snow showers, and thunderstorms possible today into Tuesday. Accumulating snow will cause mountain travel difficulties. Scattered thunderstorms today with isolated foothill and mountain storms on Tuesday. Breezy wind expected in the Central Valley today with gusty winds and poor visibility over higher terrain. Some lingering mountain showers Wednesday, dry and warmer Thursday into the weekend.
Discussion
Upper level low just off the coast along the Oregon and California drops southward and moves inland into the Bay area later today. This system will bring widespread precipitation into interior Northern California. Moderate showers are spreading across the area early this morning, and will become locally heavy as thunderstorms develop later today, especially north of I-80. Snow levels have dropped below pass levels, and at 3:30 am had reached around 6500 feet, with wet snow falling at Kingvale and accumulating snow at Donner Pass. Winter Storm Warning is on track, with 3 to 6 inches of snow down to around 6000 to 6500 feet with a foot or more over the higher peaks.
Additional Valley rainfall will range from a quarter of an inch over the San Joaquin valley to over an inch for the north end of the Valley. The foothills should generally see another half inch to an inch. Locally, thunderstorms will likely bring higher amounts. A Flash Flood Watch remains in effect through 9 pm due to the potential of very heavy rainfall associated with thunderstorms.
CAMs models show strongest ones develop over the eastern end of the Valley and foothills from around Chico Chico northward. The storms over Shasta County will be slow moving with a convergence area developing. This will increase the chances for flooding. There is also the potential for debris flows in burn areas, both in Shasta and Butte counties. In addition to heavy rain, large amounts of small hail are a threat. Shear becomes more limited through the day and may limit the development of stronger rotating cells. Temperatures will remain cool, generally peaking in the 60s across the Valley.
As the low moves further south Tuesday the region will see less shower activity but some lingering chances will still remain, mainly over higher terrain. There is a slight chance of afternoon thunderstorms, mainly over the western slopes of the Sierra and foothills. Temperatures look a little warmer, reaching the low 70s in the Valley.
Wednesday a weak ridge will try to build over Northern California. Dry weather with warmer temperatures and mostly sunny skies can be expected. The exception is for some afternoon showers over the northern Sierra and southern Cascades.
Extended Discussion (Friday through Monday)
Short wave trough brushes across NE portions of CA Friday but models keep associated precip outside of the CWA. Upper ridging then builds in over the weekend persisting into Monday. This will result in dry weather with warming temperatures. Above normal highs expected Sat/Sun, rising to about 10 degrees above normal Monday.