Chances of showers and thunderstorms in the mountains and south of I-80

May 9, 2019 – Showers and thunderstorms are possible today over the northern Sierra, potentially spreading into the northern San Joaquin Valley this evening. Chances of showers and thunderstorms continue into the weekend, mainly south of I80. A pattern change is currently projected for the middle of next week bringing cooler, potentially wetter weather across Northern California.

Discussion

Delta flow is quickly bringing low stratus clouds through Delta into Sacramento area. The Delta breeze at Travis AFB at 4 am was 27 mph, gusting to 32 mph and could strengthen a bit more later this morning. The low clouds should break up around mid-day.

A retrograding (westward moving) upper level low will bring the potential for showers and thunderstorms this afternoon over the Sierra. The HREF shows most convective activity projected in the Sierra south of I80. This will spread into the northern San Joaquin Valley and possibly as far north as the Sacramento metro area late this afternoon and into the evening. Some overnight showers are possible in those areas.

Accumulating snow is possible above 8000 feet tonight into Friday evening, with up to 4 to 8 inches over high peaks possible. High Sierra passes such as Ebbetts and Sonora passes could see some snow, but current Caltrans information is that these passes remain closed, so any travel impacts should be limited.

Higher pressure developing to the north will bring dry, northerly winds occurring over the northern Sacramento Valley. Maximum wind gusts could approach 40 mph at times from Thursday afternoon into the overnight hours. The Western Region ensemble table shows fairly impressive 850 to 700 mb northeasterly wind anomalies, up to 3 to 4 standard deviations in some locations. A Wind Advisory has been issued for the northern Sacramento Valley from 11 am this morning to 9 pm this evening. This downsloping flow will also keep relative humidity values quite low over these locations, bringing some fire weather concerns for areas with cured grasses.

Above normal temperatures are expected into the weekend, with Valley highs in the low 80s to around 90 over the northern Sacramento Valley, 5 to 10 degrees above average. The Sierra mainly south of I80 will see a lingering chance of showers and isolated thunderstorms.

Extended Discussion (Monday through Thursday)

Little change to the extended period as deterministic and ensemble guidance show a trough digging offshore. Starting Monday, we look to be on the tail end of ridge, which should bring us a day of dry weather.

The previously mentioned Pacific trough will continue to deepen offshore and move eastwards towards the west coast. As of now, have decided to main the increased cloud cover, precipitation, and decreasing high temperatures each day. Thunderstorms cannot be ruled out either; however, confidence is/was not high enough to add them into the forecast just yet.