Atmospheric River is expected to bring heavy rains across interior NorCal late this week

April 3, 2018 – Dry and warm through Wednesday. Wetter pattern returns later this week and into the weekend.

Discussion

Mostly clear skies across the region early this morning as yesterday’s trough shifts east through the northern Rockies. Current temperatures are mostly running around 4-8 degrees cooler compared to 24 hours ago and generally range from the 20s in the mountain valleys to the mid 40s to mid 50s across the Central Valley.

Mild temperatures will continue across the region today and Wednesday as weak upper ridging returns. Enough surface gradient remains for some north wind in the valley early today, but not as gusty as Monday. Flow is expected to begin to reverse to onshore this afternoon. Cloud cover will begin to increase again by tonight and Wednesday.

Little change to previous forecast for precip later this week. Expect light precip to spread over the region Thursday as initial surge of moisture and warm-advection spread up from the southwest.

Looks like heavier precipitation will develop later Friday into midday Saturday (currently looks like the southern half of the forecast area will be most at concern) when the front moves through coincident with the deepest available moisture and forcing. NAEFS continues to show 6+ positive anomaly for TPW and +8 to 9 positive anomaly for IVT during this period. Given the high snow levels (above pass levels) and increasingly favorable orographic enhancement for the west slopes of the northern Sierra Nevada, runoff concerns could develop by early Saturday. We’ll continue to monitor and fine tune the most at risk areas over the next couple of days.

Extended discussion (Saturday through Tuesday)

Atmospheric river’s impact continues over Interior NorCal Saturday with widespread moderate to heavy precipitation and strong wind. Will need to look into adding thunderstorms over the Central Valley and eastern foothills Saturday if models continue to show increased instability. Snow levels remain generally above pass levels through Saturday afternoon, then lower to 4500 to 5500 feet Saturday night into Sunday. Precip turns showery during this time frame and snow amounts at pass levels look fairly light with around 2 to 4 inches expected with local amount up to around a foot over highest terrain. Early QPF liquid estimates showing around 1.5 to 3 inches possible in the Central Valley through the event with 2 to 7 inches in the foothills and mountains. With significant amounts of rain falling over established snowpack, potential for heavy runoff and flooding concerns is increased.

Showers decrease Sunday as subsidence increases with dry weather Sunday night and Monday under upper ridging. Next Pacific frontal system brings some precip to western/northern portions of CWA Monday evening, spreading over more of the forecast area Tuesday.