December 13, 2016 – Wet weather returns to NorCal today with significant rainfall and mountain snow likely by Wednesday into Friday. Flood Watch from noon today through late Thursday night. Wind Advisory from noon Wednesday to noon Thursday. Winter Storm Watch from Thursday afternoon through Friday afternoon for West Slope Northern Sierra Nevada.
Discussion
Lots of cloud cover across interior NorCal early this morning, and radar is showing some weak returns indicative of thickening mid- level clouds. The cloud cover has limited the fog potential and kept temperatures generally in the 30s and 40s overnight.
The remainder of the week is shaping up to be rather active for NorCal with satellite imagery showing a continuous stream of clouds and deep moisture extending across the eastern Pacific from Hawaii to the West Coast. Some light precipitation will be possible over the region by this afternoon as moisture and warm- advection gradually increase.
It’s looking like the bulk of precipitation today and Wednesday should remain along and north of Interstate 80. Snow levels should initially start at around 4000-5000 feet today, but rise as an upper level ridge pushes the moisture plume northward.
Precipitation rates don’t look as high initially with this system (thru Wednesday) as the one from this past weekend, but creeks and streams are still running high. We’ve adjusted our winter weather and flood products to account for this farther north trend.
More significant impacts are expected across the region later Wednesday into Friday as the strongest portion of the storm affects the region. Precipitation rates could be rather impressive, especially over the mountains but over the Valley as well. Urban, small stream, and burn scar flooding could be a problem.
Snow levels will initially be 7000 feet or higher, but lower below pass levels as the cold front passes through. Significant snow will be possible on the back end of this storm.
Gusty south winds are also likely to develop with this storm as pressure gradients increase. With a low level jet currently progged to develop across Valley locations, wind gusts of 45-55 mph will be possible across the lower elevations and foothills. With these strong wind gusts and saturated grounds, downed trees and power outages will be possible.
Extended discussion (Saturday through Tuesday)
Upper level trough will depart the region on Saturday, giving way to ridging and dry weather that will rule into early next week. Northerly flow in the wake of the trough may lead to some breezy conditions over the weekend, especially across the coastal range and western side of the Valley. Models hinting at a weak wave by Tuesday or Wednesday of next week, but bulk of precip stays north of the area at this time. Slight warming trend expected into next week but daytime highs will still top out a few degrees below normal with chilly morning lows.