March 22, 2018 – Wet weather continues into the weekend. Snow levels will begin lowering today with significant accumulations at the higher elevations into the weekend. A Winter Storm Warning has been issued for the Sierra above 6500 feet late tonight through Thursday 11PM. Localized amounts up to 20 inches possible. Additional accumulations up to 3 feet possible Fri-Sat.
Discussion
Widespread light to moderate rain has overspread much of interior NorCal overnight as moisture transport has increased from the south in the warm sector. A few lines of heavier showers along with a few thunderstorms have been embedded in the rain, especially to the north of Sacramento. Radar has been indicating some impressive rain rates in these lines, but so far they’re moving enough to limit flooding potential. Snow levels remain very high.
It promises to be another active day of weather across the region today. The large area of rain is expected to slowly edge east out of the valley this morning as the cold front approaches, though orographics will keep precipitation going in the foothills and northern Sierra. Snow levels are expected to begin heading down this morning and impacts are likely to increase across the passes.
Thunderstorm potential is expected to increase across the area by late morning. Forecast soundings indicate greater instability than Wednesday as much of the valley may see a few hours of surface heating in the gap ahead of the cold front. Shear is not as impressive, but still sufficient for a non-zero tornado threat with any longer-lived cells (Chico area late-morning to early afternoon, and further south through the afternoon). Bigger concern will be for storms capable of producing copious amounts of small hail.
A brief break in the wet weather looks likely tonight and Friday morning before a colder system brings potentially heavier and lower snow by Friday night. Will continue winter storm watch with potential for 1-2 feet of snow possible in the northern Sierra by Sunday. Timing and amounts with this system could still change.
Extended discussion (Monday through Thursday)
Building high pressure will bring an end to any precipitation threat by Monday morning. Clearing skies and warming airmass under the ridge will bring warmer temperatures but highs are still forecast to run slightly below normal. Upper level and surface gradients indicate breezy north winds which will help to start drying things out. Temperatures continue to warm on Tuesday as upper ridge pushes inland with highs expected to warm to slightly above normal. Pressure gradients weaken slightly but breezy north winds are still expected. Even more warming is expected on Wednesday under light north wind conditions with highs throughout the valley expected to reach the upper 60s to lower 70s or several degrees above normal. Daytime highs Thursday reach to between 5 and 10 degrees above normal as high pressure settles in over the region with lighter north breezes continuing.