Another wet and windy system will move into the region today bringing moderate to heavy rain, high elevation snow and renewed localized flooding concerns. Precipitation and strong winds end tonight and early Wednesday. A break in the weather is expected before precipitation chances return toward the end of the week.
Discussion
AR is beginning to affect NorCal early this morning as precipitation has spread across the region since midnight. So far, radar and gage data indicate that rainfall has been light, less than a tenth of an inch per hour. Snow levels are holding around 5.5-6.5k ft with light rain being reported at KBLU along I-80 in the Sierra. Surface pressure gradient is beginning to tighten a bit, but winds haven’t responded much yet with southeasterly gusts still only around 15-25 mph.
It’s looking more and more likely that strong wind will be the primary impact for much of the region today as the main area of precipitation moves through during the morning.
Conditions are expected to deteriorate rather quickly this morning as moderate to heavy precipitation spreads across the area.
Strong warm-advection will lift snow levels up above the higher passes by midday with heavy snow expected to mainly impact elevations above 8500 ft during the afternoon.
Winds will steadily increase during the morning as the deepening surface low approaches the northwest coast. The strongest winds are forecast this afternoon into early this evening as the surface low moves ashore south of Cape Mendocino (995 mbs HRRR forecast) and then continues deepening as it moves up into Shasta County by late afternoon (992 mbs).
The surface gradient is forecast to reach around 8 mbs from Sacramento to Redding, and another 8 mbs from Fresno to Sacramento.
Gusts of 50-60 mph will be possible in the valley from mid to late afternoon as lower level lapse rates steepen behind the precipitation allowing stronger winds to mix down.
Winds of this magnitude may lead to more extensive damage compared to some of the more recent storms. The low is forecast to move quickly to the northeast by this evening, so while it will remain windy for several hours into the evening, the strongest winds will likely be in the rearview mirror by sunset.
As far as precipitation impacts are concerned, the heavier rain is likely up in Shasta County and over the foothills and northern Sierra.
It’s looking like the north valley convergence zone may set up again later this afternoon and evening over Shasta County, and the primary area for thunderstorms looks to be across the Motherlode late in the day, so those two areas will need to be watched closely for enhanced risk of flooding.
Elsewhere, several hours of moderate to heavy rain this morning may lead to urban and small stream flooding across portions of the valley and foothills, and prolonged heavy rain and high snow levels over the Sierra may lead to renewed issues with runoff clogged by snow banks.
Precipitation will end this evening for most areas with some showers expected to linger over the Sierra into midday Wednesday. A period of dry weather is then forecast into Friday allowing several days of respite from precipitation.
Extended Discussion (Saturday through Tuesday)
Abbreviated extended discussion as the primary operational focus is on the present, high-impact system.
A weak, quick-hitting system will impact portions California late Friday and into Saturday. At this point, ensembles and clusters point to minor impacts with light precipitation. Some individual ensemble members remain dry. Potential for another system late weekend/early next week with minor/moderate strength.