April 10, 2017 – Weak weather systems will move across the area through Wednesday. Stronger storm system possible Wednesday night and Thursday.
Discussion
Wet weather pattern expected to continue through much of the week as NorCal climbs up the list of wettest water years. 8 station index less than one inch away from the wettest in 1982-83 (since 1922) and DTS now 7th all time (since 1849-50) and 4th (since NWS records began in 1877.
First system of the week moves through the region today with lingering light showers over the coastal range and Northern zones and Northern Sierra ending this afternoon. The upper level trof has already passed to the N but the surface boundary lags behind, and at press time, was over the coastal range. IR shows some lee-side enhanced clouds over Glenn/Colusa counties as the mid level flow is perpendicular to the coastal range. This is likely to end by midday as heights rise slightly over NorCal in the wake of the short wave passing to the N and the flow turns NWly.
The second system is forecast to bring another round of precip to the area beginning on Tue and lingering into Wed. IR indicates cloud top cooling around 35N/140W ahead of a well defined vort max along 150W. Snow levels will be rising as the TPW plume makes landfall Tue/Wed. The trajectory of the moisture suggests the Shasta Drainage will be the most impacted with over an inch of precip with system #2. A period of gusty winds could reach Wind Advisory criteria in the Northern Sacramento Valley/NE foothills zones Tue nite into Wed morning.
System #3, Wed nite through Thu, looks to the strongest/wettest as a broad and deeper 5H trof develops over the Eastern Pacific on Wed and swings eastward across NorCal on Thursday. Another surge of higher TPW will work in behind the previous (Tue/Wed) plume and remain focused over NorCal as it interacts with the stronger/deeper trof. This will augment QPFs in relation to the previous systems with at least 2 to 3 inch amounts over the mountains, and around an inch in the Northern Sacramento Valley tapering to around .25 inch in the Nrn SJV. Mid tropospheric 5H temps in the minus mid to upper 20s Deg C range should steepen lapse rates and increase the potential for low-topped thunderstorms and primarily hail producers. This could lead to locally higher QPFs as well.
Snow levels should start out in the 6500-7000 ft range on Wed along the 80/50 corridor before lowering to around 5000 ft on Thu. The bulk of the QPF/Snow is expected to occur during the Wed nite period. Although the Lassen NP area is expected to see about a half of a foot of snow with system #2, the potential for 2 feet or so is there for system #3, and up to 8 to 12 inches for the 80/50 corridor. This multiple wx threat for system #3 for the middle of the week looks to be the most impactful.
Extended discussion (Friday through Monday)
Dry weather expected Friday as upper trough shifts northeast out of the area. Diminishing clouds and slightly warmer temperatures are expected, along with a shift to northerly winds. Further warming occurs on Saturday as a weak upper ridge develops, with high temperatures rising to around normal levels.
The potential is increasing for a return to wet weather Sunday into Monday. Models are in better agreement than previously for an upper trough moving into NorCal, with precipitation arriving over the area at some point during the day Sunday, and continuing through Monday. Exact precipitation timing, amounts and coverage are uncertain at this point.