January 20, 2021 – Dry weather with less windy conditions expected today and Thursday, then rain and snow chances return for the end of the week into next week.
Discussion
Clear skies and cooler temperatures are present across the region early this morning. Winds are much lighter than 24 hours ago as surface pressure gradients have relaxed and stronger winds aloft have decreased, but some areas with gusts in the 20s and 30s mph remain over the foothills and higher elevations.
Dry weather will continue today and Thursday with lighter winds. Highs will be a little cooler in the valley, but milder across higher terrain as cold advection wanes. Generally clear skies will allow for a better chance for valley frost tonight as dew points recover.
Over the course of the rest of the week, max temperatures will gradually cool through the 60s in the Valley, with temps returning to near normal by Fri with highs in the 50s in the Valley, 30s to 40s in the foothills and mtns. Min temps will cool into the 30s in the Valley, and teens and 20s in the mtns for the rest of the week.
Although there are model timing/detail issues later in the week, we are reasonably certain in the chances of rain/mountain snow on Fri in NorCal. There is enough model agreement that a seasonably cold upper trof will move into Norcal on Fri giving the CWA a chance of precip. The heaviest amounts are forecast over the Sierra (liquid amounts up to a half inch or so, and 3-6 inches or so of snow), but only a few hundredths to a tenth of an inch in the Valley. The system may linger into Sat, especially over the Sierra, but the Valley is likely to clear from N to S as the system drops Swd during the day. However, model differences create some uncertainty as to the exact timing, i.e. afternoon or evening. May see a thunderstorm or two late Friday over the southeast corner of the forecast area as daytime heating coincides with the upper trough passage.
Extended discussion (Sunday through Wednesday)
Long wave trough sets up along the West Coast allowing short wave troughs to drop into it resulting in unsettled weather through the extended forecast period. Below normal temperatures expected with snow levels in the foothills.