December 13, 2018 – Dry weather today and tonight with areas of valley fog. Light rain spreading over the north state Friday with breezy south winds and light snow over the mountains. A moderate storm system moves through NorCal late Sunday into Monday with mountain travel impacts likely. Minimal weather impacts the remainder of the week.


Upper ridging over the west coast for dry conditions this morning with just some high cloudiness spilling through the ridge. light winds in the central and northern Sacramento valley precluding morning fog there but lighter winds from about Sacramento southward allowing for some areas of morning fog in the southern Sacramento and northern San Joaquin valleys. Upper ridge will keep precipitation out of the region through tonight but increasing cloud cover should keep fog to a minimum Friday morning.

Models fairly consistent on timing of next Pacific frontal passage through NorCal on late Friday and Friday night. The precipitable water plume associated with this system is not particularly moisture with PV values of about 3/4 inch predicted. Therefore, qpf values and snow amounts with this system are not very high and impacts associated with the system are minimal.

Most areas will see breezy south winds ahead of this system Friday afternoon with the highest winds over the mountains where gusts in the 30 and 40 mph are predicted.

Both ECMWF and GFS keep lingering showers over the CWA on Saturday as a upper shortwave and lingering moisture combine to produce isolated showers over at least the northern portions of the forecast area. Next, moister Pacific frontal system currently forecast to start pushing into Norcal mid day Sunday. Precipitable water values feeding this system forecast at well over an inch so this should be a much better precipitation producer.

Models still handling this system differently as it moves inland with GFS still splitting the system taking bulk of precipitation south of the forecast area while ECMWF keeps a more consolidated solution taking heaviest precip through the central part of the state. This difference makes precipitation quantity forecasting and even timing very tricky. For this morning, weighed heavily on WPC solution which seemed to use a middle ground approach. Snow levels below 7000 feet translate to at least several inches of snow over the passes Sunday through Monday so mountain impacts are likely. Which ever model scenario works out best, most precipitation should be moving into the Sierra by Monday morning.

Extended Discussion (Monday through Thursday)

A low pressure system will be passing over northern California early Monday morning bringing rain and mountain snow to the region. The bulk of the precipitation is forecast to fall between midnight and 9AM with showers expected for the afternoon. There is some uncertainty as to what will happen for the rest of the week as the ECMWF and the GFS vastly differ.

The ECMWF tries to push another system into northwest California, which would clip our most northern counties; however, the GFS supports ridging over the area. If the GFS pans out, we’`d likely see drier and warmer weather with the potential for some valley fog. Models then try to come back into agreement for Thursday/Friday with both having a weak system advance towards the region. The GFS seems to be faster and lighter while the ECMWF is slower and brings the precip in on Friday.