March 12, 2018 – The first in a series of storms this week begins to move into the region later today. Expect periods of valley rain and mountain snow to persist each day into next weekend.
Satellite imagery shows an area of high clouds that moved across the region overnight is now exiting to the east. Areas of low clouds remain along the foothills and west slopes of the northern Sierra, and some patches of stratus are working their way inland from the coast. Additional clouds will likely develop this morning as the airmass moistens in advance of the approaching system. Temperatures are milder across most of the area and RH’s are lower, so not expecting much valley fog.
Dry and relatively mild weather will continue across most of interior NorCal today, though warm-advection and increasing moisture may squeeze out some light precipitation over the mountains in the afternoon. Precipitation will increase tonight and Tuesday as the front moves through the area.
Snow levels are expected to remain high through tonight before they start to lower Tuesday and bring travel impacts to the major trans-Sierra passes by the afternoon. Have upgraded to a Winter Storm Warning from Tuesday afternoon through Wednesday due to the likelihood of significant travel impacts due to heavy snow.
Latest operational runs of the NAM continue to show a break in precipitation over the northern Sierra Wednesday afternoon, but the GFS shows no real break with the next system already moving in by Wednesday evening. So, depending on how these trends develop, we may need to extend the Winter Storm Warning thru later in the week.
Nevertheless, the pattern will be very favorable for more heavy snow across the northern Sierra into the weekend. GEFS plumes are indicating an average of 2 feet of snowfall for KBLU by 12Z Saturday, and some of the higher terrain may see 7 feet or more by the weekend. Anyone planning travel across the northern Sierra this week would be advised to do so by Tuesday morning.
The Central Valley will also see significant rain this week with 1 to 2 inches expected by the weekend.
Extended discussion (Friday through Monday)
Upper level trough will continue to bring precipitation across the region Friday, and possibly into Saturday. The 00z iteration of the GFS brings the upper low southward along the California coast, then inland over Southern CA. The ECMWF conversely brings the trough inland over Northern CA, which would be a wetter scenario. Regardless, snow levels are currently projected to generally remain between 3000-4000 ft during this timeframe.
Drier weather is expected for the 2nd half of the upcoming weekend, though a few showers will continue to be possible mainly over the mountains. Model differences arise early next week as the GFS brings a trough inland over the Pacific NW, then drops southward into the Great Basin in a slider solution. The ECMWF keeps the low offshore, but taps into subtropical moisture and would thus be a warmer and wetter solution than the GFS. Both solutions suggest that unsettled weather will continue through at least early next week.