February 19, 2019 – Dry today with cold morning lows. Minimum temperatures near freezing in the Valley. A weak, cold system moves through Wednesday and Thursday with light mountain and foothill snow. Dry Friday with more light precipitation possible over the weekend.
Discussion
Dry northerly flow aloft again this morning between high pressure ridge over the eastern Pacific and a low pressure trough over the Great Basin. Therefore, the CWA is under fair skies this morning with light northerly winds most areas. Early morning temperatures are running similar to 24 hours ago except in areas where winds are lighter such as the northern Sacramento valley where temperatures are running several degrees lower than yesterday morning. Still expecting temperatures close to freezing in the central valley this morning so will leave frost advisory in place although winds are breezy enough that a hard freeze is unlikely. High pressure over the state will bring fair skies and a little warming today but cold airmass will keep daytime highs several degrees below normal.
A shortwave trough dropping out of the Gulf of Alaska and into the Pacific Northwest will bring a threat of light showers to the northern forecast area by Wednesday morning. This precipitation threat will spread southward across the forecast area during the day on Wednesday. Cool northerly flow will keep snow levels between 1500 and 2500 with snow levels dropping even more overnight Wednesday into Thursday. Precipitation chances shift to the Sierra Cascade range on Thursday as the upper low moves into the Great Basin. Orographics will bring the greatest amount of precipitation over the Northern Sierra but even here, snowfall totals should mainly fall short of snow advisory criteria most areas. Upper level and surface gradients will bring breezy north winds again Thursday with clearing and decreasing winds expected Friday as high pressure slips over the state.
Extended Discussion (Saturday through Tuesday)
Upper level ridging breaks down Saturday as a cold upper low pressure system drops out of the Gulf of Alaska/British Columbia and spreads into NorCal Saturday. The latest ECMWF is now trending towards bringing precipitation further south, spreading across the area by Saturday evening. Additional disturbances dropping down will continue precipitation through Sunday into early next week, mainly over the mountains. The current mainly overland track should limit moisture, so snow and rain amounts are not expected to be large. Snow levels with this cool system will are currently estimated at around 2000-3000 feet over the Sierra and as low as 1500 feet over Shasta County.