June 19, 2019 – A slow cooling trend expected through the end of the week as a low pressure system drops through the Pacific Northwest. Daytime temperatures down to near or below normal Thursday and Friday. Breezy north winds behind the departing low pressure system will bring elevated fire weather concerns at the end of the week. Warmer that normal over the weekend with cooling again early next week.
Upper level high pressure ridge over the eastern Pacific and west coast bringing fair skies to the CWA this morning. A fairly solid Delta breeze is now in place so may see some scattered coastal clouds make it into the delta this morning. Overall airmass will cool a bit today as a low pressure system drops into the Pacific Northwest.
Daytime highs today will drop a few to several degrees but still remain above normal for this time of year. Stable conditions under the ridge should keep mountain cloud cover to a minimum this afternoon. Airmass cools quite a bit more on Thursday as the upper low digs farther southward into the northern Great Basin. Daytime highs Thursday are forecast to drop to around normal.
By the end of the day Thursday, surface high pressure is forecast to nose into Oregon bringing a moderately tight northerly gradient and breezy north winds to the northern forecast area. Moderately low humidity will mean elevated fire concerns. Breezy north winds will continue through Saturday morning especially Friday and Friday night as good upper level support develops.
The main upper trough is forecast to be over the Great Basin on Friday but models hint at a final shortwave digging southward somewhere down the western Great Basin in the afternoon. GFS carves this shortwave farthest westward and paints some showers over the northern Sierra Friday afternoon. Other models are weaker with this feature. Have left showers out of Sierra forecast for now but have included ghost pops should later models become more coordinated with a deeper shortwave.
Overall airmass cools enough by Friday afternoon that daytime highs are currently forecast to come in a little below normal. High pressure pushes back in on Saturday warming temperatures back up above normal and bringing lighter surface gradients for lighter north winds.
Extended Discussion (Sunday through Wednesday)
Upper ridging over NorCal Sunday breaks down, then large upper low from GOA digs slowly southeast Monday into Wednesday. This will result in increasing synoptic cooling and Delta breeze as heights/thicknesses lower. High temperatures go from about 5 degrees above normal Sunday to about 5 degrees below normal by Wednesday.
Not expecting precip with deepening troughing next week but should see a deepening marine layer and increasing onshore flow.