October 5, 2017 – A warming trend through the end of the week with daytime highs climbing to a few degrees above normal Friday and Saturday. Dry and a little cooler with breezy north winds early next week for increased fire danger.
Discussion
Northern California currently under upper level northwest flow between a low pressure system over the northern Rockies and a high pressure ridge over the eastern Pacific. Upper level flow and a light northerly surface gradient will bring light north winds today. These winds and a warming airmass will bring a warming trend today with daytime highs expected to come in right around normal for this time of year.
Light north winds and a continued warming trend are expected on Friday as high pressure aloft pushes inland over the southwest U.S. Increasing high temperatures will push up to about 5 degrees above normal.
Saturday closes out the week very similar to Friday with above normal temperatures and relatively light northerly winds. An upper trough digging into the northern Great Basin Sunday will bring cooler temperatures especially to the northern zones.
The upper trough axis is forecast to shift east of interior northern California by Sunday morning allowing surface high pressure to nose into the Pacific northwest. Models show a fairly steep northerly surface gradient developing by mid day Sunday.
By Sunday afternoon sustained north winds down the Sacramento valley are forecast to increase into the teens. These drying north winds will bring afternoon RH values down into the lower teens. Strong north winds and low humidity will bring increased fire danger with these windy dry conditions continuing into early next week.
Extended discussion (Monday through Thursday)
Area will be sandwiched between high pressure off the West Coast in the Pacific and troughing digging from the Great Basin into the Rockies on Monday. Dry weather will rule with strong northerly flow.
Breezy winds will continue through Monday night with low daytime humidity and poor to moderate overnight recovery. This will likely lead to another day of critical fire weather conditions.
Temperatures will cool several degrees from Sunday to Monday with near normal highs expected. Pressure gradient will relax Tuesday leading to lighter winds and temperatures similar to Monday.
By Wednesday, model differences remain. Both the GFS and ECMWF show a trough approaching the Pacific NW but latest ECMWF way more aggressive with digging the wave southward while GFS shows more zonal flow. Regardless, still expecting dry weather with temperatures near normal at this point.