June 29, 2020 – Temperatures rise to around normal levels this week, with dry weather continuing. Breezy northerly wind today will gust to 20 to 30 mph in the Valley. This coupled with low humidity will bring fire weather danger into early evening.
Discussion
Upper level low pressure is moving eastward into the Rocky Mountains. Surface high pressure building into northern CA has increased the northerly surface pressure gradient between Medford and Sacramento to around 10 mb. Northerly winds have been gusting up to 20 to 30 mph overnight. This will increase fire danger within the portions of the Sacramento valley through early this evening, before winds subside.
A Delta breeze has brought a sharp demarcation between dry northerly winds over the northern, central and western Sacramento Valley and areas to the south and east. An example of this is Fairfield, which at 3 am had a relative humidity of 77 percent and southwest wind, while Vacaville had northerly winds gusting to 25 mph and a relative humidity of only 22 percent. So, good overnight humidity recovery is occurring in much of the Delta and Sacramento metro area and into the northern San Joaquin Valley. Some adjustments in area were made to the Red Flag Warning, removing some portions of the eastern side of the Sacramento Valley. The Red Flag Warning goes until 8 pm this evening.
Temperatures will warm to near normal this week, with Valley highs generally around 90-95. Weak northerly flow to begin the day on Tuesday will reverse and become southerly later in the afternoon. A trough of low pressure will rebuild itself across the coast on Tuesday night through Thursday, with increased onshore flow. Overnight lows and highs will drop a little below normal, especially in Delta Breeze influenced areas.
Extended Discussion (Friday through Monday)
Long wave trough centered off the West Coast will persist through the extended forecast period. Weak embedded short wave troughs move mainly through the PacNW with southern portions dragging across far northern portions of CA. Little impact expected for our CWA with dry weather but periods of increased onshore flow. High temperatures expected to be mostly a few degrees above seasonal normal.