July 20, 2018 – Temperatures will gradually decrease the remainder of the week with the potential for thunderstorms in the mountains. Hot weather returns next week.


Clear skies across interior NorCal early this morning. Current temperatures range from the upper 40s to mid 50s in the mountain valleys to the mid 60s to mid 70s across the Central Valley (except lower to mid 80s across the north end of the Sacramento Valley). Strong upper ridging of the past several days will weaken a bit through the end of the week as an upper low traverses southern BC with a weak trough trailing back into NorCal.

The combination of a little stronger onshore flow along with slight synoptic cooling will result in a little cooler temperatures across the region over the next few days. Additionally, an increase in monsoon moisture will bring greater cloud cover helping to edge temperatures down a bit as well.

With the return of monsoon moisture spreading further north, thunderstorm activity will become more widespread across interior NorCal through the weekend. Most of the action is expected to be diurnal in nature and mainly over the mountains, but we can’t completely rule out a slim chance for a few storms occurring elsewhere tonight and early Saturday as moisture spreads north.

The ridge is forecast to reassert itself early next week resulting in a return to hot weather and capping the extent of mountain thunderstorm activity.

Extended discussion (Tuesday through Friday)

An expansive upper level ridge will bring hot weather across the region next week. By even July standards, this upper ridge will likely be rather anomalous, with NAEFS (North American Ensemble Forecast System) guidance projecting 500mb heights to be 3-4 standard deviations higher than normal over AZ/NM. We will be on the northern fringe of this upper ridge, but with high confidence in the pattern we nudged temperatures up a few degrees across much of the extended period.

Current consensus of solutions has widespread triple digit heat across much of the Central Valley through the extended period, with temperatures potentially approaching 110 degrees again across the northern Valley.