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July 18, 2018 – Hot temperatures continue this week. Thunderstorm chances return to mainly the mountain areas late this week.

Discussion

A strong upper ridge remains centered over California this morning, bringing warm temperatures and mostly clear skies across the region. The marine layer remains widespread across the Bay Area coast this morning, with a strong breeze blowing through the Delta. This marine influence isn’t spreading very far inland this morning, as the thermally-induced trough remains situated over the Central Valley.

Temperatures across the region are running around to slightly warmer than 24 hours ago. Strong high pressure will continue to bring hot temperatures across the region today.

Highs this afternoon will likely be comparable or slightly warmer than yesterday: triple digit temperatures across the Valley, and locally up to 110 degrees in the northern Valley near Redding.

Excessive heat warnings and advisories remain in effect through this evening across the northern half of the Sac Valley and surrounding foothills.

The aforementioned thermally-induced trough will shift east of the Sierra on Thursday, allowing for a stronger onshore gradient, and deeper marine push into the Valley. This will allow for a bit of cooling in areas adjacent to the Delta Thursday, though the northern Valley will likely remain in the 105-110 range.

A bit more widespread cooling is expected Friday into Saturday, with temperatures dropping down to within a few degrees of normal. An easterly wave currently near the Gulf of California will begin to push northward over the upcoming days. Current projections keep this area of low pressure far enough offshore to limit the coverage of potential convection later this week.

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Given the increase in moisture it will likely bring however, and the usual diurnal forcing we see over the mountains, isolated thunderstorm activity will be possible over the high Sierra and Coastal Range Friday into Saturday.

Extended discussion (Sunday through Wednesday)

An upper level high pressure ridge over the southwest U.S. will bring continued warm dry weather through the extended period. Daytime highs will remain a few degrees above normal each day. Overnight lows will be a little above normal as well. Humidity will remain moderate to fairly low through the expended period especially in the northern Sacramento valley where afternoon minimum RH will drop into the teens. Light southeast flow aloft will continue to bring monsoon moisture up the Sierra and this will bring a slight threat of thunderstorms over the northern Sierra crest through the extended period.