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July 19, 2018 – Temperatures will gradually decrease late this week, with the potential for afternoon thunderstorms in the mountains. Hot weather returns next week.

Heat Advisory until 8 PM PDT this evening for Northern Sacramento Valley.

Discussion

Strong upper level ridge continues to bring fair weather across interior Northern California this morning. A few high level clouds have started to move across the region as this upper ridge slides eastward.

Extensive marine stratus continues along the California coast, and even though a good onshore gradient exists through the Delta, these cooling effects have not been felt across the interior yet. Temperatures as of this writing are still in the upper 60s to 80s with strong thermal belts in the Sierra foothills.

Another hot one is in store across much of the forecast area today. The thermally-induced trough, which was situated over the Central Valley the last couple of days, will shift east into the Great Basin today and allow stronger onshore flow to reach the interior later this afternoon and evening. These cooling effects will likely be limited to areas adjacent to the Delta today, whereas the rest of the Valley will still be in triple-digit territory.

The northern Sac Valley near Redding in particular could once again approach 110 degrees this afternoon. A Heat Advisory will remain in place into this evening for that local area.

A weak upper disturbance currently off the Baja California coast will gradually move northward the next few days. While it is projected to remain off the CA coast, it will help to draw subtropical moisture northward. Expect a modest increase in cloud cover Friday into Saturday, with perhaps a few afternoon thunderstorms over the mountains. There appears to be enough elevated instability for isolated thunderstorms even across lower elevations, but forcing is weak at best.

A healthy Delta Breeze will continue into the weekend, eventually bringing more widespread cooling across the interior. High temperatures across the Valley will be in the 90s to lower 100s, with 80s to 90s over the mountains. These weekend forecast high temperatures will be within a few degrees of normal.

Extended discussion (Monday through Thursday)

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An upper level high pressure ridge over the westerly U.S. will dominate the weather pattern throughout the extended period. Daytime high temperatures will remain several degrees above normal each day with only minor variations depending on the location of the ridge axis which models differ on slightly. Main impact of these differences will be location of any afternoon and evening thunderstorm activity. GFS has a slightly more west position on the upper ridge axis than does the ECMWF.

A more south or southeast flow aloft modeled by the GFS would bring a better threat of afternoon convection. The ECMWF solution of a more southwest flow aloft would be more conducive to a quieter more stable solution.

Have compromised for now by including a slight threat of afternoon or evening thunderstorms over the northern Sierra and leaving remainder of the forecast area clear. Both GFS and ECMWF have upper ridge east enough by the end of the week to keep any thunderstorm activity east and south of the forecast area. In short for most areas, look for hot and dry pattern to continue through the extended period.