September 26, 2019 – Cooler weather begins to return to the region today. Much cooler weather along with a chance of showers and high elevation snow over the weekend into early next week.
Clear skies cover the region early this morning. Northerly surface gradients remain tight (nearly 10 mbs MFR-SAC and 8 mbs RNO-SAC), but are slowly trending lower. Still a few gusts in the 25-40 mph range across the foothills and mountains, but not as strong as 24 hours ago.
Current temperatures are mostly a little cooler compared to 24 hours ago, but remain very mild ranging from the lower 40s in the mountain valleys to around 80 across the warmest portions of the Central Valley.
Ridge will flatten across the area today resulting in weakening offshore flow this morning and a return of the Delta Breeze later this afternoon. Temperatures will cool around 10-15 degrees compared to Wednesday across the Delta and Sacramento region with less cooling expected at the north end of the Sacramento Valley and in the mountains.
A pattern change takes hold by this weekend. Cooler temperatures are on tap as a low, originating from western Canada, moves into our area.
Shower activity is expected Saturday into early next week, mainly over the mountains with potential for some early season snowfall accumulation across the higher elevations.
Extended Discussion (Monday through Thursday)
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An upper low pressure system is forecast to be over the northern Great Basin by Monday but shortwaves rotating around the back side of the low will bring a threat of showers to the northern mountains and Sierra Cascade range.
At this time, conditions over NorCal look stable enough to preclude a risk of thunderstorms. Cool airmass over the region will keep daytime highs well below normal. The upper low shifts eastward Monday night and Tuesday bringing an end to any shower threat.
Surface high pressure pushing in behind the retreating low will bring breezy north winds Monday night and Tuesday mainly down the Sacramento valley. This more breezy condition should be fairly short-lived with models continuing to track the upper low eastward. Temperatures warm on Tuesday and Wednesday as high pressure slides over the region but remain a few to several degrees below normal.
Another Pacific trough is forecast to approach the coast around Thursday of next week. This will bring some cooling to the northern zones and bring a threat of showers to parts of the CWA around the end of next week.