June 25, 2019 – Daytime temperatures cooling over the next few days with highs on Thursday 10 or more degrees below normal. A slight chance of showers northern Shasta mountains Thursday, otherwise dry. A warming trend begins Friday with highs climbing to above normal by next Sunday. Occasionally breezy winds through the Delta and over the higher terrain mainly next few days.
Discussion
Northern California currently under southwest flow aloft between a high pressure ridge over the Great Basin and a unseasonably cool low pressure system off the Pacific Northwest coast. Small shortwave troughs rotating through this southwest flow bringing scattered high cloudiness to the forecast area but for the most part skies are fair.
A moderate delta breeze continues but coastal stratus is minimal so should not see much in the way of coastal stratus coming into the valley this morning.
Onshore flow and cooling airmass will bring cooler temperatures today. Daytime highs are expected to drop to around normal. Some areas will see increased winds ahead of the low over the next few days especially the delta and over higher Sierra Cascade elevations.
The upper low is forecast to move towards the Oregon coast on Wednesday bringing more cooling over the forecast area. Daytime highs will drop to well below normal and marine layer is likely to deepen. There may be some morning coast stratus intrusion into the delta but any precipitation should remain north and west of the forecast area.
By Thursday, the upper low begins moving into the Pacific Northwest. Daytime highs should drop to the coolest of the week and 10 to 15 degrees below normal. Although precipitation should continue to remain mainly north of the forecast area, a few showers may reach as far southward as the northern Shasta county mountains. A still deeper marine layer may bring morning coastal stratus into the delta again during the morning hours.
Models are consistent in lifting the upper low to the northeast on Friday. This should bring lighter winds and allow temperatures to warm a few degrees over Thursday although daytime highs will remain a few to several degrees below normal.
Extended Discussion (Saturday through Tuesday)
A gradual warming expected this weekend into early next week. Troughing may remain near the West Coast, but confidence is low given latest deterministic and ensemble differences. Therefore, temperatures might not be as warm as previously anticipated. Forecast highs will remain below normal through Sunday, followed by near normal temperatures early next week. By early next week, look for Valley highs in the 90s, and 60s to low 80s over the mountains.