September 5, 2019 – Isolated northern mountain thunderstorms this morning then over the Sierra this afternoon. Dry Friday and through the weekend. No significant weather through next week. A little above normal temperatures today and Friday then near to below normal over the weekend and next week.
Norcal remains under southerly flow aloft between high pressure over the four corners U.S. and a closed low off the north coast. A short wave disturbance pivoting northward in this southerly flow is currently bringing isolated showers and thunderstorms to the coast range and the northeast part of the state although most of this shower activity is outside the CWA at this time. A few rain gauges picked up some light precipitation to up to around 2 tenths of an inch. Considering precipitation and that most activity is outside the CWA, will drop Red Flag warning currently in place for the northern mountains. The shower activity is expected to move northward out of the forecast area later this morning. Attention will then shift to the Sierra Cascade range this afternoon where southerly flow and monsoon moisture will generate isolated thunderstorms. Highest moisture is still located above 700 mb so although rainfall is expected, heavy precipitation is not. The overall airmass cools slightly today as the main upper low pushes towards the north coast but daytime highs are still expected to come in a little above normal.
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The offshore upper low is forecast to open and shift into the Pacific Northwest tonight and Friday morning taking any precipitation chances with it. Airmass under brief ridging on Friday will continue daytime highs a little above normal for the end of the work week.
Another weaker Pacific trough is forecast to push through the Pacific Northwest and NorCal on Saturday and Sunday. At this time, models keep any precipitation associated with this trough to the north of the forecast area so CWA remains dry through the weekend. The main impact of this trough passage will be increased onshore flow and a fairly significant cooling of daytime highs over the weekend. Daytime highs Saturday and Sunday are forecast at a few to several degrees below normal for early September.
Extended Discussion (Monday through Thursday)
Mean trough forecast along the West Coast next week with temperatures near or slightly below average. Pattern expected to be dry except for low precipitation chances near the Oregon border.