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October 3, 2018 – Unsettled and cool weather pattern will continue through Thursday. Isolated thunderstorms likely today through this evening. Breezy north winds and lowering humidity will bring increased fire concerns over the weekend.
Upper low currently centered several hundred miles off the SoCal coast with mainly southerly flow aloft over the forecast area. A small scale disturbance pivoting northward in this southerly flow is currently bringing scattered showers over the southern portion of the CWA and over western Colusa county/eastern Lake counties where isolated thunderstorms are also occurring. Upper level diffluent flow will continue over NorCal today as the upper low edges towards the SoCal coast. Stability progs indicating enough instability for a threat of thunderstorms nearly all areas of the CWA this afternoon.
QPF is relatively light today but isolated thunderstorms could bring brief periods of heavier precipitation so will have to watch for convective development directly over burn scars for possible debris flow issues. Cooler airmass and increased cloud cover will bring a drop in daytime highs down to several degrees below normal for this time of year.
Upper trough shifts into SoCal tonight and then on into the southern Great Basin on Thursday so by Thursday afternoon precipitation threat should shift to mainly the east side of the forecast area. Conditions look more stable on Thursday so thunderstorm threat should be minimal. Daytime temperatures should cool slightly more on Thursday under a cooling airmass.
The first half of Friday looks dry at this time as northwest flow aloft and a shortwave ridge sweep across the north state. A shortwave trough dropping out of the Gulf of Alaska in this northwest flow will bring the next chance of precipitation to the northern portions of the forecast area Friday afternoon. A slight threat of showers remains in the forecast over the Northern Sierra Crest on Saturday as this next upper level system moves east of the region.
Behind this system, northerly flow develops. Conditions look quite breezy as upper level and surface pressure gradients combine to bring increased wind speeds. The northerly flow will also bring drying so wind/low RH combination over the weekend will bring increased fire concerns.
Extended Discussion (Sunday through Wednesday)
Windy Sunday. Pretty significant pattern amplification occuring this weekend with a large trough digging into western US. NAEFS ensemble data suggesting some 10 year return interval north winds setting up Saturday night into Sunday as strong and dry northerly flow sets up. 925 mb winds around 40 kts and MFR-SAC pressure gradient around 12 mbs. At this time a wind advisory looks like a good bet and have started to message this already. Winds slacken early next week ahead of the next trough forecast to move in by the middle of the week.