April 29, 2017 – Dry weather through the middle of next week with temperatures climbing to well above normal. Mountain showers possible by the end of next week.
High amplitude upper ridging is present off the West Coast with dry northerly flow. Vis Sat revealed some patchy coastal valley fog this morning otherwise skies are clear across NorCal. Surface high pressure north and east of the CWA weakens slightly today as inverted surface troughing over Interior NorCal slightly fills. Result will be less wind today with sustained speeds generally below 12 kts in the Central Valley this afternoon. AMS warms today as upper ridge axis shifts inland. High temperatures this afternoon climb into the upper 70s to lower 80s in the Central Valley, about 5 to 7 degrees above normal.
Weak short wave trough pushes through the PacNW Sunday and digs into the Great Basin. Upper ridging flattens slightly and retrogrades, resulting in little change in max temps over Interior NorCal tomorrow. Upper ridging then amplifies Monday into Wednesday with 1000-500 mb thicknesses and 5H heights trending up. EPAC surface high pressure increases into Southern Oregon as thermal troughing deepens, resulting in some breezy northerly wind again Monday into Wednesday. High temperatures climb from the lower 80s Sunday to the upper 80s to lower 90s in the Central Valley Wednesday. Max temps in the foothills and mountains will range from the upper 50s near the Sierra crest to around 80 in warmest portions of the foothills.
Extended discussion (Wednesday through Saturday)
High pressure ridging peaks over the region Wednesday, with Valley highs 10 to 15 degrees above normal, reaching into the upper 80s to lower 90s for the Valley. Temperatures should begin to trend down late in the week, but how much is uncertain. A weak system brushes through late Thursday into Friday, with a a slight chance of showers over the mountains of Shasta and western Plumas/Lassen Park. Models diverge significantly by the weekend, with the Canadian and ECMWF have a fairly sharp trough moving through, while the GFS and GFS ensembles keep a broader, weaker trough further to the west.
For now, will keep precipitation confined to the Sierra. The range of possible temperatures is large, potentially very warm to below normal. For now, will keep a blend, leaning towards slightly above normal.