September 13, 2018 – Below normal temperatures expected for the remainder of the week. Patchy smoke and haze will continue mainly near the Delta Fire.
Discussion
A longwave trough continues to hover across the West Coast, bringing cooler than normal temperatures across the region. The band of high level cirrus associated with the jet stream has begun to weaken, allowing for clearer skies across the forecast area. Even with mostly sunny skies, mild temperatures are expected today and the next several days as a cool airmass remains in place. Look for Valley high temperatures to remain in the mid 70s to lower 80s, with 50s to 70s over the mountains. These forecast temperatures are 10-15 degrees cooler than normal. Winds in the mountains will be less strong today and Friday as flow becomes more southerly and the mid level jet decreases some. Another vort max passes through Saturday afternoon but with a lack of moisture it will have little effect on us. It will bring a return of breezy winds to the Mountains though as the 700 mb jet picks back up. Overnight lows will continue to be chilly into the weekend with some patchy frost possible in high mountain valleys.
Extended Discussion (Monday through Thursday)
Weak upper trough over the west coast will keep daytime temperatures below normal early next week. Models show this trough is too dry to produce precipitation so extended forecast starts out dry. After Tuesday, models diverge fairly significantly. GFS pushes upper trough inland on Wednesday with upper ridge building over the west coast by Thursday. ECMWF digs another shortwave into the west coast trough on Wednesday and Thursday and even paints a little precipitation over the far north state Friday afternoon. Run to Run consistency is better with the GFS so have kept with this model for now. Upper ridge depicted by GFS would bring warming Wednesday and Thursday with daytime highs climbing to a little above normal by the end of the extended period.