September 27, 2018
Summary: Temperatures this afternoon on the valley floor have generally warmed into the mid to, in some cases, upper 90s. Tomorrow will again be hot by the standards of late September, but expect at least a few degrees of cooling as that sharp ridge of upper level high pressure begins to shift slowly eastward in response to a closed low located several hundred miles west of the Golden Gate. This low will begin to make its move tonight and Friday and will move inland through northern California Saturday. On Friday, the marine layer which is currently 1,700 feet deep will rapidly deepen and begin to surge through the Delta and other gaps and passes along the Coast Range. This will cause temperatures to lower down into the mid 80s. the low will be a dry one for central California but will usher in the first phase of autumn type weather.
We are still monitoring with great interest a second low moving out of the Gulf of Alaska. This low will eventually move into northern and central California. Models are still holding onto the idea of scattered showers over most of northern California and possibly parts of central California, as well, mainly from Fresno County north.
All eyes, though, are looking to the south as Hurricane Rosa continues to intensify southwest of Cabo San Lucas. This storm is expected to turn into a major hurricane late tonight and Friday. Already this afternoon, a well defined and large eye wall has developed. Modeling information has not changed much on Rosa’s predicted path and models show it weakening rapidly as it approaches the northern Baja, California coast. On its current trajectory, aided by southwesterly winds aloft, the remnants of this storm would be spread into southern California and Arizona where flooding has to be concerned. It still remains unclear exactly where the northern limit of Rosa’s moisture is going to end up. I would not be doing my job well if I didn’t consider moisture being tugged northward by a low.
The risk of showers and thunderstorms will exist over the entire area, especially Monday night through early Wednesday. For now, the amount of potential precipitation is unclear…or even whether it will rain, at all. It’s possible the northern low will affect mainly northern California with Rosa’s moisture mainly impacting southern California and Arizona, leaving the valley high and dry. Currently, a 30% chance of rain looks to be the best estimate at this time.
Temperatures, as you might expect with heavy cloud cover moving in Tuesday and Wednesday, will plummet as colder air moves in aloft and a cool marine influence spills through the Coast Range.
The chance of measurable rain should be over by Wednesday afternoon with dry weather thereafter. However, even after Wednesday, models are maintaining low pressure over the west and off shore. This pattern would result in dry weather, but temperatures would be subnormal for early October.
Forecast: Other than occasional periods of high clouds, it will be mostly clear through Sunday. Partly cloudy Sunday night. Increasing cloudiness Monday leading to a chance of showers and possibly a few thunderstorms Monday night through early Wednesday and turning much cooler. Partly cloudy Wednesday afternoon. Becoming mostly clear Wednesday night through Thursday. Expect winds to pick up later Friday afternoon and Friday night to 10 to 20 MPH with local gusts to 35 MPH with possible local areas of blowing dust along the west side.
Short Term: Lows tonight will cool into the mid 50s to the lower 60s north of Kern County and in the low to mid 60s in Kern County. Highs Friday will warm into the low to mid 90s, falling into the upper 50s to the lower 60s Friday night. Temperatures will be in the mid 80s Saturday.
Next report: Friday morning, September 28