Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

155 pm PDT Wed Oct 21 2020

Synopsis

There will be a cooling trend the next few days with night and morning coastal low clouds and fog spreading farther into the valleys. Light rain or drizzle is possible from the coast to the mountains this weekend when a weak low pressure system moves across the area. A colder low pressure system from the north will bring higher chances for precipitation Sunday and Monday. A period of moderate strength Santa Ana winds will follow late Monday into Tuesday as the trough moves east.

Discussion

For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,

12Z model trends with the trough pattern over the West show the operational ECMWF digging the weekend trough farther west than the GFS. The EC ensembles also suggest higher probabilities of this westerly track, and that solution was the primary output used today.

Lowering heights will continue our cooling trend of the past two days, and by Friday the temperatures will be right around where they should be for the end of October. Temperatures fall below normal over the weekend as a series of shortwaves trek across California.

The first shortwave is associated with a weak trough over Pacific that will move inland across southern California or northern Baja Saturday and Sunday. This system brings a minor increase in PW from 0.55" today to 0.75" on Friday, and the added moisture combined with weak lift will bring a chance for light rain or drizzle from the coast to the mountains on Saturday. However, the NBM probability of rain totals > 0.01" at a specific location from the coast to the mountains on Saturday is only 10-20%.

Lifting mechanisms with the second shortwave digging south across in intermountain west Sunday and Monday are stronger and the chances for showers are greater. This is borne out in the NBM probability of rain >0.01" increasing to 40-50%. The amount of precipitation we eventually receive will ultimately be decided by how far south and west the cold upper trof digs, but a quarter to three-quarters of an inch is possible in some areas.

This will be a cold system, much colder than what we are used to over the past few weeks, with high temperatures 10 to 20 degrees below average. Lower Desert highs on Monday will only be in the 70s. The average is 88. High Desert highs will be in the 50s. Coastal and valley highs will be in the 60s, and mountain highs in the 40s and 50s. Valley lows Tuesday morning could fall into the 40s, and there may be pockets of frost in the high desert where low will fall into the 30s. Mountain lows will be in the 20s.

The cold upper low moves east Monday night and that sets the stage for a moderate strength Santa Ana wind event late Monday and Tuesday. Monday will probably be the coolest day of the next five days, followed by warming Tuesday and Wednesday as high pressure rebuilds over California from the Pacific.

Rain Forecast

Measurable rain is possible Friday and Saturday but amounts will be light, ranging from 0.01" to 0.10" west of the mountains. Greater amounts are more likely Sunday and Monday when a stronger and more dynamic weather system moves into the area. Precip amounts will range from 0.10"-0.20" on the low end to 0.50"-0.75" on the high end.

Aviation

212100z, Coast/Valleys, Low clouds to develop after 01z Thu, with bases of 900-1300 FT MSL and tops to 2500 FT MSL, spreading up to 25 miles inland 10-12z Thu. Reduced VIS of 0-4 SM where low clouds interact with terrain. Low clouds to scatter out 17-19z Thu.

Deserts, mostly clear skies through tonight and Thursday.

Marine

No hazardous marine conditions are expected through Monday.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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