Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 191100

Area Forecast Discussion
National Weather Service San Diego CA
400 AM PDT Tue Oct 19 2021

For today through Thursday, there will be a warming trend with
weak high pressure aloft and weak onshore flow. night and morning
coastal low clouds will return tonight. For Friday through the
weekend, onshore flow will strengthen and the marine layer will
deepen. Cooling will spread inland with periods of stronger and
gusty southwest to west winds in the mountains and deserts. For
early next week, some time around Monday into Tuesday, a low
pressure system moving inland through California could bring rain.
There are timing differences among the models and greater spread
in the amounts.



.SHORT TERM (Today through Thursday)...
Weak onshore will prevail through Thursday with gradually rising
500 mb heights. A warming trend will begin today and continue
through Thursday. Thursday is expected to be the warmest day of the
week for most areas, but high temperatures on Thursday will still
be a few to around 5 degrees below average. Coastal low clouds
and fog are expected to return tonight with night and morning
coastal low clouds spreading inland into portions of the valleys.

As winds have dropped off, temperatures in the high desert have
fallen into the 30s with patchy frost possible in some areas
early this morning, especially in lower lying valley areas.


.LONG TERM (Friday through Monday)...
Low pressure centered over the northeast Pacific will strengthen
the onshore flow across California and spread moisture inland. For
southern California, the marine layer will deepen with coastal low
clouds extending onto the lower coastal slopes of the mountains at
times. There will also be periods of mid and high clouds as well.
There will be cooling spreading inland on Friday and Saturday
with not much change in high temperatures on Sunday.

Most precipitation will remain north of the area through the
weekend. However, the tail end of a low pressure system moving
inland to the north could bring enough deepening of the marine
layer for late Friday night into Saturday morning for some patchy
drizzle or light showers from the coast inland to the coastal
slopes of the mountains.

Around Monday, rain from a low pressure system moving inland along
the West Coast could spread across southern California. There
remain timing and amplitude differences with this system with the
more amplified solutions wetter. For Monday, cluster analysis
shows about half the model variance is due to timing differences
and about 20 percent due to amplitude differences. The wetter
solutions for southwestern California were driven more by ensemble
members of the GFS (about 90 percent of GFS members) and to a
lesser extent by the ECMWF (about 40 percent of ECMWF members).
Timing differences have narrowed a little over the past 24 hours
with a continuing wide spread in amounts with the wettest
solutions tapping an atmospheric river.

The 00Z deterministic ECMWF and 06Z deterministic GFS show
amounts decreasing as the rain moves eastward and southward across
southern California on Monday into Monday evening. The current
forecast is close to NBM with rainfall chances for late Sunday
night into Tuesday.


190840Z...SCT clouds will continue to dissipate through the early
morning hours leading to mostly clear skies through today into early
evening with unrestricted vis. Low clouds with bases near 1,500 FT
MSL will push ashore after 06Z Wed impacting all coastal TAF sites
and 10-20 miles inland.


Stronger onshore flow will enter the picture starting Thursday into
the weekend where winds may gusts near 20 knots each afternoon and
evening. Otherwise, no hazardous marine weather conditions are
expected through Sunday.


Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.





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