Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
108 PM PDT Friday Sep 30 2016
a low pressure trough will slowly move into California this
weekend bringing cooler and breezy weather through Monday. Strong
westerly winds are expected mainly in mountains, deserts, and
coastal waters late Sunday into Monday. Marine layer cloudiness
will become progressively more extensive each night and morning
with drizzle possible Sunday night into Monday morning. Fair
weather with a modest warming trend will follow the trough
Tuesday through Thursday.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Cumulus clouds have developed generously over the mountains, but
none have shown enough energy for a shower. Tiny patches of fog
have appeared over the coastal waters, a small clue for more to
come. Midday temps in the coastal basin are running a few degrees
lower than at this time yesterday. A trough of low pressure over
the northwest will slowly move down the coast today through
Sunday. That, and an intermittent coastal eddy circulation, will
help rebuild our marine layer and bring back coastal low clouds
and fog, extending a little farther inland each night/morning.
Tonight and Saturday morning some of this fog will probably be
dense and not very far inland. Saturday temps will be quite close
to average. As the trough moves in Sunday, breezes will pick up
everywhere, particularly in the mountains and deserts during the
afternoon and evening. Preliminary estimates on top gusts for the
windiest spots will exceed 50 mph. Coastal low clouds will head
into some inland valleys by Sunday morning, and fill the entire
coastal basin reaching up coastal mountain slopes Monday morning.
With the marine layer deepening so much, patchy drizzle is
expected Sunday night and Monday morning. This seems more probable
along farther inland extent of clouds with the additional lifting
of air by the higher terrain. Clearing on Monday appears to occur
first at the coast and last along windward mountain slopes. On
Monday it will really feel like fall should feel as daytime
temperatures in lower elevations will generally not make it out of
the 70s. The trough moves out Tuesday, but there is no big rebound
to a ridge of high pressure. A weak ridge will bring a modest
warming trend. Weak offshore flow is possible Wednesday and
Thursday, but it doesn't look extreme in the either the wind or
temperature department. Just fair and seasonal weather.
302003z...coast/valleys...unrestricted vis and few-scattered clouds at or above
10000 ft msl through 01/0600 UTC. 01/0600-1500 UTC, broken-overcast clouds
will probably gradually develop 10-15 sm inland, with bases 500-800
ft msl, tops around 900-1200 ft msl, areas of vis 3-5 sm, and local
vis at or below 1 sm. Forecast marine layer inversion strength of 5-6 deg c.
Forecast confidence for low cloud/fog occurrence and timing is low-
to-moderate, with moderate confidence in cigs/vis. Clearing of
stratus/fog 01/1500-1700 UTC.
Mountains/deserts...unrestricted vis through Saturday morning. Scattered-
broken clouds at or above 10000 ft msl gradaully diminishing tonight, becoming
sky clear by Saturday morning.
no hazardous marine weather is expected through Saturday. An area
of low pressure moving inland along the California coast will
produce unsettled conditions Sunday afternoon through Monday.
Northwesterly wind gusts during this period are expected to be
25-30 kt for the outer coastal waters, with combined seas of 6-8
ft. A Small Craft Advisory will likely be needed Sunday afternoon
through Monday for the outer coastal waters.
3-4 foot southerly swells from 190 degrees will bring 3-5 foot
surf, with occasional 6 foot sets north of Oceanside, through
Saturday. These swells may also produce strong rip and longshore
currents at the beaches. For Sunday into early next week, the
southerly swell will slowly diminish and mix with a building short
period northwest wind swell. This will result lower, but choppier
as a trough of low pressure generates strong onshore winds on
Sunday, there will be a time of low humidity below 15 percent
combining with those strong winds Sunday afternoon and evening
mainly on the Lee slopes of mountains and into parts of adjacent
deserts. However, the humidity quickly rebounds as the trough
arrives Sunday evening. Critical fire weather conditions may
happen, but only for a short time and only on isolated mountain
Lee slopes. Will continue to monitor, but at this time no
hazardous fire weather products are anticipated.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.