Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 220430

Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sun Apr 21 2019

Low pressure will drop south over Southern California tonight,
maintaining onshore flow and a deep marine layer the rest of
tonight, along with patchy fog on the coastal mountain slopes. Gusty
west to northwest winds over the mountains and deserts this evening
will turn more from the north and weaken on Monday. As the low
passes, building high pressure aloft will bring sharp warming inland
through midweek, while a shallow marine layer and onshore flow keeps
coastal areas cooler with patchy low clouds at times. Cooling
returns during the latter half of the week as a trough approaches
from the west.



Areas of low clouds were occurring, mainly along the coastal
mountain slopes and near the coast in southwest San Diego County.
Some fog was occurring on the coastal mountain slopes. Offshore
pressure gradient trends were already notable to the north as the
SAN to DAG MSLP gradient was 5.4 MB, down from 8.7 MB 24 hours ago,
while MSLP gradients were still onshore at around 8 MB from SAN to
the lower deserts. The strong west to northwest flow will become
more northerly by Monday morning as the trough axis shifts just to
our east, and this should dissipate most of the mountain stratus and
fog by around sunrise, though some stratus should continue near the
coast, though less than the last 2 days. The latest HREF shows this
well. The weak offshore flow combined with rising heights and
temperatures aloft will bring much warmer weather to inland areas
Monday and Tuesday, though coastal areas will remain under onshore
flow and the influence of water temperatures which are mostly still
around 60 deg F. That will limit the warmup there, and local WRF
shows the marine layer never dissipates entirely, though afternoons
should clear well Monday and Tuesday.

From previous discussion...

For the midweek period, temperatures will continue to rise as the
low moves east and a ridge over the EastPac drifts east across the
State. Surface winds will attempt an offshore turn during this
transition, but it appears it will only be achieved over far
northern areas of the CWA. Onshore will prevail, and an eddy over
the coastal waters may also hold the marine layer over coastal areas
with some low clouds at times, stifling any substantial warming
there. Well inland, away from marine influence, daytime temps will
soar well into the 80s and lower 90s, while the lower deserts flirt
with the century mark.

The ridge axis peaks over us late Wednesday. After that, the onshore
will increase again as a trough approaches from the Pacific. The
operational models are still getting a handle on this one. The ECMWF
has been quite aggressive with it, sparking widespread showers
across the area next Sunday, and even the GFS has been leaning
toward this solution over the past couple of runs, but timing is
uncertain. In any case, look for continued cooling into the weekend,
and even though we are leaving POPS below threshold for now, showers
are not totally out of the question.


212030Z...Coast/Valleys...Expect SCT low clouds with bases 2500-3500
ft MSL through this evening. Low clouds spreading inland again 02-
05Z Monday, with bases 1500-2500 ft MSL. Do not expect BKN ceilings
as far inland tonight so inland empire terminals will remain mostly
SCT. Skies should clear out at all the terminals 16-18Z Monday.

Mountains...West winds 20-30 kt gusting to 35-45 kt over the
mountains through this evening, with moderate to strong up/down
drafts on the desert mountain slopes. Expect mostly clear skies.

Deserts...NW winds 10-20 kt gusting to 25-30 kt, with slightly lower
wind speeds late tonight and Monday. Expect clear skies.


No hazardous marine weather is expected through Friday.


Skywarn activation will not be needed tonight or on Monday.





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