Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 230434 AAA

Area Forecast Discussion
National Weather Service San Diego CA
933 PM PDT Sat Oct 22 2016

Increasing cloudiness overnight with patchy fog near the coast. Low
pressure aloft will move east into southern California Sunday and
bring scattered mainly light showers and possibly an isolated
through Monday. A ridge will amplify over the west Tuesday and
Wednesday and bring warming but with some coastal low clouds or fog
at times. Then...a Pacific storm system could bring some
precipitation late next week to southern California.



...Evening Update...

The big story this evening, if you will, is the batch of mid to
mainly high level clouds now rapidly overspreading SoCal from the
west, well in advance of a weak upper level trough that will swing
through our area Sunday into Monday. Cloudiness overnight will keep
temps up a bit and should tamper our threat of dense fog over our
area. Have backed off the extent of dense fog overnight and Sunday
morning as a result, although patchy fog is still a possibility
overnight over the coastal waters and along the coastal strip.

High temps on Sunday will also be tempered due to the extensive
cloudiness so we have lowered temps a few degrees for Sunday. Models
are still bringing some mainly light rain showers to much of our
area from Sunday afternoon through Monday morning, with qpf still on
the light side but nonetheless welcome to the area. Periods of light
rain and sprinkles overnight Sunday into Monday morning as well with
a few thunderstorms still possible due to elevated instability.

Changes this evening to the forecast included reducing severity and
extensive fog, and increasing cloud cover, and moderating temps a
bit overnight and lowering max temps a bit for tomorrow as well.

...Remainder of Previous Forecast Discussion (issued at 200 PM
PDT Saturday afternoon)...

Water vapor satellite loop shows the center of a low around
45N/130W, or about 300 miles west of the Oregon coast, with a trough
extending SSW from there. SSMI total precipitable water (TPW) images
show about 2 inches over far southern Baja, and there is currently
some lightning south of the south tip of Baja.  Some of that
moisture will be drawn up north towards So-Cal as the upper trough
approaches. Moisture will initially be mostly above 700 mb Sunday
but lower to about 800 mb by Monday when up to 1.2 inches of TPW
will occur here. There is some elevated CAPE in the NAM and local
WRF, with WRF showing best 700-500 mb maximum unstable CAPE Sunday
night and Monday, exceeding 300 J/kg in some areas. This scenario
would most likely bring a cloud shield and sprinkles Sunday as the
dry near-surface layer will be deep enough for evaporation of
precip. For Sunday night and Monday, better coverage of showers with
more measurable rainfall would occur, especially over higher terrain
with less evaporation of rainfall. There could be isolated
thunderstorms Sunday night/Monday as well.

The system shifts east Monday night/Tuesday and moisture decreases,
so fair weather should return by Tuesday at the latest. A ridge
building over the interior west will bring a warming trend, but
onshore flow should bring at least some marine layer stratus to the
coast mainly during the nights/early mornings. Models have fairly
deep troughing off the coast late in the week, but with a lot of
differences in details. There is definitely a chance of precip for
our forecast area late in the week, and if the system has the right
track and can especially pull up moisture from the tropical East
Pacific, there could be beneficial rainfall amounts over the region.
Right now, the models indicate the best moisture staying to our


230300Z...coasts and inland valleys east to the mountains...high
clouds 20000 ft above msl or higher and variable winds prevail.
Some patchy fog/stratus overnight but the risk of dense fog is now
much the high clouds are moving in quick this evening.
Have removed the dense fog from the aviation forecast overnight but
have kept in some mention of minor VSBY restrictions. Forward into
Monday high clouds and showers or sprinkles with maybe even an
isolated thunderstorm will be the issue.

Mountains and deserts...unrestricted visibility and high clouds will
continue through 231500Z. After 231500Z increasing high and mid mid
level clouds with sprinkles or showers and maybe even a thundertorm
will be the aviation problem.


Patchy fog with visibility 1 to 3 NM at times overnight into Sunday.
Have backed off the dense fog threat from previous forecast due to
rapid overspreading of high clouds this evening. Also, there is a
chance of showers and possibly isolated thunderstorms late Sunday
through Monday, with the main impact being possible lightning.


There is a slight chance of thunderstorms Sunday evening through
Monday morning as an upper level trough lifts northeast across
Southern California. Cloud-to-ground and cloud-to-water lightning
could occur along San Diego county beaches, resulting in a danger to
beach goers and swimmers.


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





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