Area Forecast Discussion
Issued by NWS San Diego, CA
FXUS66 KSGX 231630
Area Forecast Discussion
National Weather Service San Diego CA
930 AM PDT Sun Oct 23 2016
A trough of low pressure will move through the region through Monday
and bring up moisture from the south. This will bring scattered
light showers today with a greater coverage of showers tonight and
Monday. Instability aloft will bring a slight chance of
thunderstorms today through Monday. While the cooling trend will
continue today and Monday, temperatures will stay a little above
normal for late October. A high pressure ridge will develop over the
West and bring fair weather Tuesday and Wednesday except for patchy
coastal stratus and fog, with warming by Wednesday. A rather deep
low pressure trough will bring a chance of precipitation late in the
week, though the deepest moisture will stay to the northwest.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
Moisture and instability from the south above 700 MB brought a few
very light showers, with a few stations recording a trace of
precipitation. So far, the dry layer below 700 MB has resulted in
evaporation of most of the precip. A few lightning strikes were
recorded off the Orange County Coast around 5 AM, and some in-cloud
lightning was very recently detected around 20-40 miles SW of San
Diego over the coastal waters. Patchy fog with brief visibilities
around 1/4 mile occurred in Orange County overnight. For today, the
sprinkles/light showers will continue, mainly in bands, with
isolated lightning. Some convection aloft is moving into San Diego
now, so a few lightning strikes could occur in western San
Diego County through about 1030 AM with little or no rainfall.
The trough is roughly 1000 miles west of San Diego, and the trough
axis will move through late Monday or Monday evening. Until then,
the moist unstable layer will gradually lower from its current 700-
500 MB to as low as about 800 MB tonight and Monday. This will bring
a better chance of measurable precipitation, though most areas will
have less than 1/4 inch, but a few mountain locations could have
around 1/2 inch due to being above most/all of the low-level dry
layer. Thunder could happen again tonight into Monday as NAM and WRF
show instability in the 700-500 MB layer, with WRF MU-CAPE values of
200-400 J/kg through Monday morning with some surface-based CAPE
over the mountains Monday afternoon. The system will move east of
the area late Monday night and end the precip threat then.
Temperatures will be a little bit lower today/Monday but still
remain a few degrees above seasonal averages for maximums and about
10 degrees above normal for minimums tonight.
The upper high will build over the western US, but further east than
our latitude Tue-Wed, so we will have surface onshore flow and some
marine layer stratus near the coast. Temps will remain a little
above normal, especially inland, with some warming Wednesday and
possibly Thursday. A rather deep trough will form over the east
Pacific and tap into some tropical moisture, with the possibility of
a tropical system moving by to the south/southwest of southern Baja
contributing to the moisture. Models currently have the best
moisture and precipitation to our northwest, but some precip,
possibly 1/2 inch, could occur over some of our forecast area
sometime late Thursday through Saturday.
231530Z...Patchy stratus becoming FEW-SCT with bases near 1000 ft
MSL. Patchy stratus may develop again tonight with bases near 1500
A band of light showers moving SW to NE will spread into Orange and
SW San Bernardino Counties this morning, then slowly shift east into
portions Riverside and San Diego counties this afternoon. Isolated
thunderstorms are possible beginning this afternoon and continuing
into Monday. Otherwise SCT-BKN cloud layers at/above 6000 ft MSL and
unrestricted vis through Monday morning.
Scattered light showers will move over the coastal waters later this
morning. There is also a slight chance of thunderstorms late this
afternoon through Monday, with the main impact being cloud-to-water
lightning. The Marine Weather Statement has been updated to take out
fog chances and introduce thunderstorm potential for this afternoon.
There is a slight chance of thunderstorms this evening through
Monday as an upper level trough lifts northeast across Southern
California. Cloud-to-ground lightning could occur, resulting in a
danger to beach goers and swimmers.