Area Forecast Discussion
Issued by NWS San Diego, CA
FXUS66 KSGX 240421
Area Forecast Discussion
National Weather Service San Diego CA
921 PM PDT Sun Oct 23 2016
Scattered showers and isolated thunderstorms will continue to be a
threat across Southern California through Monday as a weak low
pressure trough moves across the region. Quite mild overnight under
the cloud cover...then cooler on Monday. Fair and warmer midweek as
high pressure rebuilds aloft and marine clouds and fog are limited
to coastal areas. Another low pressure trough will bring more
clouds...cooling...and a chance of precipitation late in the week.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
It turned out to be a wet afternoon/evening over northwest portions
of the forecast area where many sites received their first
significant rains of the season. See LAXRRMSGX for details or check
our website. Thunder and lightning accompanied the heavier showers.
Radar shows showers continue to develop offshore and are drifting
NE, mainly affecting O.C. and points NE across San Bernardino
County. The thunderstorms have quieted down and rainfall is
light to locally moderate at times.
The 00Z Miramar sounding still indicated a dry layer below 6500 FT,
but did have a MU Cape value of 373. winds in the dry layer were
weak NW, but above the dry layer, South at 20-40 KTS. PW had
increased to 1.21 inch.
Through Mon...A weak shortwave will move across SoCal continuing a
threat of scattered rain showers and isolated heavier showers with
thunder possible at times. Forecast instability is marginal and the
flow aloft is rather strong, so additional rainfall is expectd to be
light to locally moderate with the most widespread rainfall in the
mountains. Very mild overnight as the flow of subtropical air
continues with considerable cloud cover. Additional rainfall of a
trace, to one-quarter to locally one-half inch through Mon evening.
For Midweek...High pressure will rebuild over the Southwest as the
trough offshore regroups with additional shortwave energy. This will
bring fair and warm days with limited marine clouds and fog nights/
Friday into next weekend...The next short wave is forecast to weaken
as it swings inland across CA on Fri with another chance of rain
across SoCal, although at this time, it appears the bulk of the rain
will fall north of the CWA. Fair and warmer again into the weekend.
Confidence...The operational runs of the GFS/ECMWF/GEM all have a
similar 500 MB pattern across the West through next weekend with
minor differences in timing of the short wave energy. This
bolsters forecast confidence.
The Pattern...A broad high pressure ridge will be centered over the
Great Plains with a trough off the West Coast for the remainder of
the month. This pattern will give CA several more bouts of
precipitation as energy ejects from the trough offshore. Another
wave could bring us more precip early next week.
Most of the rainfall today occurred in a strip from Orange County,
NE across SW San Bernardino and far NW Riverside Counties with
measurements of from one-tenth to locally one-half inch. The
shortwave that helped to trigger the activity will be out of the
picture late tonight.
PW values of from one inch to 1.5 inches are expected through Mon
morning as a brisk S-SW flow aloft continues to pump subtropical
moisture across the region. Another shortwave is rounding the base
of the trough out near 130W and will approach the area Mon and move
through Mon evening. The HiRes guidance remains stingy with amounts,
as do the Global models, but MOS POPS are high. Given the pattern
and the inability to model the instability and precip today,
tomorrow may be somewhat underdone as well.
The pattern and MOS Guidance suggest more rain is on tap for tonight
and Mon. For now, forecast amounts are conservative, but the
potential exists for more moderate/hvy convective rainfall rates in
convective bands. Confidence is low for the location of any heavier
rainfall amounts. Models show strong steering flow from the SW
continuing, so unless thunderstorm cells train over the same
location, accumulated rainfall and runoff should be modest.
240400Z...Scattered showers and isolated t-storms will continue
overnight, with the general precip band shifting south into SD
County through Monday morning. SCT-BKN cloud layers generally
between 3000 to 6000 ft MSL, lifting above 6000 ft MSL Monday
afternoon. VSBY restricted to as low as 2-3 SM in heavier
precipitation, and also especially where clouds intersect terrain,
otherwise VSBY unrestricted through Monday afternoon. Gusty and
erratic winds VCTY of any TSTMS.
Scattered rain showers and isolated thunderstorms through midday
Monday. Activity diminishing Monday afternoon. The main impact with
this activity will be cloud-to-water lightning. The Marine Weather
Statement continues the thunderstorm potential through midday
Slight chance of thunderstorms continues through midday Monday at
area beaches as an upper level trough lifts northeast across
Southern California. Cloud-to-ground lightning could occur,
resulting in a danger to all beach goers and swimmers.