Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
900 PM PDT Tuesday Sep 30 2014
low clouds and fog will develop over the coastal areas tonight. Dry
northwest flow aloft and weak onshore flow will maintain fair and
seasonal weather Wednesday. For Thursday into Sunday...high pressure
aloft and weak to locally moderate offshore flow will bring much
hotter and drier days with locally gusty northeast winds. Slow
cooling may develop near the beaches on Sunday...then spread inland
early next week as the ridge weakens and onshore flow returns.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Patchy low clouds have begun to develop along the coast this
evening. Wind observation are showing some indication of a weak coastal eddy
spinning up just as hi-res models predicted...so forecast looks on
track for more widespread low clouds in the coastal areas and
western valleys tonight.
The warming trend continues Wednesday as an upper level ridge of
high pressure begins to build over the eastern Pacific. Cold air
filtering into the Great Basin and building surface high pressure
behind a short-wave trough will generate some local gusty offshore
winds by late Wednesday morning...mainly below the Cajon Pass.
Offshore flow will likely not be strong enough to scour out the
marine layer...so expect low clouds and fog to return to the coast
Wednesday evening. Northeast to east winds will pick up Wednesday
night...with low clouds gradually being pushed off the coast. As
this happens the marine layer will become more shallow...leading to
the potential for dense fog before clouds completely clear out
Thursday morning. Offshore surface pressure gradients will peak
Thursday morning at around -10 mb san-tph. Wind gusts during the
peak will be around 35-45 miles per hour in the wind prone passes and canyons
in the inland Empire and Orange County...as well as on the favored
coastal mountain slopes and foothills in San Diego County. Winds
will weaken during the afternoon...picking up Thursday evening with a
second weaker peak Friday morning. Much weaker offshore flow is
expected for Saturday.
Along with the offshore flow...the upper level ridge of high
pressure will move overhead and strengthen through Friday. This
combination will lead to hot and very dry conditions. By Friday high
temperatures will be 10-20 degrees above normal. Although the high
begins to weaken some on Saturday...areas west of the mountains will
continue to warm due to the weak offshore flow. Widespread temperatures in
the middle 90s to low 100s likely west of the mountains
Saturday...except at the beaches where highs will generally be in
the 80s. Minor cooling will occur Sunday as the high continues to
weaken and pressure gradients trend more onshore...with the most
notable cooling along the coast. Gradual cooling continues into the
middle of next week as the ridge breaks down and onshore flow
strengthens. Low clouds and fog may return to the coastal areas by
010300z...coast/valleys...a few patches of low clouds are beginning
to develop along the coast. Clouds will continue to develop...with
ceilings at the coastal airports by 01/05-08z. Most ceilings will be
700-1000 feet mean sea level with tops to 1200 feet mean sea level. Local visible below 3 Michigan in
fog could occur along coastal mesas and other higher coastal terrain
between 08z and 16z Wednesday...with a small chance of visible below 1 Michigan.
Most areas should clear by 17z Wednesday. Low clouds will return to the
coast earlier Wednesday evening...between 02/03-06z.
Mountains/deserts...clear with unrestricted visible through Wednesday evening.
800 PM...no hazardous marine weather is expected through Sunday.
Fire weather...from previous discussion...
high pressure aloft and offshore flow with locally gusty northeast
winds will bring much warmer and drier conditions for Thursday
through Sunday. Strongest winds...with local gusts to around 40
miles per hour...are expected for late Wednesday night into Thursday
morning...followed by gradual weakening Friday and Saturday. Friday
and Saturday will be the warmest days...especially inland where
temperatures of 95 to 105 degrees are possible. Slow cooling may
begin near the coast on Sunday as the offshore pressure gradients
continue to weaken. Humidities will fall Thursday...reaching the middle
teens by middle morning. Expect lowest daytime humidities of 5 to 10
percent for inland areas Friday and Saturday with poor overnight
recovery. Humidities are expected to slowly recover for Sunday into
early next week as weak onshore flow returns. While red flag warning
criteria may not be met during this period...there will still be an
elevated threat for wildfires for Thursday into Sunday.
Skywarn activation will not be needed today.
California...heat advisory from 11 am Friday to 8 PM PDT Saturday for the
Orange County coastal areas...San Diego County coastal
Heat advisory from 11 am Friday to 8 PM PDT Sunday for the
Riverside County mountains...San Bernardino County
mountains...San Bernardino and Riverside County valleys-the
inland Empire...San Diego County mountains...San Diego
County valleys...Santa Ana Mountains and foothills.