Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
919 PM PDT Monday Aug 3 2015
Synopsis...subtropical moisture moving in from the southwest may
result in isolated showers and thunderstorms on Tuesday...mainly
over the San Francisco Bay area...and particularly in the North
Bay. Otherwise...mild temperatures are forecast to continue
through Tuesday. A modest warming trend is then expected Wednesday
and Thursday as high pressure builds across the region. Isolated
showers and thunderstorms may also occur late in the week
Discussion...as of 9:15 PM PDT Monday...the forecast was updated
this evening...concerning the potential for convective precipitation
across at least a portion of our area on Tuesday. Satellite
imagery over the eastern Pacific shows a blob of moisture in the
vicinity of the subtropical jet near 25n/135w. This moisture is
forecast to continue streaming to the east-northeast through tomorrow morning
and then track across our area during the late morning and early
afternoon hours. Meanwhile...a piece of energy is forecast to
eject out of the weak upper trough out along 135w and track across
our area by early afternoon. If the weak instability with the
incoming shortwave trough phases with the incoming subtropical
moisture over our area tomorrow...we could see some isolated
showers and thunderstorms. The 00z NAM forecasts scattered
convective precipitation across our northern coastal waters and inland
across mainly the North Bay from late Tuesday morning through middle
afternoon. The 00z GFS is just in an it shows precipitation across Napa
County and northern Contra Costa County during this same time
period. Have therefore gone ahead and added a slight chance of
showers and thunderstorms to much of the sf Bay area from late
Tuesday morning through the afternoon hours. Appears that best
chances of convective precipitation will be over the North Bay...and
perhaps locally south across the sf peninsula and into the East
Bay. If convection does develop...it will likely be elevated and
would produce only light amounts of precipitation. The fire weather
forecast was updated to reflect the chance of thunderstorms.
Moisture and middle level instability will move quickly off to our
east by late afternoon and the shower/thunderstorm threat will
end by Tuesday evening.
Increased cloud cover on Tuesday will keep inland temperatures on
the cool side of normal. The 00z NAM forecasts a high tomorrow of
only 77 at Livermore...only 3 degrees warmer than the high
forecast for sfo.
Warmer and drier weather will develop Wednesday and Thursday as a
shortwave ridge briefly develops over California. Late in the week
the trough offshore will finally move east...with an upper low
forecast to reach our area by Friday. Will need to consider adding
shower/thunderstorm chances to the forecast for Thursday night and
Friday as both the NAM and GFS show precipitation during this time
Aviation...as of 4:50 PM PDT Monday...for 00z tafs...clear skies
prevail across the region this afternoon. MVFR/IFR ceilings will
return overnight across area terminals. Moderate and locally
gusty winds will continue through this evening then ease
overnight. Moderate confidence.
Vicinity of ksfo... VFR conditions will prevail through this
evening. MVFR ceilings projected to return after 05z tonight.
Moderately strong west winds around 15 to 20 knots...with gust to
25kt will continue through this afternoon. Winds will ease to
around 10 knots overnight.
Ksfo bridge approach... similar to ksfo.
Monterey Bay area terminals... VFR conditions will prevail through this
evening. MVFR/IFR ceilings projected to return after 07z tonight.
Moderately west winds around 10 to 15 knots...with gust to 20 knots
will continue through this afternoon. Winds will ease to around 5 knots
Marine...as of 8:58 PM PDT Monday...predominately light to
moderate winds and calm seas are forecast through tomorrow. The
strongest winds will be over the northern San Francisco Bay in the
afternoon...especially near the Golden Gate Bridge and Angel
Island. Winds increase and shift towards the northwest midweek.
Public forecast: dykema
Visit US at www.Weather.Gov/sanfrancisco
Follow US on facebook and twitter at: