Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1102 PM PDT Thursday Jul 30 2015
Synopsis...a cooling trend can be expected into the weekend.
Subtropical moisture streaming into our area may produce a few
isolated showers and thunderstorms.
Discussion...as of 8:51 PM PDT Thursday...onshore winds have helped
bring temperatures down in all locations today. However average
temperatures...particularly over inland valleys...were still more
than several degrees above the late July climatic normals. Onshore
winds and its associated cooling is expected to continue into the
weekend bringing temperatures close to normal.
As onshore winds continue into the weekend middle level moisture and
instability will continue to stream northwest from socal over the top of
the marine based inversions. The first pulse of instability has
edged northward into southern Monterey and San Benito counties
this evening...but satellite and radar has not shown much more
than just a few isolated showers in those areas so far. Coastal
stratus and fog continue to be seen from the Monterey Bay northward
to Sonoma County. Precipitable water value has climbed up to 1.07"
inches on the 00z Oakland sounding...and very likely to climb
higher through the weekend. Surface dewpoint temperatures...a
measure of water vapor in the air...are presently in the 50s with
even a few lower 60s in spots. The increase in humidity will tend
to make the air feel muggy.
The present forecast outlining the timing of shower and isolated
thunderstorm chances gradually moving north over the Bay area late
tonight into Friday looks good. An end to shower and T-storm
activity is expected Saturday.
A closed middle-upper level low pressure area will be located off the
British Columbia coast early next week which will then drift slowly
southeast reaching the Pacific northwest. Broad upper level troughing will persist
over the forecast area through next week with a continuation of dry
Previous discussion...as of 3:00 PM PDT Thursday...as expected
temperatures are running cooler for almost all parts of the County Warning Area.
A few locations have seen impressive drop of 10 to 15 degrees
compared to Wednesday. After multiple days of middle 90s to lower
100s for interior spots the break is welcome.
Main forecast issue has been with the potential for convection
across our region as subtropical moisture have been moving in from
the southeast. Koak sounding this morning picked up on this with
precipitable water values exceeding 1 inch and increasing by around half an inch
compared to yesterday. Models have been consistently showing any
shower and thunderstorm activity staying our of our area.
However...current radar loop shows cells are closer to
Monterey/San Benito than they were indicating so decided to keep
a mention of isolated showers and thunderstorms over the southern
third of the County Warning Area for tonight into Friday. For the northern two
thirds of the County Warning Area convection was slightly reduced and kept just
for higher elevation spots.
A more westerly flow is forecast by Saturday which will drop any
chances for showers back to near zero. Temperatures over the weekend will
be near normal at the coast -- middle 60s to middle 70s -- with upper
70s to around 90 inland. Longer range guidance indicates quiet
weather all next week with temperatures near normal.
Aviation...as of 11:00 PM PDT Thursday... for 06z
tafs...scattered middle and high level clouds will continue to
traverse the region tonight. Showers remain possible across
Monterey and San Benito counties tonight. The chance of
thunderstorms increases and coverage expands to the north
overnight and Friday. Low clouds have begun to push inland under
a 1500 foot marine layer. IFR-MVFR ceilings expected over most
terminals overnight with clearing expected around 17z-18z Friday.
Light and locally variable overnight.
Vicinity of ksfo... VFR conditions through late tonight with IFR
ceilings expected to return after 08z tonight. Clearing is
anticipated around 17z-18z Friday. Winds will ease to 10 knots or
less overnight with winds around 15 to 20 knots expected Friday
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...IFR ceilings will prevail through
tonight. Clearing is anticipated around 18z Friday. Light and
variable winds expected overnight with a slight increase to around
10 to 15 knots Friday afternoon.
Beaches...as of 04:16 am PDT Thursday...a marginally energetic
southerly swell continues along the coast this morning.
However...unlike yesterday the swell heights are down and the
period is now 15 seconds. No beach hazard statement will be
issued...but a few beach may see some locally hazardous
conditions. Use caution if you head to the beach
today...especially south facing ones.
Marine...as of 9:00 PM PDT Thursday...light west to northwest
winds will continue across much of the region through the weekend.
The strongest winds will be located in the northern San Francisco
Bay each afternoon... especially near Angel Island.
Public forecast: canepa/Bell
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