Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
540 PM PDT sun Sep 21 2014
Synopsis...little change in the weather is forecast over the next
few days. Look for a continued fairly deep marine layer resulting
in widespread night and morning low clouds. A Pacific storm system
will bring rain to our area for Wednesday into Thursday with most
of it focused on the locations north of San Francisco. Showers are
possible from Thursday afternoon into Friday evening.
Discussion...as of 3:00 PM PDT Sunday...another round of sunny
skies inland with widespread clouds along the coast. In fact, many
coastal spots have stayed cloudy all day. Even a good portion of
San Francisco have not had any breaks from the clouds. Marine
layer stands over 2000 feet with the onshore gradient around 2.5
mb from the west. This will help to bring clouds quickly back into
our County Warning Area overnight.
Quiet weather can be expected for Monday and Tuesday as a longwave
trough slowly advances from the Pacific while the ridge continues to
progress to the east. Surface flow should remain generally
westerly along with the marine layer sticking close to 2,000 feet.
Highs will vary little each day from today's readings. Generally
middle 60s to middle 70s at the coast with middle 70s to middle 80s inland.
Things get more interesting for Wednesday and Thursday as a
Pacific storm system and associated cold front move into the pacnw
down to our region. Model guidance remains fairly consistent with
the timing and brings the leading edge of rain into the North Bay
by Wednesday afternoon and then into sf Bay region to around Santa
Cruz Wednesday night into early Thursday. One encouraging sign is
model trends have been a bit wetter compared to solutions from the
past few days, so probability of precipitation were increased for almost all spots north
of Monterey County. Some instability is also noted Wednesday night
into Thursday, so at this time thunder cannot be ruled out. The
good news is any storms that produce lightning should be wet which
will help to lessen the fire weather risk. Thunder was left out of
the grids for now, however once the higher resolution runs come in,
it may be added in. Still a little early to come up with specifics
for rainfall amounts, however based off both the operational runs
plus the ensembles, looks like parts of the North Bay could easily
pick up more than 1/2" of rain. For urban spots around the Bay
right now closer to 1/10"-1/4" looks more likely. For Santa Cruz
County southward generally less than 1/10" can be expected
although a stray shower or two could bring locally higher amounts
in the Santa Cruz Mountains. To give you an idea how much rain we
would need to catch up to normal, San Francisco is nearly 22"
behind where it should be looking back to the start of 2013!
Although rain chances will be on the decrease for the second half
of Thursday, a 120 knots jet stream is forecast to be overhead while
the longwave trough axis is still to our west. Due to that kept
showers going a bit longer than quantitative precipitation forecast is suggesting in case we get
some Post frontal activity into Thursday night and maybe even
Friday. Similar to Wednesday night into Thursday, the best chance
for rainfall will be for the northern third of the County Warning Area.
For the weekend into next week, there is less agreement between
the models with the progressive nature of the trough. Some of them
have the trough advancing to the east over the weekend with a ridge
of high pressure building in while others have an associated upper
level low becoming nearly stationary near our County Warning Area. In the case of
the second scenario, moisture would wrap around the low and
possibly move back into our County Warning Area by Sunday. Due to the large
disagreement, for now will keep a generally dry weekend/early next
week forecast going with seasonable temperatures.
Aviation...as of 5:30 PM PDT Sunday...stratus continues to extend
from the coast into the sfo Bay area from the San Carlos to the
Golden Gate. Ceilings did not clear from sfo today and does not appear
it will do so. Should see clearing Monday afternoon as 500 mb
heights rise leading to a compression of the marine layer.
Vicinity of ksfo...MVFR ceilings through 18z Monday.
Ksfo bridge approach...MVFR ceilings north of San Carlos. Ceilings
spreading south after 03z.
Monterey Bay area terminals...stratus along the Bay has now pushed
into sns. MVFR ceilings through 18-19z Monday.
Marine...as of 5:30 PM PDT Sunday...light winds and seas will
continue over the coastal waters through early to middle week. For
the middle to late week time frame moderate to large northwest swell
will enter the waters.
Climate...autumn officially begins (autumnal equinox) at 7:29 PM
PDT on Monday September 22.
... Bay north of the Bay Bridge until 9 PM
Public forecast: Bell
aviation: west pi
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