Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1105 am PDT Tuesday Sep 16 2014
Synopsis...a gradual cooling trend will continue over inland
areas today ahead of an approaching trough. This system will bring a
chance of showers to our area area late Wednesday into Thursday.
Shower chances then end and temperatures rebound for the upcoming
Discussion...as of 09:40 am PDT Tuesday...the marine layer
remains very mixed out this morning and has given way to mostly
sunny conditions region wide...even along coastal locations. This
will give way temperatures similar to yesterday along the coast
and slightly cooler inland as 850mb temperatures cool ahead of the
approaching middle/upper level trough. Made slight adjustments to
the forecast to reflect this. Otherwise...the forecast remains on
track and will be spending time looking at precipitation chances
and amounts for Wednesday night through Thursday as the trough
moves across central California.
Previous discussion...as of 3:00 am PDT Tuesday...still generally clear
across our County Warning Area early this morning although satellite products do
indicate clouds just off the coast plus a couple of bands
attempting to move inland. Surface gradient shows around 1.5 mb
directly from the west, so decided to keep a mention of clouds and
patchy fog this morning especially along the coast. Anything that
does form should rapidly burn off before noon. High pressure ridge
that brought US warm readings yesterday will be more to the east
today as a longwave trough over the North Pacific moves toward the
West Coast. Highs will generally be cooler today -- especially in
the northern third due to the proximity to the trough. Highs will
range from 60s and 70s along the coast an up north to 80s and 90
for southern interior spots.
The trough will progress to the pacnw/norcal coast by Thursday with
associated rainfall first impacting Washington/or and then into California
later in the day. As we get into Wednesday evening and night
the rain will spread to the south into our County Warning Area as the system
progresses to the south. Rain will continue into at least the
first part of Thursday before shutting off as the trough axis heads
off to our southeast.
Model trends have been a bit more bullish for the likelihood we
will see measurable rainfall and have also painted the area down
into Monterey County. Decided to up the probability of precipitation by about 10-20% for
most locations and also expanded the areas that may have rain. Amounts
still look to be very light with various forecasting tools showing
less than a 15% chance of more than 1/10" of an inch of rain down
to San Francisco. Most likely we will see lots of places picking
up just a few hundredths with a couple of gages getting around a
tenth. Due to the long period of dry weather, any rain will lead
to slick roads, so driving Wednesday night could be tricky. Temperatures
will cool for both Wednesday and Thursday due to the system.
For Thursday evening into next week dry weather returns as the
ridge of high pressure builds back across our County Warning Area. Temperatures will warm
each day as 500 mb heights exceed 590 dm and 850 mb temperatures rebound
from 11-13c up to 21-23c over the weekend. Highs will return to
the upper 60s to upper 70s at the coast with 80s and 90s inland.
Some of the models also bring an offshore wind event to our area
over the weekend by building the ridge to our north. Due to very
high fire weather concerns, that will need to be watched very closely.
Even warmer weather looks likely for next week as a stronger ridge
of high pressure builds into the western Continental U.S.. although there is
good agreement our area will be dry and warmer than normal, there
remains less agreement for the the height of the ridge plus the
locations of the axis. CPC keeps the dry and warm weather going
almost to the end of the month.
Aviation...as of 10:45 am PDT Tuesday...there was very little in
the way of low clouds this morning with only small patches in the
north and easy Bay. Stratus will return tonight as an upper trough
approaches the area but warm airmass will keep stratus from
spreading inland until late.
Vicinity of ksfo...VFR through the Airport evening rush. MVFR late
tonight. Winds will be light becoming northwest 15-18 knots after
Ksfo bridge approach...same as ksfo.
Monterey Bay area terminals...VFR through late tonight.
Marine...as of 11:00 am PDT Tuesday...light winds will continue
through much of the week. A long period southwest swell will
continue through midweek...occasionally mixing with a moderate
northwest swell Thursday and again next weekend. Southwest swell
will peak Wednesday night into Thursday.
Public forecast: rgass
aviation: west pi
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