Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1040 PM PDT Thursday Oct 8 2015
Synopsis...dry weather conditions along with above normal
temperatures will persist through this weekend as high pressure
resides over the state. Low pressure over northern Mexico will
move west out over warm to very warm Pacific waters and may be a
factor in our weather late Tuesday through Thursday of next week.
Discussion...as of 8:47 PM PDT...increasingly amplified pattern
seen over the eastern Pacific and western North America since the
start of the month. Sub-tropical ridging with strong 500 mb
heights at 590+ decameters extends NE over California with warming
of temperatures today due primarily to subsidence east of the
ridge axis. Cirrus clouds seen in the area through today as warm
air advection continues to strengthen the middle-level height ridge.
The upper low pressure system that initially developed near
southern British Columbia and the Pacific northwest last weekend...moved S-southeast
over California to the desert SW...is presently over northern
Mexico. It's taken a long trip south almost all over land...about
1600 miles straight line if numerical weather models are correct
with its forecast location being just west of the southern Baja California
peninsula this Saturday...one week after moving over California.
By the way...interesting fact...strong dynamic cooling with the low as it
passed over Monterey county's Chews Ridge (at 5 thousand feet) temperature
briefly cooled to the upper 30s late Sat night/Sun morning Oct 3-4th.
Back to present...model forecasts have been becoming a bit clearer
showing some consistency with the overall motion of the low center.
The low got a little extra kick in its step today becoming re-vitalized
by ageostrophic wind flow from the jet stream to its north over the
northern rockies and plains.
The aforementioned ridge over California will gradually flatten and weaken
a little through the weekend while the low center undercuts the
ridge becoming located over unusually warm to very warm sub-tropical
epac waters. Largely baroclinic for much of its existence the low
center will soon be over sea surface temperatures (sst) that are
toasty warm...in the 80s...in the Gulf of California and nearly
the same just west of the Baja California peninsula. The low will continue to
move westward then northwestward spending about four to five days
over warm water...that's then mostly in the 70s. Undoubtedly the
low will pick up heat energy... especially initially...in the form
of showers/T-storms /surface wind?? the low bears watching as
model forecasts have trouble handling "hybrid" weather systems
where thermodynamics and radiation play a much larger role in
system development. How much the low becomes energized with
convection is the biggest question? The low will eventually be
picked up by the middle-latitude pattern where a weakness in the
height field /mid level troughing/ typically provides an outlet.
The low may be a factor here in our weather next week...needs
watching as this may lead to fire weather issues...dry lightning?
Minor updates mostly to bump up friday's forecast high temperatures 3-4
degrees mostly South Bay to north central coast. Cirrus clouds
could also slow tonight's cooling. A little less confident about
tinkering with high temperatures forecast for the rest of the County
Warning Area as slight low level cooling is forecast going into
Friday...temperatures look good. Too early to place T-storms in forecast
for the middle of next week but again it needs watching. Decided
to include slight chance of showers as far north as Santa Cruz and
Santa Clara counties Tuesday evening to Thursday next week.
Previous discussion...by early next week... the ridge reasserts
dominance over the state... resulting in additional warming on
Monday and Tuesday. By midweek a weak shortwave trough will
flatten the ridge slightly... leading to slightly cooler
temperatures. Temperatures are expected to remain a few to several
degrees above normal through all of the ups and Downs of the
upcoming week. Meanwhile... the upper low that moved through the
area last weekend will retrograde upstream and undercut next weeks
ridge. Late next week... this upper low could once again influence
our weather... almost two weeks after it first brought windy
conditions to our area last weekend. Long term guidance is showing
some run to run and model to model consistency with introducing
precipitation to the southern portion of our forecast area as this
upper low treks back into central California. There is a large
degree of uncertainty over the timing and coverage of this
precipitation so the latest forecast package introduced only a
broad coverage of very low probability of precipitation.
Aviation...as of 10:40 PM PDT Thursday...high pressure over the
region and light winds at the surface will keep the region mainly
free of low clouds through the forecast period. High clouds
however will continue to move across the region through the
night. High confidence.
Vicinity of ksfo...VFR conditions expected through the period.
Light to locally variable winds expected overnight. High
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions expected through the
period. Light and locally variable winds expected overnight. High
Marine...as of 9:00 PM PDT Thursday...northerly winds will
continue to increase tonight especially for the southern waters.
Gentle seas will persist into the weekend with a mixed swell. A
moderate period west to southwesterly swell generated by hurricane
oho will enter the waters by tomorrow lasting into the weekend.
... Pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Public forecast: canepa
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