Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
235 PM PDT Wednesday Aug 31 2016
Synopsis...a broad upper level trough over the West Coast
will remain in place through at least the start of next week
and result in dry weather conditions along with temperatures
below seasonal averages.
Discussion...as of 2:30 PM PDT Wednesday...mostly sunny
conditions across our entire County Warning Area today as a more compressed marine
layer combined with a north-to-south surface gradient near 3 mb
was able to keep clouds to a minimum. Temperatures in most spots
have been running 3 to 6 degrees cooler than yesterday due to a
longwave trough along the West Coast that brought in lower
heights and a drop of 2c at 850 mb. Look for the marine layer to
rebound a bit overnight which will help to increase cloud cover
at the coast and adjacent valleys. Lows will be mostly in the 50s.
The synoptic pattern will undergo only minor fluctuations each day
through at least Tuesday as the the longwave trough will either be
along or near the coast. MOS guidance for many of our locations
indicates highs and low only varying by a few degrees each day (in
fact, European model (ecmwf) guidance for San Jose shows 76 or 77 for each day
Temperatures will slowly warm during the middle and end of next
week as a ridge of high pressure builds back into our region. As
the previous discussion mentioned, the models indicate a tropical
system may move west of baja and potentially bring moisture toward
our region 10 to 12 days down the Road. Confidence that far out
remains very low.
Aviation...as of 11:15 am PDT Wednesday...frayed stratus deck
offshore of the coast today with a low pressure trough aloft.
Marine layer will redevelop later today and bring about a return
of inland stratus to the region after a pair of mostly cloud free
days. Models differ on timing for this return, but there is some
indication that an early return is possible tonight by the 03-04z
time frame. Weighing the pros and cons of the set up versus the
model output, the strong onshore gradient looks to be the biggest
player for encouraging an earlier return. Thus, have added tempo
groups for most locations to include an earlier hedge time frame
for possible early return of stratus. As such, low confidence for
tonight forecast given pattern change and model uncertainty.
Otherwise, VFR today with breezy onshore winds this afternoon.
Vicinity of ksfo... VFR. Breezy, occasionally gusty, onshore winds
through the afternoon and into the night. Peak sustained wind in
the low 20kts with gusts to around 30kt possible. Low confidence
on timing of return, but currently carrying MVFR cigs returning by
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR today, light to breezy onshore winds
today. MVFR/IFR cigs tonight.
Marine...as of 03:14 am PDT Wednesday...an upper level trough
off the West Coast combined with surface high pressure off the
California coast will keep moderate to strong northwest winds
through Friday. Gale force gusts can be expected over the southern
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...sf Bay until 9 PM
Public forecast: Bell
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