Scientific Forecaster Discussion
fxus66 kmtr 221507
Area forecast discussion
National Weather Service San Francisco Bay area
807 am PDT Sat Jul 22 2017
Synopsis...warmer temperatures are expected for Saturday
afternoon, especially inland. Temperatures will then begin to
moderate on Sunday as a weak upper level trough begins to develop
off our coast. A more pronounced cooling trend appears on tap for the
first part of next week, with a deepening marine layer and
increased onshore flow. But then by the latter part of the week,
high pressure looks to build back in again resulting in a return
to warming temperatures. Aside from some possible coastal
drizzle, no precipitation is presently expected in our area
through the forecast period.
Discussion...as of 08:07 am PDT Saturday...clouds developed
overnight along the coast and continue to impact areas from Point
Reyes south along the central coast. The Fort Ord profiler
indicates the marine layer has increased a bit within the past
several hours to around 1,200 ft. San Francisco Bay area airports
have managed to stay mostly clear this morning while Monterey and
Salinas continue to be plagued with overcast conditions with
ceilings around 500 ft. Temperatures at the major airports are
within a few degrees from what they were at 24 hours ago, but
there are noteworthy changes in the hills where many locations are
running about 4 to 8 degrees warmer. Additionally, the 12z
Oakland sounding shows warming at 850 mb -- today it is 22.0 deg c
whereas yesterday it was 20.4 deg c. This will translate to 5 to
10 deg f warming at the surface for many interior areas with the
hottest locations surpassing the century mark.
No major changed planned for the forecast this morning. For
additional details, please refer to the previous discussion.
Previous discussion...as of 3:45 am PDT Saturday...skies are mostly clear
early this morning, aside from areas of coastal stratus from
about the Marin Headlands south to pt sur and extending down the
Salinas valley. Lower elevation temperatures are mostly in the
mid to upper 50s. Latest Bodega Bay and ft Ord profiler data show
some shallowing of the marine layer over the past 24 hours, along
with diminished west-northwest flow in the boundary layer and
warming of the air mass above. Consistent with this, have noted
milder temperatures up in the hills. 2:45 am reports from the Mt
tamalpais and Ben Lomond RAWS sites for example, indicated a temp
of 70 deg, 5 deg warmer at both than the same time Thursday
Latest model runs are consistent in showing significant additional
temperature increases aloft during the day today, as upper level
high pressure temporarily builds over California. Today's afternoon
Max temps look to be 5 to 10 deg warmer than those on Friday
inland, with hottest locations approaching 100 deg (and locally into
the lower 100s in the southern interior). Much less of a warm-up
is expected along the coast however, as there the Marin air
influence will continue.
Although the airmass aloft is forecast to remain similarly warm
going into Sunday, projected onset of southerly flow in the
coastal marine boundary layer will lead to some diminishment in
afternoon highs for much of our area. In contrast, however,
marine air protected portions of the southern interior will likely
be little changed, with hottest locations again reaching back into
the lower 100s.
An upper level trough is forecast to begin developing off the
northern California coast on Sunday, deepen a bit on Monday, and
before finally weakening and shifting inland across northern
California Tuesday night. This will result in deepening of the
marine layer and cooling of the air mass aloft. Expect stratus
coverage to be significantly more extensive Sunday night into
Monday morning, along with the possibility of some local coastal
drizzle. Afternoon highs on Monday look to be quite a bit cooler,
in many locations even down to below seasonal norms for the date.
Tuesday looks similar, or just a bit cooler still, as temperatures
aloft continue to diminish a bit and the marine air influence
works its way a bit farther inland. But then starting about
midweek, a return to a warming trend is projected, as upper level
high pressure begins to build back in.
Finally, we will need to continue to monitor for the possibility
that southerly flow aloft late in the weekend and into the early
part of next week might bring sufficient mid and upper level
monsoonal moisture into our area as to result in possible showers
or thunderstorms. Latest model runs continue to keep the bulk of
it to our east, but not by very much.
Aviation...as of 4:35 am PDT Saturday...for 12z tafs. Stratus has
already moved into the Salinas valley and is forming off the
coast west of the sf peninsula. Not at this time expecting cigs to
spread through to koak and ksfo. Westerly winds increasing this
afternoon with gusts over 20 kt at ksfo by around 20-21z.
Vicinity of ksfo...VFR currently and expected to prevail.
Gusty westerly winds this afternoon to over 20 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...IFR cigs over mry and sns. Expecting
clearing sometime between 15 and 17z.
Marine...as of 08:05 am PDT Saturday...high pressure over the
eastern Pacific will shift to the northwest today as a thermal
trough extends towards the central coast of California. As a
result, winds will begin to decrease for most of the coastal
waters. The northern outer waters, however, will remain gusty
through the weekend.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm until 9 am
Small Craft Advisory...sf Bay from 1 PM
Public forecast: Rowe/blier
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