Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1047 PM PDT Friday Jul 31 2015
Synopsis...a cooling trend is expected to continue into the
weekend as onshore flow increases and the marine layer deepens.
Subtropical moisture will move out of our area overnight and
isolated thunderstorms are no longer expected.
Discussion...as of 8:55 PM PDT Friday...the shortwave trough
that triggered isolated showers and thunderstorms across our area
today has moved well to our north this evening. Radar has detected
no additional convective precipitation activity since early evening. A
forecast update was recently completed to remove all mention of
showers and thunderstorms for this evening. Drier air will move
into the middle and upper levels of the atmosphere tonight and
Saturday as the middle/upper flow veers from southeast to southwest.
Thus...convective precipitation is no longer a possibility...at
least not through our current forecast period.
The cooling trend that got underway on Thursday continued today as
southerly flow and a deeper marine layer allowed marine air to
infiltrate farther inland. The Livermore airport's high of 87
today was 20 degrees cooler than wednesday's high of 107.
Temperatures are expected to continue their downward trend through
the weekend...albeit at a slower pace...as the upper ridge over
the western Continental U.S. Shifts eastward and a trough remains off the
West Coast. Also...low level onshore flow is forecast to increase
a bit more over the weekend. Most inland valley locations will see
temperatures at least slightly below normal through the weekend.
Mild temperatures will then continue through next week as the
upper trough offshore advances on the West Coast.
Fort Ord profiler data indicate that the marine layer has
deepened past 2000 feet this evening. Would therefore expect
widespread low clouds and patchy fog to develop overnight and into
early Saturday. Most inland areas will clear by midday tomorrow
and Sunday...but expect at least areas of low clouds to persist
near the ocean through the afternoon hours.
Aviation...as of 10:50 PM PDT Friday...plenty of low clouds are
impacting the terminals tonight. The current GOES MVFR probability
product is showing stratus penetrating both San Francisco and
Monterey bays as well as northern portions of the Salinas valley.
According to The Fort Ord profiler the marine layer has become
pretty solid at a depth of 2000 feet. With the moist boundary
layer and this deep marine layer in place tomorrows stratus
burnoff could stick around a little bit longer than typical.
Vicinity of ksfo...bkn-ovc015 is in place over the terminals
tonight. Ceilings are expected to deteriorate to ovc008 by 1200z.
First guess at the morning burn off is 1830z.
Confidence is moderate.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...currently ovc009 at Monterey
conditions are expected to continue to deteriorate to around 600
feet by 1000z with a period of 3sm br bkn003 possible between 1200
Confidence is moderate.
Marine...as of 8:50 PM PDT Friday...a weak onshore surface
pressure gradient will maintain generally light winds and gentle
seas through the weekend. Although...generally light winds are
expected to prevail please keep in mind that the afternoon sea
breeze can be gusty this time of the year.
... Bay until 11 PM
Public forecast: dykema
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