Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1046 PM PDT Thursday Jul 10 2014
Synopsis...an upper level low pressure off the coast will produce
a slight chance of late night thunderstorms over the extreme
northeast portions of the North Bay. Otherwise...little change is
expected through Friday with low clouds overnight giving way to
mostly sunny conditions during the day for most locations. High
pressure over the Desert Southwest will build over California
starting on Saturday...resulting in a warming trend over the
weekend...especially for inland areas where locally hot conditions
will develop by Sunday.
Discussion...as of 8:50 PM PDT Thursday...southerly flow and a
relatively deep marine layer combined to keep inland temperatures on
the cool side of normal today...as much as ten degrees cooler than
normal in the North Bay valleys. Meanwhile...enough afternoon
clearing at the coast allowed most coastal areas to warm close to
Short-term focus continues to be on the possibility of
thunderstorms over the far northeastern portion of our forecast
area tonight. Water vapor satellite imagery currently shows a
weak upper low west of point area. Circulation around this low is
drawing middle level moisture north-northwest out of east-central California.
Some of this middle level moisture is expected to clip Napa County
later tonight. But the important question is whether there will be
sufficient middle-level instability to trigger elevated convection.
The 00z NAM is consistent with previous model runs in forecasting
pretty decent middle-level instability across the northern Sierra
and far northern California mountains overnight as a weak shortwave
rotates around the upper low. However...the instability gradient
to the southwest of these unstable areas is very tight with the
middle levels forecast to be much more stable over our area. Only
northeast Napa County is clipped with some marginal tt values late
tonight...with MUCAPE values even less impressive. Therefore...the
odds of thunderstorm development far enough to the south and west
to affect any portion of the North Bay later tonight appear very
low. But will keep the slight chance of thunderstorms in the
forecast for northeastern Napa County for the time being. By
Friday morning the upper low offshore will have lifted to the
north...causing the flow in the middle and upper levels to veer to
the SW and advect a much drier and more stable airmass across our
entire region...thereby dropping convection chances down to near
Little change is forecast as we move into Friday. Southerly flow
will persist and the marine layer will likely remain around 2000
feet deep. Much warmer weather will develop inland over the
weekend as the broad upper ridge currently over the southwestern
U.S. Builds westward over California. 850 mb temperatures are forecast to
increase by as much as 6 degrees c by Sunday. More importantly...the
building upper ridge will greatly reduce the depth of the marine
layer over the weekend and thereby significantly decrease or
eliminate the influence of cool marine air on areas well inland.
Some of the inland valleys will warm by as much as 15 degrees from
Friday into Sunday. Forecast highs for the inland valleys on
Sunday will probably need to be nudged at least a few degrees
higher depending on what the rest of the 00z model output shows.
Similar to recent warm-ups...the upcoming warming trend over the
weekend will mostly be limited to inland areas since onshore flow
is forecast to persist. Night and morning low clouds and fog will
mostly be confined to areas near the ocean over the weekend and
into early next week.
The upper ridge is forecast to weaken slightly early next
week...which will allow slightly cooler air to filter inland by
midweek. Otherwise...conditions next week should be typical for
Aviation...as of 10:46 PM PDT Thursday...the weather pattern is
very similar to that of last evening with a low pressure center
positioned just off of the coast maintaining southerly flow for
the area. The marine layer has deepened substantially stratus has
moved well inland. The southerly flow is however keeping stratus
away from sfo and Oak for the time being.
Vicinity of ksfo...VFR is expected to prevail through at least
midnight. MVFR ceilings are in the forecast beginning 1000z. First
guess for morning stratus burn off 1800z.
Confidence is moderate
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings have return to the terminals
for the evening. Ceilings are expected to fall to ovc008 at both
terminals around midnight. First guess for morning stratus burn
off is 1900z at kmry and 2000z at ksns.
with an upper level low moving towards the area late tonight and
some moisture present above the surface...have included a slight
chance of thunderstorms in extreme northeast Napa County for
tonight. Chances will remain very low and so will not issue any
red flag warning products at this time but wanted to highlight
awareness to the low end threat of storms in a small portion of
the forecast area. After this threat tonight...warm and dry
weather returns with interior areas becoming hot again by the
Marine...as of 10:46 PM PDT Thursday...a weak low pressure system
remains positioned just off the coast this evening. This low will
maintain southerly winds across the coastal waters through Friday.
Northwest winds will return Saturday as the low moves inland and
high pressure builds off the West Coast.
... Bay north of the Bay Bridge until 11 PM
Public forecast: dykema
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