Scientific Forecaster Discussion
fxus66 kmtr 232352
Area forecast discussion
National Weather Service San Francisco Bay area
452 PM PDT Fri Jun 23 2017
Synopsis...cooling trend started today for most areas as onshore
flow returns. The far interior valleys will remain seasonably
warm through this evening. The overall trend this weekend and into
next week will be a return to more seasonable temperatures along
with more extensive night and morning low clouds followed by sunny
Discussion...as of 01:28 PM PDT Friday...after a prolong period
of heat, the much talked about cool down began today. Afternoon 24
hour trends are still impressive with a solid 10 to 20 degree
drop from Thursday to Friday. Some of the biggest drops are:
Santa Rosa 17 degrees, Livermore 15 degrees and Oakland 15
degrees, Los Altos 19 degrees and Spring Valley 21 degrees.
However, despite the cool down still want to highlight the
cumulative effects of the prolonged heat combined with highs
exceeding 90 degrees impacting sensitive groups. Far interior
locations might be cooler than Thursday, but will likely end up in
the mid and upper 90s today. Therefore, a heat advisory remains in
effect through this evening for interior East Bay, mountains of
Monterey cty and interior San Benito.
The cool down noticed today will continue through the weekend. No
heat advisories/warnings will be needed. In fact, night and
morning low clouds will be common place as temperatures return to
near normal by next week.
One major change to the forecast was to include a slight chance
of thunderstorms over Monterey/San Benito late tonight into
Saturday. Confidence is rather low, but if it verifies impacts
could be huge, especially from a fire weather perspective. For
several days now, models have been keying in on the
moisture/convection near baja tracking north. Latest models do
bring the mid-upper level moisture (700-500mb) northward tonight
and early Saturday. In addition to the moisture advection, upper
level instability increases. Mu cape through the layer increases
with modified total totals greater than 30 all indicating upper
level lift. At this point only giving it a 15 pct chance of
occurring, but still wanted to add it to the forecast.
Medium and longer range models still indicate later next week and
towards the end of the month show more pronounced ridging with
potential for another round of weak offshore flow and associated
warming. CPC has the 6-10 forecast above normal temps for California.
Aviation...as of 4:50 PM PDT Friday...for 00z tafs. Widespread
low clouds and fog continue at the coast and were beginning to
developing locally inland by late Friday afternoon. Marine layer
remains relatively shallow and so IFR ceilings probably won't
develop around sf Bay until after evening rush. However, IFR
ceilings will likely develop near Monterey Bay by early evening.
Onshore flow has increased so there will likely be earlier and
more widespread development of low cigs overnight. In addition, a
weak upper level disturbance approaching from the south on
Saturday morning will produce some mid/high level clouds on
Saturday and delay clearing of low clouds due to deepening marine
layer. Moderate afternoon and evening onshore winds.
Forecast confidence: moderate
Vicinity of ksfo...VFR conditions expected to persist through the
evening. IFR cigs expected to return overnight tonight, sometime
around 10z at ksfo, and sooner for koak. Westerly winds around 20
knots through early evening, and then diminishing. Low clouds
expected to clear by around 17z Saturday.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...similar to yesterday, low cigs likely to
develop at ksns a few hours before at kmry due to light southerly
flow along the coast. IFR ceilings then expected overnight and
through much of Saturday morning with later clearing on Saturday
compared to today.
Marine...as of 10:23 am PDT Friday...generally light southerly
winds over the coastal waters today. Onshore winds will be breezy
through the Golden Gate gap and across the northern San Francisco
Bay. Northwest winds return this weekend. Generally decreasing
seas through the coming days.
Small Craft Advisory...sf Bay until 9 PM
Public forecast: mm
Visit US at www.Weather.Gov/sanfrancisco