Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1051 PM PST Sat Jan 31 2015
Synopsis...high pressure will maintain dry and mild weather
across the region through the middle of next week. Late in the
week a Pacific weather system will gradually spread rain southward
into the San Francisco Bay area. Mild temperatures are expected
throughout the week.
Discussion...as of 9:05 PM PST Saturday...a record dry January
ended on a record warm note. Dry offshore flow and an unseasonably
warm airmass combined to produce record high temperatures
throughout the San Francisco Bay area. Nine daily high temperature
records were set or tied today including at downtown San
Francisco...Oakland and San Jose.
Offshore winds in the hills have been subsiding since this
afternoon...but light offshore flow still continues in most areas.
Surface pressure gradients...which had been trending light onshore
by late afternoon...have since gone flat or reverted to light
offshore. So there is currently not much evidence to support a
significant inland influx of low level moisture overnight...even
through low clouds have already formed along the San Mateo County
coast. The NAM and WRF models do indicate a trend towards greater
low level moisture near the coast...into the North Bay...and
around sf Bay tonight and into Sunday morning. This will likely
result in patchy low clouds and fog by daybreak Sunday. But do not
expect widespread fog and low clouds...especially for inland
Lack of offshore flow will mean slightly cooler temperatures on
Sunday. Record highs are not likely tomorrow...although a few sf
Bay locations like San Jose might get close to record warmth.
Temperatures will remain above normal. After patchy low clouds/fog
clear off...skies will be mostly sunny except for occasional high
Not much change is forecast through the first half of the work
week. The upper ridge over California will flatten slightly as a couple of
weather systems move over the ridge and into the Pacific northwest. Precipitation
with these systems will remain to our north through at least early
Thursday...but we can expect to see occasional middle and high level
clouds. Temperatures will remain above normal.
The medium range models have been consistent the past few days in
developing an atmospheric river event by late next week and into
next weekend...with significant precipitation potential for
northern California. The GFS in particular is showing a sharp demarcation
between large rainfall amounts north of the Golden Gate...rapidly
tapering off to no measurable rain south of the sf Bay area. The
European model (ecmwf) forecasts precipitation to reach much farther south...but
with much lighter amounts across northern California. This type of warm
advection/atmospheric river pattern typically produces the results
that the GFS is forecasting. However...the GFS is having a tough
time deciding exactly where to place the interface between
significant precipitation and almost no precipitation...as this demarcation has
been shifting north and south with each successive model run. Will
need to wait for better model consistency and agreement before
forecast confidence can develop concerning heavy rain potential
and exact locations. For now it looks like the North Bay may be
the only part of our forecast area with the potential for
significant rainfall late next week and into next weekend.
Aviation...as of 10:45 PM PST Saturday...VFR except for a few
patches of fog or low clouds by early Sunday morning.
Vicinity of ksfo...VFR. Tempo MVFR visibility possible Sunday
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...clear through this evening. Patchy
low clouds possible Sunday morning. VFR for most of Sunday.
Marine...as of 10:45 PM PST Saturday...northwest wind and seas are
forecast for the rest of the weekend and into early next week with
high pressure over the waters. The winds will become southeasterly
over the coastal waters middle to late week with a chance of rain
developing over the northernmost waters by Thursday.
Public forecast: dykema
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