Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1043 PM PDT sun Oct 19 2014
Synopsis...a cold front will spread rain across most of the San
Francisco Bay area from late tonight into Monday evening. Dry
weather will return by Tuesday. Warming is then forecast for the
second half of the week as high pressure strengthens over central
and Southern California.
Discussion...as of 08:45 PM PDT Sunday...low clouds have been
slow to develop this evening ahead of the approaching front
keeping skies mostly clear over the County Warning Area. Increasing stratus
expected with some drizzle is possible along the immediate coast
tonight. The cold front approaching the West Coast is now bringing
rain as far south as Astoria Oregon and will continue to spread
further south overnight. The 00z models continue to trend drier
and areas south of the Golden Gate are likely to see very light
precipitation. South of the Santa Cruz Mountains the models still
indicate isolated rainfall though likely not enough for many
Made a quick update to account for current conditions and short
term trends...otherwise forecast is on track.
Previous discussion...as of 3:00 PM PDT Sunday...outside of a few pockets
of clouds at the coast generally sunny conditions across our area.
Temperatures are running a bit behind from yesterday with a
slightly stronger westerly flow at the surface plus lower 500 mb
heights as a longwave trough nears the coast.
Rain returns to our County Warning Area late tonight as the trough axis and
associated front move to the pacnw/norcal region. Similar to the
past couple of events, the bulk of the rain will remain north of
our area with the southern edge forecast to mostly impact the
North Bay. Easily more than 1/3" could fall over the North Bay
hills by Monday afternoon. Bigger question remains as you get
farther to the south. 18z NAM run just in continues to be trending
drier compared to the 1z (which was drier than previous guidance).
This is in line with GFS/European model (ecmwf) which also keeps most of the rain
north of sf. Precipitable water values are forecast to be over an inch through
tomorrow morning, so feel justified in keeping at least a chance
of rain going for the entire sf Bay region. However, rainfall
totals will likely be unimpressive -- a tenth or less. For Santa
Clara County southward, odds favor no rain falling. Any
precipitation will end by Monday night.
Quiet weather is forecast for the remainder of the work week as a
ridge of high pressure builds in behind the departing storm.
Temperature guidance from the European model (ecmwf) MOS shows widespread 70s to the
coast with upper 70s to middle 80s inland toward the end of the
week, so temperatures were pushed up a bit for Thursday into Saturday.
Another longwave trough advance to the West Coast by the early part
of next week, however virtually all associated rainfall will be to
Aviation...as of 10:40 PM PDT Sunday...coastal stratus is slowly
spreading over the sfo Bay area. Clouds should continue to
increase as frontal system approaches. Best chance of any rain
will be middle morning through early afternoon when the front moves
Vicinity of ksfo...becoming MVFR. Winds remaining under 10 knots but
switching more southwest on Monday.
Ksfo bridge approach...same as ksfo.
Monterey Bay area terminals...stratus has stayed in northern mry Bay
leaving mry and sns clear at this time. More offshore stratus
moving towards mry and sns with IFR ceilings. Ceilings may be high enough
to be MVFR.
Marine...as of 10:45 PM PDT Sunday...a frontal system will move
into the northern waters early Monday morning. Winds will be
light southerly in the northern waters and light west to northwest
in the southern waters through Tuesday. Winds will switch to
northwest over all waters Wednesday through Friday. A large
northwesterly swell generated by a storm in the Gulf of Alaska
will arrive at the coast tonight. Long period forerunners will be
arriving tonight with the larger waves arriving Monday.
Public forecast: ac/Bell
aviation/marine: west pi
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