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Area forecast discussion 
National Weather Service San Francisco Bay area
1009 PM PST Friday Nov 28 2014

Synopsis...periods of rain are forecast from tonight through the
weekend. Rainfall through the weekend will mostly be light to
moderate in intensity...with brief localized heavy rain possible.
Rain chances will diminish on Monday...but then a potentially
stronger weather system will bring more rain to the region from
Monday night through early Wednesday. Rain could be heavy on
Monday night and Tuesday and will be accompanied by strong and
gusty winds.

&&

Discussion...as of 9:00 PM PST Friday...rain began to spread into
the North Bay late this afternoon and has become more widespread
north of the Golden Gate this evening. Amounts thus far have
generally been less than 0... some locations in
Sonoma County have picked up between 0.25-0.50".

00z models show the frontal rain band reaching San Francisco by
midnight tonight and then slowly sagging south through the rest of
the sf Bay area late tonight...and possibly reaching Santa Cruz by
daybreak Saturday. Rain rates look to be mostly light to
moderate...although the Sonoma County Airport has already reported
a brief episode of heavy rain. The best potential for heavier rain
rates will be late tonight into early Saturday morning when a
shortwave trough moving in from the west-southwest enhances the frontal
boundary. Based on the 00z NAM and 00z GFS...this enhancement will
most likely occur across the sf peninsula and into the east and
South Bay sometime between about 4:00-8:00 am Saturday. Once this
shortwave moves to our east late Saturday morning...the frontal
boundary is forecast to stall out across the Monterey Bay
area and weaken rather rapidly...resulting in precipitation tapering
off on Saturday afternoon.

The next shortwave trough will approach from the southwest on
Saturday night and result in renewed rainfall late Saturday night
into Sunday. The models are all over the place as far as precipitation
intensity and location of heaviest rainfall on Sunday. So forecast
confidence is low on the potential for Hydro impacts later this
weekend. In any case...both the 00z NAM and 00z GFS show only
moderate rainfall amounts on Sunday and so Hydro issues should
remain minor. Winds through the weekend will not be a factor.
Winds will increase somewhat on Sunday...but mainly over the
coastal waters. Winds over land should remain below advisory
levels through Sunday night.

Monday continues to look like a relatively dry day as a shortwave
ridge temporarily builds over California...downstream from a deep upper
low offshore. That low will gradually approach the California
coast early next week. It's this system that has the potential to
produce the most widespread and heaviest rainfall over the next
several days...and will definitely be the one to produce the
strongest winds. Rain will begin to spread across the area on
Monday night...with the bulk of the precipitation expected to occur on
Tuesday and Tuesday night when periods of heavy rain are possible.
Thunderstorms may also occur during this time frame...although the
models keep most of the significant instability over the coastal
waters.

Southerly winds will increase on Monday in advance of the
approaching system...but mainly over the coastal waters. Locally
strong and gusty winds will develop over land by late Monday night
and especially on Tuesday and Tuesday evening. Strongest winds
will be near the immediate coast and in the coastal hills where
gusts to 50 miles per hour are possible. Wind advisories may be needed from
late Monday night through Tuesday evening. The hazardous weather
outlook was updated this evening to draw attention to this period
of strongest winds.

Showers will continue into Wednesday as the upper low accelerates
to the northeast. The upper trough axis is forecast to move inland
by late Wednesday morning and so showers will probably taper off
on Wednesday afternoon.

Rain totals for this entire event...from this evening through the
middle of next week...are expected to be impressive...but have
been difficult to predict with a great deal of confidence due to
model inconsistencies. In general...widespread rain totals of
between 1 and 3 inches are likely in most urban locations...with 3
to 5 inch totals certainly attainable in the coastal hills by the
middle of next week. If enhanced frontal rain bands stall in any
one location for any length of time...localized amounts of six
inches or more are possible. One factor that may inhibit rainfall
intensity in the coastal hills early next week is wind direction.
Because the surface low will be approaching from almost due west
late Monday and Monday night...low level winds will initially
have an easterly component which will mean greatly reduced
orographic enhancement of rainfall in the coastal hills. Low level
winds will eventually veer to the south and southwest...much more
favorable directions for lift along the coastal ranges...by late
Tuesday and Tuesday night...but then only across the southern
part of the area.

&&

Aviation...as of 10:00 PM PST Friday...a Pacific storm and the
associated surface front are pushing through the region this
evening and will continue into Saturday am. Short term models
suggest precipitation band could enhance early tomorrow morning... so
greatest impact is likely to be during that same period with MVFR
to IFR ceilings and visbys accompanying the heaviest rain. Southerly
winds and wet runways will occur during the morning hours. Light
rain showers into the afternoon.

Vicinity of ksfo...precipitation band now moving into the vicinity.
Cloud base will lower to around 2000ft above ground level with solid rain by 10-12z
Saturday. South- southeast winds up to 10 kts. Brief gusts up to
20 knots may accompany frontal passage.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...brief advection fog earlier this
evening has diminished. VFR expected through remainder of the
night. MVFR late in the forecast with precipitation developing at or
just before sunrise.

&&

Marine...as of 09:49 PM PST Friday...southerly winds will
increase over the coastal waters tonight and Saturday as a storm
system develops off the coast. Winds will be gusty Saturday and
Sunday producing hazardous conditions with a fresh steep swell.
The storm system will also bring periods of moderate to heavy
rain. Unsettled weather is expected for much of next week as more
storm systems move through the region.

&&

Mtr watches/warnings/advisories...
...

&&

$$

Public forecast: dykema
aviation: drp
marine: mm

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