Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
425 am PST Friday Nov 28 2014
Synopsis...a frontal system will bring rain to the North Bay today
and will spread southward across the San Francisco and Monterey Bay
areas through Saturday. Unsettled conditions will persist through
early next week in advance of a stronger upper level low forecast to
impact the region by the middle of next week.
Discussion...as of 04:06 am PST Friday...a mix of radiation and
advection fog being reported around the North Bay...Monterey Bay
and portions of the San Francisco Bay this morning. Areas of
fog...dense at times...will likely continue through the early
morning with conditions slowly improving after sunrise.
Otherwise...temperatures are generally in the 40s region-wide this
morning along with weak surface winds.
Our attention turns to the north where a deeper plume of moisture
continues to advect into northern California with rain currently
pushing onshore. This feature is forecast to slowly drop south
through the next 24 hours as a middle/upper level disturbance sweeps
into northern California. Rain will develop over the North Bay as
early as this afternoon and will spread southward across the San
Francisco Bay area later in the day. The rain is expected to
increase in intensity tonight across the San Francisco Bay area
southward through the Santa Cruz Mountains when a secondary
disturbance moves through. By Saturday morning...light rain will
likely push down into the Monterey Bay region where the greatest
rainfall amounts will likely be confined to the coastal ranges.
Another round of heavier precipitation is then expected Saturday
night into Sunday as another impulse approaches the region...likely
from the Santa Cruz Mountains southward down the Big Sur coast. It
is Worth keeping in mind that given the lack of strong middle/upper
level support...it remains difficult to pinpoint the exact location
and timing of these heavier episodes of precipitation.
The latest forecast models continue to show a decrease in activity
from Sunday night through Monday as a weak middle/upper level ridge
builds in ahead of a stronger middle/upper level low forecast to
develop offshore. While a break is expected...lingering showers will
be possible over much of the region with an ample amount of moisture
aloft. The main middle/upper level system is then forecast progress
toward the coast and will likely bring another round of widespread
rainfall to the San Francisco/Monterey Bay region as it pushes
inland Tuesday into Wednesday. Breezy and at times gusty winds
remain a possibility Tuesday and Wednesday depending on the strength
and track of the surface low pressure associated with the upper
Will keep a mention of showers through the remainder of next week as
the European model (ecmwf)/CMC continue to show unsettled conditions over the region
as weak disturbances ride along the mean flow. When all said and
done...we could pick up 3"-5" across the coastal ranges and 1"-3" in
most urban and valley locations through midweek.
Aviation...as of 4:23 am PST Friday...areas of fog continue to
develop as seen on satellite and area metar observations. The fog
layer is mostly shallow...even here at the National Weather Service office we could see
a couple stars at times through a brief thinning of the fog layer.
The 12z Oakland sounding is interesting showing a low precipitable
water (pw)...only 0... not much different from last
evening's similarly dry sounding. A deep layer of dry air through
most of the atmosphere is allowing for decent radiational cooling.
All of this adds up to moderate to high confidence IFR conditions
Vicinity of ksfo...conditions deteriorating at ksfo. IFR will
settle in with visibility from 1/2 to 2 miles and a ceiling
between 200 and 600 feet until 18z. VFR is forecast to return from
late morning hours through the afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...deteriorating conditions here as
well with IFR forecast until 18z.
Marine...as of 04:06 am PST Friday...weak high pressure off the
Southern California coast will persist today. Southerly winds will
develop during the day as a frontal system approaches the area.
These southerly winds will increase Sunday or Monday as another
storm system approaches California.
Public forecast: rgass
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