Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
141 PM PST Tuesday Nov 25 2014
Synopsis...dry weather and seasonably warm temperatures will
persist through Thanksgiving day as a result of high pressure over
the region. Unsettled weather will return to the North Bay as early
as Friday with rain slowly spreading southward through Saturday.
Widespread precipitation is then expected Sunday into Monday as a
stronger middle/upper level system pushes through the region.
Discussion...as of 01:40 PM PST Tuesday...the forecast through
midweek remains tranquil as high pressure dominates the middle/upper
level pattern over much of the West Coast. Generally sunny
skies...dry weather and temperatures above seasonal averages will
persist as a result. This pattern also supports decent travel
conditions heading into Thanksgiving day with no precipitation or
widespread fog over the San Francisco/Monterey Bay area.
The middle/upper level ridge will begin to flatten on Thursday and
weaken further through late week as the upper level jet drops
southward toward northern California. In addition...a deeper plume
of moisture will advect into northern California and bring
precipitation to that part of the state as early as Thursday night.
Rain is then forecast to slowly spread southward Friday and Friday
night bringing rain to portions of the North Bay. Overall...the best
chance for rain on Friday will be north of the Golden Gate Bridge.
Precipitation will then slowly spread southward Friday night into
Saturday as a series of middle/upper level disturbances push into
northern California and the Pacific northwest. There remains some
uncertainty in the model solutions with respect to how far south the
rain will advance on Saturday. With that said...will keep the best
chances for precipitation north of the Santa Cruz Mountains Friday
night into Saturday. The heaviest rainfall is likely to be confined
to the North Bay where a few inches will be possible. It is Worth
nothing that the southern third of the forecast area may stay dry
Rain will then spread across the entire region Sunday and Monday as
the forecast models show a stronger middle/upper level low and
associated trough approaching the West Coast. Widespread moderate to
at times heavy rain will be possible given the increased middle/upper
level dynamics and stronger jet aloft. The GFS and European model (ecmwf) both differ
on the timing of the frontal passage...and thus the heaviest band of
precipitation. However...it looks like Sunday night into Monday
appears to be the best window for this to occur. Breezy to windy
conditions are also expected Sunday night into Monday when the core
of the surface low approaches the region...leading to the
possibility of downed trees and power lines.
The middle/upper level trough axis will then shift inland Monday night
into Tuesday with lingering showers possible across much of the
region. While it remains very difficult to predict rainfall totals
at this point...it is Worth mentioning that the GFS/ECMWF/CMC all
show some portions of the region receiving 3 to 5 inches of rainfall
from Friday through Monday night. The latest wpc 1-7 day forecast
depicts 3 to 5 inches over the North Bay...2 to 3 inches across the
Santa Cruz Mountains and 2 to 2.5 inches for the satan Lucia
mountains. Meanwhile...inland areas could pick up anywhere from 1/2
inch in the southern inland valleys to nearly 2 inches in the urban
areas of the San Francisco Bay area. Again...it is important not to
focus too much on these rainfall amounts at this time given the
expected fluctuations in the forecast models in the coming days.
Conditions dry out by the middle of next week with a ridge building
back across the region in wake of the exiting middle/upper level system.
Aviation...as of 9:50 am PST Tuesday...dry airmass continues
over the area for VFR conditions today. Some patchy fog could
develop near Santa Rosa near sunrise tomorrow.
Vicinity of ksfo...VFR.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 09:45 am PST Tuesday...high pressure will move
into the Great Basin tonight. Winds will become light east to
southeast by Wednesday as a frontal system moves into the Pacific
northwest. Southerly winds will increase by the end of the week as
a stronger storm approaches the area.
Public forecast: rgass
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