Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
905 PM PDT Wed Oct 26 2016
Synopsis...rain will move into the area Thursday morning then
increases in coverage and intensity late Thursday afternoon
through early Friday morning with periods of heavy rain likely,
especially for the coastal hills in Monterey County. Showers will
taper Friday afternoon. Unsettled weather expected to continue Saturday
before another front arrives from the northwest with rain likely
late Saturday night and Sunday. Halloween afternoon and evening
looks mostly dry with another system possible by late Monday into
Discussion...as of 9:05 PM PDT Wednesday...temperatures topped
out in the 60s along the coast, 70s inland and low to mid 80s in
the warmest inland valleys today under mostly sunny skies. A
significant change in the weather is expected tomorrow as the next
in a series of storms take aim at the region.
Latest model runs indicate rain will begin as early at Thursday
morning becoming widespread across the forecast area through the
day. Doppler radar is already picking up return well off the coast
at this hour with all indications pointing to an early morning
start to the rain over the North Bay Thursday. Moisture
associated with hurricane Seymore spinning about 775 miles west
southwest of the southern tip of baja Mexico will become in
entrained in the southerly flow of the frontal boundary moving
toward the central coast. Precipitable water values of over 1.5"
are forecast to move in over Monterey County Thursday afternoon
and evening. Periods of heavy rainfall are anticipated Thursday
night into Friday as an upper level closed low pulsates around
the longwave trough collides with the plume of moisture as it
moves onshore over the Big Sur coast. These enhanced dynamics will
translate into rainfall totals of 2 to 4 inches over the Santa
Lucia Range. Along with increased rainfall rates this shortwave
will bring an increased chance of thunderstorms over the area
south of Big Sur. A Flash Flood Watch remains in effect for the
soberanes and the chimney fire burn scar.
From previous discussion...rainfall totals have been adjusted
upward from the overnight package for most spots especially for
Monterey County. Totals by Friday afternoon will likely range from
1/2" to 1" for many urban spots to more than 4" for the Santa
Lucia Range. Thankfully winds will not be as much of an issue
compared to previous events.
Rain will switch to showers with a few forecast to be around into
Saturday. By Saturday an upper level low and surface low will move
toward the norcal coast directly from the west. Unlike the first
system, the location of the low will bring higher rainfall amounts
to the North Bay with 1-2" likely in parts of the North Bay
mountains. Rainfall will be less for the remainder of our region
with another 1/3 to 2/3" around sf Bay with small amounts for
inland spots to the south. Santa Cruz Mountains could pick up
another inch. Rainfall will generally end late on Sunday with just
a few showers forecast for Monday.
In many cases, we will be fortunate with the first two systems and
both of them will focus the bulk of rainfall on different parts of
our County Warning Area. That should mitigate some of the flooding potential.
For those with Halloween plans, Monday should stay mostly dry for
the afternoon and evening before a third system quickly moves through
Monday night into Tuesday. This feature will be moving in from the
northwest with the center of the low expected to go over Monterey County
or San Luis Obispo County. Due to the speed of the system plus precipitable water
values not exceeding 0.95" through the period rainfall amounts
should be less than the first two systems.
Model solutions then diverge for the second half of next week
with some brining in a fourth system while the others indicate dry
weather. CPC 8 to 14 day outlook does favor wetter than normal
conditions, so active weather remains in the cards at least
through November 9th.
Aviation...as of 6:51 PM PDT Wednesday...little to no change from
the previous discussion. Moderate-high confidence VFR this evening.
Winds are light onshore. Ceilings will deteriorate and rain chances
will increase towards the end of the taf package as a frontal boundary
shifts further southward. MVFR ceilings and visibilities develop
Vicinity of ksfo...VFR with winds shifting to southeast later this
evening. Vcsh beginning 12z with MVFR cigs developing around 18z
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions this evening with IFR-LIFR
cigs possible later tonight and Thursday morning. Low confidence.
Marine...as of 2:51 PM PDT Wednesday...gradually decreasing
southerly winds will prevail through tonight and into Thursday
ahead of an approaching frontal boundary. Then split flow will
develop across the coastal waters... with northerly winds north of
Point Reyes and southerly winds persisting south of Point Reyes.
Light to moderate northwest seas will gradually subside through
the work week before a larger northwest swell train arrives early
into the weekend.
Public forecast: CW
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