Technical NOAA Weather Discussion

FXUS66 KMTR 171218

Area Forecast Discussion
National Weather Service San Francisco Bay Area
459 AM PDT Tue Oct 17 2017

.SYNOPSIS...After two very warm days, a gradual cooling trend is
forecast for today and tomorrow. The pattern will then change as
a system drops down from the north and brings generally light
rainfall to parts of the area late Thursday into Friday.

&& of 3:00 AM PDT Tuesday...After two days which saw
very warm temperatures all of the way to the coast, a cooling
trend will start as the ridge of high pressure that has been
overhead weakens and is briefly replaced by a zonal flow. Both
today and tomorrow will see temperatures drop 3 to 8 degrees. So,
instead of widespead 80s to mid 90s, we will see highs back in
the 60s and 70s by Wednesday.

At the same time, we have been closely watching the latest
guidance as a weak disturbance is forecast to move from just off
the coast toward the far southern part of our region later today.
GOES16 is picking up this feature and already shows some cooling
cloud tops in the area. Models remain all over the board with
what will happen with this feature. Local WRF and HRRRX both
indicate light precipitation over the souther third of Monterey
County and possibly into the southern portion of San Benito
County. However, a slew of other models keep the moisture to the
south and indicate only some clouds. Based on both the HRRRX and
WRF keeping any convection in far southern parts of our CWA,
decided to go with 15 PoPs but keep it confined from around Big
Sur and points to the east and south. Biggest concern would be for
dry lighting if we do get anything to form. Any precipitation
should be off to the east by the evening.

All signs still point to a brief but substantial pattern change as
a system drops down from the Gulf of Alaska and brings rain into
the North Bay by late on Thursday. In general guidance has backed
off a bit with rainfall amounts compared to 24 hours ago on the
global models. However, 0Z and 06Z NAM do keep precipitation going
as the associated front moves through. Still looks like between
.10" and .20" for many spots especially for San Francisco Bay
northward. The rest of our region will likely see lesser amounts.
Winds will be locally breezy due to the frontal passage, however
speeds should mostly stay under 20 mph.

A ridge of high pressure in the Pacific will rebuild to the coast
over the weekend before becoming nearly stationary near our CWA
next work week. 500 MB heights could approach an impressive 597 DM
suggesting another round of heat is likely.

&& of 4:59 AM PDT Tuesday...Weather from today into
Wednesday will be in transition from light offshore flow to light
onshore flow, and there may be patchy coastal stratus and fog
returning to the immediate coast this evening. Smoke from a newly
developing wildfire in the Santa Cruz Mountains this morning may
reduce slant-wise and horizontal visibilities southward to the
Monterey Peninsula terminals today; the WRF`s model output shows
lower level winds N-NE near 10 knots today over the Santa Cruz
Mountains then onshore flow across the Monterey Bay this evening
should help improve visibilities. However, at about this time
VFR/MVFR at KMRY and KSNS will also transition to tempo IFR cigs
in stratus mid-late evening, IFR is likely tonight and Wednesday
morning. For Bay Area terminals VFR/MVFR for the period.

Vicinity of KSFO...VFR. Wind light and variable until 22z then NW-N
near 10 knots into the evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR, however depending on areal coverage of
a newly developing wildfire in the Santa Cruz Mountains visibilities
may lower to MVFR at times today as indicated in the 12z tafs. Stratus
cigs are likely tonight, tempo IFR 04z-08z is followed by IFR cigs
tonight and Wednesday morning.


.FIRE of 10:35 PM PDT Monday...A pattern change is
on the way, with a general cooling, moistening trend and the
possibility of wetting rains arriving this week. Winds will begin
to shift and become more onshore tonight, helping to bring the
cooler temps, moister air mass, and improved air quality for many
areas. Temperatures will decrease a few to several degrees each of
the next 3 days, returning to near normal temperatures by mid
week. Humidities will also increase given the cooler temperatures
and onshore flow. Some uncertainty regarding the upcoming rains.

A convective complex off of the Baja California coast located on
the southwestern flank of the exiting ridge will bring a slight
chance of thunderstorms southern Monterey and San Benito counties
from late this morning through the evening.

Model data now indicates light non-wetting warm sector precip
arriving late morning Thursday, while the main band of cold
frontal precipitation arriving Thursday evening through Friday
morning. This system will bring wetting rains to some areas but
not others, with the best chance of wetting rains over the North
Bay mountains and hills, where fires are most active. Much less
precipitation is expected farther southward and/or at lower

&& of 4:28 AM PDT Tuesday...High pressure over the
eastern Pacific will gradually weaken and move south this week. A
low pressure trough and cold front from the Gulf of Alaska will
move across the California coastal waters later Thursday, and
light rain will accompany the frontal passage. A moderate to long
period southerly swell will mix with northwesterly swell this
afternoon and tonight. A very large northwesterly swell train, the
largest of this season so far, is then forecast to arrive Thursday
night through Friday.





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Prepared by Weather at: Tue Oct 17 06:30:03 PDT 2017
From the National Weather Service