National Weather Service Raw Text Product

Displaying AFOS PIL: AFDMTR Received: 2017-10-12 11:44 UTC

FXUS66 KMTR 121144

Area Forecast Discussion
National Weather Service San Francisco Bay Area
444 AM PDT Thu Oct 12 2017

.SYNOPSIS...Gusty north winds will impact high elevation locations
through the early morning. Another round of gusty winds is 
possible in the hills once again Friday night into Saturday. 
Otherwise, look for dry conditions and near normal temperatures 
through the end of the work week. A warming trend is expected over
the weekend.


.DISCUSSION...As of 3:00 AM PDT Thursday...Northern California is
positioned between an offshore 585DM high pressure ridge and the
base of an upper trough that swept a dry cold front through the
region yesterday. This synoptic scale pattern has resulted in a
rapid tightening of the northerly pressure gradient and enhanced
northerly winds aloft. The winds have been strongest over the
Diablo Range so far this evening, with Mount Diablo reaching a
peak gust of 53mph, while winds over the open waters have topped
out at 45mph (buoy 13). Elsewhere, wind gusts in the range of 30
to 40 mph have been reported at other high elevation or open 
water locations, while areas closer to the valley floors remain 
light to calm. This disparity in winds at higher elevations versus
the valley floors is due to the nocturnal inversion decoupling 
the upper atmosphere from the surface boundary layer. Local short- 
range model suggests downsloping winds may penetrate this boundary
and bring locally breezy winds closer to valley floors through 
the remainder of the overnight hours. The northerly winds will 
begin to subside by around sunrise then weaken significantly by 
mid to late morning. In general, these winds are weaker and less
widespread than the winds that initially spread the North Bay
fires earlier this week. 

These dry, gusty northerly winds will be sufficient to create 
critical fire weather conditions in an area already facing 
numerous ongoing wildfires, thus, a red flag warning is in effect 
through 5PM for locations above 1500 feet. Individuals living in 
the North Bay, especially in areas just south of where there are 
active fires, should heed the advice of local officials regarding 
evacuations or other emergency preparation. Additionally, smoke 
from these fires will be advected southward towards the immediate 
Bay Area, bringing extremely poor air quality to the major 
metropolitan areas located there. Close windows, limit outdoor 
activities, limit indoor cooking and vacuuming, and refer to your 
local air quality district for more information. 

Short to mid range models indicate that an energetic vorticity 
lobe will round the base of the aforementioned trough and cause it
to dig towards the Great Basin late Friday. As a result, another
round of dry, gusty northerly winds are expected across the area,
but especially over the higher terrain in the North Bay. Latest
guidance suggests these winds peaking Friday night into early
Saturday, bringing critical fire weather conditions, especially to
areas already battling wildfires. These winds will be stronger and
more widespread than the ongoing winds early this morning. We 
will monitor this upcoming situation closely and issue any 
pertinent products as needed. 

Otherwise, near normal daytime temperatures through the end of 
the work week with a slight warming trend through the weekend and 
into early next week. Cool overnight lows are expected with mostly
clear skies (other than the smoke). 

&& of 04:43 AM PDT Thursday...As wildfires continue 
to burn in the North Bay expect smoke, haze, and possibly ash to 
continue to periodically reduce visibilities. Currently all sites 
MVFR to VFR conditions. Any cigs will be from layers of smoke 
rather than clouds. 

Models are still indicating low level wind shear to continue
through early morning for most sites. Otherwise westerly winds
this afternoon gusting to around 20 kt.

Vicinity of KSFO...VFR, with a smoke layer remaining at around
3500 to 5000 ft. Generally light westerly winds this morning to
become gusty to around 20 kt this afternoon and evening. Low level
wind shear still possible until 16z this morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR/VFR conditions due to haze. Winds 
mainly light and variable, possibly light E-SE at KMRY and KSNS. 
Westerly winds return by late this morning. Westerly winds gusting
at 15 to 20 kt this afternoon possible at KSNS.


.FIRE WEATHER...As of 4:35 AM PDT Thursday...Northerly flow aloft
in between trough to the northeast and offshore ridge is currently
bringing gusty northerly winds to the greater San Francisco Bay
Area. The strongest winds are occurring at higher elevations,
particularly those at 1500 feet or above. Mount Diablo gusted up
to 53mph earlier in the night and continues to consistently gust
over 45 mph, while other ridges and peaks continue to report
northerly wind gust into the 30s. Localized downsloping occurred
for locations just south of these ridges and peaks, however, the
atmosphere largely decoupled overnight, allowing for generally
light winds for the vast majority of urban locations with
occasional gusts mixing down to the surface. 

Short term model data shows wind speeds beginning to taper off 
after sunrise before subsiding by a larger magnitude during the 
late morning. Despite the subsiding winds, critical fire weather 
conditions will continue to exist through the afternoon as 
relative humidity values drop into the upper single digits to low 
teens in some locations. These poor overnight humidities and gusty
northerly winds may have contributed to a flare up in the fires 
between 230 and 400am per GOES16 fire weather and infrared 
imagery. A Red Flag Warning is in effect for elevations above 1000
feet in the across the region until 5 pm Thursday. 

Another round of gusty offshore winds and dry conditions is 
anticipated to arrive late Friday into early Saturday. The dry, 
gusty offshore winds arriving overnight Friday into Saturday are 
likely to be stronger than the current ongoing winds and cover a 
larger swath of land, with localized gusts over the tallest peaks
and ridges possibly approaching 60 mph. 

&& of 02:27 AM PDT Thursday...Gusty northwesterly winds
will continue through most of the week as strong high pressure
builds over the eastern Pacific. Strong winds will result in steep
fresh swell across the waters. Expect smoke from the North Bay
wildfires to reduce visibilities at times especially over the


     .Tday...Wind Advisory...CAZ507-511
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay from 9 AM




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