Weather Service Report

FXUS66 KMTR 182357

Area Forecast Discussion
National Weather Service San Francisco Bay Area
357 PM PST Sat Nov 18 2017

.SYNOPSIS...Dry and seasonably cool weather can be expected
through the weekend. The next system will bring rain chances early 
Monday, mainly across the North Bay. Additional rainfall is 
possible at times in the North Bay for much of next week, but most
of our area will likely see dry and warm weather.


.DISCUSSION...As of 1:15 PM PST Saturday... Satellite imagery 
shows mostly clear skies with a few passing high clouds over 
California this afternoon. The mostly clear skies and cold air 
advection aloft allowed temperatures to cool into the upper 20s to
mid 40s throughout the region, with the coolest locations 
farthest inland/away from the moderating influences of the 
Pacific/San Francisco Bay waters. Temperatures this afternoon are 
running near to slightly below seasonal readings for this time of 
the year, with reports in the upper 50s to mid 60s. A few to 
several degrees of additional warming are possible through the 
remainder of the daylight hours today. Seasonal temperatures and 
dry conditions are forecast to continue through the remainder of 
weekend as high pressure builds offshore of Baja, Southern and 
Central California.

This building 591DM high pressure ridge will partially 
deflect/delay a weak trough descending from the Gulf of Alaska 
late this weekend. Over the last 24-36 hours, forecast models 
have been trending towards lower precipitation amounts for our
area as the aforementioned ridge deflects the approaching trough
farther out of area each run. Latest model data shows the North
Bay picking up the most rain from this storm system, as per the
norm, but amounts are coming in lower and lower. The latest storm
total precipitation model consensus brings in amounts of just
.05-.25 for the North Bay (locally up to .33 at the wettest spots),
with areas south of the Golden Gate lucky to see a few 
hundredths. Heavier precipitation is expected farther north, from 
Mendocino county and beyond, as the ridge dominates from the San 
Francisco Bay Area and southward.

Previous model runs also showed a secondary, weak system mid to
late next week, however, given the push towards a stronger ridge,
this system is now expected to be even weaker. Instead, it looks
like we will see a dirty ridge type of scenario, with moisture
riding over the top of the ridge as long as it exist.
Precipitation will lingering along the northern periphery of the
ridge off and on through the upcoming week, occasionally bringing
light rain to the northern portions of Napa and Sonoma counties.  

This ridge will continue to be the most prominent feature in our
regional weather until next weekend, when it begins to wane and
shift downstream. At the same time, forecast models show a well
organized upper low in the Gulf of Alaska, which could bring
widespread precipitation Sunday or Monday, Nov26-27. Models do
differ on the strength of the upper low, with the Euro showing
500MB heights at 5150M and the GFS at 5260M, as well as the
timing, with the GFS sluggish and wetter and the Euro much more 
progressive and drier. Be sure to stay tuned as next weekends 
storm system comes into better focus.


.AVIATION...As of 3:57 PM PST Saturday...The air mass is mainly 
dry and stable supporting VFR for the 00z taf cycle. Few-sct 
cirrus clouds tonight then high clouds thicken by late morning 

Vicinity of KSFO...VFR. Light wind tonight and Sunday.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...VFR. Wind becoming SE 5-10 knots tonight
and Sunday morning then light W Sunday afternoon and evening.

&& of 3:49 PM PST Saturday...High pressure just off the
coast of Central California will continue to generate light northerly
winds through Sunday evening. Winds are then forecast to switch out
of the south by late Sunday night or early Monday ahead of a frontal
boundary. Winds over the southern waters will remain light to moderate
out of the northwest. Light to moderate seas will continue through
the weekend.





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