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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 171534

Area Forecast Discussion
National Weather Service San Francisco Bay Area
834 AM PDT Tue Sep 17 2019

.SYNOPSIS...Dry and mild weather conditions are expected across
the region today. A weather system approaching from the northwest  
may bring scattered showers to primarily the North Bay late
tonight and Wednesday, and will also keep temperatures on the 
cool side of normal through Thursday. Warmer weather is forecast 
on Friday and Saturday, with slight cooling then expected for 

&& of 08:30 AM PDT Tuesday...The ongoing forecast 
this morning remains on track with mainly clear sky conditions and
temperatures in the 50s to lower 60s. Looking for a mild day with
mostly sunny conditions as temperatures warm into the 70s to 
lower 80s inland to upper 60s along the immediate coastline. Will 
be monitoring the latest observations and forecast data coming in 
today with regards to the approaching frontal system now pushing 
inland across the Pacific Northwest. This system will drive a 
weakening frontal boundary across the region early Wednesday with 
light precipitation possible from the Santa Cruz Mountains 
northward. For additional details, please see the previous 
forecast discussion below.


.PREV DISCUSSION...As of 3:10 AM PDT Tuesday...Drier air has made its 
way into our region after the departure of yesterday's frontal 
system. Skies remain mostly clear early this morning with only 
patchy low clouds and a few thin high clouds drifting by. Expect 
mostly sunny skies today along with mild temperatures. Highs are 
forecast to be mostly in the 70s to lower 80s, except 60s along 
the immediate coast. 

Early morning satellite imagery shows an upper trough off the 
coast of the Pacific Northwest, moving to the southeast. Like the
system that brought rain to our region yesterday, this one also 
has a fairly decent moisture tap with precipitable water values as
high as 1.5 inches. However, this incoming system is forecast to 
have more of an inland trajectory, which means less potential for
widespread rainfall across our area when the system moves through
late tonight and Wednesday. Based on a consensus of model data, 
showers may begin in the North Bay shortly after midnight tonight 
and then spread as far south as the San Francisco Peninsula and 
East Bay on Wednesday morning. Isolated showers may linger into 
Wednesday afternoon. Rainfall totals are generally forecast to be
less than a tenth of an inch, although the 06Z HRRR forecasts 
isolated amounts up to a half inch in the hills of the North Bay. 
This system will usher a cooler airmass into our region, and high
temperatures on Wednesday will mostly be in the upper 60s and 
70s. Locally breezy conditions will develop near the coast and in 
the hills on Wednesday afternoon as northwest winds increase.

The upper trough is forecast to move into the Great Basin by
Thursday morning and drier northerly flow will develop over our
area at that time. Earlier model runs indicated the potential for
gusty northerly winds in our hills late Wednesday night and 
Thursday morning, but most recent models forecast only moderate 
wind speeds. Temperatures will begin to warm in portions of our 
area on Thursday, but still remain at or below seasonal averages.

An upper ridge is forecast to develop along the West Coast by late
Friday and onshore flow will diminish and perhaps transitions to
light offshore. These developments will result in warmer weather 
on Friday and Saturday with high temperatures warming into the 70s
and lower 80s near the coast, and 80s and lower 90s inland. 

The models generally agree that the ridge will weaken and shift to
our east by Sunday as a trough approaches from the northwest. 
Thus, Sunday will likely be at least a few degrees cooler than
Saturday, with low clouds potentially returning to coastal areas 
as onshore flow redevelops. 


.AVIATION...As of 04:30 AM PDT Tuesday...For 12z tafs. VFR 
conditions through this evening. Expect increasing cloud layers 
this evening and overnight as well as marginal LLWS over the North
and East Bay. At this time, looks weak enough to be left out of 
the tafs, but will re-evaluate with the next model runs. MVFR 
conditions may develop towards the end of the taf period along 
with increasing shower chances late tonight and tomorrow morning. 
Generally light winds increasing and becoming onshore this 
afternoon 10-15 kt with locally higher winds at KOAK and KSFO.

Vicinity of KSFO...VFR through late tonight. Increasing cigs
expected this evening and overnight with possible MVFR conditions
early tomorrow morning. W winds around 10 kt becoming around 15 
kt this afternoon with gusts 20-25 kt. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions through late tonight
before possible MVFR/IFR cigs develop over the peninsula by early
tomorrow morning. Generally light winds becoming onshore around
10-15 kt this afternoon. 

&& of 08:30 AM PDT Tuesday...Light to locally gusty 
west to northwest winds will prevail over the waters through the 
middle of the week. The strongest winds will be generally confined
along and south of the coastal points, as well as through the 
coastal gaps and over the bays. A front will move through Tuesday 
night and Wednesday bringing a slight chance of showers to the 
northern waters tonight and into Wednesday morning. Northwesterly 
winds will increase at the end of the week. Mixed seas will 
continue with a moderate period northwest swell and a light 
southerly swell.


     .Tday...SCA...Mry Bay from 2 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 2 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 2 PM




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