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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 181742

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1042 AM PDT Wed Sep 18 2019

.SYNOPSIS...A cold front will push across the San Francisco Bay 
Area this morning before dissipating over the Central Coast this 
afternoon. Light rain showers will be possible through midday 
across the northern portion of the region with clearing conditions
later in the day. A warming and drying trend is then forecast for
late this week and into the upcoming weekend as high pressure 
builds in from the eastern Pacific.

&& of 08:45 AM PDT Wednesday...12z Oakland sounding
revealed a moist air mass overhead with precipitable water at 
1.20 inches, which is above the 90% percentile for the date. A 
cold front has pushed into the Bay Area this morning with light 
rain developing as a result. Showers have moved through the North 
Bay with light rain now beginning to move into San Mateo and 
Alameda/Contra Costa Counties. Rainfall amounts have been light, 
with generally only a few hundredths of an inch reported so far in
most populated locations and locally higher amounts up to 0.20 in
the mountains of Sonoma County. The largest impacts from the rain
this morning will be wet and slick roadways across the Bay as the
front pushes through. Models project the frontal boundary to 
continue to weaken as it moves south, with showers dissipating as 
they reach the Santa Cruz Mountains and Central Coast. Thus, 
locations along Monterey Bay and points south are expected to 
remain dry today. Showers across the Bay Area should end by midday
with partly cloudy skies this afternoon. High temperatures today 
will be a couple degrees below normal as cool air moves in behind 
the front, with highs mainly in the upper 60s and 70s. The current
forecast remains on track and no updates are anticipated this 
morning. For additional details, see the previous discussion.


.PREV of 02:29 AM PDT Wednesday...A weakening frontal 
boundary continues to approach the North Bay this morning and will
push into the San Francisco Bay Area through sunrise. Latest 
regional radar data and observations show precipitation rates of 
0.01" to around 0.10" of rainfall per hour across northern 
California. Expecting similar amounts of precipitation as the 
frontal boundary moves southward through the early morning. Rain 
showers along the cold front are forecast to dissipate by mid/late
morning as the boundary approaches the Central Coast. Thus, 
locations south of Santa Cruz are less likely to see measurable 
rainfall with this system. With this said, the greatest impact to 
the region will be the potential for wet and slick roadways over 
the greater San Francisco Bay Area early this morning. 

With some clearing in wake of the frontal passage by this afternoon, 
temperatures are forecast to range from the upper 60s along the 
immediate coastline to 70s inland. Cannot rule out a few of the 
warmest inland locations reaching 80 deg F this afternoon. Overall, 
not expecting much of a cool down from this cold front. Similar 
temperatures are forecast for Thursday as the core of the mid/upper 
level low shifts inland and high pressure begins to build over the 
eastern Pacific. 

A more robust warming trend is then forecast for Friday and Saturday 
as the ridge to the east builds toward the West Coast allowing for 
warming temperatures aloft. More widespread 80s are forecast by 
Friday across the interior with the potential for lower 90s by 
Saturday in the region's warmest inland locations. At the coast, not 
expecting much in the way of a marine layer with temperatures likely 
to warm into the lower to middle 70s on Friday and middle to upper 
70s by Saturday. Locations such as Santa Cruz will also likely see 
daytime temperatures reaching into the lower to middle 80s by 
Saturday as well. 

Temperatures trend slightly lower Sunday into Monday as another 
mid/upper level system drops southward into the Pacific Northwest 
and inland to the east of the San Francisco Bay Area. At this time, 
dry conditions look most likely with this system with temperatures 
remaining near seasonal averages. The operational models are 
trending about 5 degrees cooler for Monday compared to the NBM, yet 
will not make significant changes at this time. Depending on the 
exact track of the aforementioned system, the development of 
offshore flow looks possible early next week which may result in an 
increase in temperatures and much drier conditions. This pattern 
will be closely monitored in the coming days for the potential for 
increased fire concerns. It is important to remember that our Fire 
Season is not yet over as widespread wetting rains have not occurred 
across our region. 


.AVIATION...As of 10:42 AM PDT Wednesday...For 18Z TAFs. A weak 
cold front continues to pass through the San Francisco Bay Area 
this morning, resulting in a a mixture of MVFR to VFR ceilings at 
the terminals. As the afternoon progresses, skies will go from 
BKN/OVC to more FEW/SCT at or above 3,000 ft AGL in the post- 
frontal environment with stratocumulus/cumulus development. 
Challenging forecast with overnight regarding the potential for 
low clouds. Think there will be enough mixing to prevent 
widespread low clouds, but still possible to see MVFR and even IFR
ceilings at some terminals.

Vicinity of KSFO...Ceilings are beginning to lift at SFO as of the
18Z TAF publication time with VFR conditions. After the front
passes through, winds will transition from a light south/southwest 
direction to the west around 15 kt with FEW-SCT stratocumulus/cumulus

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR/VFR conditions this morning,
becoming VFR later in the day. Light S/SE winds will become
onshore as the day progresses.

&& of 08:17 AM PDT Wednesday...Generally light and 
variable to locally moderate northwest winds will prevail today. 
The strongest northwest winds will be focused along and south of 
Point Sur. Northwest winds strengthen across the waters by 
Thursday afternoon becoming moderate for most of the waters. Mixed
seas will continue with a moderate period northwest swell and a 
light southerly swell.






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