Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS66 KMTR 170543
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1043 PM PDT Mon Oct 16 2017

.SYNOPSIS...Look for another warm day regionwide today. A gradual
cooling trend is forecast for Tuesday and Wednesday. 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.

&&

.DISCUSSION...as of 8:00 PM PDT Monday...Unseasonably warm
temperature and dry conditions prevailed across most of the 
forecast area earlier today under the apex of a broad moderate
amplitude ridge. Temperatures were generally 8 to 12 degrees above
normal. Onshore winds have begun to cool the coastal areas as of
late this afternoon/evening. For example, Monterey cooled from the
mid 80s to mid 60s and humidities rapidly increased in the course
of around an hour this evening when the winds shifted to become 
onshore. This goes to show the importance of wind direction on 
temperature and humidity. 

Updated the afternoon forecast package to include a mention of a
slight chance of dry thunderstorms primarily over Monterey, Santa
Cruz, and San Benito county for Tuesday afternoon. There is a 
surge of moisture around 600mb coupling with some elevated 
instability on the southwestern flank of the exiting ridge. 
Satellite imagery already shows an area of cooler cloud tops in an
area the size of Texas that the National Hurricane Center gives a
30% chance of forming into a tropical cyclone within the next 48 
hours. Should this complex strengthen into a cyclone, we would
likely need to make updates to the forecast, so please keep close
attention to any updates regarding this. 

A regionwide cooling and dampening trend will begin Tuesday and 
peak on Thursday with increasing onshore flow and the arrival of 
the upper trough in Northern California. This upper trough will 
also spread light amount of precipitation across much of the 
greater San Francisco Bay Area, with the highest amounts of rain 
forecast over the areas that need it most, the North Bay hills and
mountains in a period from late Thursday through early Friday.
Some very light warm sector precipitation will precede the 
heavier band of cold frontal precipitation earlier in the day 
Thursday.
 
Despite some run to run model consistency issues regarding 
precipitation amounts, data suggests wetting rains should fall 
over most of the North Bay fires. Wetting rains are those that are
sufficient in helping to suppress fires, generally with 
accumulations in excess of a 1/10" of rain. The latest forecast 
includes a range of 1/4" to 1/3" for the coastal and mountains 
regions of the North Bay which surpass this thresholds. Valley 
locations in the North Bay are forecast to receive a little less 
than the surrounding hills and mountains, generally from around 
1/6" to 1/4", with similar amounts in the coastal Santa Cruz 
mountains to Ocean beach. Elsewhere, precipitation amounts will 
taper off with distance inland and southward. San Francisco Bay 
Shoreline areas (ie downtown SF, Oakland, San Jose) can expect 
closer to 1/10"-1/6", with a few to several hundredths of an inch 
anticipated for locations in the far East Bay and Monterey Bay. 

In the wake of the trough, high pressure will build back into the
region heralding in a return to warmer temperatures, drier
weather, and less onshore influence. 

&&

.AVIATION...as of 10:38 AM PDT Monday...For 06z TAFs. Generally
VFR conditions persist over the region with light winds overnight.
FU/HZ will also likely impact the North Bay terminals through the
forecast period as well. High clouds will also continue to stream
over the region. Onshore winds will then increase generally after
18z on Tuesday region wide.

High confidence. 

Vicinity of KSFO...VFR with light winds through early Tuesday
morning. More predominate W/NW winds will return Tuesday 
afternoon with winds at or around 10 KT.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with light winds through the early
Tuesday morning. More predominate W/NW winds will return Tuesday 
afternoon. 

&&

.FIRE WEATHER...as 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 to Santa Cruz, Monterey, and San Benito
counties from late tomorrow 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 
elevations. 

&&

.MARINE...as of 10:35 PM PDT Monday...Generally light to moderate
northerly winds will persist over the coastal waters through late
week. A cold front is forecast to pass through the region late 
Thursday evening from the northwest. The front will bring light 
rain from northwest to southeast Thursday evening through Friday 
morning. A long period southerly swell train will arrive late 
tonight and will mix with a robust long-period northwesterly swell
late Thursday evening.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...None.

$$

PUBLIC FORECAST: DRP
AVIATION: RGass
MARINE: RGass
FIRE WEATHER: DRP

Visit us at www.weather.gov/sanfrancisco

National Weather Service Glossary of Abbreviations