Scientific Forecaster Discussion
fxus66 kmtr 180555
Area forecast discussion
National Weather Service San Francisco Bay area
1055 PM PDT Tue Oct 17 2017
Synopsis...cooling is expected on Wednesday and Thursday as
onshore flow strengthens. A weak early season weather system is
forecast to bring light rain to much of the region from Thursday
afternoon through Thursday night. Dry weather is expected to
return by Friday. A warming trend will begin over the weekend.
Warm and dry weather is expected regionwide early next week.
Discussion...as of 8:55 PM PDT Tuesday...widespread high cloud
cover resulted in slight cooling across most of our region today.
Cooling was more pronounced near the ocean where light onshore
developed. Showers and thunderstorms developed off the south-
central coast west of Point Conception today and radar showed weak
returns across far southern Monterey County late this afternoon
and early this evening. However, there is no indication any
rainfall reached the ground.
Cooling will become more widespread on Wednesday as onshore flow
strengthens by afternoon. However, inland areas and the hills are
expected to remain warmer than normal.
More pronounced cooling will occur on Thursday, especially inland,
as clouds associated with a weak incoming weather system
overspread the area. The 00z NAM is consistent with the previous
NAM in developing light rain along the Sonoma County coast by
midday Thursday and then spreading rain across most of the North
Bay during the afternoon and early evening. The weakening frontal
boundary will then press south of the Golden Gate by late
Thursday and bring scattered amounts of light rain to the rest of
the sf Bay area Thursday evening and as far south as Monterey
County by later Thursday night. The models continue to trend drier
with this system and a blend of recent model output indicates
most locations will pick up less than a tenth of an inch on
Thursday and Thursday night. Locally higher amounts of a quarter
to a third of an inch are possible across northwest Sonoma
County. Nearly all models now indicate rain will end by daybreak
Friday. A forecast update was completed earlier this evening to
increase rain chances in the North Bay on Thursday afternoon and
to remove rain chances from all areas on Friday.
Friday is expected to be a dry and cool day. A warming trend will
get underway over the upcoming weekend as an upper ridge begins to
build over California. That ridge will then strengthen early next
week and light offshore flow will develop. This will result in warm
and dry conditions regionwide next Monday and Tuesday with
widespread highs in the 80s, even near the ocean. In fact, if
current model projections hold, our weather next Monday and
Tuesday will end up being similar to what occurred this past
Sunday and Monday.
Aviation...as of 10:50 PM PDT Tuesday...for 06z tafs. Remnant mid
and upper level clouds will continue to stream in over the region
as a disturbance moves east to the south of the area. Occasional
reductions in vsbys can be expected as wildfires continue to burn
in the region. Winds will continue to ease becoming light and
variable overnight. Patchy low clouds will redevelop along the
immediate coast with little to no impact on area terminals.
Vicinity of ksfo...VFR conditions will prevail through the
forecast period with scattered to broken high clouds moving
across area terminals through tonight. Moderate onshore winds will
become light easterly to variable overnight.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions will prevail through early
Wednesday morning with broken to overcast high clouds moving
across area terminals. Light onshore winds will become light
and variable overnight.
Fire weather...as of 7:35 PM PDT Tuesday...increasing high
clouds resulted in slight cooling across portions of the district
today. Also, relative humidity values have been increasing in the
valleys and along the lower coastal slopes as light onshore flow
begins to develop. However, the airmass in the hills above 1000
feet remains relatively warm and very dry. Poor humidity
recoveries are expected in the hills tonight. Fortunately, winds
will remain light. More widespread cooling will take place on
Wednesday as westerly, or onshore, winds increase in the
afternoon. This will result in better humidity recoveries
Wednesday night into Thursday morning and more significant cooling
by Thursday. A weak early season weather system is forecast to
bring light rain to the North Bay from Thursday afternoon through
Thursday evening and to much of the rest of the district on
Thursday night. The models have been trending drier with this
system. Latest forecast rainfall amounts for Thursday and Thursday
night range from a tenth to a quarter of an inch across northwest
Sonoma County and northern Marin County, and generally less than
a tenth of an inch in all other areas. Most rainfall is expected
to end by Friday.
A warming trend will begin over the upcoming weekend as a high
pressure ridge begins to build over California. This ridge will
strengthen into early next week and result in warm and dry
conditions districtwide on Monday and Tuesday.
Marine...as of 9:00 PM PDT Tuesday...an upper level disturbance
located off the coast of Santa Barbara will continue east just
south of our coastal waters. This system has been associated with
light precipitation and lightning. There is still a chance of
thunderstorms over the southern waters between now and midnight. A
cold front is forecast to move across the waters late Thursday.
This system may bring light rain. A moderate to long period mixed
northwesterly and southerly swell has arrived with buoys reporting
periods of 18 to 19 seconds across the coastal waters. Later this
week a very large northwesterly swell train is forecast to move
across the area. This large swell will create hazardous
Public forecast: dykema
fire weather: dykema
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