Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 010539
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1039 PM PDT Sun Apr 30 2017
.SYNOPSIS...A significant warming trend is forecast from Monday
through Wednesday as high pressure builds inland over the
Great Basin and offshore flow develops. Widespread 80s to mid 90s
are expected Tuesday and Wednesday afternoons, with warmest
inland locations in the southern interior potentially approaching
100. A cooling trend is then expected for the latter part of the
week as the upper ridge over California is replaced by a trough.
Rain showers are possible as we head into next weekend with an
unseasonably deep upper low then approaching our region.
.DISCUSSION...As of 8:15 PM PDT Sunday...Sunshine prevailed
across our district this afternoon, with generally clear skies
continuing this evening. Satellite imagery shows just a few
patches of stratus over the coastal waters. Max temps today were
generally similar to or even a bit cooler than yesterday, as
passage of a very weak upper level system temporarily interrupted
the warm-up of the air mass over us.
Models remain in good agreement that the large scale upper level
ridge axis will shift eastward towards the coast and then build
northward into the Pacific Northwest over the first part of the
week. As this occurs, flow will turn more offshore and lower
tropospheric temps increase. This afternoon`s 00Z KOAK sounding
had an 850 mb temp of 14.5C. Consensus of latest model output is
that this will increase to around 17C by 00Z Tuesday, 18C by 00Z
Wednesday and 19.5C by 00Z Thursday. This is getting well up
there; the all-time record high KOAK sounding 850 mb temp for May
3 is 20.4C. At the same time, the increased offshore flow and
subsidence should result in greater reflection of the warm air
mass aloft in the surface temperatures.
Key question and concern then is just how warm it will get. Have
looked back at basic meteorological analyses of some past cases
of record temperatures this time of year, and present situation
looks only a tad less robust. Two that have considered in
particular are those of April 29, 1981 and April 26, 2004. In the
former, Downtown San Francisco and Santa Rosa reached 89 and 98
deg respectively, and in the latter 91 and 93 deg. A bit warmer
aloft in these past events than current model forecasts in present
situation, but only by a degree or two.
Upshot is concern that warmup over the first part of the week
could be even larger than what inherited forecasts indicate,
especially coastal areas. For now, with only limited new model
output, will confine updates to the tonight and tomorrow time
frame. Am also going to issue a Hazardous Weather Outlook,
highlighting that temps look to potentially get much warmer than
any experienced thus far this year.
Finally, will note that early season offshore warm events like
this characteristically end with a southerly stratus surge.
Although higher resolution model output doesn`t go out
sufficiently far to be confident of the timing, latest GFS output
(18Z Sunday run) does hint it could be as soon as late Wednesday
night into Thursday morning.
.From Previous Discussion...The moderate pressure gradient will
lead to locally breezy conditions for the overnight hours the next
two nights, in particular late tonight in Monday as 925 MB speeds
are expected to possibly top 40 KT for a brief period going from
the East Bay Hills to the Santa Cruz Mountains. Due to the very
short duration, a wind advisory was not issued. Urban spots will
mostly see speeds under 15 MPH.
The ridge will head off to the east as a longwave trough near the
coast starting Thursday night. An associated low will dive down
to either near our coast (ECMWF) by the weekend or into southern
Nevada (GFS both operational and ensemble mean). For now kept the
low end PoPs going in the forecast from the previous shift. Either
solution will bring cooler conditions and drop most spots back
into the 60s.
.AVIATION...As of 10:35 PM PDT Sunday for 06z TAFs. VFR conditions
are expected to prevail at most terminals through the period. The
two potential terminals with issues are KMRY and KSNS, which may
see early morning stratus. Winds have subsided this evening and
will increase again by Monday afternoon.
Vicinity of KSFO...VFR. Winds have relaxed since sunset and expect
the trend to continue overnight. Could see few to scattered low
clouds early Monday morning, but not expecting marine stratus to
impact the terminal at this time. Breezy conditions expected again
Monday out of the west to northwest at 15 to 25 kt by the
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR conditions expected for most of the
period. There is a chance both KMRY and KSNS could see some early
morning stratus that would result in MVFR/IFR ceilings and
visibilities. Latest TAF package has actually decreased the cloud
cover based on current trends and model guidance. Winds will
become westerly Monday afternoon at 10 to 15 kt.
.MARINE...as of 10:23 PM PDT Sunday...High pressure will continue
to generate strong northwest winds across the coastal waters
overnight tonight and persist through most of the forecast
period. winds may subside slightly by Thursday. In addition, the
strong winds will also continue to cause very rough seas due to
fresh swells with short periods.
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pigeon Pt to Pt Pinos 0-10 nm
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
PUBLIC FORECAST: Blier/Bell
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