Scientific Forecaster Discussion
fxus66 kmtr 260405
Area forecast discussion
National Weather Service San Francisco Bay area
905 PM PDT Tue Apr 25 2017
Synopsis...a weak system passes inland to the north dragging a cool
front southeastward through the Bay area later Wednesday. This will
result in light rain mainly from the Santa Cruz Mountains northward
Wednesday. A warming and drying trend will then get underway beginning
Thursday and likely continuing into the upcoming weekend as high
pressure builds over the eastern Pacific. Breezy northerly winds are
also possible from Thursday into Friday, especially in the hills.
Discussion...as of 9:05 PM PDT Tuesday...temperatures are generally
a few to several degrees milder than they were this time last evening.
Cloud cover has been variable through the evening and winds are from
the west to northwest.
Cloud cover will increase tonight associated with an appreciable plume
of precipitable water tracking east-southeast across the eastern Pacific and
entering the California coastal waters; values near 1.30 inches by
early Wednesday morning as forecast by the GFS model is fairly close
to oakland's April all-time maximum (1.36 inches) for all Oakland
soundings in the period 1948-2014. A nearly stationary, strong and
closed mid-upper level high is centered over the Pacific straddling
the 30n latitude between approx 130w and 150w longitude. We're located
under the eastern extent of this high while a zonal jet stream enters
the Pacific northwest. If this much above normal amount of moisture
streaming over the Pacific was coupled with moderate-strong dynamics
we'd most certainly be looking at an appreciably wet day coming up,
but this is not quite the case as the primary mechanism at work will
be a cool frontal boundary sweeping into the moist air enhancing upward
vertical motion and resulting in mainly light rain. The eastern extent
of the ridging currently developing over the Bay area will be briefly
weakened by this cool frontal passage Wednesday. Gusty/breezy northwest winds
develop Thursday-Friday. The pattern turns warmer and dry late this
week and weekend with steadily building 500 mb heights as the eastern
Pacific Ridge steadily moves toward norcal.
Previous discussion...as of 01:20 PM PDT Tuesday...deep moisture aloft
continues to advect inland across the region this afternoon and
has resulted in widespread mid/high level cloudiness. All of this
is in advance of a weak mid/upper level trough that will push
inland into the Pacific northwest late tonight into Wednesday.
This system may bring some very light precipitation to the
northern portion of our region, mainly from the Santa Cruz
Mountains northward early Wednesday morning. Cannot rule out some
lingering showers into the early afternoon from the East Bay hills
down into the Santa Lucia mountains, yet chances for widespread
measurable rainfall remain minimal as this system lacks needed
mid/upper level support as it pushes inland to our north. With
this passing system, temperatures will be seasonably cool and
temperatures will range from the lower 06s coast to middle 60s
inland through Wednesday. Some of the warmer, interior locations
of the central coast may warm into the lower 70s.
A warming trend will then get underway by Thursday as dry conditions
return along with mostly sunny skies. As a stronger mid/upper level
low drops southward out of Canada into The Rockies and high pressure
develops offshore late in the week, temperatures will warm into the
70s inland. In addition, breezy to locally gusty winds will develop
due to the tightening pressure gradient from Thursday into Friday.
Further warming of the air mass aloft will result in lower 80s
during the afternoon hours this weekend while onshore winds near the
coast keep temperatures in the 60s to 70s.
Temperatures moderate slightly by early next week as the mid/upper
level ridge weakens as a system pushes inland into the Pacific
northwest. With that said, the main storm track will remain well
north of the region and dry weather conditions are likely into the
beginning of may.
Aviation...as of 4:45 PM PDT Tuesday...mid and high clouds
continue to stream over the area this afternoon with some areas of
lower ceilings possible early this evening around ksfo. Expect
clouds to increase overnight and into Wednesday. VFR/MVFR
conditions through the period with -ra possible at ksts and vcsh
for Bay area terminals on Wednesday.
Vicinity of ksfo...webcams show increasing cumulus this afternoon
which prompted the tempo bkn016 group in the 00z taf issuance for
the next few hours. Will monitor the observations and amend if
necessary. Breezy west winds will decrease within several hours
after sunset. The mid/high clouds will continue to increase
overnight and into Wednesday with opportunity for MVFR ceilings
after about 15z with showers in the vicinity. Confidence on timing
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions this evening with breezy
onshore winds through sunset. Increasing clouds will likely bring
MVFR ceilings to terminals overnight or early Wednesday morning.
Marine...as of 7:49 PM PDT Tuesday...high pressure off the West
Coast continues to result in moderate to locally strong winds. The
strongest winds are associated with a coastal jet near the coast
from Monterey Bay southward. Winds and seas will increase over
all coastal waters Wednesday night as high pressure strengthens.
... Bay until 9 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 9 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Public forecast: canepa
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