Scientific Forecaster Discussion
fxus66 kmtr 210557
Area forecast discussion
National Weather Service San Francisco Bay area
1057 PM PDT Wed Sep 20 2017
Synopsis...a cool, upper level trough will dominate the western
portion of the country through the workweek maintaining cooler
than average temperatures and breezy conditions. Aside from the
potential for isolated showers offshore or near the coast tonight
through Thursday, dry weather conditions will prevail. A gradual
warming trend is then expected to begin this weekend and persist
into early next week.
Discussion...as of 9:05 PM PDT Wednesday...a weak cold frontal
boundary remains draped across north-central California this
evening. Kmux radar showed isolated light showers over Santa Cruz
and Santa Clara counties earlier this evening, but that activity
has since dissipated.
Infrared satellite currently shows an area of convective precipitation
off the Oregon coast associated with a cold upper trough that is
digging south. Several lightning strikes are being detected
within this active precipitation area. The trough is forecast to
move southeast and inland across northern California late tonight
and Thursday morning. Most models indicate that precipitation
will remain well offshore from our area or well to our east late
tonight and Thursday. However, the hrrr forecasts scattered
shower activity along the Sonoma County coast around sunrise
Thursday morning and then isolated showers developing over the
Santa Lucia mountains in Monterey County by late Thursday morning.
An evening forecast update included expanding slight shower
chances late tonight and Thursday morning, primarily over the
coastal waters and along the Sonoma County coast. Most of our
forecast area is expected to remain dry tonight and Thursday.
A cooler airmass will sweep across our area with the trough on
Thursday with 850 mb temps now forecast to drop as low as 4 deg c.
Thus, we can expect cooler temperatures in most areas tomorrow. In
addition, breezy conditions will develop in the afternoon,
especially near the coast where local wind gusts of up to 40 mph
are possible late Thursday afternoon and evening. Winds will then
diminish late Thursday night into Friday. This cool airmass will
mean chilly nights for the remainder of the week and into the
early part of the weekend. Overnight lows will drop into the 40s
in most inland valleys, and some of the colder valleys may see
a few upper 30s. The coolest nights are expected to be
Thursday night and Friday night.
From previous discussion...high pressure is then forecast to
build over the eastern Pacific and nudge inland as the mid/upper
level trough currently impacting the region shifts to the east.
This will result on drying conditions and a warming air mass
aloft. Look for temperatures to warm back to near seasonal
averages through the weekend and likely back above average by
early next week. Thus, widespread 80s to lower 90s are currently
forecast by Tuesday and Wednesday of next week. Cannot rule out
brief offshore winds as well at times which will help bring the
warmer conditions to the coast.
Aviation...as of 10:54 PM PDT Wednesday for 06z tafs...models
indicate a colder and drier airmass will settle into the area
overnight which will likely result in VFR conditions. Have once
again scaled back the amount of low clouds overnight. Moderate
onshore flow will ease overnight. Moderate confidence.
Vicinity of ksfo...VFR conditions will prevail through tonight.
Moderate onshore flow will ease overnight. Moderate confidence.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...IFR cigs prevail through tonight over
kmry. Ksns will likely remain VFR but confidence is low. Onshore
flow around 10 kt will prevail through tonight. Low confidence.
Marine...as of 9:00 PM PDT Wednesday...brisk winds are forecast
across the coastal waters tonight and Thursday in the wake of a
cold front that moved across the waters earlier today. A moderate
northwesterly swell will continue through the later half of the
week and into the weekend.
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 3 am
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 3 am
Small Craft Advisory...sf Bay until 3 am
Public forecast: dykema/rgass
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