Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
951 am PST sun Nov 29 2015
Synopsis...cold temperatures again tonight. Weak storm system
fizzles in North Bay tomorrow. Slight warming trend into midweek
then a wet and windy storm system early Thursday.
Discussion...as of 9:00 am PST Sunday... this morning saw another
round of frost/freezing temperatures. As mentioned yesterday...
overnight mixing winds would be the driving factor in localized
temperature flucuations... and thats exactly what has happened.
The North Bay saw calm winds... and as a result is running 10
degrees or more cooler than yesterday morning. Several reports of
temperatures in the upper teens to lower 20s are coming in from
the North Bay... especially around sebastapol... Graton...
Healdsburg and Petaluma. On the flip side... winds were slightly
stronger around the Monterey Bay area... especially in Monterey
itself... and as a result... temperatures there are 5 to 10
degrees warmer than yesterday morning. The winds are having such a
tremendous impact on temperatures primarily because everything has
been in a steady state as a Rex block is over the western Continental U.S..
the California coast has remained in dry north to northeast flow
on the Lee side of this feature.
The Rex block over the western Continental U.S. Will be on its last legs
today and looks to be ejected from the area and east of The
Rockies by Monday. Temperatures today and tonight will still be
cold and similar to yesterday... however we should begin to see a
slight warming trend into midweek. Todays forecast focus will be
primarily on temperatures tonight... so keep an eye on if another
freeze/frost product is issued shortly after lunchtime as has been
the case the last few days.
A shortwave disturbance will approach the region later tomorrow
but this feature will fizzle out as it hits the dry stagnant air
mass in the wake of the Rex block. A few stray rain showers are
possible over the North Bay tomorrow evening.
Medium term models have come into general agreement that a
stronger storm system will move into the region by sometime early
Thursday. This storm system currently looks sufficient to bring
breezy winds and moderate amounts of rain to the region. Secondary
forecast shift focus will be on further tuning precipitation and wind
amounts associated with this storm system. Long term models show
another storm system approaching about a week out from now... next
Sunday. Will need to keep an eye on this... however... the midweek
storm system looks to be the strongest of the upcoming 3
Previous discussion...as of 3:12 am PST Sunday...its a cold late
November night across northern California with -4 at Truckee and
single digits from South Lake to Mammoth. Closer to home plenty of
readings in the upper 20s and 30s with further drop in
temperatures through sunrise. Frost and freeze headlines in place
through 9 am and will let those ride. Plan right now will be to
let those expire at 9 am and then the day shift will reassess if
another round of frost/freeze headlines is needed. In the meantime
another sunny and seasonably cool afternoon is in store with highs
mainly in the 50s for this afternoon.
MOS guidance suggest another potentially cold night into Monday
morning. However a weak system approaching may induce just enough
cloud cover and onshore winds by Monday morning. As noted above
day shift will look over the observed lows and 12z guidance before
deciding if another round of headlines is necessary.
Weak little boundary will fall apart as it hits the ridge and dry
airmass on Monday. NAM and GFS runs keep spitting out some very
light quantitative precipitation forecast and will show some slight chance probability of precipitation mainly over the
North Bay Monday with better chance of any liquid out over the
ocean waters near pt Arena. This is no big deal and any raindrops
observed should just be considered a bonus at this point.
Tuesday and Wednesday look dry as we'll be in-between systems with some
slightly warmer nights.
Main forecast focus is then on potentially moderate to strong cold
front due to arrive Thursday. Models in pretty good agreement for
a quick moving system sometime Thursday. Early quantitative precipitation forecast estimate shows
0.25-0.75 on average with 1 inch totals very possible. GFS model
shows a 1 inch plume of tpw moving onshore with the front.
Combination of high tpw...strong cold advection and potentially
favorable orographics could all line-up although some hint of
upper trough axis going negative which would bring winds more
south than southwest into the coastal range. Anyway still lots of
model iterations to go through but GFS/European model (ecmwf) and Canadian all on
board with a Thursday frontal passage.
System looks to be fast moving with only some lingering showers
Thursday night before high pressure and dry weather returns
Aviation...as of 9:51 am PST Sunday...an upper level ridge will
build over the area through tonight. Dry conditions will continue
with the wind flow primarily light offshore.
Vicinity of ksfo...VFR. High confidence.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR. High confidence. Light southeast winds
kmry but up to 10-12 knots ksns this morning.
Marine...as of 8:22 am PST Sunday...light winds will continue
over the coastal waters today...except local moderate easterly
winds near the Golden Gate. Winds will turn southerly over the
northern waters tonight as a weak frontal system approaches. A
stronger Pacific weather system will generate moderate to strong
southerly winds by midweek. Seas will be light today and then
build tonight into Monday.
Public forecast: drp
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