Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
750 PM PST Monday Dec 9 2013
Discussion...as of 7:50 PM PST Monday...no need to update
forecast at this time. Freeze warning officialy GOES into effect
at 10 PM for all locations except the city. This morning we broke
a record at San Jose and for the most part we had been forecasting
temperatures this morning (monday) to be the coldest of the week. No
doubt another very cold night is in store but there are already
some signs of a slight moderation in cold temperatures. Skies are clear
and winds relatively light at the surface with some breezy
offshore winds in the hills. Its intersting to note that the sfo
to wmc gradient is 15.4 mb...usually a sure sign of strong winds.
But most of that gradient is over the Sierra with only 1 mb
offshore from SAC to sfo. Anyway this and latest NAM model data
suggest even lighter winds tonight compared to last night in the
hills. 00z koak sounding shows fairly impressive airmass warming
through the entire depth of the troposphere over the last 24 hours
and on the order of 8 degrees celsius warmer at the 850 mb level.
Finally 24 hour dewpoint trends are currently 2 to 8 degrees
warmer than last night and generally in the 20s (where as last
night many sites were in the teens). So this all points to another
cold night but likely a few degrees warmer than last nights down
right brutally cold night in some locations (19 at Napa and Santa
Rosa comes to mind).
Anyway after another cold start to the morning with more frost on
the wind shields we will likely see another sunny and fairly nice
afternoon with highs from the middle to upper 50s. Full sunshine along
with very light winds will make for a pleasant Tuesday afternoon.
Middle-shift will be re-evaluating the need for a freeze warning on
Tuesday night into Wednesday morning. Looking at the new 00z NAM MOS
guidance it would appear zone 508 which encompasses much of the
populated area around the Bay may not reach criteria and be able
to be pulled from subsequent warnings. The same thinking holds
true with zones near the ocean as continued airmass warming and
ridging aloft will lead to continued seasonably cool but not
necessiarly sub-freezing nights. First Glace suggest at least one
more night warnings will be necessary for the usual North/East and
South Bay valleys as well as portions of the Salinas valley.
Pretty good bet that we should be done with freeze warnings by
Thursday morning with more airmass warming and mild afternoons
under high pressure.
Weak system brushes the coast with a return to onshore flow by
Friday. Somewhat unfortunately all long range guidance is now
suggesting dry weather for the foreseeable future. From a water
resource perspective this is bad news with Little Hope for
any type of signficicant rain or mountain snow for at least the
next 10 days and likely beyond which is supported by the latest
CPC 14 day outlook.
Previous extended discussion from 3 PM Monday...
All signs point to gradual warming through the week as a ridge of
high pressure starts to build into our County Warning Area and 850 mb temperatures rise.
By Thursday temperatures will be back close to normal (in fact lows for
Thursday morning will be above freezing for almost all spots).
Computer models do indicate a system will move into the pacnw on
Thursday with the southern extent of moisture expected
to brush across norcal late Thursday into Friday. Decided to
reduce probability of precipitation for our County Warning Area with virtually all of the ensemble and
operational models indicating dry conditions.
Ridge of high pressure centered over the eastern Pacific will
remain in charge for the end of the week through all of next week
which will keep the storm track well to our north. No sign of a
precipitation break though as the ensembles and operational runs keep any
rainfall basically out of California. 6-10 and 8-14 day outlooks
have below normal rainfall with above normal temperatures projected. If
the 384 hour GFS depiction for accumulated precipitation verifies,
we will make it to Christmas without any precipitation.
Aviation...as of 3:56 PM PST Monday...dry. Light winds.
Vicinity of ksfo...VFR.
Ksfo bridge approach...VFR.
Monterey Bay area terminals...VFR.
Climate...record low temperatures for the next two days along with
the year of occurrence.
Dec 9th Dec 10th
Kentfield......... 19 (1972) 22 (1932)
San Rafael........ 23 (1972) 23 (1972)
Napa.............. 19 (1932) 18 (1932)
San Francisco..... 30 (1972) 32 (1972)
sfo Airport....... 24 (1972) 29 (1972)
downtown Oakland.. 26 (1972) 29 (1972)
Oakland Airport.... 26 (1972) 27 (1972)
Richmond.......... 22 (1972) 28 (1972)
Livermore......... 18 (1972) 20 (1972)
Mountain View..... 28 (1951) 29 (1972)
San Jose.......... 25 (2013) 24 (1972)
Gilroy............ 20 (1972) 23 (1978)
Monterey.......... 23 (1972) 27 (1972)
Santa Cruz........ 23 (1972) 27 (1972)
Salinas........... 24 (1972) 31 (1972)
Salinas Airport... 25 (1972) 30 (1972)
King City......... 16 (1956) 21 (1956)
... Warning...all zones except San Francisco
Public forecast: rww/Bell
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