Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
940 am PST Monday Dec 9 2013
Discussion...as of 9:00 am PST Monday...although we have received
a slew of cold temperature reports this morning only San Jose at
this time has broken a record for the date (25 which breaks the
old one by a degree). Another round of cold weather is expected
tonight although all signs point to a few degrees of warming as
temperatures begin to warm aloft. Will be making one change to the
freeze warning and add all coastal zones back into it except for
San Francisco for tonight. Look for upper 20s to middle 30s at the
coast tonight with generally 20s inland. Warmup still expected
through the week with temperatures returning back to near normal for most
spots around Thursday.
Previous discussion...by 3 am temperatures had fallen to the
freezing mark or below across most inland areas...and even in some
coastal spots such as Half Moon Bay and Monterey. Only at
locations close to San Francisco Bay were temperatures still above
freezing...mostly in the middle and upper 30s. Light northeast
breezes around the sf Bay area have been helping to keep temperatures up
a bit overnight...particularly close to the Bay. In the
hills...northeast winds are currently gusting in excess of 20 miles per hour
and locally up to 35 miles per hour. But a strong nocturnal inversion should
continue to keep these stronger winds confined to the hills for
the remainder of the night. This will allow temperatures below the
low level inversion to drop at least a few more degrees before
sunrise.Freeze warnings continue across the entire forecast
area...except San Francisco...until 9 am this morning.
The current surface pressure gradient from wmc to sfo is a robust
13.6 mb. This offshore gradient is responsible for the current brisk
offshore winds in the hills and for advecting a very dry airmass
into the region. Current surface dew points are only in the teens
and lower 20s. A combination of very dry air...and some warming of
the airmass aloft...will result in warmer daytime highs today.
Most locations should top out near the 50 degree mark today. Temperatures
today will still be well below normal...but warmer than the chilly
middle 40s most of our area saw yesterday.
An upper ridge over the eastern Pacific will nose inland across
central California today and tonight...and 500 mb heights will rise
past 570 dm. 850 mb temperatures will warm past 5 degree c by
tonight...about ten degrees c warmer than 850 mb temperatures were over
the weekend. But even with this warming aloft...widespread
freezing temperatures are expected once again tonight due to light
winds and a very dry airmass which will allow for extremely
efficient radiational cooling. Most inland valleys are forecast to
cool into the 20s once again tonight. Will continue with a freeze
warning for all inland areas...as well as the sf Bayside
zone...from 10 PM tonight until 9 am Tuesday. Coastal zones are
currently not included in tonight's freeze warning...as slight
moderation in temperatures is expected near the ocean. But expect some
coastal zones to be close to meeting freeze warning
criteria...especially the northern Monterey Bay zone. Will need to
re-evaluate whether any coastal zones should be included in the
freeze warning tonight once the 12z model data comes in. By
Tuesday night...the only locations likely to drop below freezing
are the inland valleys.
Daytime temperatures will continue to rise through the week as a
weak upper ridge remains over California. High temperatures will warm well
into the 50s by midweek...with some lower 60s likely as well.
Minimum temperatures will also continue to moderate...but at a
slower pace...as the airmass begins to moisten.
A weak Pacific weather system is forecast to approach the West
Coast around Thursday...but both the GFS and European model (ecmwf) forecast this
system to ride up over the ridge and drop all precipitation north of our
area. Have reduced probability of precipitation across the North Bay from late Thursday
into Friday...but have not removed them entirely given that the
00z Canadian still shows some light precipitation over the North Bay late
Thursday and Thursday night.
Upper level ridging is forecast to strengthen starting at the end
of the week...resulting in dry and mild weather for the upcoming
weekend. No significant rainfall is in site during the next 7 to
Aviation...as of 9:40 am PST Monday...very little change from
previous forecast. VFR conditions through the period with
relatively light winds. Did remove fog mention for ksts as
boundary layer relative humidity appears to remain dry.
Vicinity of ksfo...VFR. Light winds.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR. Light winds.
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)
Public forecast: Bell/dykema
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