Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1037 PM PDT Wednesday Jul 29 2015
Synopsis...warm overnight temperatures are expected once again
tonight for elevations above 1500 feet. Temperatures will begin
to cool slightly on Thursday...but muggy conditions are expected
Thursday and Friday as increasing monsoon moisture moves in from
the south promoting the possibility of showers and isolated
thunderstorms for the end of the week.
Discussion...as of 9:45 PM PDT Wednesday...there were very large
temperature differences from the coast to inland areas today. For
instance Half Moon Bay reported a high of 71 while it was 36
degrees hotter (107) at Livermore Airport 38 miles inland. Inland
valleys as well as the hills and mountains were quite warm to hot
today. The East Bay interior valleys had some of the hottest high
temperatures that in some cases exceeded desert SW heat...the high
in Livermore was hotter than 105 in Las Vegas and 106 in Laughlin
today. On the coast a southerly surge supported by a smx-sfo 1.7
mb pressure gradient brought clouds...fog and cooler weather to
the immediate coastline northward to the Mendocino coast. There's
some evidence that the marine layer is deepening a little since
earlier today...and 24 hour temperature changes indicate most if not all
places are cooler compared to last evening. However the area's
weather is now in a state of fairly large transition...the
trickiest part now is forecasting high temperatures for Thursday
and even Friday...highly dependent on the marine layer
depth...strength of the onshore winds...interaction with the middle
level instability, et cetera.. for the next day or two
temperatures...particularly inland...will be exceptionally
difficult to pin down. As hot as some temperatures got today no
record highs were tied or exceeded. Only about a 1/2 dozen record
highs on Thursday are a little cooler than today's...but based on
guidance most if not all of them will be out of reach. Recent sea
surface temperatures have undergone some decent cooling in the last week
or so...while moist surface to lower levels of the airmass is
forecast to remain over the waters into the weekend...supporting
additional stratus and fog.
Complicating matters a bit more will be the arrival of middle level
moisture and instability from socal...southern areas first Thursday
then spreading area-wide late Thursday night into Friday. Total totals
into the lower 30s spreads northward during this time...albeit a
little slow at first...but the highest values are over the Bay area
by Friday. The present forecast timing for shower and T-storm chances
By the weekend any shower or T-storm activity quickly shifts east
out of the area. 500 mb heights will steadily fall and onshore
winds will continue to bring temperatures closer to the climatological
averages for early August. Overall...on the larger synoptic to hemispheric
scale the middle latitude pattern continues to resemble a fall or even
a winter-time pattern in the sense of the large number of long-wave
troughs located around the northern hemisphere. Dolores' remnant
tropical moisture for instance was quickly swept out of the area
back on July 20th...persistence of northwest coastal winds until the most
recent wind reversal are a couple examples of this. It's difficult
to place a lot of confidence in the numerical models recently with
this large scale fluctuation.
Previous discussion...as of 2:45 PM PDT Wednesday...the current visible
satellite image continues to show stratus working its way up the
coast this afternoon with mostly clear skies over the land areas
of the County Warning Area. Cumulus clouds are popping up over Southern California
as monsoon moisture pushes towards the forecast area. As forecast
temperatures around the forecast area are running on the hot
side...especially for inland areas. It is currently 100 in Walnut
Creek...104 at Livermore Fire Department and 101 in Morgan Hill.
Its another story along the coast where low clouds are keeping it
on the cool side. At weather forecast office Monterey it is currently 71...Capitola is
currently 67 and 62 in Halfmoon Bay.
The current satellite water vapor image is showing a ridge of
high pressure along the West Coast with a dome of high pressure
centered over roughly Oklahoma...monsoon moisture is wrapping
around this high and is being advected into Southern California.
As mentioned in the previous afd a new low pressure center is
developing around 35 north and 140 west. 1200z gfs40 and European model (ecmwf)
have initialized well with these synoptic weather features and
remain in good agreement through the forecast period.
Both models forecast the dome of high pressure currently centered
over Oklahoma sliding westward Thursday tapping into the monsoon
moisture pool advecting this moisture into southern Monterey and
San Benito counties. The low pressure center currently positioned
at 35 north and 140 west slides closer to the coast by Friday and
enhances the monsoon moisture push into our forecast
area...allowing the moisture to push further northward into Napa
and Sonoma counties.
The nam12 and gfs40 both move some vorticity energy through the
area Thursday night into late Friday morning...the models differ
in the timing of this feature. This energy will provide lift and
along with some instability will provide support for showers and
thunderstorms around the forecast area beginning Thursday
afternoon through Friday evening. Initially the shower and
thunderstorm threat will begin in Monterey and San Benito County
but will spread north Friday into Napa and Sonoma counties.
By the weekend the European model (ecmwf) and gfs40 show a low pressure system
pushing into British Columbia...merging with the previously
mentioned Pacific low. This feature will suppress The Four
Corners high and result in southwest flow which will bring an end
to the shower and thunderstorm threat over the weekend.
Aviation...as of 10:36 PM PDT Wednesday...stratus has filled in
across the coastal waters with low clouds expected to push inland
through coastal gaps and valleys tonight. The marine layer has
deepened to around 1600 feet marine layer per The Fort Ord profiler.
IFR to LIFR ceilings expected across most terminals overnight. Winds
will be light through tonight.
Vicinity of ksfo...VFR conditions will continue through late
tonight with IFR ceilings expected after 10z. Ceilings are expected to
scatter out around 17z. Light winds.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...IFR ceilings will lower to LIFR after
07z. Temporary vlifr conditions expected between 12z-16z Thursday
morning. Ceilings are expected to scatter out around 17z-18z. Light
Marine...as of 8:58 PM PDT Wednesday...a thermal trough situated
over the region will maintain light southerly winds across the
coastal waters through Thursday morning. Winds will then veer to
the west Thursday afternoon. A mixed swell will also continue to
impact the coastal waters through late week.
Public forecast: canepa/Larry
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