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. 

&& 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 
looks good. 

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 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. 

&& 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 

&& 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. 


Mtr watches/warnings/advisories... 


Public forecast: canepa/Larry 
aviation: CW 
marine: CW 

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