187 
fxus66 kmtr 060119 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area California 
440 PM PDT Sat Jul 5 2008 


Discussion...as of 3:20 PM PDT Saturday...main focus with this 
afternoons package is the upcoming hot daytime and mild nighttime 
temperatures. 


It sure was a slow process...but the widespread stratus this morning 
did finally burn off for most locations. However...there are a few 
strong holds near the San Mateo coast and southern Monterey Bay. 
The feet Ord profiler still has a marine inversion pegged at a 1000 
feet. The north-s(sfo-acv) gradient did creep up to 5 mb this afternoon 
which probably lead to some of the clearing. The onshore gradient is 
barely at 2 mb...so any lingering stratus is clinging to the 
immediate coast. The weaker onshore flow also helped interior 
locations warm as much as 12 degrees over Friday/S maximum temperatures. 


The longwave pattern over the region continues to indicate an upper 
level trough off the West Coast slowly moving eastward. There is even 
a weak boundary associated with it that is moving though norcal 
at this time. This eastward progression should continue through the rest of 
the afternoon. In addition to the favorable gradients...the weak 
boundary should help to further mix out the stratus. 


Short term(tonight-tuesday)... 
expect stratus to return tonight behind the departing boundary. 500 
mb heights for Sunday 12z will be similar to Saturday/S so a marine 
layer around a 1000 feet should work. Had reports of drizzle this 
morning so added it for coastal locations near mry Bay and pt Reyes. 


Sunday through Tuesday...a rather beefy upper level ridge (greater 
than 590 dam) over the epac will gradually build eastward of California. 
Each successive day will see slightly warmer temperatures and less 
chance for night and morning stratus. In fact...by Tuesday am the 
nam12 has a slight offshore flow...which would all but knock out 
the stratus. Therefore...have cut back on stratus north of sf peninsula 
and kept it off the coast southward. 


As for temperatures...Sunday into Monday will be largest jump in 
temperatures over the next few days. By Monday...many inland 
locations will be the upper 90s to lower 100s. As mentioned on the 
previous discussion...MOS typically does a horrible job with these 
situations and will continue to go above MOS through the period. 
Just for reference...850 mb temperatures are forecast to be in the 
upper 20s(c) Monday evening. Tuesday temperatures will warm even 
more...especially near the coast...with interior locations at least 
100 degrees. Besides daytime temperatures...thermal belts will 
definitely become established with many locations above 1500 feet 
remaining above 65 degrees by Monday and Tuesday night. 


Long term(wednesday-saturday)... 
the beginning of the extended looks to be the peak of the heat wave 
as the ridge axis will be parked over head. At this time 500 heights of 591- 
592 dam with 850mb temperatures approaching 30c are being forecast. The 12z 
models did come in slightly weaker than previous runs...but either 
way this set up looks hot. Models do forecast offshore flow through 
time period as well...but much of the guidance keeps the gradient 
less than 10 mb. Would like to see higher offshore gradient before 
wind or fire weather becomes a concern. Thermal belts will only 
strengthen Wednesday and Thursday night with some locations remaining 
in the 70s all night long. 


The million dollar question will be when does the heat end? 
Experience says do not believe the models initial break down of the 
ridge. Typically medium range models tend to flatten the ridge... 
especially during a heat wave...too early. Therefore...the forecasted 
trough approaching the coast by Friday might be overdone slightly. 
That is not to say there will not be any cooling with the trough. 
At this time will keep the best cooling on Friday for the 
coastline. Did keep many interior locations hot...but not as hot 
as Thursday. By Saturday...the ridge should flatten enough to 
allow for widespread relief. 


Besides heat...will also have to monitor any sub-tropical moisture 
that may get tapped into Wednesday night through Friday. The GFS has 
flow at 500 mb becoming more S to southeast Wednesday night with a slight 
increase in middle and upper level moisture. However...with the ridge 
axis to the north moisture will have a tough time making it 
northward. Also...elevated lapse rate (700-500mb and 500-300mb) 
just are not that impressive. Would like to see better moisture 
and steep lapse rates before dry thunderstorms become an issue. 


For the up coming heat wave...an excessive heat outlook (sfospsmtr) 
will remain for much of the region. This will likely be upgraded to 
watch overnight tonight. If the situation continues on its current 
path...parts of the County Warning Area will likely see heat warnings by midweek. 
&& 


Aviation...as of 4:40 PM PDT Saturday...low clouds and areas of fog 
are expected to return to most terminals tonight. One change from 
the previous taf package is based off of collaboration with cwsu 
and latest high resolution WRF model...looks like the clouds will 
not return to sfo until around 10z. Stronger sfo-acv gradient 
also GOES along with this line of thinking. In addition...looks like 
it will not be in the Bay as long as usual so by 16z conditions 
should be improving markedly. There is even a chance that sjc will 
not get any clouds overnight...however did not feel confident enough 
to completely go against the current package and climatology. Gusty 
westerly winds of up to 30 kts should begin to taper off after 03z. 
For the Monterey Bay area...low clouds and fog should return during 
the evening hours and may drop conditions back to under 1/2 a mile 
with ceilings around 001. May even be a little bit of drizzle...so left 
that in as a possibility. Skies for sns and mry should clear starting 
around 18z. 
&& 


Fire weather...as of 6:00 PM PDT Saturday...main concern will 
be the upcoming hot and dry weather early next week. Upper level 
high building over the area will compress the marine layer down to 
500 feet or below by Monday night so that all of the basin 
complex fire will be above the marine layer. Temperatures in the 
mountains including the los padres nf area are expected to reach the 
middle 90s to near 105 Tuesday through Thursday...while minimum 
humidities will drop well into the single digit range. Minimum 
temperatures are expected to drop only into the middle 60s to middle 70s at 
night so relative humidity recoveries will be very poor. 


Biggest question will be the winds during this heatwave. At this 
time...it looks like the surface winds will be light or very lightly 
offshore so wind conditions are not expected to worsen. But this 
needs to be closely monitored. 


&& 


Mtr watches/warnings/advisories... 
... Coastal waters except Pigeon pt to pt pinos 
0 to 20 nm 
Small Craft Advisory...sfo Bay 


$$ 


Public forecast: mehle 
aviation/marine: Bell/Larry 
fire weather: west pi 


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