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fxus66 kmtr 051220 
afdmtr 


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


Discussion...as of 4:40 am PDT Saturday...primary forecast concerns 
center around next week's heat...particularly from Tuesday through 
Thursday...and whether hot temperatures will develop all the way to 
the coast. 


Typical Summer weather is expected in the short-term with a 
persistent marine layer expected near the coast and with mostly 
clear and warm weather forecast inland. An upper trough passage early 
on Friday temporarily disrupted the marine layer...but Fort Ord 
profiler data indicate the marine inversion has redeveloped and the 
marine layer is relatively shallow at just under 1000 feet. The 
shallow nature of the marine layer has resulted in somewhat limited 
inland development of low clouds early this morning...mainly around 
sfo Bay and locally inland from Monterey Bay. More widespread stratus 
coverage is likely by sunrise. But given the shallow marine layer... 
expect all inland areas to clear by late morning. Temperatures will 
warm several degrees in most areas today...the result of decreased 
onshore flow and a warming airmass aloft. The warming trend will 
continue into Sunday as an upper ridge over the eastern Pacific 
begins to build over California and onshore flow weakens further. 


Very robust warming is forecast to occur from Monday through 
Wednesday as the upper ridge strengthens overhead and the flow begins 
to trend offshore. Both the GFS and European model (ecmwf) have backed off slightly on 
the strength of the upper ridge with the latest model run. However... 
recent model runs...of the GFS in particular...are indicating more 
in the way of offshore low level flow by Tuesday and Wednesday. 
This would suggest that very warm temperatures will develop all 
the way to the coast by Tuesday and especially Wednesday of next 
week. MOS guidance typically under-forecasts heat events such as the 
one coming next week...at least more than a few days out anyway. 
Latest MOS numbers are probably too cool from Monday through 
Wednesday...particularly for areas near the coast. Have decided to go 
with warmer temperatures than MOS for most of our forecast area next 
week...both for highs and overnight lows. The NAM is showing almost 
no tendency towards offshore flow next week...at least through 
Tuesday morning. Both the NAM and GFS forecast the upper ridge axis 
to be just offshore next Tuesday...an upper level pattern most 
conducive to development of offshore surface flow. So...will favor 
the GFS surface pattern next week and go with rapidly warming temperatures 
near the coast from Monday through Wednesday. Looks like best 
offshore flow conditions will occur Tuesday night into Wednesday 
morning and so Wednesday should be the warmest day near the ocean. 
Inland areas will continue hot into Thursday. Widespread middle 
to upper 90s are forecast for inland areas next Tue-Thu...with the 
warmer inland areas expected to warm past 100 and perhaps as high as 
110 in places like Pinnacles National Monument and near Lake 
Berryessa. Coastal temperatures will likely climb into the 80s by Tuesday 
and possibly past 90 by Wednesday. 


Heat advisories are a good possibility by Tuesday and we may need an 
excessive heat warning by Tuesday or Wednesday. Since the hottest 
weather is still three days away...will continue with an excessive 
heat outlook for now (under special weather statement) and consider 
transition to a watch product within the next 24 hours. 


Both the GFS and European model (ecmwf) indicate the upper ridge will rapidly break 
down late next Thursday into Friday as a trough moves into the Pacific 
northwest. This will result in much cooler weather by the end of next 
week. 


&& 


Aviation...as of 5:10 am PDT Friday...stratus covers the immediate 
coast...the Salinas valley and parts of the Bay area...in particular 
the East Bay. This morning the stratus layer appears to be thinner on 
satellite. Area profiler data supports this as the marine layer has 
become significantly compressed since last evening. Additional low 
level subsidence and warming is forecast today and Sunday further 
keeping the marine layer capping inversion compressed. As a result 
stratus ceilings are mostly in the IFR range this morning. Similarly 
tonight along the immediate coast expect IFR ceilings and 
visibilities. 


Bay area terminals...so far most of the stratus is over the East Bay 
and some is over the South Bay...a typical night-time Summer stratus 
pattern. Since the marine layer is about 800-1000 feet deep ocean 
based stratus has a difficult time extending inland over the coastal 
terrain thereby limiting its coverage. A relatively thin layer of 
stratus with less areal coverage often leads to earlier burn-off 
times. Ksfo and koak should be VFR by 17z...ksjc between 16z-17z. If 
ksts gets stratus it should clear out by 16-17z as well. Otherwise 
trends indicate stronger onshore pressure gradients from sfo-SAC this 
afternoon and again Sunday afternoon. This combined with a compressed 
marine layer can produce locally gusty onshore winds in the afternoon 
and early evening. 


Central coast terminals...stratus is filling in over Monterey Bay and 
the Salinas valley. IFR ceilings will likely scatter out at kmry and 
ksns between 17z-18z. Light onshore winds will likely roll the 
stratus back in this evening. IFR is expected tonight. 


&& 


Fire weather...as of 4:30 am PDT Saturday...biggest concern 
continues to be weather conditions near the basin complex...both in 
the short term (locally gusty north-northeast winds) and also 
as we move into the first half of next week (very warm and dry 
conditions with offshore flow). For most of the area...the wind flow 
remains out of the northwest...but local north-northeast winds have 
developed at the portable RAWS station near the basin complex burn 
site. That RAWS station...at an elevation of 2500 feet...currently 
indicates a north-northeast wind of 18 miles per hour with gusts to 27 miles per hour. Also relative humidity 
recoveries at that location have been very poor overnight...remaining 
below 30 percent. Latest high resolution WRF model output indicates 
moderate NE winds will persist at the basin complex burn site 
through the morning with winds subsiding slowly in the afternoon. The 
marine layer will remain shallow at 1000 feet or less and thus most 
of the fire will remain in the warm and dry airmass above the 
inversion. Not much change is expected tonight and Sunday morning 
with a continued shallow marine layer and locally gusty northerly 
winds. Winds should ease Sunday afternoon into Monday...but 
temperatures will warm up. Hot temperatures with very low humidities 
are expected to develop early next week as a strong high pressure 
moves in aloft. Temperatures will warm well into the 90s during the 
first half of next week and probably over 100. In addition... 
indications are that widespread offshore winds will develop as early 
as Sunday night and likely by Monday night. At this point it appears 
that offshore winds will mostly be light...but locally stronger winds 
are possible. Relative humidity recoveries will be very poor at night through most 
of next week. Relief from the hot and dry conditions probably won't 
come until Thursday night. 


&& 


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


$$ 


Public forecast: dykema 
aviation/marine: canepa 
fire weather: dykema 


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