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Area forecast discussion 
National Weather Service Sacramento California 
345 am PST Friday Nov 21 2008 


Discussion... 
the northerly mfr-SAC pressure gradient has been increasing to 8 mbs 
overnite in the Post trough/frontal environment over interior norcal. 
The northerly gradient has resulted in a light to moderate north wind 
which has steadily been lowering dewpoints/humidities in the 
interior of norcal. This is precluding fog development in the 
Interior Valley...except for some fog or low cloud development in 
the wind sheltered areas in the mountain valleys/basins of Plumas 
Colorado...and in the lower elevations of the motherlode where the northwesterly 
winds from the valley become upslope winds and converge with the 
downslope winds from the higher elevations of the West Slope of the 
Sierra Nevada. The adiabatic and mixing effects are also resulting in 
warmer temperatures than last night mainly in the northern SAC valley. In the Post 
frontal environment today...the middle level cold air advection will weaken...and we 
look for a milder day mainly over the northern SAC valley where clouds and 
light rain occurred into early afternoon yesterday. Elsewhere...we are 
looking for more of a persistent temperature regime and have gone above 
much of the MOS guidance numbers. A look at BUFKIT forecast 
soundings indicate a rather low mixing height today...which will 
limit momentum Transfer of the higher wind speeds aloft...as the 
lower level warm air advection results in a strong cap or inversion preventing deep 
mixing. Also...by afternoon the higher wind speeds aloft are forecast to 
decrease significantly. Kbbx and kdax vwps show 25/40 kts of winds 
at 2000 feet respectively at 09z. 


Synoptic pattern for norcal has a Flat Ridge over norcal today with 
high level moisture passing S of our forecast area...and moisture 
several hundred miles offshore along 140w associated with the next 
upstream trough that is forecast to track further north than the last 
system...and brush extreme norcal late tonight and early Sat. We 
have confined our mention of low probability of precipitation/low quantitative precipitation forecast to Sat morning for the 
Shasta Colorado mountains for now. A lingering light northerly wind is expected to 
limit the return of higher dewpoints precluding fog throughout much 
of the valley. 


Developing/deepening upper trough over the eastern Pacific on sun leads to an 
amplifying ridge along the West Coast and a pleasant late autumn day 
for norcal...that is after morning fog dissipates in the southern SAC... 
northern San Joaquin Valley...and Delta areas. With the upper ridge over the 
area...surface winds are forecast to turn southerly again allowing 
dewpoints to recover Sat night and Sun morning allowing formation of 
fog. 


After sun...the forecast picture is quite unclear with model 
differences and spreads offering a wide variety of possible 
outcomes. We have discounted the Canadian model for Monday with its 
open wave bringing precipitation to norcal...while the GFS/European model (ecmwf) develop a 
closed low over the eastern Pacific and keeping precipitation out west of the interior 
of norcal...altho...oops!...the 06z GFS now spreads precipitation into the 
interior. So...we will follow the lead from the 0716z HPC discussion 
.."at least one robust cluster exists...Consisting of the 12z/20 
European ensemble mean...the 00z/21 European model (ecmwf)...and the 00z/21 UKMET." 
Translation...we expect some precipitation in norcal...eventually...but not 
necessarily in the early part of next week. But our confidence/ 
certainty rise during the latter part of the week...as a series of 
about 3 low pressure systems increase the chances that at least one 
of these systems...if not all 3...will tip the rain buckets in the 
interior of norcal. Jhm 


&& 


Aviation... 
VFR conditions which are present over most of the area will continue 
through today with mostly clear skies. An exception to this is local 
IFR/LIFR this morning in a narrow north-south band of low clouds/fog 
along the Sierra foothills south of kaun. Northwesterly winds have 
banked the low clouds up against the foothills. These winds are also 
preventing the development of fog in the valley this morning. There 
is some Mountain Valley fog currently adjacent to Chester and Quincy 
and also around Lake Tahoe which will last into middle morning. The 
northerly winds in the valley will increase this morning with local 
gusts to 20-25 knots possible...then decreasing in the afternoon and 
becoming light southerly tonight. Ek 


&& 


Sto watches/warnings/advisories...none. 


&& 


$$