FXUS66 KSTO 221804
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
955 AM PST TUE DEC 22 2009
.DISCUSSION...
CLOSED LOW OVER ORE FORECAST TO DROP SSEWD ACROSS WRN NV THIS
AFTERNOON AND TOWARDS KLAS BY AROUND 06Z WED. BLENDED TPW PRODUCT
SHOWS PWS A LITTLE OVER A THIRD OF AN INCH IN THE VCNTY OF THE UPPER
LOW...WITH MUCH LOWER PWS A LITTLE OVER A TENTH OF AN INCH OVER THE
SIERNEV. SUBSIDENCE BEHIND YESTERDAY'S COLD FRONT AND UPPER TROF
AXIS HAS LOWERED PWS AND CLEARED CLOUDS FROM N TO S OVER THE SIERNEV
NORTH OF I-80...AND AREAS S OF I-80 WILL BE CLEARING BY LATE
MORNING. THEN...MORE CLOUDS AND ISOLATED TO WIDELY SCATTERED SNOW
SHOWERS MAINLY NEAR THE CREST ARE POSSIBLE THIS AFTERNOON AS THE
CLOSED LOW DIGS SSEWD E OF THE SIERNEV CREST. LITTLE QPF/SNOW
ACCUMULATION IS EXPECTED THIS AFTERNOON. IN ADDITION...THE NWLY AND
STILL CYCLONIC FLOW FROM THE COAST WILL BE SPREADING STRATO-CU AND
SCATTERED SHOWERS SEWD INTO THE COASTAL RANGE TODAY PER THE WRF
MODEL. MUCH OF THIS CLOUDINESS WILL NOT MAKE INTO THE WRN SAC VLY
DUE TO THE DOWNSLOPE/NWLY FLOW ERODING CLOUDS.
WITH THE CLOSED LOW DROPPING SSEWD...NWLY SFC WINDS ARE EXPECTED TO
INCREASE DUE TO PRESSURE RISES ACROSS ORE BEHIND THE CLOSED LOW.
LAPSE RATES ARE FORECAST TO BE STEEP ENOUGH UP TO 800 MBS OR SO TO
ALLOW FOR DOWNWARD MOMENTUM TRANSFER AND LOCALLY GUSTY NWLY SURFACE
WINDS THIS AFTERNOON IN THE VALLEY. STRONG CAA WILL RESULT IN A
CHILLY DAY ACROSS NORCAL WITH MORNING RAOBS SOME 6 TO 19 DEG COLDER
THAN YESTERDAY. THE COLDER TEMPS WILL BE IN THE MTN AREAS.
HIGH PRESSURE WILL BUILD OVER THE ERN PAC IN THE WAKE OF THE CLOSED
LOW ON WED...AND THEN IS FORECAST TO SHIFT TOWARDS THE COAST ON THU.
ENOUGH WIND IS FORECAST TONIGHT TO KEEP THE BOUNDARY LAYER MIXED TO
PRECLUDE FROST/FOG FROM FORMING WED MORNING. THIS WILL NOT BE THE
CASE THE NEXT COUPLE OF MORNINGS AS THE RIDGE AXIS SHIFTS CLOSER TO
THE COAST ON THU...AND THEN INLAND ON FRI. SO THU MORNING SHOULD
BEGIN FROSTY/FOGGY...AND SIMILAR CONDITIONS ARE EXPECTED ON XMAS
MORNING AS THE RIDGE AXIS SHIFTS INLAND. OTHERWISE DRY WX EXPECTED
FROM WED-XMAS OVER INTERIOR NORCAL. JHM
&&
.EXTENDED DISCUSSION (SATURDAY THROUGH TUESDAY)...
UPPER LEVEL RIDGING GIVES WAY TO AN APPROACHING TROUGH ON SATURDAY
AND SUNDAY. MORNING FOG IS POSSIBLE OVER THE SOUTHERN SACRAMENTO AND
NORTHERN SAN JOAQUIN VALLEYS AND THE DELTA FOR SATURDAY MORNING
BEFORE THE RIDGE MOVES OUT. CLOUDS INCREASE THROUGH THE DAY AS A
SLIGHT CHANCE OF PRECIPITATION SPREADS INTO THE COASTAL MOUNTAINS
LATE SATURDAY AFTERNOON/EARLY EVENING AND SPREADS OVER THE REST OF
THE AREA OVERNIGHT. SNOW LEVELS DROP TO AROUND 5000 FEET BY SUNDAY.
THIS SPLITTING TROUGH CONTINUES TO BE HANDLED DIFFERENTLY BY THE
MEDIUM RANGE MODELS. THE ECMWF HAS A DEEPER TROUGH AND IS WETTER
THAN THE GFS AND THE CANADIAN MODELS. WILL PUT IN A SLIGHT CHANCE OF
PRECIPITATION INTO TUESDAY AS THIS SYSTEM BRINGS MULTIPLE SHORTWAVES
THROUGH. NOT EXPECTING A HEAVY QPF EVENT AT THIS POINT. EK
&&
.AVIATION...
UPPER LEVEL TROUGH SHIFTS TO THE EAST TODAY INTO TONIGHT WITH A
RIDGE OF HIGH PRESSURE BUILDING BEHIND IT. VFR CONDITIONS IN THE
VALLEY TODAY WITH CLEAR SKIES AND NORTHERLY WIND GUSTS UP TO 28 KTS
WITH SURFACE HIGH PRESSURE BUILDING INTO OREGON. THE DRY NORTHERLY
WINDS WILL LIMIT FOG DEVELOPMENT IN THE VALLEY IN THE MORNING.
LINGERING ISOLATED TO SCATTERED SHOWERS OVER THE MOUNTAINS WITH SNOW
LEVELS DOWN TO AROUND 2500 FEET DURING THE DAY TODAY FOR AREAS OF
LIFR/IFR CONDITIONS WITH IMPROVING CONDITIONS AND SHOWERS
DIMINISHING TONIGHT.
&&
.STO WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
