FXUS66 KSTO 242328
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
330 PM PST THU DEC 24 2009
.DISCUSSION...
UPPER LEVEL RIDGE CONTINUES TO BE THE DOMINANT FEATURE IMPACTING
NORTHERN CALIFORNIA WEATHER. THIS RIDGE IS CENTERED JUST OFF THE
WEST COAST AND IS SLOWLY PUSHING EASTWARD. AT THE SURFACE HIGH
PRESSURE IS CENTERED OVER CENTRAL NEVADA CREATING A DECENT
NORTHEASTERLY GRADIENT THROUGH THE SIERRA. THIS GRADIENT IS EXPECTED
TO WEAKEN OVER THE NEXT FEW DAYS BUT WILL STILL CAUSE STRONG
KATABATIC WINDS TO BLOW DOWN SW/NE SIERRA CANYONS. TEMPERATURES THIS
AFTERNOON ARE GENERALLY IN THE 50S TO LOWER 60S THROUGH THE VALLEY
AND IN THE 40S OVER HIGHER ELEVATIONS.
SYNOPTIC PATTERN CHANGES ONLY SLIGHTLY OVER THE NEXT 12 TO 24 HOURS
AS THE UPPER RIDGE SHIFTS OVERHEAD. THE ENHANCED SUBSIDENCE WOULD
LIKELY PROMOTE MORE WIDESPREAD FOG TONIGHT...BUT RH FORECASTS IN
ADDITION TO THE RESIDUAL OFFSHORE GRADIENT MAY INHIBIT DENSE FOG
FROM FORMING. WITH CONTINUED MOSTLY CLEAR SKIES AND THE BOUNDARY
LAYER DECOUPLING AT LOWER ELEVATIONS...EXPECT ANOTHER CHILLY NIGHT
THROUGH THE VALLEY WITH TEMPERATURES DIPPING DOWN INTO THE UPPER 20S
TO LOWER 30S ONCE AGAIN. THE COLDER MOUNTAIN BASINS WILL ONCE AGAIN
DIP INTO THE TEENS. THERMAL BELTS WILL DEVELOP IN FOOTHILL LOCATIONS
KEEPING TEMPS MODERATED.
MAIN CHALLENGE WITH THE FORECAST REMAINS TO BE THE DEVELOPING STORM
CURRENTLY IN THE CENTRAL PACIFIC SOUTH OF THE ALEUTIAN ISLANDS. THIS
SYSTEM IS EXPECTED TO SPREAD CLOUDINESS OVER THE FORECAST AREA
SATURDAY AS IT PUSHES THE UPPER RIDGE EAST. THE GFS AND NAM HAVE
WAVERED IN THEIR SOLUTIONS AND NOW FAVOR THE SYSTEM SPLITTING AS IT
MOVES INLAND SATURDAY NIGHT...LEAVING MOST OF THE FORECAST AREA DRY.
THE 12Z CANADIAN MODEL HAS SHIFTED IN THIS DIRECTION AS
WELL...LEAVING THE ECMWF AS THE ONLY MODEL THAT CONSOLIDATES
ENOUGH ENERGY TO BRING PRECIPITATION INTO THE NORTHERN INTERIOR
SATURDAY NIGHT INTO SUNDAY. IN LIEU OF THIS DEVELOPMENT...WILL
CONTINUE TO HEDGE THE PRECIPITATION FORECAST UNTIL THE MODELS ARRIVE
IN BETTER AGREEMENT. IF PRECIPITATION DOES FALL...TOTALS WOULD BE
MODEST AT BEST AS THERE IS NO SIGNIFICANT MOISTURE TAP. SNOW LEVELS
WOULD ALSO HOVER IN THE 4000 TO 5000 FT RANGE...THOUGH SOME OF THE
USUAL COLDER MOUNTAIN BASINS WOULD SEE LIGHT SNOW. DANG
.EXTENDED DISCUSSION (SUNDAY NIGHT THROUGH FRIDAY)...A SHORT WAVE
UPPER LOW DROPS SOUTHEAST ALONG THE CALIFORNIA COAST LATE SUN
UNDERCUTTING THE BROAD UPPER RIDGE CENTERED OVER THE ROCKIES. ALL
MODELS TAKE MOST OF THE MOISTURE AND ENERGY ASSOCIATED WITH THIS
SHORT WAVE FEATURE INLAND WELL SOUTH OF THE DISTRICT AND KEEP
CONDITIONS DRY THROUGH ABOUT TUE. A SHORT WAVE RIDGE BUILDS UPSTREAM
IN THE WAKE OF THE SHORTWAVE. A PACIFIC FRONT MOVING INTO THIS RIDGE
WEAKENS SUBSTANTIALLY BUT STILL PROVIDES A CHANCE OF PRECIPITATION
TUE NIGHT INTO WED. LATE IN THE WEEK MODELS HINT AT A STRONGER SHORT
WAVE AFFECTING NORTHERN CA...THOUGH TIMING AND INTENSITY ARE STILL
VARYING SUBSTANTIALLY FROM RUN TO RUN. PRECIP WITH THESE SYSTEMS ARE
UNIMPRESSIVE AND THE MAIN EFFECTS WILL BE MODERATING NIGHTTIME
TEMPERATURES WITH SOME VALLEY FOG AND LIGHT PRECIP IN HIGHER
ELEVATIONS. DK2
&&
.AVIATION...
GENERALLY VFR CONDITIONS ACROSS THE AREA THROUGH CHRISTMAS DAY
EXCEPT FOR AREAS OF MVFR CONDITIONS DUE TO BR AND ISOLD LIFR
CONDITIONS DUE TO FOG AFT 08Z UNTIL AROUND 18Z IN THE SACRAMENTO
AREA AND SOUTH TO THE NORTHERN SAN JOAQUIN VALLEY. LOCAL NORTHEAST
WIND GUSTS UP TO 35 KTS IN THE SIERRA THROUGH FRIDAY MORNING.
&&
.STO WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
