FXUS66 KSTO 230511
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
900 PM PST TUE DEC 22 2009
.EVENING DISCUSSION UPDATE...
A FEW LINGERING FLURRIES ALONG THE SIERRA NEVADA CREST FROM LAKE
TAHOE SOUTHWARD BUT THESE CONTINUE TO DIMINISH AND MOVE SOUTHWARD AS
THE LOW PUSHES EAST. FOG NOT EXPECTED TO BE A PROBLEM OVERNIGHT DUE
TO SOME LIGHT BUT NOT NON-EXISTENT WINDS AND DRYING ATMOSPHERIC
CONDITIONS BUT WOULD NOT COUNT IT OUT COMPLETELY IN FAVORED AREAS
ESPECIALLY NEAR WATERWAYS.
NO NOTABLE CHANGES TO CURRENT FORECAST PACKAGE. TEMPERATURES WILL
BE NEAR TO A LITTLE BELOW FREEZING TONIGHT IN MOST VALLEY
LOCATIONS. A LITTLE WARMER TOMORROW NIGHT DUE TO FOG REDEVELOPMENT.
.AFTERNOON DISCUSSION...
CLOSED UPPER LOW NOW OVER NWRN NV AND IS FORECAST TO DROP SSEWD
ACROSS WRN NV THIS THIS EVENING AND TOWARDS KLAS BY AROUND 06Z WED.
SATELLITE/MOSAIC RADAR INDICATES CLOUDS AND ASSOCIATED SHOWERS
SPREADING SWD ALONG AND WEST OF THE CREST OF THE SIERNEV. BLENDED
TPW PRODUCT SHOWS PWS UP TO .25 INCH IN THE VCNTY OF THE UPPER
LOW...WITH MUCH LOWER PWS A LITTLE OVER A TENTH OF AN INCH OVER THE
REST OF THE SIERNEV IN OUR CWA WHERE SKIES ARE CLEAR. WITH VERY
LITTLE PW...WE ONLY EXPECT LIGHT SHOWERS OR FLURRIES THIS EVENING...
WITH THE POSSIBILITY OF FLURRIES NEAR THE SIERNEV CREST LINGERING
AFTER MIDNITE SOUTH OF I-80 AS THE SURFACE RIDGE AXIS BUILDS ACROSS
WRN NV AND INITIATES UPSLOPE FLOW ON THE E SIDE OF THE SIERNEV WHICH
MAY CAUSE A LITTLE SPILLOVER JUST WEST OF THE CREST. LINGERING
CYCLONIC FLOW OVER THE WRN PORTION OF THE CWA SPREAD SOME STRATO-CU
AND SCATTERED SHOWERS SEWD INTO THE COASTAL RANGE DURING THE DAY...
BUT NOW THAT THE UPPER LOW HAS MOVED FARTHER S INTO NV...THE
CYCLONIC FLOW HAS WEAKENED AND DOWNSLOPE FLOW IS ERODING MUCH OF THE
CLOUDINESS IN OUR CWA.
WITH THE CLOSED LOW DROPPING SSEWD...NWLY SFC WINDS HAVE INCREASED
DUE TO PRESSURE RISES ACROSS ORE BEHIND THE CLOSED LOW. LAPSE RATES
WERE STEEP ENOUGH UP TO 800 MBS TO ALLOW FOR DOWNWARD MOMENTUM
TRANSFER AND LOCALLY GUSTY NWLY SURFACE WINDS DURING THE AFTERNOON
IN THE VALLEY. AS THE SURFACE RIDGE BUILDS ACROSS WRN NV LATER
TONIGHT...THE NAM 925 MBS WINDS INDICATE A MODERATE NLY BARRIER JET
WILL DEVELOP ON THE W SIDE OF THE SAC VLY. ENOUGH WIND IS FORECAST
TONIGHT TO KEEP THE BOUNDARY LAYER MIXED TO PRECLUDE FROST/FOG FROM
FORMING WED MORNING OVER MUCH OF THE VALLEY ESPECIALLY ON THE W
SIDE. THE E SIDE IS NOT PRONE TO AS MUCH WIND IN THIS PATTERN...BUT
THE DRY AIR MASS IS ALSO LIKELY TO PRECLUDE MUCH FOG/FROST.
HOWEVER...WITH LIGHTER WINDS ON THE E SIDE OF THE VLY...THIS AREA
WILL BE SUBJECT TO LESS MIXING AND COLDER MIN TEMPS.
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.
ALTHOUGH THERE WILL BE ENOUGH WIND TONIGHT TO KEEP THE BOUNDARY
LAYER MIXED TO PRECLUDE FROST/FOG FROM FORMING WED MORNING...THIS IS
NOT LIKELY TO 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 RIDGE SHIFTS TO THE EAST ON SATURDAY AS AN UPPER LEVEL
TROUGH DIGS OFF OF THE WEST COAST. PATCHY FOG AND LOW CLOUDS ARE
POSSIBLE SATURDAY MORNING OVER THE VALLEYS.THE MODELS STILL VARY ON
THE STRENGTH AND TRACK OF THIS SYSTEM FOR SUNDAY. THE 12Z GFS SPLITS
THE SYSTEM AND KEEPS MOST OF THE PRECIPITATION OUT OF THE AREA. THE
12Z ECMWF IS STRONGER AND BRINGS A LOW DOWN THROUGH CALIFORNIA
BRINGING PRECIPITATION SUNDAY INTO MONDAY. THE CURRENT FORECAST
LEANS TOWARDS CLIMO AND THE ECMWF WITH A THREAT OF PRECIPITATION TO
INTERIOR NORTHERN CALIFORNIA SUNDAY INTO MONDAY. THE MODELS SHOW
ANOTHER SYSTEM COMING IN LATE TUESDAY FOR A CONTINUED THREAT OF
PRECIPITATION ON TUESDAY. TEMPERATURES ARE EXPECTED TO BE COOLER
(WITH NEAR TO BELOW NORMAL TEMPERATURES) OVER THE WEEKEND INTO EARLY
NEXT WEEK. HSO
&&
.AVIATION...
REGION UNDER A NORTHERLY FLOW REGIME AS HIGH PRESSURE MOVES INTO THE
WEST COAST. VALLEY WINDS RANGING 5-12 KTS WITH NRN SAN JOAQUIN
VALLEY ON THE HIGHER END OF THAT SPECTRUM. WIND SPEEDS WILL LESSEN
OVERNIGHT BECOMING 3-8 KTS. HIGH PRESSURE RIDGE WILL BRING VFR CONDS
AND LIGHT WINDS NOW INTO SATURDAY MORNING. THE INFLUX OF DRY
NORTHERLY AIR TODAY SHOULD PREVENT ANY WIDESPREAD FOG DEVELOPMENT
WEDNESDAY MORNING...HOWEVER PATCHY FOG EXPECTED TO RETURN EARLY
THURS MORNING. JBB
&&
.STO WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
