FXUS66 KSTO 212340
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
330 PM PST MON DEC 21 2009
.DISCUSSION...
THE WINTER SOLSTICE OCCURRED AT 947 AM PST THIS MORNING...AND ON
CUE...PARTS OF INTERIOR NORCAL WILL EXPERIENCE SOME WINTRY WX
THROUGH TONITE. A COLD FRONT WILL BE MOVING ACROSS THE I-80 CORRIDOR
THROUGH THIS EVENING...AND THE UPPER LEVEL TROF ALONG THE NRN CA
COAST WILL MOVE ACROSS THE AREA LATER TONITE. THERE IS NOT A LOT OF
MOISTURE WITH THIS SYSTEM AS PWS HAVE BEEN SHUNTED SWD BY THE
APPROACHING TROF. GOES BLENDED PW AND AND MORNING RAOBS INDICATE PWS
GENERALLY FROM .50 TO .60 INCHES...AND MUCH DRIER AIR BACK UPSTREAM
WITH PWS LESS THAN .25 INCHES W OF 130W. MORNING RAOBS INDICATE CAA
OCCURRING ABOVE 900 MBS...AND AS THE UPPER TROF MOVES INLAND THE 700
MBS TEMPS ARE FORECAST TO LOWER TO -12 TO -16 DEG C OVER INTERIOR
NORCAL EARLY TUE MORNING. CURRENT SNOW LEVELS FROM 5500 FT TO 6000
FT SHOULD LOWER TO 3000 TO 4000 FT OVERNITE...LOCALLY LOWER IF PCPN
CONTINUES INTO TUE MORNING...BUT PCPN IS EXPECTED TO WIND DOWN THEN.
BUFKIT FORECAST QPFS INDICATE PCPN WILL VARY FROM .30 INCH AT RDD...
ABOUT .80 INCH AT BLU...AND ONLY A FEW HUNDREDTHS IN THE INTERIOR
VLY. OROGRAPHICS ARE WEAK TO MODERATE AT BEST FOR THE SIERNEV WITH
THE 12Z GFS CROSS SECTIONS SHOWING THE BEST UVM OVER THE SIERNEV
FROM 00Z-06Z TUE...WEAKENING BY 12Z TUE. THUS...PCPN IS EXPECTED TO
INCREASE IN AREAL COVERAGE AND INTENSITY THROUGH TONITE OVER THE
SIERNEV AND THUS WE HAVE A WINTER WX ADVSRY FOR SNOW IN EFFECT FOR
THE SIERNEV AS HIGHER ELEVATIONS MAY SEE 7-11 INCHES OF SNOW...WITH
AT LEAST 2-4 INCHES EXPECTED AROUND 5000 FT. CURRENT IR SATELLITE
IMAGERY SHOWS CLOUD TOPS ENHANCING OVER THE FAR NRN SIERNEV IN THE
WAA ZONE AHEAD OF THE COLD UPPER LEVEL TROF...WITH RADAR SHOWING
BANDS OF ECHOES OVER THE ERN HALF OF THE CWA. A SECOND AREA OF PCPN
ALONG THE COAST LOOKS MORE SIGNIFICANT ON RADAR AS THE UPPER TROF
APPROACHES THAT AREA. THIS BAND OF PCPN IS FORECAST TO MOVE ACROSS
OUR FORECAST AREA AND INTO THE SIERNEV TONIGHT.
A COLD NORTH WIND IS EXPECTED TO BLOW ON TUE AS THE UPPER TROF MOVES
TO THE E...AND THE UPPER LOW DIGS TOWARDS KLAS. THIS IS A GOOD CAA
BRISK NLY FLOW PATTERN FOR INTERIOR NORCAL. LOW CLOUDS MAY LINGER
OVER MOST OF INTERIOR NORCAL ON TUE DUE TO CYCLONIC FLOW AND CAA
BEHIND THE TROF. BUT BY TUE NITE AND WED...THE LOWER LEVELS ARE
EXPECTED TO DRY OUT SIGNIFICANTLY RESULTING IN CLEAR SKIES AND COLD
MIN TEMPS WED MORNING. GUIDANCE SUGGESTS THERE WILL BE ENOUGH WIND
AND MIXING WED MORNING TO PRECLUDE MUCH FROST/FOG AS HIGH PRESSURE
BUILDS OVER ORE. BUT NOT SO ON SUBSEQUENT MORNINGS.
A HIGHLY AMPLIFIED RIDGE OVER THE ERN PAC IS FORECAST TO SHIFT
TOWARDS THE COAST AND INLAND WED THRU FRI WITH MODERATING TEMPS...
AND STRENGTHENING TEMP INVERSIONS. THIS WILL LIKELY RETURN AREAS OF
FOG TO THE VLY WITH RELATIVELY PLEASANT WX ABOVE THE INVERSIONS
THROUGH XMAS DAY. AS SURFACE HIGH PRESSURE FROM ORE BUILDS INTO THE
GREAT BASIN THU MORNING...WINDS ARE FORECAST TO WEAKEN SIGNIFICANTLY
AND FROST/FOG IS EXPECTED TO BE MORE WIDESPREAD THAN ON WED MORNING.
GIVEN THE DEVELOPING ELY GRADIENT...DOWNSLOPE/KATABATIC WINDS OVER
THE SIERNEV WILL HAVE A MODERATING EFFECT...WHILE THE DECOUPLING OF
WINDS IN THE INTERIOR VLY RESULT IN LIGHT WINDS AND STRONG
RADIATIONAL COOLING. ELY FLOW OR OFFSHORE FLOW WILL CONTINUE ON FRI
AS A 1040 MBS HIGH IS FORECAST TO BUILD INTO ID AND THE GREAT BASIN.
THIS WILL CONTINUE COLD AND DRY CONDITIONS AT NIGHT FOR THE SIERNEV
WITH MODERATING DAYTIME TEMPS. HIGH CLOUDINESS SHOULD BE MORE
PREVALENT OVER INTERIOR NORCAL THU/FRI AS HIGH CLOUDS ARE EXPECTED
TO SPILL OVER THE RIDGE. JHM
&&
.EXTENDED DISCUSSION (FRIDAY NIGHT THROUGH MONDAY)...
THE RIDGE OF HIGH PRESSURE OVER THE AREA CHRISTMAS DAY WILL SHIFT
EAST ON SATURDAY AS AN UPPER LEVEL TROUGH DIGS OVER THE EASTERN
PACIFIC. THE MODELS STILL VARY WITH THE TRACK OF THIS SYSTEM WITH
THE 18Z OPERATIONAL GFS TRACKING IT NORTH AND THE ECMWF TRACKING IT
SOUTH. THE GFS ENSEMBLE SPREAD IS OVER 50 M WITH THIS SYSTEM
INDICATING HIGH UNCERTAINTY. ALTHOUGH EITHER WAY COOLER TEMPERATURES
(COMPARED TO CHRISTMAS DAY) ARE EXPECTED FOR THE WEEKEND INTO EARLY
NEXT WEEK. THE CURRENT FORECAST LEANS MORE TOWARDS CLIMATOLOGY AND
THE OPERATIONAL GFS WITH A THREAT OF PRECIPITATION ACROSS THE AREA
FOR SUNDAY AND MONDAY. HSO
&&
.AVIATION...
LOW CIGS CONT TO PLAGUE THE VALLEY FLOOR RANGING BKN-OVC 400-1400
FEET. VSBYS ALSO LOW FOR MUCH OF THE VALLEY AROUND 1-3 SM. EXPECT
TAF SITES TO CONTINUE FLUCTUATING BETWEEN LIFR-IFR-MVFR THRU TUES
MORNING. IMPROVING CONDS WILL BEGIN AFT 12Z WITH VFR CONDS STARTING
SOMETIME BETWEEN 18-22Z. WINDS GENERALLY BETWEEN SOUTHWEST TO
SOUTHEASTERLY FLOW WITH SPEEDS AROUND 6-12 KTS. WINDS WILL BECOME
NORTHWESTERLY AND SHOULD STRENGTHEN TO 10-20 KTS WITH GUSTS UP TO 30
KTS BY TUES MORNING...RIDGETOP WINDS GUSTING 30-40 KTS. JBB
&&
.STO WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FOR SNOW UNTIL 4 AM PST TUESDAY FOR WESTERN
PLUMAS COUNTY...LASSEN PARK AREA AND WEST SLOPE OF NORTHERN SIERRA
NEVADA.
&&
$$
