FXUS66 KSTO 261758
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
950 AM PST THU NOV 26 2009

.DISCUSSION...
CLOUD COVER FROM THE OFFSHORE SYSTEM IS BEGINNING TO SPREAD INTO 
INTERIOR NORCAL AS APPROACHING LOW PRESSURE WEAKENS THE STRONG RIDGE 
OVER THE AREA. THE INCREASE IN CLOUD COVER AND SLIGHT SYNOPTIC 
COOLING WILL RESULT IN MAX TEMPS SOME 3 TO 10 DEG COOLER TODAY OVER 
THE SAC VLY...WITH 1 TO 6 DEG OF COOLING ELSEWHERE. EVEN SO...MAX 
TEMPS WILL STILL BE A LITTLE ABOVE NORMAL...AND THE WX WILL 
COOPERATE BY REMAINING TRANQUIL FOR THE HOLIDAY. 

THE EFFECTS OF THE OFFSHORE SYSTEM ON NORCAL LOOK QUITE PROBLEMATIC 
AT THIS TIME...ESPECIALLY THE QPFS...AS A COMPARISON OF THE MODEL 
QPFS RESULTS IN A VARIETY OF SOLUTIONS...YOU COULD PICK YOUR OWN... 
RELATIVELY DRY...OR WETTER. NOT TOO SURE A COMPROMISE SOLUTION IS 
THE WAY TO GO WITH THIS SYSTEM...AS THE STORM EVOLUTION AND TRACK 
WILL BE A MAJOR FACTOR...THUS A LOT OF UNCERTAINTY. WHAT WE HAVE 
NOTICED ON IR IMAGERY TO BOLSTER SOME CONFIDENCE IS THE 
STRETCHING/SPLITTING OF THE TROF ALONG 45N/130W...OFF THE WA/OR 
COAST. SO IT LOOKS LIKE THE SPLITTING TROF SCENARIO WILL 
MATERIALIZE...NOW TO DETERMINE THE TIMING AND TRACK OF THE CLOSED 
LOW. THE 12Z MODELS SEEM TO BE IN GOOD AGREEMENT THAT THE LOW WILL 
CLOSE OFF VCNTY OR JUST W OF SFO BY 00Z SAT...AND THEN DROP SWD AND 
AFFECT SOCAL ON SAT. COMPARISON OF QPFS FROM THE GFS INDICATE THE 
PCPN WILL BEGIN/END SOONER...BEGINNING LATER TONITE AND ENDING BY 
18Z 11/28 OR SAT MORNING. FOR THE AMOUNTS...WE ARE FAVORING THE 
LOWER QPFS AS THE WAA PRECIP AHEAD OF THE COLD FRONT SHOULD BE 
RELATIVELY LIGHT...THIS IS DUE TO THE VERY LOW TWP AIR OFF THE COAST 
AND TO THE S OF THE FRONTAL BAND. THE SWLY FLOW FOR THE MOST PART 
WILL BE DRY...AND THUS SLOW TO MOISTEN THE ATMOSPHERE. 700 MBS WINDS 
OF 30-45 KTS ARE FORECAST TONITE...THEN BACKING AND WEAKENING DURING 
THE DAY ON FRI AS THE AIR MASS FINALLY MOISTENS. THE BACKING FLOW 
WILL RESULT IN A MORE MERIDIONAL PW PLUME AND WINDS PARALLELING THE 
SIERNEV. WITH THE SYSTEM CLOSING OFF ON FRI...THE DEFORMATION AXIS 
ALONG THE CA/OR BORDER SHOULD BE DROPPING SWD INTO THE PLUMAS CO 
AREA BY 00Z SAT...INTO THE 80-50 CORRIDOR BY 06Z SAT...AND THEN S OF 
OUR AREA BY 12Z SAT AS NELY TO ELY FLOW DEVELOPS OVER INTERIOR 
NORCAL AS THE CLOSED LOW FORMS TO OUR SW. AT THIS TIME...FAVORING 
THE BETTER RES MODEL QPFS...THE NAM AND WRF WHICH HAVE THE MAX QPFS 
ALONG THE DEFORMATION ZONE...THUS A NARROW BAND OF 3-6 TENTHS OF AN 
INCH PCPN FROM PLUMAS CO...SWWD TO E OF SAC...AND INTO THE SCK-MOD 
AREA. THE HIGHER AMOUNTS IN PLUMAS CO DUE TO OROGRAPHICS ENHANCING  
QPFS ALBEIT FOR A SHORT TIME PERIOD. 

FRI WILL BE MUCH COOLER AS THICKNESSES AND 850 MBS TEMPS LOWER SOME 
16 DAM AND 9 DEG C...AT LEAST SOME 10-20 DEG F OVER MUCH OF THE 
AREA. CHILLY NLY BREEZES EXPECTED ON SAT AS THE FLOW TURNS DRIER FOR 
THE VLY...WHILE ELY FLOW TO THE NE OF THE CLOSED LOW WILL RESULT IN 
SOME BACKWASH CLOUDS AND LIGHT PCPN SPILLING OVER THE E SIDE OF THE 
SIERNEV CREST...AND POSSIBLY W OF THE CREST SAT MORNING. THE ELY 
FLOW IS FORECAST TO WEAKEN SUFFICIENTLY BY SAT AFTN.    JHM 

.PREVIOUS DISCUSSION...
THANKSGIVING 2009 IS HERE AND IS EXPECTED TO BE THE LAST IN THE 
STRING OF DRY AND RELATIVELY MILD DAYS THAT WE/VE EXPERIENCED THIS 
WEEK. THE APPROACHING SYSTEM OFFSHORE NOW INSIDE OF 140W AND HAS 
NUDGED THE STRONG RIDGE OF HIGH PRESSURE EASTWARD THAT HAS BEEN 
PERSISTENT OVER THE REGION. TEMPERATURES TODAY WILL BE COOLER THAN 
THE PAST COUPLE OF DAYS AS THE OVERALL AIRMASS COOLS A BIT IN 
RESPONSE TO THE GRADUAL LOWERING OF HEIGHTS ALOFT AND HIGH CLOUDS 
INCREASE AHEAD OF THE APPROACHING SYSTEM...BUT THE BIGGER FACTOR 
WILL BE THE DECREASE IN OFFSHORE FLOW AND MUCH WEAKER SUBSIDENCE 
OVER THE AREA THAN WHAT WE/VE SEEN THE PAST 48 TO 72 HOURS. AREAS 
LIKE REDDING AND FAIRFIELD THAT HAVE BENEFITED FROM THE NORTHERLY 
DOWNSLOPE FLOW RESULTING IN 70+ DEGREE AFTERNOON TEMPERATURES WILL 
SEE A SIGNIFICANT DROP TODAY AS FLOW REVERSES TO LIGHT ONSHORE...BUT 
TEMPS WILL STILL BE A LITTLE ABOVE NORMAL.

MORE SIGNIFICANT CHANGES IN THE WEATHER ARRIVE LATER TONIGHT AND 
FRIDAY AS THE OFFSHORE SYSTEM SPLITS WITH MUCH OF THE ENERGY NOW 
FORECAST TO SLIDE SOUTHEASTWARD DOWN THE INTERIOR OF CALIFORNIA... 
REACHING SOCAL OVER THE WEEKEND. THIS IS A DRAMATIC SWITCH IN THE 
MODEL FORECAST SOLUTIONS COMPARED TO 24 HOURS AGO WHEN IT LOOKED 
LIKE MOST OF THE ENERGY WOULD REMAIN IN THE NORTHERN STREAM. 
THEREFORE THE FORECAST HAS BEEN ADJUSTED TO REFLECT COOLER AND MORE 
UNSETTLED WEATHER RETURNING LATER TONIGHT AND FRIDAY. QPF IS STILL 
EXPECTED TO BE RATHER LIGHT GIVEN THE ACCELERATED MOVEMENT TO THE 
SOUTH FRIDAY AS STRONG RIDGING DEVELOPS UPSTREAM OVER THE EASTERN 
PACIFIC. HOWEVER...ENOUGH SNOW IS EXPECTED OVER THE NORTHERN SIERRA 
NEVADA FRIDAY AFTERNOON AND EVENING TO POTENTIALLY LEAD TO SOME 
TRAVEL IMPACTS ACROSS I-80 AND HIGHWAY 50 SO QPF TRENDS WILL BE 
WATCHED FOR POSSIBLE ADVISORY ISSUANCE WITH LATER FORECAST PACKAGES.

STRONG NORTHERLY SURFACE FLOW FORECAST TO DEVELOP IN THE WAKE OF THE 
SYSTEM LATER FRIDAY NIGHT INTO SATURDAY AS COLD ADVECTION INCREASES 
AND STRONG SUBSIDENCE NORTHERLY UPPER LEVEL FLOW DEVELOPS. THIS WILL 
RESULT IN A COOL AND BREEZY DAY ACROSS THE REGION SATURDAY... 
ESPECIALLY IN THE MOUNTAINS WHERE TEMPERATURES WILL BE AROUND 20 
DEGREES COOLER THAN THOSE EXPECTED TODAY. BREEZES WILL CONTINUE 
SUNDAY WITH TEMPERATURES FORECAST TO MODERATE A BIT.

&&

.EXTENDED DISCUSSION (MONDAY THROUGH THURSDAY)...
A HIGH AMPLITUDE RIDGE WILL BE IN PLACE OVER NORTHERN CALIFORNIA 
THROUGH THE PERIOD. STRONG SURFACE HIGH PRESSURE MAY BE OVER THE 
GREAT BASIN WITH OCCASIONAL GUSTY WINDS THROUGH CANYONS ALONG THE 
WEST SLOPE OF THE SIERRA. STRONG INVERSIONS COULD DEVELOP MONDAY AND 
TUESDAY FOR LATE NIGHT AND EARLY MORNING FOG WITH HAZY AFTERNOON 
SUNSHINE. TEMPERATURES SHOULD BE NEAR SEASONAL NORMALS. THE LATEST 
MODELS ARE FARTHER EAST WITH ENERGY DROPPING DOWN THE EAST SIDE OF 
THE RIDGE...KEEPING SHORTWAVES OVER THE PLAIN STATES. WE MAY BE IN 
STORE FOR MORE FOG THAN WE HAVE SEEN RECENTLY NEXT WEEK. 

PIECES OF ENERGY PULLED FROM THE LOW OVER THE GULF OF ALASKA MAY 
EVENTUALLY CARVE OUT A TROF NEAR 130W TOWARD THE END OF NEXT WEEK 
BUT THE RIDGE APPEARS TO HOLD FOR NOW. THE EXTENDED MODELS ARE 
HINTING THAT THE WESTERLIES WILL UNDERCUT THE RIDGE AT SOME POINT 
BEYOND DAY 7. FOR NOW OUR FORECAST REMAINS DRY THROUGH THE PERIOD. 
SG 

&&

.AVIATION...
STRONG HIGH PRESSURE IN PLACE OVER THE GREAT BASIN WILL WEAKEN BY 
AFTERNOON AS A COLD FRONT MOVES ONTO THE CALIFORNIA COAST. HIGH 
CLOUDS AHEAD OF THIS SYSTEM WILL LIMIT FOG DEVELOPMENT IN THE 
VALLEYS. EXPECT MOSTLY VFR CONDITIONS WITH LIMITED MVFR 3-5SM BR AT 
KSMF...KSAC...KMHR AND KSCK UNTIL 18Z...THEN VFR BKN-OVC200-250. 
GUSTY EAST WINDS 25 TO 35 MPH CONTINUE THROUGH CANYONS ALONG THE W 
SLOPES OF THE SIERRA. A BAND OF RAIN WILL MOVE INTO THE SHASTA 
COUNTY AREA AFTER 06Z TONIGHT. RAIN WILL NOT IMPACT THE SMF OR SAC 
TERMINALS UNTIL EARLY FRIDAY MORNING. SG

&&

.STO WATCHES/WARNINGS/ADVISORIES...NONE. 

&&

$$