FXUS66 KSTO 271217 RRA
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
400 AM PST FRI NOV 27 2009
.DISCUSSION...
FORECAST ISSUES TODAY ARE VALLEY RAIN AND MOUNTAIN SNOW FROM THE
COLD FRONT...AND GUSTY NORTH TO NORTHEAST WINDS THAT DEVELOP
TONIGHT. A SPLITTING TROF THAT DEVELOPS INTO A CLOSED LOW OVER
CENTRAL CALIFORNIA WILL BRING LIGHT RAIN SHOWERS TO THE VALLEYS
TODAY. SHOWERS HAVE ALREADY BEEN OBSERVED AT REDDING AND RED BLUFF.
THE 06Z WRF CAME IN LIGHTER WITH THE QPF IN THE VALLEYS...BRINGING
SOME SHOWERS INTO SACRAMENTO BY 10 AM OR SO. BUT IT HAS BEEN
CONSISTENT IN STALLING THE FRONT ALONG THE WEST SLOPES LATER THIS
AFTERNOON AS A VORT MAX ROTATES INTO THE AREA BEFORE DEVELOPING INTO
THE CUTOFF LOW. PERSISTENT SHOWERS ARE LIKELY IN THE NORTHEAST
CORNER OF THIS LOW...FROM AUBURN TO PLACERVILLE TO THE WEST SLOPES
OF THE SIERRA. OVER 0.75 INCHES OF RAIN MAY FALL ALONG THE FRONTAL
BAND BUT MOST WILL BE IN THE FOOTHILLS ABOVE 1000 FEET. MEANWHILE
STRONG UPPER DIVERGENCE DEVELOPS IN THE SIERRA AHEAD OF THE 500MB
-25C COLD POOL THIS AFTERNOON. WHILE MOISTURE AND OROGRAPHICS ARE
LIMITED...MODELS OFTEN UNDERPLAY THE COLD AIR ADVECTION THAT CAN
OCCUR IN THIS SORT OF PATTERN. EXPECT A PERIOD OF MODERATE TO HEAVY
SNOW TO FALL IN LASSEN PARK TO I-80 AND HIGHWAY 50 BY NOON OR
SO...AND SPREAD SOUTH TOWARD CARSON PASS LATER IN THE AFTERNOON. WE
ISSUED A WINTER WEATHER ADVISORY FOR 4 TO 8 INCHES OF SNOW
ACCUMULATION AND ICY ROADS OVER THE PASSES TODAY AND THIS EVENING.
THERE IS A LOT OF DRY AIR IN THE MID LEVELS OF THE KOAK SOUNDING AND
EVAPORATIVE COOLING WILL LIKELY CAUSE LOWER SNOW LEVELS NEAR 4500
FEET THIS MORNING BEFORE THEY RISE TO 6000 FEET OR SO LATER TODAY.
SNOW LEVELS WILL DROP RAPIDLY WITH THE COLD FRONT BACK TO 4000 TO
4500 FEET. WHILE AN ISOLATED LIGHTNING STRIKE CAN NOT BE RULED OUT
NEAR THE UPPER LOW...WE DID NOT INCLUDE TS IN THE FORECAST FOR NOW
AS STABILITY PROFILES DO NOT APPEAR FAVORABLE.
THE GFS IS MORE PROGRESSIVE MOVING THE LOW INTO THE COLORADO RIVER
VALLEY BY 18Z TOMORROW. WRAP AROUND SNOW SHOWERS ARE CERTAINLY
POSSIBLE NEAR THE SIERRA CREST SATURDAY. ADDITIONALLY...AS THE 110KT
JET SLIDES SOUTH AND STRONG SURFACE HIGH PRESSURE BUILDS INTO
SOUTHERN OREGON...GUSTY NORTH TO NORTHEAST WINDS DEVELOP TONIGHT AND
SATURDAY. THE 00Z NAM-12 HAD A 14MB SURFACE GRADIENT FROM MFR TO SAC
EARLY SATURDAY AND THERE WILL ALSO BE A 7MB GRADIENT ACROSS THE
SIERRA. THE MODEL SHOWED 35-40KTS OF WIND AT 850MB AND HAD 35KTS OF
WIND 30M AGL. 700MB WIND BARBS OF 50 KTS WERE NOTED ALONG THE
SIERRA. THE FRESH SNOWFALL COULD POSE CONSIDERABLE BLOWING AND
DRIFTING PROBLEMS SATURDAY OVER THE PASSES. IN THE VALLEYS...A WIND
ADVISORY TYPE EVENT (SUSTAINED WINDS OF 25 MPH AND GUSTS TO 45 MPH)
IS LIKELY ESPECIALLY ALONG THE WEST SIDE OF THE VALLEY AND THROUGH
CANYONS ALONG THE WEST SLOPE OF THE SIERRA. SG
.EXTENDED DISCUSSION (TUESDAY THROUGH FRIDAY)...
UNCERTAINTY OF FORECAST INCREASES TOWARD THE END OF THE EXTENDED
PERIOD. ALL THE MEDIUM RANGE MODELS SHOW THE STRONG MEAN WEST COAST
RIDGE PERSISTING THROUGH THE EXTENDED PERIOD OF THE FORECAST...BUT
DIFFER IN TIMING AND HANDLING OF SIGNIFICANT SHORT WAVE ENERGY
MOVING INTO THE RIDGE TOWARD THE END OF NEXT WEEK. GFS IS MOST
AGGRESSIVE DIGGING ANOTHER SYSTEM SOUTHEASTWARD INTO CALIFORNIA FROM
THE GULF OF ALASKA LATE IN THE WEEK WHILE THE GEM AND ECMWF ARE
WEAKER AND SLOWER. MODELS CONTINUE TO HINT AT MORE SIGNIFICANT
CHANGES OUT AROUND DAY 10 AS THE WEST COAST RIDGE RETROGRADES TO
140W AND FORMS A STRONG BLOCK OVER THE GULF OF ALASKA/WESTERN BC
WHICH MAY EVENTUALLY BE UNDERCUT SETTING THE STAGE FOR A POTENTIALLY
MORE PROLONGED WET PERIOD FOR NORTHERN CALIFORNIA.
&&
.AVIATION...
COLD FRONT NOW ENTERING FAR NORTHWEST CALIFORNIA. LEADING EDGE OF
BAND OF PRECIPITATION ALONG AND AHEAD OF THE FRONT WILL REACH A LINE
FROM ABOUT KSTS TO KCIC BY 12Z AND WILL CONTINUE SOUTHEAST REACHING
KSAC AROUND 15Z-16Z AND KBLU BY AROUND 18Z. VFR CONDITIONS EXPECTED
TO PREDOMINATE IN THE VALLEY WITH CIGS AROUND 060 WITH OCCASIONAL
PERIODS OF MVFR CIGS AND VSBYS POSSIBLE WITH HEAVIER SHOWERS AS THE
FRONTAL BAND MOVES THROUGH. CONDITIONS WILL DETERIORATE TO IFR IN
SHSN AND BR ACROSS THE NORTHERN SIERRA NEVADA THIS MORNING AND
CONTINUE THROUGH ABOUT 06Z TO 12Z SATURDAY. SN LVLS AROUND 4000 FT.
NLY SFC FLOW DEVELOPS BEHIND THE COLD FRONTAL PASSAGE THIS AFTERNOON
AND EVENING AND WILL STRENGTHEN SIGNIFICANTLY AFT 12Z SATURDAY.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY 8 AM THROUGH 6 PM TODAY WESTERN PLUMAS
COUNTY/LASSEN PARK.
WINTER WEATHER ADVISORY 10 AM THROUGH 10 PM TODAY WEST SLOPE
NORTHERN SIERRA NEVADA.
&&
$$
