FXUS66 KSTO 270510
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
910 PM PST THU NOV 26 2009
.DISCUSSION...WATER VAPOR IMAGERY SHOWS A TROUGH OF LOW PRESSURE
MOVING INTO THE PACIFIC NORTHWEST THIS EVENING...WITH THE TAIL END
OF THE COLD FRONT NOW PUSHING INTO NORTHERN CALIFORNIA. COMPOSITE
RADAR IMAGERY SHOWS THIS WELL...WITH RAIN EXTENDING FROM NORTH OF
THE OREGON AND CALIFORNIA BORDER DOWN INTO MENDOCINO COUNTY.
MEANWHILE...THE 00Z OAKLAND AND MEDFORD SOUNDINGS SHOWED A
SIGNIFICANT DRY LAYER ABOVE 700 MBS AND BELOW 400 MBS...WHICH WE
WILL NEED TO OVER COME FIRST. THEREFORE...BELIEVE IT WILL BE
UNLIKELY TO SEE MUCH IF ANY PRECIPITATION BEFORE 06Z OR MAYBE EVEN
08Z THIS EVENING...AS THE PRECIPITATION BAND SLOWLY MOVES INTO THE
NORTHERN INTERIOR LATE TONIGHT AND ON FRIDAY. EXPECT THE BRUNT OF
THE PRECIPITATION TO MOVE ACROSS THE NORTHERN INTERIOR ON FRIDAY
DURING THE DAY...AS A PIECE OF ENERGY BREAKS FROM THIS SYSTEM AND
FORMS A LOW OVER OR IN THE VICINITY OF THE BAY AREA TOMORROW
AFTERNOON. THIS MAY BE A LITTLE FASTER THAN OUR CURRENT FORECAST IS
SHOWS. HOWEVER...MODELS ARE HAVING SOME DIFFICULTY WITH THIS LOW
AND THE TIMING. IT MAY ALSO MAKE FOR AN INTERESTING AFTERNOON...AS
SOME MODELS ARE INDICATING ENOUGH INSTABILITY TO SEE A LIGHTNING
STRIKE OR TWO.
IN THE MEANTIME...EXPECT PRIMARILY CLOUDY SKIES THIS EVENING...WITH
TEMPERATURES RUNNING WITHIN A FEW DEGREES OF WHERE THEY WERE LAST
NIGHT. WILL DO A QUICK UPDATE THIS EVENING TO REFLECT A QUICKER
TIMING OF PRECIPITATION ACROSS THE VALLEY TOMORROW. PALMER
.PREVIOUS DISCUSSION (ISSUED AT 330 PM PST)...CLOUD COVER FROM THE
OFFSHORE SYSTEM WILL CONTINUE TO THICKEN OVER INTERIOR NORCAL
TONIGHT AHEAD OF THE APPROACHING LOW PRESSURE AS IT NEARS THE COAST.
GFS N-S CROSS SECTION OVER INTERIOR NORCAL SUGGESTS IT WILL REMAIN
DRY IN THE SUB-CLOUD THIS EVENING...EVENTUALLY MOISTENING
SUFFICIENTLY TO SUSTAIN PCPN AFTER 06Z IN THE INTERIOR OF
NORCAL...NORTH OF THE I-80 CORRIDOR.
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. ALTHOUGH TODAY'S
MORNING MODELS SEEM TO BE IN GENERAL AGREEMENT IN CLOSING OFF A LOW
VCNTY OF THE SFO BAY AREA...THEN DROPPING IT SWD INTO SOCAL ON SAT.
THE IR IMAGERY SHOWS THE STRETCHING/SPLITTING OF THE TROF ALONG
45N/130W...OFF THE WA/OR COAST...WITH THE SRN PORTION OF THE TROF
JUST BEGINNING TO TURN SELY AS THE UPSTREAM JET SPEED MAX APPROACHES
FROM THE WEST.
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 HIGHER
TPW ALONG 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. THE BETTER RESOLUTION
MODEL QPFS...NAM AND WRF HAVE THE RELATIVELY NARROW NE-SW ORIENTED
BANDS OF 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 IS EXPECTED. THE HIGHER AMOUNTS IN PLUMAS CO
DUE TO OROGRAPHICS ENHANCING QPFS ALBEIT FOR A SHORT TIME PERIOD. WE
ESTIMATE A NARROW BAND OF 3-7 INCHES OF SNOW IS POSSIBLE MAINLY N OF
THE I-80 CORRIDOR IN THE 12 HR PERIOD FROM FRI 18Z TO 06Z SAT. WILL
UPDATE SPS FOR THIS SCENARIO. FRI WILL BE MUCH COOLER AS THICKNESSES
AND 850 MBS TEMPS LOWER SOME 16 DAM AND 9 DEG C...AT LEAST SOME
10-17 DEG F OVER MUCH OF THE HIGHER TERRAIN...AND 5 TO 7 DEG IN THE
VALLEY.
AS THE CLOSED LOW DROPS S OF THE AREA FRI NITE AND SAT...SKIES WILL
CLEAR FROM N TO S AS DRIER NELY FLOW DEVELOPS AND SUBSIDENCE
INCREASES OVER INTERIOR NORCAL. HOWEVER...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. PRESSURE RISES TO OUR N AND E ARE FORECAST
TO INCREASE NLY TO NELY GRADIENTS TO AS MUCH AS 15 MBS FROM MFR-SAC
BY SAT MORNING. THIS WILL RESULT IN BRISK/BREEZY CAA DOWNSLOPE
WINDS...AND POSSIBLE FORMATION OF A NLY BARRIER JET ON THE W SIDE OF
THE SAC VLY...ALONG WITH GUSTY DOWNSLOPE WINDS IN NE-SW ORIENTED
CANYONS.
THE CLOSED LOW IS FORECAST TO DROP S INTO BAJA/OLD MEX ON SUN AND A
POSITIVELY TILTED RIDGE IS FORECAST TO BUILD INLAND. ANTICYCLONIC
FLOW ALOFT WILL RESULT IN OFFSHORE FLOW. DRY WX WITH COLD NITES AND
MILD DAYS ARE EXPECTED IN THIS PATTERN. JHM
.EXTENDED DISCUSSION (MONDAY THROUGH THURSDAY)...
RIDGE OF HIGH PRESSURE REMAINS OVER THE AREA THROUGH TUESDAY
BRINGING NEAR NORMAL TEMPERATURES AND DRY WEATHER. LOCALLY GUSTY
EASTERLY WINDS ARE EXPECTED ON SUNDAY NIGHT ESPECIALLY THROUGH THE
CANYONS ALONG THE WEST SLOPE OF THE SIERRA. THE GUSTY EASTERLY WINDS
SHOULD DECREASE DURING THE DAY MONDAY AS THE SURFACE HIGH OVER THE
GREAT BASIN WEAKENS AND MOVES EAST. RADIATIONAL COOLING AND LIGHT
WINDS OVER THE VALLEYS MONDAY AND TUESDAY WILL BRING PATCHY TO AREAS
OF LATE NIGHT AND MORNING FOG IN THE CENTRAL VALLEY AND POSSIBLY THE
MOUNTAIN VALLEYS. THE 12Z AND 18Z OPERATIONAL GFS SHOWS A CUT OFF
LOW APPROACHING THE CALIFORNIA COAST TUESDAY INTO WEDNESDAY BUT IS
THE ONLY MODEL SHOWING THIS SO LEFT THE FORECAST DRY FOR NOW.
&&
.AVIATION...HIGH CLOUDS INCREASING FURTHER THIS EVENING WITH MID
CLOUDS SPREADING IN AFTER MIDNIGHT AHEAD OF AN APPROACHING SYSTEM.
MVFR VIS AND CIGS POSSIBLE WITH -SHRA STARTING AFTER 08Z OVER THE
NORTHERN MOUNTAINS AND THEN SPREADING INTO THE NORTHERN SACRAMENTO
VALLEY OVERNIGHT AND CONTINUING THROUGH FRIDAY. AREAS OF GRADUALLY
DECREASING VIS IN BR FOR THE SOUTHERN SACRAMENTO AND NORTHERN SAN
JOAQUIN VALLEYS THROUGH 12Z...FOLLOWED BY POSSIBLE RAIN SHOWERS WITH
MVFR AND LOCAL IFR POSSIBLE INTO THE AFTERNOON. MOUNTAIN SNOW ABOVE
4500 FEET THROUGH THE DAY AND INTO THE EVENING FRIDAY WILL BRING
LOCAL IFR/LIFR CONDITIONS.
&&
.STO WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
