F1 tornado, Jenner, California, January 27, 2005

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
230 PM PST THU JAN 27 2005

.DISCUSSION...IT HAS BEEN A MAINLY CLOUDY AND COOL AFTERNOON ACROSS
THE BAY AREA...WITH TEMPERATURES IN A NARROW RANGE FROM THE MID 50S
TO AROUND 60 EXCEPT SOME MID/UPPER 40S AT THE HIGHEST ELEVATIONS.
VISIBLE IMAGERY SHOWS THE THICKENING CIRRUS FIELD IN THE WARM
ADVECTION ZONE AHEAD OF THE INCOMING FRONTAL SYSTEM...NOW LOCATED
ABOUT 100NM OFFSHORE AND MOVING AT A RAPID NEAR 40KT PACE. ALTHOUGH
IR IMAGERY IS NOT THAT IMPRESSIVE...CONVECTIVE CELLS ARE FORMING
ALONG THE BAND AND AMSU RAIN RATES INDICATE A FEW PIXELS NEARING
0.50 HOUR INSIDE 130W AT 1907Z. PRECIPITABLE WATER VALUES IN THE
PLUME ARE AROUND 0.75...WITH THE JUICIEST OF AIR HOLDING SOUTH OF
30N ALONG THE SUBTROPICAL JET AND NOT BEING TAPPED IN THE SPLIT FLOW.
UPPER LOW SEEN ON WATER VAPOR IMAGERY NEARING 40N/130W SEEMS TO HAVE
PEAKED IN INTENSITY...AS THE NEGATIVELY TILTED TROF AXIS SWINGS
TOWARD THE BAY AREA.

EXPECT A QUICK HITTING/MODERATE RAIN AND WIND EVENT OVERNIGHT...
WITH PEAK WIND AND RAIN EXPECTED BETWEEN 07Z-13Z. 925 WIND FIELD OFF
NAM INDICATES 40KT JUST OFF THE COAST BY 06Z AS SURFACE MOISTURE
CONVERGENCE INCREASES. 850MB WINDS HOWEVER ARE NOT ALL THAT
IMPRESSIVE...AND WITH LITTLE UPPER JET SUPPORT EXPECT WINDS TO
REMAIN BELOW ADVISORY CRITERIA. THEREFORE...WILL HIGHLIGHT
INCREASING WINDS LATE TONIGHT WITH A FEW GUSTS TO 40MPH IN
ZONES/GRIDS. STRONGEST WINDS ARE EXPECTED ALONG IMMEDIATE COAST AND
HIGHEST ELEVATIONS...BUT EASE UP BY 18Z AS LIGHTER WESTERLY FLOW
TAKES OVER. STACKED SYSTEM WILL MOVE NORTHEASTWARD FRI...WITH
WEAKENING SURFACE LOW REACHING OR COAST BY 00Z SAT. CONSIDERING
RAPID MOVEMENT OF SYSTEM AND ONLY MODERATE MOISTURE ENTRAINMENT...
BELIEVE QPF WILL BE MANAGEABLE ACROSS THE AREA WITH MAXIMUM AMOUNTS
AROUND 2 NORTH BAY WETTEST LOCATIONS AND NEARLY THAT PERHAPS IN
SANTA CRUZ/SANTA LUCIA MOUNTAINS WITH SOME HELP FROM 700MB SW FLOW
UP TO 40 KTS. OTHERWISE...A GENERAL 0.5 TO 1.25 IS EXPECTED. SURELY
NOT A CONCERN FOR MAIN-STEM RIVERS...BUT BRIEF URBAN FLOODING AND
SOME RISES IN SMALL CREEKS IS POSSIBLE EARLY FRI MORNING.

ONCE THE FRONT SWINGS THRU THE AREA...COLDER AIR ALOFT AND STEEP MID-
LEVEL LAPSE RATES WILL ARRIVE FRI AFTERNOON. BEST CHANCE FOR
ISOLATED THUNDERSTORMS WILL BE IN NORTH BAY...CLOSEST TO DYNAMICS
FROM THE UPPER LEVEL ENERGY SWINGING AROUND. IN FACT...BY MID-DAY
THE IMMEDIATE BAY AREA COULD BE MAINLY DRY WITH SHOWERS STILL
PESTERING THE SOUTHERN CWA AND ISOLATED SHOWERS/THUNDERSTORMS RE-
DEVELOPING IN THE NORTH. BY FRIDAY NIGHT HOWEVER...SHOWERS WILL BEGIN
TO DECREASE IN ALL AREAS IN THE WAKE OF DEVELOPING NVA AND
SUBSIDENCE. SURFACE RIDGING BUILDS IN ON SATURDAY...AS MID LEVEL
ENERGY CLOSES OFF THE LOW OVER SOCAL. THIS JET ENERGY ON THE BACK-
SIDE OF THE TROF SWINGS SOUTHWARD...WITH ANOTHER WEAK VORT AND WARM
ADVECTION ZONE RIDING ALONG IT. THIS FEATURE WAS PREVIOUSLY PROGGED
TO COME CLOSE ENOUGH TO THE COAST TO PROMOTE A FEW SHOWERS...BUT
LATER MODEL RUNS INDICATE THE MID LEVEL MOISTURE FIELD WILL STAY
OFFSHORE AND HAVE THUS REMOVED ANY POPS FOR SATURDAY. INSTEAD...
EXPECT INCREASING SUNSHINE TO DEVELOP AND SLIGHTLY MILDER
TEMPERATURES AS THE THICKNESS TROF LIFTS OUT. PATTERN BEGINS TO
AMPLIFY BY SUNDAY AS THE CLOSED LOW SETTLES INTO THE 4 CORNERS AND
RIDGE POPS UP OVER THE EASTERN PACIFIC.

EXTENDED PERIOD SHOULD FEATURE A RETURN TO DRY WEATHER AND
NEAR/ABOVE NORMAL TEMPERATURES IN MOST AREAS NEXT WEEK. ONLY
EXCEPTION WILL BE FOG PRONE AREAS...ONCE THE VALLEY FOG DEVELOPS AND
BEGINS TO ADVECT EASTWARD. HPC CHARTS STAY CLOSE TO THE ENSEMBLE
MEAN PREDICTIONS THRU DAY 7...WITH BELOW NORMAL HEIGHTS OVER THE
SW...RIDGING OVER THE PACIFIC NORTHWEST AND MOST OF THE ARCTIC AIR
BOTTLED UP NORTH OF THE USA. THE MILD AND DRY PATTERN FOR US MAY WELL
HOLD INTO THE 8-14 DAY PERIOD AS ENSEMBLES SUGGEST THE SOUTHWEST LOW
EVENTUALLY LIFTING OUT AND AN EASTERN TROF DEVELOPING. THIS COULD
RETURN US TO THE POSITIVE PNA PATTERN...WHICH HAS BEEN IN PLACE
FOR MUCH OF JANUARY. UNDER THIS REGIME...MOST OF THE WEST COAST IS
WARM AND DRY AND THE EAST IS COLD AND OCCASIONALLY STORMY.

&&

.MARINE...SEAS WILL INCREASE TO NEAR 15 FEET IN THE COASTAL
WATERS BY TONIGHT RESULTING IN HIGH SURF CONDITIONS THROUGH SATURDAY.
SWELL WILL PEAK AT AROUND 16 FEET FRIDAY AFTERNOON.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
       TGT...SCA...PT ARENA TO PT PIEDRAS BLANCAS INCLUDING SFO AND
       MRY BAYS.
       TNGT THRU SAT...HIGH SURF ADVISORY ALONG THE ENTIRE COAST.

&&

PUBLIC FORECAST: SOROKA
MARINE/AVIATION: HORNE
BAPSU:           SELLERS

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO (ALL LOWER CASE)

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