FXUS66 KMTR 220432
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
832 PM PST MON DEC 21 2009

.DISCUSSION...AS OF 8:32 PM PST MONDAY...SHOWERS CONTINUE ACROSS THE
CWA THIS EVENING AS A TROUGH OF LOW PRESSURE DIVES SOUTH INTO
CALIFORNIA. COLD AIR ADVECTION IS EXPECTED BEHIND THE TROUGH REDUCING
OVERNIGHT TEMPERATURES BY 5 TO 10 DEGREES BELOW LAST NIGHTS LOW
TEMPERATURES. THIS COLD AIR ADVECTION WILL ALSO RESULT IN INCREASING
WINDS OUT OF THE NORTH. THESE INCREASING WINDS WILL ELIMINATE THE
POSSIBILITY OF BOTH FROST OR PROLONGED FREEZING TEMPERATURES
TONIGHT.

THE 0000 UTC NAM12 IS INDICATING WIDESPREAD 925 MB WINDS IN THE 35 TO
40 KNOT RANGE WITH THE OPERATIONAL GFS INDICATING SIMILAR VALUES.
A HIGH WIND WATCH WAS CONSIDERED BUT UPON FURTHER REVIEW OF MODEL
GUIDANCE IT WAS DECIDED NOT TO ISSUE ONE. SEVERAL CROSS SECTIONS
UTILIZING THE NAM12 WERE CARRIED OUT NORMAL TO THE THERMAL WIND.
THESE CROSS SECTIONS REVEALED THAT ADVISORY/WARNING LEVEL WINDS
WILL NOT MIX DOWN BELOW THE 2500 FOOT ELEVATION THRESHOLD.
ADDITIONALLY...ISENTROPIC ANALYSIS ALONG THE 295 K PLAIN DOES NOT
INDICATE ANY SIGNIFICANT ISENTROPIC DOWN GLIDE WHICH WOULD TRANSPORT
MOMENTUM TO LOWER LEVELS OF THE ATMOSPHERE. IN FACT...WINDS AT THE
295 K LEVEL RUN NEARLY PARALLEL TO THE ISENTROPIC SURFACE.
REGARDLESS...WINDS WILL INCREASE OUT OF THE NORTH NORTHWEST THIS
EVENING AND MAY REACH 30 TO 35 KNOTS OVER THE RIDGES AND HIGHER
TERRAIN ACROSS THE CWA.

WINDS WILL DECREASE TUESDAY EVENING AS HIGH PRESSURE BUILDS OVER THE
EASTERN PACIFIC AND WEST COAST. THIS HIGH PRESSURE WILL RESULT IN
DRY PLEASANT CONDITIONS DURING THE NEXT FEW DAYS. TEMPERATURES WILL
HOWEVER DIVE INTO THE MID TO UPPER 30S OVERNIGHT UNDER MOSTLY
CLEAR SKIES. THE CLEAR SKIES WILL ALSO SUPPORT AREAS OF FOG IN THE
VALLEYS WEDNESDAY AND THURSDAY NIGHT.

&&

.AVIATION...AS OF 4:40 PM MONDAY...SCATTERED SHOWERS WILL CONTINUE
FOR THE NEXT FEW HOURS (TILL 04Z) AS THE FRONT SLOWLY MOVES SOUTH
ACROSS THE REGION. COLD AIR CUMULUS WILL FILL IN BEHIND THE FRONT
WITH GUSTY WINDS EXPECTED AFTER MIDNIGHT. CONDITIONS WILL SLOWLY
IMPROVE OVERNIGHT AS CLOUDS SCATTER OUT AND DRIER AIR FILTERS IN
BRINGING AN END TO ANY SHOWERS.

VICINITY OF KSFO...SCATTERED SHOWERS EXPECTED TO TAPER THIS EVENING
WITH GRADUALLY IMPROVING CONDITIONS. BROKEN MVFR CIGS EXPECTED
UNTIL 04Z. CIGS WILL LIFT AND SCATTER OVERNIGHT. WEST WINDS WILL
REMAIN IN THE 10 TO 15 KT RANGE WITH GUSTS TO 20 KT EXPECTED AFTER
04Z.

KSFO BRIDGE APPROACH...SAME AS KSFO.BROKEN MVFR CIGS EXPECTED UNTIL
04Z. CIGS WILL LIFT AND SCATTER OVERNIGHT. LIGHT WEST WINDS
ANTICIPATED TROUGH THE FORECAST PERIOD.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
   .TNGT...SCA...PT ARENA TO PIGEON PT 0-60 NM STARTING 8 PM.
        ...SCA...PIGEON PT TO PT PIEDRAS BLANCAS 0-60 NM
                 STARTING 1 AM.

$$

PUBLIC FORECAST: LARRY
AVIATION/MARINE: CW

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO