FXUS66 KMTR 220618
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
1015 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 10:15 PM MONDAY...THE LAST BAND OF SHOWERS IS
MOVING SOUTH ACROSS THE SOUTHERN HALF OF THE FORECAST AREA. RESIDUAL
SHOWERS ARE POSSIBLE ACROSS THE REGION HOWEVER THE MAIN SHOWERS
ACTIVITY HAS PASSED THE BAY AREA. COLD AIR WILL FILL IN BEHIND THE
FRONT RESULTING IN GUSTY WINDS. CONDITIONS WILL IMPROVE OVERNIGHT AS
CLOUDS SCATTER OUT AND DRIER AIR FILTERS IN BRINGING AN END TO ANY
SHOWERS.

VICINITY OF KSFO...SHOWER ACTIVITY HAS ALL BUT COME TO AN END. BROKEN
VFR CIGS OVER 7000 FT EXPECTED TO LIFT AND SCATTER OUT AFTER 11Z.
GUSTY WEST WINDS IN THE 15 TO 25 KT RANGE WITH GUSTS TO
30 KT EXPECTED UNTIL 11Z.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...A FEW SCATTERED SHOWERS ARE POSSIBLE OVER
THE NEXT HOUR OR TWO. EXPECT IMPROVING CONDITIONS TONIGHT WITH BROKEN
MVFR-VFR CIGS SCATTERING OUT OVERNIGHT. GUSTY WEST WINDS IN THE 10 TO
15 KT RANGE WITH GUSTS TO 20 KT EXPECTED UNTIL 12Z.

&&

.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