FXUS66 KMTR 060458
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
858 PM PST SAT DEC 5 2009
.DISCUSSION...AS OF 8:55 PM PST SATURDAY...A RELATIVELY COOL AIRMASS
IN PLACE...ALONG WITH SLOW CLEARING OF LOW CLOUDS...RESULTED IN A
COOL DAY ACROSS OUR REGION TODAY. AFTERNOON HIGHS WERE GENERALLY IN
THE LOWER TO MID 50S...ABOUT 5 DEGREES BELOW NORMAL. EVEN COOLER
WEATHER IS ON THE WAY.
DRIER AIR SWEPT ACROSS THE REGION THIS AFTERNOON AND WITH CLEAR
SKIES AND LOWER DEWPOINTS THIS EVENING...TEMPERATURES ARE FALLING
MUCH MORE QUICKLY THAN LAST EVENING. BY 8 PM TEMPERATURES HAD ALREADY
FALLEN INTO THE MID AND UPPER 30S IN THE NORTH AND EAST BAY
VALLEYS. CURRENT DEWPOINTS VARY QUITE A BIT IN THOSE VALLEY
AREAS...ANYWHERE FROM THE MID 20S NEAR LIVERMORE TO THE MID 30S IN
THE NORTH BAY. THUS...IT'S DIFFICULT TO GET A GOOD HANDLE ON JUST
HOW FAR TEMPS WILL DROP OVERNIGHT. LATEST MOS GUIDANCE INDICATE
OVERNIGHT VALLEY LOWS WILL MOSTLY BE AROUND THE 30 DEGREE MARK...WITH
THE COLDEST SPOTS DROPPING INTO THE UPPER 20S. CLEARLY THERE WILL BE
WIDESPREAD FROST IN THE VALLEYS TONIGHT...BUT PROBABLY NOT A HARD
FREEZE IN MOST AREAS.
THE HIGH AMPLITUDE RIDGE CENTERED ALONG 135W THIS EVENING
CONTINUES TO ELONGATE...SETTING THE STAGE FOR COLD AIR TO DROP SOUTH
OUT OF CANADA AND INTO THE WESTERN UNITED STATES...AND FOR THE
WESTERLIES TO FINALLY BREAK UNDER THE RIDGE AND INTO CALIFORNIA.
OUR AREA WILL FIRST HAVE TO DEAL WITH THE COLD AIR...ALONG WITH LOW
SNOW LEVELS. A SHORTWAVE DROPPING DUE SOUTH OUT OF WESTERN CANADA IS
NEAR SEATTLE THIS EVENING. VERY COLD AIR IS ASSOCIATED WITH THIS
SHORTWAVE WITH 850 MB TEMPS OF MINUS 10 C CURRENTLY SPILLING S ACROSS
THE CANADIAN/WASHINGTON BORDER. THE MODELS AGREE QUITE WELL IN
DEVELOPING A CLOSED UPPER LOW OVER WA AND OR OVERNIGHT AND THEN
SLOWLY DROPPING THAT LOW SOUTH INTO FAR NORTHERN CALIFORNIA BY LATE
SUNDAY. MEANWHILE...AS THE RIDGE ELONGATES AND BEGINS TO PINCH OFF
OVER ALASKA...A TROUGH WILL CARVE OUT TO THE WEST OF THE UPPER LOW...
ALLOWING THE FLOW TO DEVELOP SIGNIFICANT OVER-WATER TRAJECTORY BY
SUNDAY NIGHT. SO...ALTHOUGH THE SYSTEM APPEARS VERY DRY ON SATELLITE
THIS EVENING...IT WILL GRADUALLY PICK UP MOISTURE OVER THE NEXT 24 TO
36 HOURS AS THE FLOW EXTENDS FARTHER OVER THE WATER. IN ADDITION...
SOME MOISTURE IS FORECAST TO BEGIN SLIPPING UNDER THE RIDGE BETWEEN
30N AND 40N...BUT IT APPEARS THAT MOST OF THAT MOISTURE WILL END UP
OVER SOUTHERN CA. IN ANY EVENT...THE SYSTEM IS FORECAST TO PICK
UP SUFFICIENT MOISTURE TO SPREAD PRECIPITATION OVER OUR ENTIRE
FORECAST AREA SUNDAY NIGHT AND MONDAY. THE SYSTEM WILL SLOWLY
ACCUMULATE MORE MOISTURE AS IT SAGS SOUTH AND SO THE SOUTHERN HALF
OF OUR FORECAST AREA WILL LIKELY SEE HIGHER PRECIPITATION AMOUNTS.
PRECIP TOTALS THROUGH MONDAY AFTERNOON WILL RANGE FROM A QUARTER TO
THREE QUARTERS OF AN INCH...WITH UP TO AN INCH OR MORE POSSIBLE IF
THE WETTER NAM VERIFIES. OBVIOUSLY SUCH RAINFALL TOTALS ARE NOT
IMPRESSIVE...BUT THE IMPORTANT THING TO WATCH HERE WILL BE SNOW
LEVELS. CURRENT MODEL OUTPUT INDICATE SNOW LEVELS WILL DROP TO 1000
TO 2000 FEET ACROSS THE SF BAY AREA BY MONDAY MORNING...AND TO 2000
TO 3000 FEET ACROSS THE MONTEREY BAY AREA BY EARLY MONDAY AFTERNOON.
IF HEAVIER CONVECTIVE ELEMENTS DEVELOP ALONG THE COLD FRONT...THEN
SNOW LEVELS COULD DROP AS LOW AS 500 FEET AT TIMES. IN ADDITION...IF
CLOUDS BEGIN TO OVERSPREAD THE NORTH BAY EARLY ENOUGH TOMORROW TO
PREVENT MUCH DAYTIME WARMING...SNOW LEVELS COULD START OUT LOWER THAN
EXPECTED ON SUNDAY NIGHT ONCE THE PRECIP BEGINS. THESE FACTORS WILL
NEED TO BE CONSIDERED WHEN DECIDING IF A WINTER WEATHER ADVISORY
MIGHT BE NEEDED FOR SOME OF OUR MOUNTAIN ZONES LATE SUNDAY NIGHT INTO
MONDAY. IT'S BEGINNING TO LOOK LIKE THERE WILL BE SUFFICIENT MOISTURE
AND COLD AIR TO DROP ACCUMULATING SNOW ON SOME OF OUR HIGHER
ELEVATION ROADWAYS SUCH SKYLINE BOULEVARD AND THE SUMMIT OF HIGHWAY
17.
A FEW SHOWERS MAY LINGER INTO EARLY MONDAY EVENING...BUT FOR THE
MOST PART PRECIP SHOULD END BY SUNSET MONDAY. THE QUESTION THEN WILL
BE HOW FAR TEMPERATURES DROP ON MONDAY NIGHT. THE AIRMASS OVER THE
AREA WILL BE COLD ENOUGH TO PRODUCE A HARD FREEZE IN OUR VALLEYS
MONDAY NIGHT...AS WELL AS FROST CLOSE TO THE OCEAN. 850 MB TEMPS
ARE FORECAST TO BE AS LOW AS MINUS 5-6 C. HOWEVER...THERE MAY BE
ENOUGH LINGERING LOW LEVEL MOISTURE TO HOLD TEMPS UP A BIT. IN ANY
CASE...MONDAY NIGHT WILL ALMOST CERTAINLY BE THE COLDEST NIGHT OF THE
SEASON THUS FAR AND WILL KEEP THE FREEZE WATCH IN PLACE FOR OUR
VALLEY ZONES FROM LATE MONDAY NIGHT INTO TUESDAY MORNING.
IT APPEARS AS THOUGH TUESDAY AND MOST OF WEDNESDAY WILL BE DRY AND
COOL AS THE RIDGE REBUILDS TEMPORARILY JUST OFFSHORE. THEN...THE
WESTERLIES WILL FINALLY BREAK UNDER THE RIDGE LATE WEDNESDAY INTO
THURSDAY. THE FIRST IMPULSE THAT BREAKS THROUGH LOOKS TO BE FOCUSED
MOSTLY ON SOUTHERN CA AND DOES NOT APPEAR TO BE OVERLY WET. BUT THE
MEDIUM RANGE MODELS AGREE THAT ONCE THE BREAKTHROUGH FINALLY DOES
OCCUR...THE STAGE WILL BE SET FOR AT LEAST TWO OR THREE SUBSEQUENT
SYSTEMS TO FOLLOW LATE NEXT WEEK...INTO THE WEEKEND...AND VERY
LIKELY INTO THE FOLLOWING WEEK AS WELL. AND...THESE SUBSEQUENT
SYSTEMS ARE EXPECTED TO BE WETTER AND WINDIER. IT'S STILL TOO FAR OUT
AND THERE IS STILL TOO MUCH MODEL UNCERTAINTY TO TRY TO PIN DOWN
TIMING BEYOND THE MIDDLE OF NEXT WEEK.
&&
.AVIATION...AS OF 4:00 PM PST SATURDAY...A COLD FRONT IS SLIDING
SOUTH INTO MONTEREY AND SAN BENITO COUNTIES. SKIES HAVE CLEARED OVER
THE SFO BAY AREA AND WILL CLEAR FROM THE SOUTHERN MRY BAY AREA
01-02Z. NORTHEAST WINDS DEVELOP ALOFT BEHIND THE FRONT. THIS
WILL KEEP THE SFO BAY AREA CLEAR THROUGH TONIGHT. THE MRY BAY AREA
IS NOT AFFECTED AS MUCH AND COULD STILL SEE PATCHY LOW CLOUDS
EARLY SUNDAY. WINDS ARE EXPECTED TO INCREASE THROUGH EARLY EVENING
AS COLDER UNSTABLE AIR ALLOWS WINDS ALOFT TO MIX TO THE GROUND.
WINDS SHOULD DECREASE AFTER MIDNIGHT.
VICINITY OF KSFO...VFR CONDITIONS THROUGH SUNDAY. WEST TO NORTHWEST
WINDS WILL INCREASE THROUGH EARLY EVENING TO 15 KT WITH POSSIBLE
GUSTS AS HIGH AS 20 KT. WINDS DECREASING TO 10 KT OR LESS AFTER 12Z.
KSFO BRIDGE APPROACH...SAME AS KSFO.
MRY BAY AREA TERMINALS...MVFR CIGS WILL PREVAIL THROUGH 01-02Z.
SKIES WILL CLEAR AFTER 04Z BUT PATCHY CLOUDS MAY REDEVELOP AFTER
12Z.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.TNGT...SCA PT ARENA TO PT PIEDRAS BLANCAS 10-60 NM
SCA PT ARENA TO PT REYES 0-10 NM
.LATE MON NIGHT-TUE MORNING...FREEZE WARNING FOR VALLEY ZONES
AND N BAY MOUNTAINS.
$$
PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: LARRY/W PI
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO
