000
FXUS66 KMTR 101734
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
1034 AM PDT FRI OCT 10 2008
.DISCUSSION...AS OF 10:00 AM PDT FRIDAY...SKIES ARE CLEAR ACROSS THE
DISTRICT WITH TEMPERATURES IN THE 50S. A STRONG COLD AND
DRY UPPER LEVEL VORT MAX IS APPROACHING FROM THE NORTH AND IS
EXPECTED TO MOVE THROUGH THE DISTRICT THIS AFTERNOON AND EVENING.
LATEST MODEL OUTPUT INDICATES AN ASSOCIATED 500 MB WIND MAX OF
ABOUT 105 KT...REMARKABLY STRONG ESPECIALLY FOR THIS EARLY IN THE
SEASON. AS IT PASSES THROUGH...THE COMBINATION OF STEEPENING LAPSE
RATES AND INCREASING SUBSIDENCE ARE EXPECTED TO RESULT IN INCREASING
DOWNWARD MIXING OF HIGHER MOMENTUM AIR FROM ALOFT. AT THE SAME
TIME...THE NW TO SE SURFACE PRESSURE GRADIENT IS PROJECTED TO
INCREASE AS PRESSURES RAPIDLY FALL OVER SOUTHERN NEVADA WITH LATEST
NAM OUTPUT PROJECTING AN SLP DIFFERENCE BY 00Z SAT OF 25 MB BETWEEN
THE FAR NW COAST OF CALIF AND THIS LOW CENTER.
SIMILAR SCENARIOS IN THE PAST HAVE RESULTED IN SOME NOTABLE STRONG
SURFACE WIND EVENTS...WITH THE BASIC SYNOPTIC SETUP REMINISCENT
FOR EXAMPLE OF THOSE ASSOCIATED WITH THE 29 NOV 1991 SAN JOAQUIN
VALLEY I-5 DUST STORM WIND EVENT AND THE 03 APR 1999 SAN
FRANCISCO/MONTEREY BAY REGION COASTAL HIGH WIND EVENT. THAT SAID...
THERE ARE A COUPLE OF POTENTIALLY MITIGATING FACTORS. IN THE PRESENT
CASE...THE UPPER LEVEL VORT MAX ISN`T PROJECTED TO PASS THROUGH
UNTIL LATE THIS AFTERNOON AND EVENING...RATHER THAN EARLIER IN THE
DAY WHERE MAX DYNAMICALLY INDUCED DOWNWARD MOMENTUM TRANSPORT
FROM ALOFT WOULD BE BETTER COUPLED WITH PEAK IN DIURNAL HEATING AND
ASSOCIATED STEEPER PBL LAPSE RATES. ALSO...AIR ALOFT WITH CURRENT
SYSTEM ISN`T AS COLD AS IN THESE PAST CASES. HAVE ALSO NOTED THAT
SURFACE WINDS...ASIDE FROM RIGHT ALONG THE NORTH BAY COAST...HAVE
GENERALLY BEEN LIGHTER THAN HAD BEEN ANTICIPATED THE PAST 24 HRS
DESPITE SIGNIFICANT INCREASE THAT DID OCCUR IN THE SURFACE PRESSURE
GRADIENT. NET RESULT IS THAT WILL MAINTAIN INHERITED WIND
ADVISORIES...BUT CURRENT THINKING IS THAT WINDS MUCH MORE LIKELY TO
REACH ADVISORY STRENGTH RIGHT ALONG THE COAST...BY THE COASTAL
GAPS...AND IN THE HILLS THAN ELSEWHERE. WINDS SHOULD THEN DECREASE
OVERNIGHT AT LOWER ELEVATIONS AS THE LOW-LEVEL AIR MASS STABILIZES...
BUT WITH NAM 925 MB WIND OUTPUT INDICATING LOCALLY STRONG AND GUSTY
WINDS CONTINUING IN THE HILLS THROUGH EARLY SATURDAY MORNING.
OVER THE WEEKEND...THE UPPER LEVEL LOW WILL LIFT TO THE NE WITH DRY
NORTHERLY FLOW ALOFT AND A GRADUALLY WARMING AIR MASS ALOFT OVER
OUR DISTRICT. SOME LOCALLY GUSTY WINDS MAY STILL OCCUR AFTERNOON AND
EVENING HOURS...BUT OTHERWISE IT LOOKS LIKE BEAUTIFUL FALL WEATHER.
THE ONE CONCERN IS WHETHER EARLY MORNING MIN TEMPS COULD GET LOW
ENOUGH TO RESULT IN FROST IN THE COLDER VALLEY LOCATIONS.
ESPECIALLY FOR THE NORTH BAY VALLEYS...THIS WOULD BE A NEAR-RECORD
EARLY OCCURRENCE...BUT CONTINUE TO BELIEVE THE POSSIBILITY CAN`T BE
RULED OUT. LATEST GFS MOS GUIDANCE FOR EXAMPLE IS PROJECTING MIN
TEMPS OF 36 AND 34 FOR KAPC FOR SAT AND SUN MORNINGS RESPECTIVELY...
AND 32 FOR BOTH MORNINGS FOR KPRB. SUSPECT THERE WILL STILL BE TOO
MUCH VERTICAL MIXING FOR ANY FROST OCCURRENCE SAT MORNING...BUT DOES
LOOK LIKE A BIT OF FROST COULD OCCUR IN THE COLDEST VALLEY
LOCATIONS EARLY SUNDAY MORNING. WILL LOOK AT THIS FURTHER FOR THE
AFTERNOON PACKAGE WHEN ALL NEW MODEL GUIDANCE HAS ARRIVED. NO SIGN
OF RAIN FOR AT LEAST THE NEXT WEEK.
&&
.AVIATION...AS OF 10:35 AM PDT FRIDAY...THE LATEST VISIBLE SATELLITE
IMAGE IS SHOWING CLEAR SKIES ACROSS THE SAN FRANCISCO BAY AREA. VFR
CONDITIONS ARE EXPECTED TO CONTINUE THROUGH THE FORECAST PERIOD. THE
NORTH SOUTH SURFACE PRESSURE GRADIENT IS SLIGHTLY LESS THAN
YESTERDAY WHILE THE ONSHORE GRADIENT IS SLIGHTLY GREATER...THIS
WILL RESULT IN STRONGER WESTERLY WINDS FOR THE SAN FRANCISCO BAY
AREA THAN THOSE OBSERVED YESTERDAY. WINDS ARE EXPECTED TO PEAK IN
THE AFTERNOON WITH DAY TIME HEATING.
.MONTEREY BAY AREA TERMINALS...THE LATEST VISIBLE SATELLITE IMAGE IS
SHOWING CLEAR SKIES ACROSS THE MONTEREY BAY AREA TERMINALS. VFR
CONDITIONS ARE EXPECTED TO CONTINUE THROUGH THE FORECAST PERIOD.
WESTERLY WINDS ARE EXPECTED TO BE SLIGHTLY STRONGER THAN YESTERDAY
AFTERNOON DUE TO A STRONGER ONSHORE PRESSURE GRADIENT AND A WEAKER
NORTH SOUTH PRESSURE GRADIENT.
&&
.MARINE...AS OF 10:35 AM PDT FRIDAY...GALE FORCE WINDS ARE EXPECTED
TO CONTINUE OVER THE COASTAL WATERS THROUGH LATE TONIGHT AS LOW
PRESSURE REMAINS IN THE GREAT BASIN AND HIGH PRESSURE BUILDS OVER
THE EASTERN PACIFIC. THE STRONGEST WINDS ARE EXPECTED IN THE NORTHERN
OUTER WATERS THIS AFTERNOON. THESE WINDS WILL PRODUCE LARGE STEEP
FRESH SWELL ACROSS THE THE COASTAL WATERS. ALTHOUGH WINDS ARE
EXPECTED TO DIMINISH LATE TONIGHT THEY WILL STILL REMAIN IN SMALL
CRAFT ADVISORY CRITERION FOR THE MAJORITY OF THE FORECAST PERIOD.
&&
.FIRE WEATHER...AS OF 10:00 AM PDT FRIDAY...RED FLAG WARNING IN
EFFECT THROUGH SATURDAY. FOR THE TIME BEING NO CHANGES WILL BE MADE
TO THE ONGOING WARNINGS. VERIFYING OVERNIGHT DATA SHOWS ALL THE
HILL SITES VERIFIED ALONG WITH THE NORTH BAY VALLEYS. AS NOTED
ABOVE WINDS ARE EXPECTED TO INCREASE AGAIN LATER THIS AFTERNOON AND
OVERNIGHT.
WILL RE-ASSESS ALL THE WARNINGS THIS AFTERNOON BUT RELUCTANT TO
MAKE TOO MANY CHANGES DURING THE EVENT. AT THIS TIME IT WOULD APPEAR
REASONABLE TO RUN THE WARNINGS OVERNIGHT INTO AT LEAST SATURDAY
MORNING WITH THE HILLS OF THE NORTH AND EAST BAY BEING THE WINDIEST
SPOTS. BY SATURDAY AFTERNOON WINDS WILL BE DECREASING FROM THE
NORTHEAST BUT MORE NOTED DRYING WILL OCCUR AS WELL.
LATEST MODEL OUTPUT INDICATES PROLONGED MODERATE NORTHEAST WINDS
ACROSS THE NORTH AND EAST BAY HILLS WITH SATURDAY NIGHT INTO SUNDAY
AND EVEN POSSIBLY SUNDAY NIGHT INTO MONDAY. THIS OCCURS AS
TEMPERATURES START TO WARM BACK UP DURING THE AFTERNOONS. WILL AT
LEAST HAVE TO CONSIDER A WATCH OR ANOTHER RED FLAG FOR THE NORTH AND
EAST BAY HILLS DURING THIS TIME FRAME BUT WILL RIDE OUT THE CURRENT
EVENT BEFORE COMMITTING.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.TDA...GLW...ALL COASTAL WATERS.
SCA...S.F.BAY AND MONTEREY BAYS THIS AFTERNOON.
SCA...ROUGH SAN FRANCISCO BAR.
WIND ADVISORY...ALL ZONES THROUGH TONIGHT.
.TODAY THROUGH SATURDAY...RED FLAG WARNING ALL ZONES.
$$
PUBLIC FORECAST: BLIER
AVIATION/MARINE: LARRY
FIRE WEATHER: RWW
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO
