840
FXUS66 KMTR 121130
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
430 AM PDT MON MAY 12 2008
..MUCH WARMER WEATHER EXPECTED DURING THE SECOND HALF OF THE WEEK
DISCUSSION...AS OF 3:00 AM MONDAY
NOT MUCH CHANGE IS EXPECTED IN
THE SHORT-TERM. COOL AND BREEZY WEATHER WILL HANG ON FOR ONE MORE
DAY BEFORE A WARMING TREND GETS UNDERWAY.
AN UPPER TROUGH IS CURRENTLY SETTLING IN OVER THE NORTHERN ROCKIES
AND GREAT BASIN. A SHORTWAVE EMBEDDED IN THE COOL NORTHWESTERLY FLOW
ON THE BACK SIDE OF THIS TROUGH HAS SPREAD AN AREA OF HIGH CLOUDS
OVER OUR AREA EARLY THIS MORNING. THE HIGH CLOUDS HAVE MADE IT
DIFFICULT TO ASCERTAIN THE EXTENT OF LOW CLOUD COVER...BUT BASED ON
SURFACE OBSERVATIONS LOW CLOUDS APPEAR CONFINED TO AREAS AROUND
MONTEREY BAY. EXPECT LOW CLOUDS TO REMAIN PATCHY GIVEN THE GOOD LOW
LEVEL MIXING GOING ON...ESPECIALLY NORTH OF MONTEREY BAY. HIGH CLOUDS
ARE EXPECTED TO GRADUALLY THIN BY AFTERNOON...LEAVING US WITH MOSTLY
SUNNY AND BREEZY CONDITIONS FOR THE BALANCE OF THE DAY. CURRENT
SURFACE GRADIENTS INCLUDE A ROBUST 8 MB DOWN THE COAST FROM ACV TO
SFO AND 15 MB FROM SFO TO LAS VEGAS. THESE STRONG GRADIENTS WILL
PERSIST THROUGH THE DAY AND WE CAN EXPECT NORTHWEST WIND GUSTS OF 30
MPH...OR LOCALLY HIGHER...THIS AFTERNOON ALONG THE COAST AND THROUGH
COASTAL GAPS. TEMPERATURES WILL REMAIN BELOW NORMAL ACROSS MOST OF
OUR AREA DUE TO A RELATIVELY COOL AIRMASS ALOFT AND A STRONG ONSHORE
FLOW OFF A CHILLY OCEAN. ONLY THE NORTH BAY WILL SEE TEMPS NEAR
NORMAL...THANKS TO THE STRONG N-S GRADIENT HELPING TO LIMIT ONSHORE
FLOW THERE. WINDS WILL EASE OVERNIGHT TONIGHT AS GRADIENTS RELAX. LOW
TEMPERATURES TONIGHT WILL BE VERY CHILLY FOR MID-MAY...IN THE MID TO
UPPER 30S IN THE COOLEST INLAND VALLEYS.
WINDS WILL VEER TO THE NORTH LATE TONIGHT AND TUESDAY AS AN UPPER
RIDGE BUILDS OFFSHORE ALONG 130W AND A SURFACE HIGH BEGINS TO
DEVELOP OVER NORTHERN NEVADA. THE AIRMASS WILL WARM SEVERAL DEGREES
ALOFT BY TUESDAY...AND ONSHORE FLOW WILL BE GREATLY REDUCED. THE
RESULT WILL A WARMUP OF 7 TO 12 DEGREES ON TUESDAY. THE WARMUP ON
TUESDAY WILL PUSH TEMPS UP ABOVE NORMAL...BUT THE BRUNT OF THE
WARMING TREND WILL BE YET TO COME. THE UPPER RIDGE WILL BUILD
STRONGLY NEAR THE COAST WEDNESDAY AND THURSDAY AND BY THURSDAY
AFTERNOON THE GFS AND ECMWF FORECAST A 595 H5 HIGH CENTER SOMEWHERE
NEAR THE OREGON COAST. THE FLOW ALOFT WILL TURN FROM NORTH TO
NORTHEAST AND SURFACE FLOW WILL BECOME LIGHT OFFSHORE BY LATE
TUESDAY NIGHT AND THEN STRONGLY OFFSHORE BY LATE WEDNESDAY
NIGHT. IN FACT...BY LATE WEDNESDAY NIGHT (12Z THURSDAY) THE GFS
FORECASTS A 15 MB OFFSHORE GRADIENT FROM WMC TO SFO. THE ECMWF AND
NAM ARE NOT QUITE AS STRONG...BOTH FORECASTING A 12 MB OFFSHORE
GRADIENT. BUT THE NAM FORECASTS 40 KNOT NORTHEAST WINDS AT 925 MB
ACROSS THE SF BAY AREA ON WEDNESDAY NIGHT. A WIND ADVISORY
MAY BE NECESSARY WEDNESDAY NIGHT AND THURSDAY MORNING FOR ELEVATIONS
ABOVE 1000 FEET IN THE NORTH AND EAST BAY.
TEMPERATURES WILL WARM INTO THE UPPER 80S TO MID 90S INLAND ON
WEDNESDAY...AND THEN WELL INTO THE 90S ON THURSDAY. PORTIONS OF
INTERIOR MONTEREY AND SAN BENITO COUNTIES COULD EASILY HIT THE
CENTURY MARK BY THURSDAY AS THE MODELS FORECAST 850 MB TEMPS TO RISE
TO 26 DEG C IN THOSE AREAS BY THEN. COASTAL TEMPERATURES WILL WARM
RAPIDLY AS WELL...SPURRED UPWARD BY LIGHT OFFSHORE FLOW ON
WEDNESDAY MORNING AND THEN BY STRONG OFFSHORE FLOW BY THURSDAY
MORNING. SAN FRANCISCO COULD EASILY WARM INTO THE MID OR UPPER 80S
ON THURSDAY AND PERHAPS EVEN REACH 90. THIS WILL BE IN STARK
CONTRAST TO THE CHILLY UPPER 50S AND LOWER 60S THE CITY HAS SEEN THUS
FAR DURING THE MONTH OF MAY.
NOT MUCH CHANGE IS EXPECTED ON FRIDAY AS THE UPPER RIDGE HOLDS FIRM.
COASTAL TEMPS WILL LIKELY COOL A FEW DEGREES AS OFFSHORE FLOW
WEAKENS...BUT INLAND AREAS SHOULD CONTINUE TO WARM. COOLING SHOULD
GET UNDERWAY OVER THE UPCOMING WEEKEND AS THE RIDGE BEGINS TO BREAK
DOWN...BUT MOST COOLING WILL INITIALLY BE CONFINED TO COASTAL AREAS
AND COASTAL VALLEYS. COOLING NEXT WEEKEND WON'T BE AS RAPID IF THE
ECMWF VERIFIES AS THAT MODEL MAINTAINS THE RIDGE FOR A LONGER PERIOD
OF TIME COMPARED TO THE GFS.
WILL CONTINUE TO HIGHLIGHT THE HOT AND DRY CONDITIONS FOR THE SECOND
HALF OF THE WEEK WITH A SPECIAL WEATHER STATEMENT AND HAZARDOUS
WEATHER OUTLOOK (HWO). WILL ALSO ADD LOCALLY WINDY CONDITIONS TO THE
HWO FOR THE NORTH AND EAST BAY HILLS WEDNESDAY NIGHT/THURSDAY
MORNING.
AVIATION...AS OF 4:30 AM MONDAY
KMRY AND KSNS REPORT MVFR CEILINGS
WHILE BAY AREA TERMINALS REPORT VFR. LOW LEVEL COOLING AND A DRYING
NORTHERLY LOW LEVEL WIND FLOW SHOULD COMBINE TO SCATTER OUT THE
STRATUS CLOUDS AT KMRY AND KSNS BY 15Z. VFR IS FORECAST TONIGHT AND
TUESDAY. ONSHORE WINDS WILL LIKELY BE LOCALLY GUSTY THIS AFTERNOON
AND EVENING.
FIRE WEATHER...AS OF 3:15 AM MONDAY
MUCH WARMER AND DRIER WEATHER
WILL DEVELOP OVER THE FIRE WEATHER DISTRICT DURING THE SECOND HALF
OF THE WEEK. DAYTIME RELATIVE HUMIDITY VALUES WILL DROP INTO THE
TEENS BY WEDNESDAY AND INTO THE SINGLE DIGITS IN THE DRIEST INLAND
AREAS THURSDAY AND FRIDAY. OF CONCERN IS LOCALLY STRONG AND GUSTY
NORTHEAST WINDS THAT ARE EXPECTED ACROSS THE NORTH AND EAST BAY
HILLS WEDNESDAY NIGHT INTO THURSDAY. THESE GUSTY WINDS WILL BE
ACCOMPANIED BY VERY POOR RELATIVE HUMIDITY RECOVERIES. LIGHTER
OFFSHORE WINDS ARE EXPECTED BY LATE THURSDAY INTO FRIDAY.
MTR WATCHES/WARNINGS/ADVISORIES
.TDA...SCA...PT ARENA TO PT PIEDRAS BLANCAS AND SF BAY.
PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: CANEPA
FIRE WEATHER: DYKEMA
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO
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