Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000
FXUS66 KMTR 180419
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
919 PM PDT WED SEP 17 2014

.SYNOPSIS...A PACIFIC WEATHER SYSTEM WILL BRING A CHANCE OF LIGHT
RAIN TO MUCH OF THE REGION TONIGHT AND THURSDAY...MAINLY ACROSS
THE NORTH AND ALSO IN COASTAL AREAS. THERE IS ALSO A SLIGHT
CHANCE OF THUNDERSTORMS ON THURSDAY...MAINLY ACROSS THE NORTH AND
EAST BAY. TEMPERATURES WILL COOL BELOW SEASONAL AVERAGES THROUGH
THE REMAINDER OF THE WORK WEEK. DRY CONDITIONS WILL RETURN AND
TEMPERATURES WILL REBOUND DURING THE UPCOMING WEEKEND AS LOW
PRESSURE MOVES OFF TO THE EAST.

&&

.DISCUSSION...AS OF 9:15 PM PDT WEDNESDAY...AN UPPER TROUGH IS
LOCATED OFF THE NORTHERN CALIFORNIA COAST ALONG 130W THIS EVENING.
AN ASSOCIATED COLD FRONT SWEPT INTO NORTHWEST CALIFORNIA LATE THIS
AFTERNOON AND EARLY EVENING....PRODUCING WIDESPREAD RAINFALL NORTH
OF POINT ARENA AND EVEN SCATTERED THUNDERSTORMS NEAR EUREKA. THE
FRONT HAS SINCE WEAKENED QUITE A BIT...BUT LIGHT RAIN HAS MADE IT
AS FAR SOUTH AS OCCIDENTAL AND SANTA ROSA WITHIN THE PAST HOUR.
THE 00Z NAM IS CONSISTENT WITH PREVIOUS NAM OUTPUT IN SPREADING
LIGHT RAIN ACROSS MOST OF THE NORTH BAY OVERNIGHT...AS WELL AS
INTO ALL OF OUR COASTAL AREAS BY LATE TONIGHT. CURRENT FORECAST
LOOKS ON TRACK EXCEPT HAVE TAKEN OUT MENTION OF THUNDERSTORMS FOR
TONIGHT AS IT APPEARS THERE IS NO LONGER ANY CONVECTION ALONG THE
FRONT...AND THE MODELS SHOW VERY LITTLE INSTABILITY FOR THE
REMAINDER OF TONIGHT.

SHOWER ACTIVITY WILL CONTINUE INTO THURSDAY AS THE UPPER TROUGH
GRADUALLY DIGS SOUTHEAST ALONG THE COAST. INSTABILITY WILL
INCREASE ON THURSDAY AS A COLDER AIR MOVES IN ALOFT. BASED ON
LATEST MODEL GUIDANCE...THE BEST CHANCE OF ISOLATED THUNDERSTORMS
ON THURSDAY WILL BE ACROSS THE NORTH AND EAST BAY.

THE TROUGH IS FORECAST TO DROP SOUTH ON THURSDAY NIGHT AND FRIDAY
AND EVENTUALLY FORM A CUTOFF LOW NEAR POINT CONCEPTION. EARLIER
RUNS OF THE NAM HAD INDICATED THAT SHOWERS AND THUNDERSTORMS MIGHT
STILL BE A POSSIBILITY ACROSS THE FAR SOUTHEASTERN PART OF OUR
FORECAST AREA INTO FRIDAY...BUT NOW NOW THE NAM IS ON BOARD WITH
THE OTHER MODELS IN KEEPING ALL PRECIP OUT OF OUR AREA AFTER
THURSDAY EVENING.

850 MB TEMPS ARE FORECAST TO DROP BY ABOUT 8 DEG C BY TOMORROW AND
SO WE CAN EXPECT WIDESPREAD COOLING WITH TEMPERATURES FALLING
BELOW NORMAL IN MOST AREAS. TEMPS WILL THEN GRADUALLY WARM FROM
FRIDAY THROUGH THE WEEKEND AS THE LOW FIRST SINKS SOUTH OF OUR
AREA AND THEN MOVES OFF TO THE EAST.

DURING PAST FEW DAYS THE MEDIUM RANGE MODELS HAD BEEN FORECASTING
A RATHER ROBUST WARMING TREND DURING THE FIRST HALF OF NEXT WEEK
WHEN A STRONG UPPER RIDGE WAS EXPECTED TO DEVELOP OFF THE
CALIFORNIA COAST. BUT THE LATEST MODELS NOW SHOW ANOTHER UPPER
TROUGH APPROACHING THE WEST COAST WITH RAIN POSSIBLE ACROSS AT
LEAST THE NORTHERN PORTION OF OUR AREA BY TUESDAY NIGHT OR
WEDNESDAY OF NEXT WEEK.

&&

.AVIATION...AS OF 6:40 PM PDT WEDNESDAY...CLOUDS WITH INCOMING LOW
PRESSURE AREA ARE QUICKLY APPROACHING THE COASTLINE WITH LIGHT
RAIN REPORTED AS FAR SOUTH AS GUALALA. LIGHT RAIN WILL SPREAD
ACROSS THE NORTH BAY THIS EVENING...AND THE REST OF THE AREA
OVERNIGHT. MVFR CIGS FORECAST TONIGHT WITH BRIEF LOCAL PATCH OR
TWO OF IFR CIGS POSSIBLE BUT IN GENERAL THE INCOMING TROUGH WILL
BRING COOLER LOW-MID LEVEL TEMPERATURES (ERODING AIR MASS
STABILITY) AND A PICK UP IN SURFACE WINDS AND MIXING.

VICINITY OF KSFO...VFR IS EXPECTED TO HOLD FOR MOST OF THIS EVENING.
THE LOWER TO MID LEVEL CLOUDS WILL SPREAD NO LATER THAN MIDNIGHT.
MVFR CIGS WITH LIGHT RAIN POSSIBLE LATE AT NIGHT. WET RUNWAYS.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...VFR HOLDING TOGETHER SO FAR BUT
POTENTIAL IS THERE FOR LOW CLOUD DEVELOPMENT AHEAD OF THE INCOMING
LOW. CLOUD LAYERS TONIGHT FORECAST TO BE PRIMARILY MVFR.

&&

.MARINE...AS OF 6:35 PM PDT WEDNESDAY...VERY LARGE LONG PERIOD
SOUTHWESTERLY SWELL WILL IMPACT THE WATERS THROUGH THURSDAY. A LOW
IS CENTERED 350 MILES WEST OF UKIAH. THIS LOW WILL MOVE THROUGH
THE NORTHERN WATERS ON THURSDAY RESULTING IN LIGHT SOUTHERLY WINDS
WITH A CHANCE OF SHOWERS. THERE WILL BE A SLIGHT CHANCE OF
THUNDERSTORMS THIS EVENING NORTH OF POINT REYES AND IN THE SAN
FRANCISCO BAY THURSDAY. WINDS WILL BECOME LIGHT TO MODERATE
NORTHWEST FRIDAY THROUGH SUNDAY AFTER THE LOW MOVES INLAND.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TNGT...NONE.

&&

$$

PUBLIC FORECAST: DYKEMA
AVIATION: CANEPA
MARINE: W PI/CANEPA

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