Dense Fog in Washington, September 28, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
950 PM PDT THU SEP 27 2012

.SYNOPSIS...CLOUDS WILL GRADUALLY INCREASE ACROSS THE AREA TONIGHT
THROUGH FRIDAY AHEAD OF A COLD FRONT OFFSHORE. THE TAIL END OF THIS
FRONT WILL MOVE THROUGH THE EXTREME NORTHERN PORTION OF THE AREA
FRIDAY EVENING. LOOK FOR HIGH PRESSURE ALOFT TO REBUILD OVER THE
REGION THIS WEEKEND.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS JUST SOME HIGH CLOUDS OVER
WESTERN WASHINGTON THIS EVENING WITH A COLD FRONT WELL OFFSHORE NEAR
135W AND AN UPPER LEVEL RIDGE TO THE EAST. FOG/STRATUS IMAGERY NOT
SHOWING ANYTHING AS OF 04Z.

UPPER LEVEL RIDGE WILL CONTINUE TO MOVE EAST TONIGHT WHILE THE COLD
FRONT OFFSHORE MOVES SLOWLY EASTWARD. SURFACE GRADIENTS WILL REMAIN
LIGHT OVERNIGHT INTO FRIDAY MORNING. HAS AS BEEN THE CASE FOR THE
LAST COUPLE OF DAYS THE WINDS FOR 850 MB DOWN TO THE SURFACE ARE
ALSO LIGHT. THIS WILL KEEP AREAS OF FOG IN THE FORECAST UNTIL
AROUND 18Z FRIDAY.

MODEL SOLUTIONS DIFFER A BIT ON THE STRENGTH OF THE FRONT AS IT
APPROACHES WESTERN WASHINGTON WITH THE GFS STRONGER THAN THE NAM
SOLUTION. SATELLITE IMAGERY RIGHT NOW SHOWS CLOUD TOP TEMPERATURES
COOLING ALONG THE FRONT. IN ADDITION THE COOL UPPER LEVEL TROUGH
BEHIND THE FRONT IS NOT DIGGING BUT MOVING EASTWARD. THIS WILL ALLOW
THE FRONT TO MAINTAIN SOME OF IT STRENGTH AS IT REACHES SOUTHERN
BRITISH COLUMBIA AND THE NORTHERN PORTION OF WESTERN WASHINGTON. IT
IS STILL A GLANCING BLOW WITH MOST OF THE SYSTEM MOVING INLAND NORTH
OF WESTERN WASHINGTON BUT WITH THE FRONT SHOWING SIGNS OF STAYING
INTACT WILL UP THE POPS ALONG THE NORTH COAST INTO THE LIKELY
CATEGORY FOR LATE FRIDAY AFTERNOON INTO FRIDAY EVENING. FOR THE
REMAINDER OF THE AREA JUST SOME HIGH LEVEL CLOUDS UNTIL THE
AFTERNOON HOURS ON FRIDAY.

FRONT MOVING THROUGH FRIDAY NIGHT WITH INCREASING ONSHORE FLOW
BEHIND THE FRONT FOR LATE FRIDAY NIGHT INTO SATURDAY MORNING KEEPING
SKIES CLOUDY OVER THE AREA. UPPER LEVEL RIDGE STARTS TO REBOUND
SATURDAY AFTERNOON WITH 500 MB HEIGHTS APPROACHING 580 DM BY 00Z
SUNDAY RESULTING IN SOME SUNSHINE FOR WESTERN WASHINGTON.

UPPER LEVEL RIDGE CONTINUES TO BUILD SATURDAY NIGHT INTO SUNDAY WITH
THE 500 MB HEIGHTS IN THE MID 580'S SUNDAY AFTERNOON. SURFACE
GRADIENTS GO FLAT ONCE AGAIN LEADING TO ANOTHER ROUND OF AREAS OF
FOG IN THE MORNING. MORE 70S ARE POSSIBLE FOR THE LAST DAY OF
SEPTEMBER. FELTON

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS.
CONFIDENCE IN THIS PERIOD REMAINED LOW DURING THIS PERIOD DUE TO
GENERALLY POOR CONSISTENCY AND LACK OF AGREEMENT IN THE MEDIUM RANGE
MODELS. HOWEVER LEANING TOWARDS THE IDEA OF THE UPPER TROF DIGGING
TO OUR EAST DURING THIS PERIOD INSTEAD OF OVER THE PACIFIC
NORTHWEST. THIS WOULD TEND TO KEEP OUR WEATHER DRY BUT ALLOW TEMPS
TO GRADUALLY LOWER TO SLIGHTLY BELOW NORMAL.

&&

.CLIMATE...TODAY WAS THE 18TH DAY THIS MONTH WITH A HIGH TEMPERATURE
OF 70 DEGREES OR MORE AT SEATTLE-TACOMA AIRPORT. THE NORMAL NUMBER
OF DAYS WITH HIGHS 70 PLUS IN SEPTEMBER IS 15. SINCE RECORDS STARTED
AT SEA-TAC IN 1945 THERE HAVE ONLY BEEN 10 SEPTEMBERS WITH 20 DAYS
OR MORE OF HIGHS 70 PLUS. THE RECORD IS 25 DAYS SET IN 1967.

THE CHANCES FOR A 70 PLUS HIGH TEMPERATURE IN SEATTLE DECREASE
SIGNIFICANTLY IN OCTOBER. THE NORMAL NUMBER OF DAYS IN THE MONTH
WITH A HIGH OF 70 PLUS IS 2. THE RECORD FOR THE ENTIRE MONTH IS 8
DAYS SET IN 1991. THERE HAVE BEEN A TOTAL OF 129 DAYS WITH HIGHS 70
PLUS IN OCTOBER AT SEA-TAC AND 60 PERCENT OF THOSE...77...HAVE
OCCURRED IN THE FIRST WEEK.

RAINFALL AT SEA-TAC SO FAR THIS MONTH IS 0.03 INCHES. AT THIS POINT
THIS IS THE THIRD DRIEST SEPTEMBER ON RECORD SURPASSED ONLY BY 1975
AND 1991 WHERE ONLY A TRACE WAS RECORDED. FELTON

&&

.AVIATION...WEAK WESTERLY FLOW ALOFT WILL BECOME SOUTHWESTERLY
TONIGHT. THE AIR MASS IS STABLE AND GENERALLY DRY. AN APPROACHING
COLD FRONT WILL SPREAD HIGH CLOUDS INTO WESTERN WASHINGTON LATE
TONIGHT. HOWEVER MAY SEE PATCHY FOG AND IFR CONDITIONS EARLY FRIDAY
MORNING. CLOUDS WILL THICKEN AND LOWER FRIDAY AFTERNOON AS THE FRONT
MOVES INLAND. THIS SYSTEM IS FAIRLY WEAK BUT MAY SEE LIGHT RAIN
ALONG THE COAST...WITH JUST A FEW SPRINKLES ELSEWHERE. 33

KSEA...LIGHT NORTHERLY SURFACE WINDS TONIGHT WITH VFR CONDITIONS.
PATCHY FOG WILL BE IN THE VICINITY ONCE AGAIN EARLY FRIDAY. EXPECT
INCREASING MID/HIGH CLOUDS FRIDAY AFTERNOON. 33

&&

.MARINE...AFTER LIGHT FLOW TONIGHT...A WEAK COLD FRONT ON FRIDAY
WILL BRING MODERATE ONSHORE FLOW. WINDS OVER THE STRAIT OF JUAN DE
FUCA WILL INCREASE AND WILL LIKELY REACH SMALL CRAFT ADVISORY
CRITERIA FRIDAY NIGHT. WEST SWELLS WILL BUILD TO 10 FEET.

WINDS WILL DIMINISH SOMEWHAT ON SATURDAY AND BE LIGHT ON SUNDAY.
HIGH PRES OVER THE NORTHEAST PACIFIC WILL MAINTAIN NORTHWESTERLY
FLOW EARLY NEXT WEEK. BY TUESDAY OR SO THERE COULD BE SMALL CRAFT
ADVISORY WESTERLIES IN THE STRAIT AND NORTHWESTERLIES OVER THE
SOUTHERN COASTAL WATER ZONES. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML (ALL LOWERCASE)