FXUS66 KSEW 300448
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
850 PM PST SUN NOV 29 2009
.SYNOPSIS...WEAK HIGH PRESSURE WILL REMAIN OVER THE AREA THROUGH
MONDAY MORNING. A WEAK COLD FRONT WILL BRING A CHANCE OF LIGHT RAIN
TO THE AREA MONDAY AFTERNOON AND EVENING. DRY AND SUNNIER WEATHER
WILL DEVELOP TUESDAY AS A 1032 MB HIGH CENTER BUILDS NEAR THE B.C.
ALBERTA BORDER THROUGH WEDNESDAY. ANOTHER WEAK FRONT MAY BRING A
CHANCE OF RAIN TOWARD THE END OF THE WEEK. COLDER WEATHER IS
POSSIBLE FOR THE WEEKEND AND BEYOND AS THE WEATHER PATTERN CHANGES.
&&
.SHORT TERM...SOUTHERLY BREEZE AND SLY PRESSURE GRADIENTS PERSIST SO
THAT FAVORS LOW STRATUS INSTEAD OF FOG. I CUT BACK THE AMOUNT OF FOG
IN THE FCST THRU MON MORNING AND JUST KEPT PATCHY FOG IN SOME OF THE
ZONES TONIGHT. AT 8PM OLM-BLI WAS +1.4MB AND HAD INCREASED SLIGHTLY
OVER THE PAST FEW HOURS...STILL...IT WILL BE A HODGE PODGE OF VERY
LOW STRATUS AND PATCHY FOG IN THE AREAS THAT CLEAR OUT THIS EVENING.
THE FREEZING LEVELS FOR THE MOUNTAIN FORECAST LOOKED A NOTCH TOO
HIGH THRU TUE NITE...SO I LOWERED THOSE DOWN A BIT. THE COLD FRONT
FOR MONDAY AFTERNOON IS NOT MUCH OF A FRONT BUT IT SHOULD LOWER THE
SNOW LEVEL MON NITE...LOWER THAN WHAT IS IN THE FCST RIGHT NOW AND A
COUPLE INCHES OF SNOW INT HE CASCADES WILL FRESHEN UP THE SLOPES.
AFTER THE COLD FRONT MOVES THRU HIGH PRES AND COOL DRY AIR WILL
FRESHEN UP THE AIR MASS INTO TUESDAY...ESPECIALLY OVER THE CASCADES.
THE NAM SHOWS 850MB TEMPS COOLING FROM ARND +4C OVER THE CENTRAL
CASCADES MONDAY MORNING TO ARND -4C BY TUESDAY MORNING...PERHAPS
EVEN AS COOL AS -7C ALONG THE EAST SLOPES OF THE CASCADES BY TUE
NITE. SO AFTER THAT A QUICK SHOT OF SNOW SHOWERS IN THE CASCADES
THRU MON EVENING THERE WILL BE A TRANSITION TO OFFSHORE FLOW TUESDAY
AS THE 1032MB HIGH CENTER BUILDS NEAR THE B.C. ALTA BORDER. THAT IS
A BIG CHANGE FROM THE LOW PRES THAT IS UP THERE NOW AND THIS
PERSISTENT SLY FLOW WITH THESE PERSISTENT AREAS OF LOW STRATUS OVER
WRN WA. THE FCST FOR PARTLY TO MOSTLY SUNNY SEEMS A GOOD BET TUE/WED
AND TUE NITE SHUD BE CLEAR WITH A GOOD CHANCE OF A FROST. A FEW
CHANGES TO THE ZONES AND GRIDS HAVE ALREADY BEEN PUT IN...MORE
TWEAKS ARE IN THE OFFING OVERNIGHT BUT THE GENERAL FCST TOWARD
SUNSHINE TUE WED IS IN GOOD SHAPE. 19
.LONG TERM...LATEST GFS CONTINUES THE IDEA OF SNEAKING ONE MORE WEAK
COLD FRONT THRU THE AREA LATER THIS WEEK AND THEN AMPLIFYING THE
RIDGE JUST OFFSHORE IN RESPONSE TO A COUPLE OF HUGE STORMS THAT
DEVELOP IN SUCCESSION OUT ARND 170W. THAT SWITCHES THE WEATHER
PATTERN TO COLD AND DRY OVR WRN CANADA AND THE PACNW BY THE WEEKEND.
A LITTLE EARLY TO SOUND THE ALARMS ON THIS BUT FOR THE MOMENT THE
GLOBAL MODELS HAVE BEEN TRENDING IN THE SAME DIRECTION AND THE GFS
CERTAINLY DEVELOPS AWFULLY COLD AIR OVER THE PACNW A WEEK FROM NOW.
SOMETHING TO WATCH. 19
&&
.HYDROLOGY...A COLD FRONT ON FRIDAY WILL BRING SOME LIGHT RAIN TO
THE AREA. UP TO A HALF INCH OF RAIN COULD FALL ACROSS THE NORTH WITH
LESSER AMOUNTS ELSEWHERE. THIS IS NOT ENOUGH PRECIPITATION TO CAUSE
FLOODING CONCERNS.
GENERALLY DRY WEATHER THE REST OF THE WEEK WITH NO FLOODING
EXPECTED. MODELS INDICATE THE POSSIBILITY OF A TRANSITION TO COLDER
WEATHER NEXT WEEKEND WITH ONLY LIGHT PRECIPITATION.
NO FLOODING IS EXPECTED ON THE GREEN RIVER THROUGH NEXT WEEKEND.
&&
.AVIATION...HIGH PRESSURE IS NOW AT THE SURFACE AND ALOFT OVER
WESTERN WASHINGTON. WHILE SOUTHERLY PRESSURE GRADIENTS ARE RATHER
TIGHT NEAR THE CANADIAN BORDER...KEEPING CONDITIONS STIRRED UP OVER
FAR NORTH PARTS OF THE AREA...GRADIENTS ARE RATHER FLAT FROM
SNOHOMISH COUNTY TO JEFFERSON COUNTY SOUTHWARD. LOW STRATUS IS
CONCENTRATED ALONG THE COAST...THRU THE CHEHALIS GAP AND OVER
WESTERN PORTIONS OF PUGET SOUND...WHILE IT IS BECOMING MORE
SCATTERED TO BROKEN FROM THE EAST PUGET SOUND LOWLANDS INTO THE
SOUTHWEST INTERIOR. THIS RATHER CLEAR AREA SHOULD FILL IN WITH FOG
SHORTLY AS STRONG LATE NOVEMBER RADIATIONAL COOLING ALLOWS
TEMPERATURES TO COOL TO THE DEWPOINT. TEMPERATURE AND MOISTURE
PROFILES THROUGH THE BOUNDARY LAYER FAVOR THE FORMATION OF RATHER
THICK AND DEEP FOG IF THE STRATUS ABOVE DOES CLEAR.
STRATUS WILL LIFT MIDDAY AS A FRONTAL SYSTEM DROPS SOUTH FROM
BRITISH COLUMBIA GIVING INCREASED MIXING AND A SOME LIGHT
PRECIPITATION.
CONDITIONS ALOFT DRY AND ANY CLEARING WITH LIGHT OFFSHORE FLOW AND
MORE RADIATIONAL COOLING MAY GIVE MORE FOG LATE MON NIGHT INTO TUE
AS INDICATED BY THE NAM12.
KSEA...WITH LIGHT SLY FLOW TURNING SE WILL WATCH FOR ANY CLEARING IN
THE STRATUS DECK THIS EVENING TO PROVIDE SUFFICIENT RADIATIONAL
COOLING TO ALLOW A LOW STRATUS OR FOG DECK TO FORM BY 11Z AND
PERSIST THRU 21Z MON. LIGHT SLY PRES GRADIENTS FAVOR THE FORMATION
OF A VERY LOW STRATUS WITH TOWER VIS 1/4SM AND GROUND VIS 3/4SM BR
OVC002. EXPECT CIGS TO RISE TO ABOUT 025 DURING THE AFTERNOON HOURS
MON AS THE FRONT APPROACHES THEN MOVES THROUGH. LIGHT S/SW WINDS
WILL TURN TO NORTHERLY AROUND 02Z TUE. ALBRECHT
&&
.MARINE...HIGH PRES IS CENTERED OVER THE WATERS THIS EVENING BUT
SMALL CRAFT ADVISORY CONDITIONS PERSIST OVER THE FAR NORTHERN
COASTAL WATERS...THE WEST ENTRANCE TO THE STRAIT...AND THE FAR
NORTHERN INLAND WATERS NEAR THE CANADIAN BORDER.
A HIGH 18 FT WNW SWELL HAS MOVED INTO THE WATERS AND WILL SLOWLY
DECAY THRU MON. EXPECT THE COMBINATION OF SEAS TONIGHT AND
INCREASING WIND MON WITH A PASSING COLD FRONT TO MAINTAIN SMALL
CRAFT ADVISORY CONDITIONS ALONG THE COAST THRU MON. WILL LIKELY NEED
TO ISSUE SMALL CRAFT ADVISORIES CENTRAL AND EAST STRAIT AND POSSIBLY
ADMIRALTY INLET LATER TONIGHT AS PRES GRADIENTS RESPOND TO THE
PASSING COLD FRONT.
HIGH PRESSURE WILL BUILD OVER BRITISH COLUMBIA MONDAY NIGHT THEN
SLIDE SOUTHEAST INTO EASTERN WASHINGTON DURING THE DAY WEDNESDAY.
THIS WILL GIVE LIGHT TO MODERATE OFFSHORE FLOW TO THE WATERS OVER
THE FIRST HALF OF THE WEEK. ALBRECHT
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.
.SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE TO THE
STRAIT OF JUAN DE FUCA...AND NORTHERN INLAND WATERS THROUGH
MONDAY.
$$
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