FXUS66 KSEW 072315
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST MON DEC 7 2009
.SYNOPSIS...STRONG SURFACE HIGH PRESSURE OVER THE INTERIOR OF
WESTERN CANADA HAS PUSHED MODIFIED COLD DRY ARCTIC AIR INTO WESTERN
WASHINGTON. THESE CONDITIONS ARE EXPECTED TO CONTINUE THROUGH
MUCH OF THIS WEEK WITH THE JETSTREAM WELL SOUTH OF THE PACIFIC
NORTHWEST INTO CALIFORNIA. THE COLD AIR MASS IS ANTICIPATED TO
SLOWLY MODERATE LATER THIS WEEK.
&&
.SHORT TERM...COLD AIR MASS IN PLACE OVER WASHINGTON WILL BE THE
MAIN STORY FOR THE NEXT THREE DAYS AT LEAST. NO RECORDS ARE LIKELY
TO FALL...BUT OVERNIGHT LOWS WILL BE IN THE TEENS AND 20S...WITH
DAYTIME HIGHS STRUGGLING TO REACH THE MID 30S. WITH THIS COLD AIR IN
PLACE ANY PRECIPITATION WOULD BE SNOW...BUT THE CHANCE OF SNOW IS
LOW THROUGH THURSDAY.
TONIGHT THERE SHOULD BE A PERIOD OF CLOUDINESS AS SOME MID AND HIGH
LEVEL CLOUDS MOVE SOUTH FROM BRITISH COLUMBIA. THIS BAND OF CLOUDS
IS APPARENT IN SATELLITE IMAGERY AND LOOKS FAIRLY THICK...SO HAVE A
SIX HOUR PERIOD OF MOSTLY CLOUDY 06Z-12Z WITH PARTLY CLOUDY EACH
SIDE OF THAT. DETAILS MAY NOT BE EXACTLY CORRECT BUT THAT IS THE
GENERAL IDEA.
MOS POPS ARE IN THE LOW CHANCE CATEGORY FOR NORTHERN ZONES FOR EACH
OF THE THREE PERIODS TUESDAY THROUGH WEDNESDAY. HAVE CHOSEN TO KEEP
THE FORECAST DRY DURING THIS TIME AS THE GENERAL PATTERN DOES NOT
CHANGE SIGNIFICANTLY. THE SURFACE FLOW WILL BE LIGHT TO MODERATE
OFFSHORE...WITH DRY NORTHWEST FLOW ALOFT. APPARENTLY MOS IS PICKING
UP ON WEAK DISTURBANCES COMING THROUGH THE FLOW ALOFT BUT THE MODELS
THEMSELVES ARE DRY...SO WILL KEEP THE FORECAST DRY.
SIMILAR STORY THURSDAY WITH THE SAME PATTERN AND A MOSTLY DRY
FORECAST. HOWEVER A WEAK SYSTEM STARTS TO SHOW UP IN THE MODELS...
ESPECIALLY NORTHERN ZONES. KEPT THE CHANCE OF FLURRIES IN THE
FORECAST THURSDAY NIGHT AND FRIDAY IN THE NORTH. BURKE
.LONG TERM...THE FORECAST BECOMES CLOUDY FROM THE NORTH ON FRIDAY
AND BY FRIDAY NIGHT THERE IS A CHANCE OF SNOW EVERYWHERE. THERE
COULD BE RAIN ON THE COAST BUT HAVE KEPT THE FORECAST MOSTLY SNOW
WITH JUST A FEW MENTIONS OF RAIN OR SNOW. THERE IS ALSO A STRONGER
STORM MOVING INTO OREGON ABOUT SATURDAY WHICH WILL LIKELY BRING A
LITTLE PRECIPITATION INTO SOUTHERN ZONES. EXTENDED POPS ARE HIGH FOR
THIS EVENT...LIKELY OR BETTER...AND THERE IS DEFINITELY A CHANCE
THAT MEASURABLE SNOW COULD FALL AT THE START OF THE PRECIPITATION.
IN FACT SOME ZONES COULD SEE SNOW ALL DAY. WILL WAIT FOR A FEW MORE
MODEL RUNS BEFORE RAMPING UP THE ANTICIPATION.
SUNDAY WILL BE SHOWERY BUT SURFACE FLOW WILL HAVE TURNED ONSHORE AND
TEMPERATURES WILL BE MOSTLY IN THE 40S. SNOW SHOULD BE CONFINED TO
THE MOUNTAINS BY SUNDAY. ON MONDAY THE EURO AND GFS DISAGREE WITH
THE GFS BRINGING A WET SYSTEM INLAND AND THE EURO HOLDING IT
OFFSHORE UNTIL TUESDAY. HAVE INDICATED A CHANCE OF SHOWERS...WHICH
IS MORE LIKE THE EURO. BURKE
&&
.HYDROLOGY...WITH THE COLD DRY WEATHER PATTERN IN PLACE THIS WEEK..
NO RIVER FLOODING WILL OCCUR. ANY PRECIPITATION OVER THE WEEKEND IS
LIKELY TO BE LIGHT...AND CONTINUED COLD WEATHER WILL MAKE FLOODING
THEN UNLIKELY AS WELL. THE GREEN RIVER WILL NOT FLOOD. BURKE
&&
.AVIATION...A MID LEVEL DECK OF CLOUDS WILL SPREAD SOUTH FROM B.C.
THIS EVENING. LIGHT OFFSHORE FLOW WILL KEEP THE LOW LEVEL AIR MASS
VERY DRY. THERE IS A WEAK LOW PRES AREA OFF VANCOUVER ISLAND AND
CLOUDS ARE DEVELOPING OVER THE B.C. COASTAL WATERS FROM VANCOUVER
ISLAND TO THE CHARLOTTES...BUT IF SNOW SHOWERS DEVELOP IT IS LIKELY
THAT THEY WILL STAY JUST OFFSHORE AND WRN WA WILL SEE ONLY THE MID
LEVEL CLOUDS. BEHIND THIS SHORTWAVE EXPECT CLEARING BY TUE AFTERNOON
THRU NITE. 19
KSEA...INCREASING MID LEVEL CLOUDS THIS EVENING WITH LIGHT NE
BREEZE.
&&
.MARINE...STILL BREEZY NE WIND SAN JUANS AND BLI NORTH...SO WILL
KEEP UP A SMALL CRAFT ADVISORY THERE. PRESSURE IS FALLING OVER THE
WHOLE REGION AND THE HIGH OVER B.C. HAS WEAKENED TO A MERE 1027MB
CENTER THIS AFTERNOON...BUT IF THE PRESSURE STARTS TO FALL FASTER
OVER THE COASTAL WATERS THE GRADIENT COULD COME UP ENUF FOR EAST
WINDS TO INCREASE THRU THE STRAIT AGAIN...BUT FOR NOW I HAVE
E15-20KT FOR THE WEST ENTRANCE TO THE STRAIT AND NO ADVISORY THERE.
THE MESO MODELS SHOW JUST SMALL CRAFT ADVISORY STRENGTH FRASER
OUTFLOW TO 25KT GIVING OUT TUE MORNING WITH RATHER LIGHT WINDS AND
GRADIENT LATER TUE THRU WED. THE OVERALL WEATHER PATTERN SEEMS
FAVORABLE FOR MODERATE FRASER OUTFLOW WED-FRI HOWEVER THE WRFGFS
EXTENSION ESTABLISHES A SE WIND THRU OUR SO CALLED SALISH SEA
STARTING ON WED THRU THE END OF THE WEEK. I HAVE NOT LOADED THAT
LATEST GUIDANCE INTO THE WIND GRIDS HOWEVER...RATHER LIGHT NE FLOW
SEEMS MORE LIKELY...PERHAPS AS STRONG AS NE 20KT FROM THE FRASER
INTO THE SAN JUANS AT TIMES THRU THE END OF THE WEEK. 19
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY NORTHERN INLAND WATERS.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
