Debris Flow in Washington, January 31, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
945 AM PST THU JAN 31 2013

.SYNOPSIS...A WEAK FRONT WILL MOVE THROUGH WESTERN WASHINGTON TODAY.
HIGH PRESSURE AND LIGHT OFFSHORE FLOW WILL BE OVER THE REGION FRIDAY
AND SATURDAY. WEATHER SYSTEMS WILL AFFECT THE REGION NEXT WEEK.

&&

.SHORT TERM...COR SENT FOR TYPOS AND UPDATED HYDRO SECTION. A
WEATHER SYSTEM WILL MOVE THROUGH WESTERN WA TODAY. PRESSURE FALLS
OVER THE PAST FEW HOURS HAVE BEEN SLIDING DOWN VANCOUVER ISLAND AND
PEAKED AT -2MB/3HRS NEAR TOFINO AT 14Z AND NOW THE PRES FALLS ARE
FADING NEAR KUIL. TATOOSH HAS HAD A DECENT BREEZE OF S25KT AND THERE
AREA AREAS OF RAIN ON THE RADAR PUSHING THROUGH WESTERN WA THIS
MORNING. THE LOW CLOUDS FOG AND DRIZZLE OF YESTERDAY HAVE GIVEN WAY
TO A TYPICAL CLOUDY DAY WITH AREAS OF RAIN WHICH IS PERHAPS NOT AS
GLOOMY. THE FRONT DOESN'T LOOK LIKE MUCH ON RADAR OR SATELLITE AND
THE PRECIP SHOULD DRY UP LATER THIS AFTERNOON OR THIS EVENING. THE
NAM SHOWS THE RATHER WEAK LOW PRESSURE TROUGH AND AREAS OF PRECIP AT
4PM GIVING WAY TO 1030MB HIGH PRES BY DAYBREAK FRIDAY AND A DRYING
AIR MASS. THE NAM TIME HEIGHT CROSS SECTION FOR SEATTLE SHOWS A
MAINLY DRY TIME HEIGHT CROSS SECTION STARTING ABOUT MID MORNING
FRIDAY AND LASTING TIL DAYBREAK SUNDAY. LIGHT OFFSHORE FLOW FRI AND
SAT WITH 1030MB HIGH PRES EAST OF THE CASCADES AND LOWER PRESSURE
OFFSHORE...BUT AREAS OF LOW CLOUDS AND FOG ARE CERTAINLY A GOOD BET
ALONG THE I-5 CORRIDOR AND ESPECIALLY IN THE PUGET SOUND BASIN
FRIDAY NIGHT AND SATURDAY MORNING. HAVE NOT LOOKED CAREFULLY AT THE
MESOSCALE MODELS FOR THAT BUT THERE IS ALREADY SOME NIGHT AND
MORNING FOG IN THE FORECAST. OTHERWISE THE END OF THE WEEK OUGHT TO
BE AT LEAST PARTLY SUNNY WITH HIGH TEMPS ABOVE NORMAL...AND THE BEST
BET FOR SUN AND WARMTH IS IN THE FOOTHILLS AND ON THE COAST. 19

.LONG TERM...THE 12Z GFS HAS A SYSTEM AND 500MB HEIGHT FALLS MOVING
THROUGH THE RIDGE SATURDAY NIGHT AND EARLY SUNDAY AND THEN THE DIRTY
FLAT RIDGE BULGING UP AGAIN SUNDAY AFTERNOON. THE 12Z NAM HAS A
LITTLE LIGHT RAIN OR DRIZZLE...MAINLY COAST AND NORTH ON SUNDAY.
THAT IS THE QUICK LOOK AT THE NEW RUN...THERE ARE DECENT HEIGHT
FALLS BY TUE EVENING AND AGAIN WED NIGHT SO THE GFS IS LOOKING LESS
WIMPY WITH A COUPLE OF FRONTS NEXT WEEK. 19

&&

.HYDROLOGY...THE NEWAUKUM CRESTED JUST BELOW 9 FEET OVERNIGHT AND
THE CHEHALIS RIVER AT CENTRALIA IS FORECAST TO CREST JUST BELOW 58
FEET THIS AFTERNOON. THAT IS BELOW THE THRESHOLD HEIGHT OF INTEREST
FOR THE CONSTRUCTION PROJECT ALONG I-5 IN CENTRALIA.
NO RIVER FLOODING IS EXPECTED FOR THE NEXT 7 DAYS. 19

&&

AVIATION...A WEAK UPPER TROF WILL MOVE ACROSS THE AREA TODAY FOR WLY
FLOW ALOFT. AN UPPER RIDGE WILL BUILD OVER THE REGION TONIGHT AND
THE FLOW ALOFT WILL BECOME MORE NLY. LIGHT ONSHORE FLOW TODAY WILL
BECOME FLAT TONIGHT.

CIGS WILL BE THE MAIN FORECAST PROBLEM TODAY. OVERALL...CIGS ARE
EXPECTED TO BE MAINLY IN THE 3-5K FT RANGE BUT THERE WILL BE AREAS
OF CIGS NEAR 1K FT. EXPECT LOCALIZED VSBYS OF 1-3SM -RA BR AND CIGS
NEAR 500 FT.

KSEA...CIGS SHOULD GENERALLY BE IN THE 1-2K FT RANGE TODAY BUT MAY
BRIEFLY LIFT TO NEAR 7K OR 8K FT THIS MORNING. WINDS WILL BE SLY
7-12 KT.

&&

.MARINE...
ONSHORE OR SOUTHERLY FLOW WILL PREVAIL TODAY. AN AREA OF HIGH PRES
WILL SETTLE OVER THE AREA TONIGHT FOR FLAT OR VERY WEAK FLOW. EXPECT
THE FLOW TO BECOME LIGHT OFFSHORE OR NORTHERLY ON FRI. EASTERLY FLOW
WILL STRENGTHEN ON SATURDAY DUE TO BUILDING HIGH PRES E OF THE
CASCADES AND LOWER PRES OVER THE NE PACIFIC.

&&

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

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.