Debris Flow and High Wind in Washington, February 22 to 26, 2012

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
940 AM PST WED FEB 22 2012

CORRECTED TYPO IN THE SYNOPSIS.

.SYNOPSIS...SHOWERY WEATHER WILL PREVAIL OVER THE AREA TODAY IN THE
WAKE OF THE COLD FRONT THAT MOVED THROUGH EARLIER. HIGH PRESSURE
ALOFT WILL PREVAIL OVER THE REGION TONIGHT AND THURSDAY FOR DRIER
WEATHER. A FRONTAL SYSTEM WILL BRING MORE RAIN AND MOUNTAIN SNOW TO
THE AREA ON FRIDAY.

&&

.NEAR TERM...CONDITIONS IMPROVED FASTER THAN WHAT THE MODELS
INDICATED. THE SHOWERS HAVE BECOME MOSTLY SCATTERED ACROSS THE
CWA THIS MORNING...PROMPTING AN UPDATE TO THE FORECAST. IN
FACT...SOME AREAS WERE SEEING SUNNY SKIES THIS MORNING.

.SHORT TERM...
MOST OF THE SHOWER ACTIVITY WILL BE FOCUSED IN THE PUGET SOUND
CONVERGENCE ZONE OVER KING COUNTY BUT WITH THE STRONG FLOW ALOFT
EXPECT THIS FEATURE TO GET PUSHED INTO THE CASCADES. SNOW LEVELS...
WHICH STUBBORNLY HELD AROUND THE 4000-5000 FOOT RANGE IN THE
CASCADES OVERNIGHT WILL DROP TO AROUND 2500 FEET THIS AFTERNOON.
OROGRAPHIC FLOW INTO THE CASCADES WILL WEAKEN BUT 850 MB WINDS
REMAINING IN THE 30-40 KNOT RANGE TODAY. WILL STAY WITH THE SNOW
ADVISORY FOR THE CASCADES WITH THE CENTRAL CASCADES LIKELY GETTING
AS MUCH AS A FOOT OF NEW SNOW BY MIDNIGHT TONIGHT. THE HIGHEST
ACCUMULATIONS WILL BE SOUTH OF STEVENS PASS.

UPPER LEVEL RIDGE CONTINUES TO BUILD OFFSHORE TONIGHT WITH SURFACE
HIGH PRESSURE BUILDING INTO WESTERN WASHINGTON. THE COMBINATION OF
THESE TWO FACTORS WILL BRING ABOUT AN END TO THE SHOWER ACTIVITY FOR
THE LOWLANDS. 850 MB WINDS STILL AROUND 30 KNOTS THIS EVENING INTO
THE CASCADES SO WILL KEEP THE SNOW ADVISORY GOING UNTIL MIDNIGHT
TONIGHT. OROGRAPHIC FLOW WEAKENING AFTER MIDNIGHT INTO THURSDAY
MORNING.

A LITTLE BREAK IN THE ACTION ON THURSDAY WITH THE UPPER LEVEL RIDGE
OFFSHORE AND SURFACE HIGH PRESSURE OVER THE AREA. THE BREAK WILL BE
SHORT LIVE AS THE NEXT SYSTEM TO REACH THE AREA APPROACHES THURSDAY
NIGHT. WARM FRONT BRUSHES BY WESTERN WASHINGTON LATER THURSDAY NIGHT
INTO FRIDAY MORNING WITH THE PRECIPITATION OUT AHEAD OF THE TRAILING
COLD FRONT SPREADING ACROSS THE AREA BY NOON FRIDAY. COLD FRONT
MOVING THROUGH WESTERN WASHINGTON FRIDAY NIGHT. SNOW LEVELS SHOULD
REMAIN BELOW PASS LEVELS AHEAD AND WITH THE FRONT. THIS SYSTEM HAS
THE POTENTIAL TO BE A BIG SNOW PRODUCER FOR THE MOUNTAINS. FELTON

.LONG TERM FROM THE PREV DISCUSSION...
EXTENDED MODELS IN GOOD AGREEMENT AGAIN THIS MORNING WITH A COOL
UPPER LEVEL TROUGH MOVING INTO WESTERN WASHINGTON BEHIND THE COLD
FRONT LATE FRIDAY NIGHT INTO THE WEEKEND. NO REAL ORGANIZED SYSTEM
MOVING INTO THE AREA BUT WITH THE UNSTABLE AIR MASS SHOWERS WILL POP
UP ESPECIALLY DURING THE DAYLIGHT HOURS. 1000-850 MB THICKNESS
VALUES START OUT OVER 1300 METERS BUT BY SATURDAY NIGHT DROP BELOW
1300 METERS AND STAY BELOW 1300 METERS THROUGH MONDAY. BOTH THE
ECMWF AND THE GFS SHOW A TROUGH AXIS MOVING THROUGH WESTERN
WASHINGTON SUNDAY AFTERNOON. THIS COULD HELP FOCUS THE SHOWER
ACTIVITY AT THAT TIME. CURRENT FORECAST HAS A BROAD BRUSH CHANCE OF
RAIN AND SNOW SHOWERS FOR THE LOWLANDS. WILL STAY WITH THAT IDEA IN
THE MORNING PACKAGE. MODEL SOLUTIONS START TO DIVERGE ON DAY 7 WITH
THE ECMWF BRINGING ANOTHER FRONT INTO THE AREA WHILE THE GFS IS
STRONGER WITH THE UPPER LEVEL RIDGING AND KEEPS THE FRONT OFFSHORE.
WILL CONTINUE THE CHANCE POPS FORECAST FOR DAY 7. FELTON/05

&&

.HYDROLOGY...
ANOTHER 1 TO 3 INCHES OF PRECIPITATION IS LIKELY IN THE CENTRAL
CASCADES TODAY BUT MUCH OF THIS WILL FALL AS SNOW AS THE SNOW LEVELS
WERE ALREADY COMING DOWN THIS MORNING AND SHOULD BE NEAR 2500 FEET
BY THIS AFTERNOON. ADDITIONALLY...THE PRECIP HAS ALREADY BECOME
SHOWERY IN CHARACTER OR OF THE HIT OR MISS VARIETY W OF THE
CASCADES. DECREASING RAINFALL AND LOWERING SNOW LEVELS WILL ALLOW
THE UPPER REACHES OF MOST RIVERS TO CREST THIS MORNING...AND CRESTS
WILL FLOW DOWNSTREAM LATER TODAY AND TONIGHT.

THE RIVERS FLOWING OFF THE WEST SLOPES OF THE CENTRAL CASCADES HAVE
RISEN MOST SHARPLY. THE PILCHUCK...STILLAGUAMISH...TOLT...AND
SNOQUALMIE RIVERS HAVE REACHED FLOOD STAGE. FLOOD WARNINGS ARE IN
EFFECT...CALLING FOR MINOR TO MODERATE FLOODING ON THOSE RIVERS.
FLOOD WARNINGS HAVE ALSO BEEN ISSUED FOR THE SKYKOMISH RIVER...WHICH
IS RISING MORE SLOWLY THAN EXPECTED EARLY THIS MORNING...AND THE
REACH OF THE PUYALLUP RIVER NEAR ORTING...WHICH IS CLOSE TO REACHING
FLOOD STAGE.

OTHER RIVERS STILL FACE A THREAT OF FLOODING...AND ADDITIONAL
WARNINGS COULD BE ISSUED. THE GREATEST THREAT IS PROBABLY ON THE
SNOHOMISH RIVER...WHICH IS FORMED BY THE CONFLUENCE OF THE SKYKOMISH
AND SNOQUALMIE RIVERS. NWRFC GUIDANCE STILL SHOWS THE SNOHOMISH
CRESTING IN THE MODERATE FLOOD RANGE. HOWEVER...GIVEN THE SLUGGISH
BEHAVIOR OF THE SKYKOMISH -- WHICH IS THE MAIN DRIVER FOR THE
SNOHOMISH CREST -- I AM GOING TO HOLD OFF ON ISSUING A FLOOD WARNING
FOR THE SNOHOMISH FOR NOW.

THE LATEST RUN OF THE HYDROLOGIC MODELS ALSO SUGGESTS THERE COULD BE
MINOR FLOODING ALONG REACHES OF THE COWLITZ AND GREEN RIVERS. RIVERS
FLOWING OFF THE OLYMPIC MOUNTAINS...ESPECIALLY THE BOGACHIEL...ALSO
BEAR CLOSE WATCH. RIVERS OF THE CHEHALIS BASIN AND THE NORTH
CASCADES...NAMELY THE NOOKSACK AND THE SKAGIT...APPEAR TO FACE A
MINIMAL THREAT OF FLOODING.

THE FLOOD WATCH THAT IS IN EFFECT FOR MOST ALL OF WESTERN WASHINGTON
WILL REMAIN IN EFFECT UNTIL THE HEAVY PRECIPITATION EVENT HAS
DEFINITELY ENDED.

ANOTHER FRONTAL SYSTEM WITH LOWER SNOW LEVELS WILL MOVE THROUGH THE
FORECAST AREA FRIDAY THROUGH SATURDAY. IT WILL BE A SNOW PRODUCER
FOR THE MOUNTAINS...AND IT WILL NOT PRESENT A FLOODING CONCERN. A
COLDER AND SOMEWHAT DRIER PATTERN WILL PREVAIL THROUGH THE WEEKEND
AND EARLY NEXT WEEK...AND AGAIN NO FURTHER FLOODING IS EXPECTED.
MCDONNAL/05

&&

.AVIATION...STRONG ONSHORE FLOW THROUGH THIS EVENING WILL GRADUALLY
DIMINISH OVERNIGHT AS HIGH PRES OFFSHORE BUILDS INTO THE COASTAL
WATERS. MOISTURE ASSOCIATED WITH A STRONG COLD FRONT AND ASSOCIATED
SHORT WAVE TROUGH ALOFT THAT SWEPT SOUTHEAST ACROSS THE AREA EARLY
THIS MORNING IS DECREASING FROM THE NORTH. BY MID AFTERNOON MOST
AREAS WILL SEE SCATTERED SHOWERS...MAINLY NEAR THE TERRAIN AND WITH
A PUGET SOUND CONVERGENCE ZONE FROM ABOUT SEATTLE NORTH TO ABOUT THE
KING-SNOHOMISH COUNTY LINE. CEILINGS THAT WERE MAINLY IFR FROM THE
CENTRAL PUGET SOUND NORTHWARD WILL BECOME VFR WITH CIGS GENERALLY
ABOVE 030.

EXPECT ONSHORE FLOW...SHOWERS...AND REMNANT CONVERGENCE ZONE
ACTIVITY TO DIMINISH BEGINNING ABOUT 03Z.

KSEA...DRIER AIR ALOFT IS FILTERING INTO THE TERMINAL FROM THE
NORTH...AND THIS SHOULD RESULT IN CEILINGS LIFTING TO SCT-BKN025
BKN-OVC040 BY 20Z. EXPECT SCATTERED SHOWER ACTIVITY...MAINLY FROM
ABOUT THE TERMINAL NORTHWARD AS A CONVERGENCE ZONE PARKS OVER
NORTHERN PORTIONS OF SEATTLE AND SEATAC TO THE SOUTH OF THE ZONE.
SOUTH TO SOUTHWEST WINDS 15 KNOTS WITH GUSTS TO 25 KNOTS ARE
EXPECTED 21Z-03Z. WINDS WILL DIMINISH BEGINNING 03Z TO SOUTH 5-9
KNOTS THROUGH TONIGHT. CEILINGS TONIGHT MAINLY 035-045. ALBRECHT

&&

.MARINE...STRONG ONSHORE FLOW BEHIND A COLD FRONT THAT MOVED THROUGH
THE AREA EARLY THIS MORNING WILL CONTINUE THROUGH THIS EVENING. THE
ONSHORE FLOW WILL GRADUALLY DIMINISH OVERNIGHT AS A SURFACE RIDGE
OVER THE OFFSHORE WATERS SHIFTS EAST INTO THE COASTAL WATERS. THE
RIDGE WILL SHIFT INTO THE INTERIOR LATE THU.

A WARM FRONT MOVING INTO THE COASTAL WATERS LATE THURSDAY NIGHT WILL
INCREASE SOUTH TO SOUTHEAST FLOW. GALES ARE POSSIBLE ON THE COAST
LATE THU NIGHT INTO FRI MORNING. THE ASSOCIATED COLD FRONT WILL MOVE
ACROSS THE AREA LATER FRI AFTERNOON OR NIGHT. STRONG ONSHORE FLOW IS
POSSIBLE AGAIN LATER SAT INTO SAT NIGHT AS A DEEP TROUGH FORMS OVER
OR TO THE EAST OF THE WATERS. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS AND WATCHES ARE IN EFFECT FOR PARTS OF THE
     CWA.

     WINTER WEATHER ADVISORY ABOVE 2500 FEET UNTIL MIDNIGHT PST
     TONIGHT FOR THE CASCADES.

PZ...GALE WARNINGS ARE IN EFFECT FOR THE ADMIRALTY INLET AND
     CENTRAL/EASTERN STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING
     WATERS.

     SMALL CRAFT ADVISORY FOR ROUGH BAR UNTIL 4 PM PST THURSDAY
     FOR THE GRAYS HARBOR BAR.

&&

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