Avalanche in Washington, April 27, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT SUN APR 27 2014

.SYNOPSIS...A COLD UPPER LEVEL TROUGH WILL MOVE THROUGH WESTERN
WASHINGTON PRODUCING SHOWERS AND A POSSIBILITY OF THUNDERSTORMS
WITH SMALL HAIL. SNOW SHOWERS ARE EXPECTED IN THE MOUNTAINS. THE
TROUGH WILL MOVE EAST TONIGHT. AN UPPER RIDGE WILL BUILD OVER THE
PACIFIC NORTHWEST MONDAY THROUGH THURSDAY FOR DRY AND MUCH WARMER
WEATHER. COOLER AND CLOUDY WEATHER IS FORECASTED FOR LATE IN THE
WEEK AS THE LOW LEVEL FLOW TURNS BACK ONSHORE.

&&

.SHORT TERM...A VERY STRONG JET STREAM CENTERED ALONG 43N IS AIMED
AT THE SOUTHERN OREGON COAST. WESTERN WASHINGTON IS SEEING
WIDESPREAD SHOWERS AS A VIGOROUS AND COLD UPPER LEVEL TROUGH SWINGS
THROUGH THE AREA AND BRINGS RATHER STRONG LOW LEVEL ONSHORE FLOW.
THE CENTRAL PUGET SOUND AREA IS SEEING MUCH LESS SHOWER ACTIVITY AS
WESTERLY FLOW ALOFT INTERACTS WITH THE OLYMPICS GIVING SOME RAIN
SHADOWING. TEMPERATURES ARE GENERALLY IN THE 40S.

THE AIR MASS IS UNSTABLE. 500 MB TEMPERATURES ARE AROUND -32C AND
850 MB TEMPERATURES ARE -2C TO -3C. THE SKINNY CAPE PROFILE WITH
CAPE VALUES 300-600 J/KG COMBINED...MODERATE TO LOW SPEED SHEAR AND
LOW DIRECTIONAL SHEAR FAVORS ISOLATED PULSE THUNDERSTORMS -TYPICAL
PACIFIC NORTHWEST SPRING ONE STROKE WONDERS- THAT PRODUCE SOME SMALL
HAIL OR GRAUPEL. CONVECTIVE TEMPERATURES ARE ONLY IN THE MID TO
UPPER 40S AND SHOULD BE EASILY REACHED. WHILE ISOLATED THUNDERSTORMS
CAN OCCUR JUST ABOUT ANYWHERE...THE BEST CHANCE FOR MORE LONG
LASTING AND ORGANIZED THUNDERSTORMS WILL BE THIS AFTERNOON AND
EVENING WITH A PUGET SOUND CONVERGENCE ZONE THAT WILL LIKELY BUILD
WESTWARD FROM THE CASCADES AS FLOW GRADUALLY RELAXES. THE LOCATION
AND PERSISTENCE OF ANY CONVERGENCE ZONE THAT DEVELOPS WILL BE
CRITICAL FOR SNOWFALL AMOUNTS IN THE CASCADES ABOVE ABOUT 3000 FEET
THIS AFTERNOON. THE 00Z WRFGFS 4KM SOLUTION AND THE 12Z NAM12
SOLUTION BOTH POINT TO THE POTENTIAL OF A QUICK 6-10 INCHES OF SNOW
AROUND STEVENS PASS THIS AFTERNOON AND EVENING. THE MIDNIGHT SHIFT
ISSUED A SPECIAL WEATHER STATEMENT FOR THIS OVERNIGHT. A WINTER
WEATHER ADVISORY WILL BE ISSUED IF THE CONVERGENCE ZONE DEVELOPS
NEAR THE PASSES. THE ZONE FORECASTS AND GRIDS WERE UPDATED TO
INCLUDE A SLIGHT CHANCE OF TSTMS AND SMALL HAIL THIS AFTERNOON AND
EVENING OVER THE ENTIRE FORECAST AREA.

THE UPPER LEVEL TROUGH WILL MOVE EAST TONIGHT RESULTING IN SHOWERS
TAPERING OFF AND THE CONVERGENCE ZONE DISSIPATING AROUND DAYBREAK.
TEMPERATURES TONIGHT WILL DROP INTO THE MID 30S IN NORMALLY COLDER
OUTLYING VALLEYS.

AN UPPER RIDGE WILL START BUILDING OVER THE AREA LATER MONDAY
THROUGH TUESDAY. THERE ARE STILL SOME TROUBLING MODEL SOLUTIONS THAT
SHOW A WARM FRONT MOVING INTO WESTERN OREGON THEN DISSIPATING AS IT
MOVES INTO OUR AREA EARLY TUESDAY MORNING...BUT OVERALL THE MODEL
CONCENSUS APPEARS TO BE THAT PRECIPITATION WILL REMAIN OFFSHORE AND
LIGHT AS UPPER RIDGING RAPIDLY BUILDS OVER THE AREA AND HIGH
PRESSURE BUILDING OVER SE BRITISH COLUMBIA AND EASTERN WASHINGTON
GIVES INCREASING OFFSHORE FLOW. DRYING AND WARMING INDICATED BY THE
GOING FORECAST LOOKS GOOD.

OTHER THAN THE UPDATE FOR A SLIGHT CHANCE OF TSTMS THRU THIS
EVENING...CURRENT FORECASTS LOOK GOOD. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...00Z EXTENDED
MODELS CONTINUING WITH THE VERY WARM WEATHER SCENARIO FOR WEDNESDAY
AND THURSDAY. STRONG OFFSHORE FLOW AT THE LOWER LEVELS WITH EVEN THE
850 MB WINDS DUE EASTERLY AT 10 TO 20 KNOTS WEDNESDAY AND WEDNESDAY
NIGHT. MODEL 850 MB TEMPERATURES WARM SIGNIFICANTLY...TO PLUS 11-13C
ON WEDNESDAY AND THEN GO EVEN A COUPLE OF DEGREES WARMER ON
THURSDAY. 500 MB HEIGHTS REMAIN IN THE 580-585 DM RANGE BOTH DAYS.
WILL STAY WITH THE FORECAST OF 70S TO LOWER 80S ON WEDNESDAY AND A
COUPLE OF DEGREES WARMER ON THURSDAY. CURRENT FORECAST HAS A RECORD
TYING 81 DEGREES FOR SEATTLE ON THURSDAY.

MODEL CONSISTENCY FALLS APART ON FRIDAY AND CONFIDENCE IN THE
FORECAST IS NOT VERY HIGH AT THIS POINT. ALL OF THE MODELS WANT TO
WEAKEN THE UPPER LEVEL RIDGE BUT THE SIMILARITIES STOP THERE. THE
GFS IS GOING FOR A GOOD MARINE PUSH ON FRIDAY. THE ECMWF ALSO TURNS
THE LOW LEVEL FLOW ONSHORE BUT MUCH WEAKER AND LATER ON FRIDAY. THE
CANADIAN KEEPS THE SURFACE GRADIENTS OFFSHORE WITH A NEGATIVELY
TILTED TROUGH APPROACHING THE OREGON COAST. CURRENT FORECAST IS A
MESH OF THE GFS/ECMWF WITH SOMEWHAT OF A MARINE PUSH ON FRIDAY WITH
COOLER TEMPERATURES. MODEL SOLUTIONS CONTINUE TO BE INCONSISTENT FOR
SATURDAY WITH THE ECMWF SWINGING A SHORTWAVE OUT OF AN UPPER LEVEL
LOW OFF THE COAST OVER THE AREA WHILE THE CANADIAN HAS THE LOW
FURTHER SOUTH MOVING THE SHORTWAVE INTO OREGON. THE GFS HAS THE LOW
MUCH FURTHER NORTH LEAVING WESTERN WASHINGTON IN ZONAL FLOW WITH
STRONG ONSHORE GRADIENTS. WITH THE LACK OF CONSISTENCY WILL STAY
WITH THE CURRENT FORECAST WHICH GOES WITH THE ECMWF SOLUTION WITH A
CHANCE OF SHOWERS ON SATURDAY. FELTON

&&

.HYDROLOGY...RIVER FLOODING IS UNLIKELY FOR THE NEXT 7 DAYS.

AT 830 AM...THE NORTH FORK OF THE STILLAGUAMISH NEAR ARLINGTON WAS
AT 4.2 FT/2325 CFS. THE NORTH FORK OF THE STILLAGUAMISH RIVER NEAR
OSO WAS AT 216.1 FT/1535 CFS. THE POOL EAST OF THE SLIDE WAS 279.3
FT.

THE RIVER WILL RISE THROUGH THIS EVENING BEFORE LEVELING OFF AGAIN
THEN RECEDING ON MONDAY. THE RISE IN THE RIVER IS EXPECTED TO BE
ABOUT 400 TO 700 CFS.

ABOUT 0.25 INCHES OF RAIN FELL OVER THE SR 530 SLIDE AREA LAST NIGHT
WITH THE FRONTAL SYSTEM. ANOTHER 0.25 TO 0.40 INCHES CAN BE EXPECTED
TODAY INTO TONIGHT. WHILE IT IS MOST LIKELY THAT CONVERGENCE ZONE
ACTIVITY WILL REMAIN SOUTH OF THE SLIDE AREA...THERE IS A SLIGHT
CHANCE THAT THE CONVERGENCE ZONE WILL SET UP OVER THE SLIDE AREA
LATE THIS AFTERNOON INTO THIS EVENING WHICH WOULD ENHANCE THE
PRECIPITATION AMOUNTS BY ANOTHER 0.25-0.50 INCHES. THERE IS A THREAT
OF A THUNDERSTORM THROUGH ABOUT 8 PM PDT THIS EVENING. ALBRECHT

&&

.AVIATION...AN UPPER TROF OVER THE AREA WILL CONTINUE MOVING SLOWLY
E. THE TROF WILL BE E OF THE AREA BY MON MORNING. WLY FLOW ALOFT AND
LOW LEVEL ONSHORE FLOW.

THE AIR MASS WILL BE UNSTABLE ENOUGH TODAY FOR ISOLATED TSTMS...
PROBABLY MORE NUMEROUS WITHIN THE PUGET SOUND CONVERGENCE ZONE
/PSCZ/ THAT WILL DEVELOP THIS AFTERNOON NEAR THE KING/SNOHOMISH
COUNTY BORDER. THE TSTMS WILL LIKELY BE ACCOMPANIED BY SMALL HAIL.
EXPECT THE AIR MASS TO STABILIZE ABOUT SUNSET.

CIGS WILL BE MAINLY IN THE 4-6K FT RANGE EXCEPT IN THE 2-3K FT RANGE
WITH SHWRS/TSTMS. MTNS WILL BE OCCASIONALLY OBSCD.

KSEA...THERE IS ABOUT A 30 PERCENT CHANCE OF A TSTM AT THE TERMINAL
TODAY...MOST LIKELY BETWEEN 2000 UTC AND 0100 UTC MON. THE TSTMS
WILL MOVE FROM THE SW TO NE. MEANWHILE...ANTICIPATE SLY WINDS OF
10-15 KT WITH OCCASIONAL GUSTS NEAR 20 KT TODAY.

&&

.MARINE...
ONSHORE OR WESTERLY FLOW WILL PREVAIL TODAY IN THE WAKE OF AN
OCCLUDED FRONT THAT MOVED ACROSS THE AREA LAST NIGHT. HIGH PRESSURE
WILL BUILD OVER THE AREA ON MON FOR LIGHT NORTHERLY FLOW. EXPECT
OFFSHORE FLOW TO CONTINUE ON TUE.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL
       WATERS...CENTRAL STRAIT OF JUAN DE FUCA...AND THE PUGET
       SOUND/HOOD CANAL.

     SMALL CRAFT ADVISORY IN EFFECT FOR ROUGH BAR UNTIL 6 AM
       MONDAY FOR THE GRAYS HARBOR BAR.

&&

WWW.WEATHER.GOV/SEATTLE

FOR THE GRAPHICAL AFD...VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
1054 AM PDT SUN APR 27 2014

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1045 AM     HAIL             5 WSW PUYALLUP          47.17N 122.38W
04/27/2014  E0.25 INCH       PIERCE             WA   TRAINED SPOTTER

&&

GRUB