Tornado in Washington, May 27, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
924 AM PDT FRI MAY 27 2011

.SYNOPSIS...THE UPPER LEVEL TROUGH OVER WESTERN WASHINGTON TODAY
WILL PRODUCE SHOWERS AND A CHANCE OF THUNDERSTORMS. SCATTERED
SHOWERS WILL PERSIST SATURDAY AS THE TROUGH MOVES SOUTH OVER OREGON.
A WEAK UPPER LEVEL RIDGE WILL BRING DRIER WEATHER WITH A LITTLE MORE
SUNSHINE ON SUNDAY. A LARGE UPPER LEVEL LOW WILL PARK OFF THE WEST
COAST NEXT WEEK...KEEPING A CHANCE OF SHOWERS IN THE FORECAST FOR
WESTERN WASHINGTON.

&&

.SHORT TERM...AN EAST-WEST ORIENTED UPPER LEVEL TROUGH IS RIGHT OVER
W WA THIS MORNING. SATELLITE IMAGERY SHOWS A FAIRLY LONG FETCH OF
UNSTABLE LOOKING CUMULUS OFFSHORE HEADED DUE E TOWARD THE WA COAST.
SATELLITE CLOUD TOP TEMPS ON SHOW THE COLDEST TOPS ALREADY UP TO
-21C SO THE CHANCE OF THUNDERSTORMS FOR THIS AFTERNOON LOOKS ON
TRACK. RADAR SHOWS ONLY SCATTERED SHOWERS OVER THE AREA AT 830 AM...
BUT SHOWERS SHOULD INCREASE IN COVERAGE THIS AFTERNOON AS THE AIR
MASS BECOMES MORE UNSTABLE. 06Z GFS SHOWS LIFTED INDICES GETTING
DOWN TO AROUND -1 OVER THE SW INTERIOR AND PUGET SOUND AT 00Z THIS
AFTERNOON. 12Z NAM HAS INDICES BETWEEN 0 AND -1 OVER THE SAME
GENERAL AREA.

THE UPPER LEVEL TROUGH CLOSES OFF IN TO A LOW TONIGHT AND EVENTUALLY
MOVES SE OVER OREGON ON SATURDAY. MOST UNSTABLE AIR GOES WITH IT SO
NO THUNDER IS EXPECTED FOR W WA. HOWEVER MODELS SHOW SHOWERS
CONTINUING PROBABLY WITH HIGH CHANCE OR LIKELY POPS. NAM BUFR
SOUNDING FOR KSEA SHOWS AN EQUILIBRIUM LEVEL JUST BELOW 15KFT SO
CONVECTION WILL BE HEALTHY.

MODELS PAINT A MORE PESSIMISTIC PICTURE FOR SUNDAY...EVEN THOUGH A
WEAK UPPER LEVEL RIDGE WILL BE MOVING OVER THE AREA SUNDAY
AFTERNOON. MODERATE SW ONSHORE FLOW TONIGHT WILL BECOME MORE
WESTERLY SATURDAY EVENING...BUT STILL STRONG ENOUGH TO KEEP THE
LOWER AIR MASS GENERALLY MOIST THROUGH MOST OF SUNDAY. NAM BUFR
SOUNDING SHOWS A PERSISTENT CLOUD BASE AROUND 5000 FT THROUGH MOST
OF SUNDAY...WITH ENOUGH SHALLOW CONVECTION IN THE AFTERNOON TO
SUPPORT CUMULUS DEVELOPMENT. MODEL-TIME HEIGHT SECTIONS ARE EVEN
MORE PESSIMISTIC WITH THE PRESENCE OF THIS MOISTURE LAYER. WILL BACK
OFF THE MOSTLY SUNNY FORECAST IN THIS AFTERNOONS PACKAGE...AND WILL
PROBABLY GO WITH ...BECOMING PARTLY SUNNY...FOR SUNDAY AFTERNOON.
EVEN THAT MAY BE A LITTLE OPTIMISTIC. KAM

.LONG TERM...GFS/ECMWF ARE IN GOOD AGREEMENT WITH THE LARGE SCALE
PATTERN FOR NEXT WEEK. ANOTHER UPPER LEVEL LOW WILL END UP LINGERING
OFF THE W COAST FOR MOST OF NEXT WEEK...MAINTAINING SOMEWHAT MOIST
AND UNSTABLE S FLOW ALOFT OVER W WA THROUGH THE PERIOD. S FLOW ALOFT
DEVELOPS BY MONDAY WITH PERHAPS ENOUGH DIFFLUENCE OVER THE CASCADES
FOR THUNDERSTORMS TO FORM. BOTH GFS/ECMWF SHOW A SHORTWAVE TROUGH
ROTATING AROUND THE MAIN LOW AND UP OVER W WA ON TUESDAY...FOR
WIDESPREAD SHOWERS. THE PROBLEM IS THAT THE MODELS GENERALLY ARE NOT
CONSISTENT IN HANDLING THE SMALLER SHORTWAVE TROUGHS SO CONFIDENCE
IN THE CURRENT MODEL SOLUTION FOR TUESDAY IS LOW. WE WILL PROBABLY
SEE SOME SORT OF SHORTWAVE AND A RAINY DAY AT SOME POINT NEXT
WEEK...THE QUESTION IS WHEN. FOR NOW THE BROADBRUSH CHANCE OF
SHOWERS FORECAST WILL SUFFICE UNTIL THE SMALLER DETAILS ARE
RESOLVED. KAM

&&

.AVIATION...A WEAK TROUGH OF LOW PRESSURE AT THE SURFACE AND ALOFT
SITS FROM A LOW NEAR 51N 133W SE INTO THE WASHINGTON AND OREGON
OFFSHORE WATERS. THIS TROUGH WILL LIFT NE THROUGH THE AREA THIS
AFTERNOON INCREASING SHOWER ACTIVITY ACROSS THE AREA IN THE MOIST
AND UNSTABLE AIR MASS. CONVECTIVE INDICES ARE NOT OVERLY IMPRESSIVE
FOR THIS AFTERNOON AND EVENING WITH LIFTED INDICES AROUND -0.2 AND A
SKINNY CAPE OF ABOUT 250 J/KG. BUT CONVECTIVE TEMPERATURES ARE
RATHER LOW AT 52F...SO AN ISOLATED TSTM WITH THE TROUGH PASSAGE
22Z-02Z CANNOT BE RULED OUT. TERMINALS WILL NOT SHOW TSTMS AT THIS
POINT. THE AIR MASS WILL REMAIN RATHER MOIST TONIGHT AND WILL REMAIN
SOMEWHAT UNSTABLE.

KSEA...SOUTHERLY WIND 10-12 KT WILL RISE TO SW 12G25KT 20Z-03Z THEN
EASE TO S 8-10 KT TONIGHT. CIGS 015-025 WILL RISE TO BKN035-040 BY
19Z AS THE BOUNDARY LAYER HEATS UP WITH LATE MAY INSOLATION. CIGS
MAY DROP BACK TO 015-020 13Z-17Z SAT AS THE LOWER LEVELS COOL.
ALBRECHT

&&

.MARINE...A WEAK TROUGH OF LOW PRESS EXTENDS SE FROM A WEAK LOW NEAR
51N 133W. THIS TROUGH WILL LIFT NE THROUGH THE WATERS THIS
AFTERNOON. MODERATE TO STRONG ONSHORE FLOW THROUGH THIS EVENING WILL
GRADUALLY RELAX TONIGHT. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR
THE CENTRAL AND EAST STRAIT FOR WESTERLY FLOW AND ACROSS PUGET SOUND
AND HOOD CANAL FOR SOUTH TO SOUTHWEST FLOW.

A RIDGE OF HIGH PRES ALONG ABOUT 138W COMBINED WITH LOWERING PRES E
OF THE CASCADES WILL KEEP ONSHORE FLOW GOING OVER THE WEEKEND. SMALL
CRAFT ADVISORY CONDITIONS WILL BE MOST LIKELY IN THE CENTRAL AND
EAST STRAIT OF JUAN DE FUCA DURING THE LATE AFTERNOON AND EVENING
HOURS. SMALL CRAFT ADVISORY CONDITIONS FOR NW WINDS WILL BE POSSIBLE
OVER THE COASTAL WATERS AND PERHAPS IN ADMIRALTY INLET...ESPECIALLY
DURING THE LATE AFTERNOON AND EVENING HOURS.

LOW PRES AT THE SURFACE AND ALOFT WILL DROP SE INTO THE OREGON
OFFSHORE WATERS MON. EXPECT WINDS OVER THE WATERS TO BECOME VARIABLE
TO 15 KT EARLY NEXT WEEK AS THE LOW DROPS TO THE SW OF THE AREA.
ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.
     SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL.

WWW.WEATHER.GOV/SEATTLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
1158 AM PDT FRI MAY 27 2011

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

1157 AM     HAIL             5 WSW PUYALLUP          47.17N 122.38W
05/27/2011  M0.25 INCH       PIERCE             WA   TRAINED SPOTTER

&&

GRUB