Severe Thunderstorm Warning 4, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT WED JUL 2 2008

.SYNOPSIS...SOUTHERLY FLOW ALOFT WILL BRING A MOISTER AND UNSTABLE
ATMOSPHERE TO WESTERN WASHINGTON THROUGH THURSDAY. A BROAD UPPER
LEVEL LOW PRESSURE TROUGH WILL CROSS THE REGION ON INDEPENDENCE DAY.
ANOTHER TROUGH WILL PASS BY ON SUNDAY. FLAT UPPER LEVEL HIGH
PRESSURE RIDGING SHOULD THEN DOMINATE FOR THE FIRST HALF OF NEXT
WEEK.

&&

.SHORT TERM...AN ACTIVE EVENING SHAPING UP ACROSS PARTS OF WESTERN
WASHINGTON WITH THUNDERSTORMS AFFECTING THE
CASCADES...FOOTHILLS...AND PORTIONS OF THE LOWLANDS INCLUDING THE
PUGET SOUND REGION. STORMS THAT FORMED EARLIER IN THE DAY OVER THE
CASCADES WERE LIKELY MORE SURFACE BASED IN ORIGIN DURING THE MAX
DAYTIME HEATING. HOWEVER...WITH THE UPPER TROUGH MOVING CLOSER TO
THE COAST...SHORT WAVES EMBEDDED IN SLY FLOW ARE LIFTING ACROSS THE
AREA PROVIDING ADDITIONAL DYNAMICS. MODELS SHOW DEEP SLY FLOW
THROUGH TONIGHT AND THURSDAY MORNING LEAVING THE DOOR OPEN TO
ELEVATED CONVECTION JUST ABOUT ANYWHERE. RADAR IMAGERY SHOWS ECHOS
INCREASING AGAIN OVER THE SOUTH WASHINGTON CASCADES WHICH WILL
LIKELY PROPAGATE NORTH OVER THE NEXT COUPLE HOURS. UPDATED THE ZONES
EARLIER TO INCLUDE A CHANCE OF SHOWERS AND THUNDERSTORMS OVER ALL OF
WESTERN WASHINGTON THROUGH TONIGHT.

BOTH THE GFS/NAM SHOW A STRONGER SHORTWAVE TO LIFT ACROSS THE AREA
THURSDAY MORNING. THIS SHOULD KEEP THE THREAT OF ELEVATED CONVECTION
GOING ACROSS MOST THE AREA. THE AFTERNOON IS TRICKY WITH A LACK OF
ANY WELL DEFINED UPPER LEVEL VORT FEATURES AND COOL LOW LEVEL
ONSHORE FLOW. UPPER LEVEL DYNAMICS WOULD PROBABLY BE NEEDED IN THE
AFTERNOON FOR ADDITIONAL THUNDERSTORMS OVER THE LOWLANDS. KEEPING
THUNDERSTORMS LIMITED TO THE MOUNTAINS THURSDAY AFTERNOON/EVENING
SEEMS REASONABLE.

THE GFS/ECMWF BOTH SHOW A COUPLE OF SHORT WAVES TO ROLL THROUGH THE
AREA ON INDEPENDENCE DAY. SOME SHOWER ACTIVITY SEEMS INCREASINGLY
LIKELY OVER PARTS OF WESTERN WASHINGTON BUT WHERE AND EXACTLY WHEN
ARE UNCLEAR. MERCER

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...BOTH THE GFS AND ECMWF
SEEM TO AGREE THAT FLAT UPPER RIDGING WILL BUILD IN FOR THE FIRST
HALF OF NEXT WEEK WITH 500 MB HEIGHTS RISING INTO THE 580+ DAM
RANGE. THIS WILL CAUSE A REBOUND OF INTERIOR LOWLAND TEMPERATURES
BACK TO NEAR 80. SHALLOW MARINE CLOUDS WILL BE CONFINED TO THE
IMMEDIATE COASTLINE.  HANER

&&

.AVIATION...ONE BAND OF THUNDERSTORMS FROM THE HOOD CANAL BRIDGE TO
N OF SNOQUALMIE PASS AT 03Z WILL MOVE NORTH TO THE CANADIAN BORDER
AROUND 0430Z. RADAR AND LIGHTNING DETECTION DATA SHOW ANOTHER BAND
OF TSTMS DEVELOPING E OF KKLS AND MOVING NORTHWARD. THESE STORMS
SHOULD MOVE THROUGH THE CENTRAL PUGET SOUND AREA BETWEEN 05Z AND 07Z.

STRONG PRESSURE GRADIENTS DEVELOPED THIS EVENING WITH KOTH-KSEA
GRADIENTS BRIEFLY EXCEEDING 5 MB. INFLOW THROUGH THE STRAIT AND
CHEHALIS GAP IS ALSO ALLOWING LOW LEVEL MARINE AIR TO ADVECT INLAND
OVERNIGHT. GIVEN THE VERY HIGH IN SITU DEWPOINTS 55-63 DEG F THIS
EVENING...IT APPEARS THAT COASTAL STRATUS...NOW AT KHQM AT OVC006
WILL SPREAD INLAND ALL THE WAY TO KSEA AND KPAE. CEILINGS MAY BE
LIFTED 200-300 FT WITH THE UNSTABLE MID LEVEL ATMOSPHERE...BUT IT
DOES APPEAR THAT MANY OF THE TERMINALS WILL SEE IFR CONDITIONS
13Z-17Z TIME FRAME. WHILE MODEL SOUNDINGS DO NOT SHOW THESE LOW
CONDITIONS...THEY APPEAR LIKELY GIVEN GRADIENTS AND DEWPOINTS AND
ARE INDICATED IN MODEL OUTPUT STATISTICS FROM THE NGM AND ETA. LOW
CLOUDS WILL EVAPORATE BY 18Z IN THE INTERIOR LEAVING ABUNDANT CLOUDS
060-100 DURING THE AFTERNOON AND EVENING HOURS AND A CHANCE OF
SHOWERS.

KSEA...WIND PREDOMINATELY SLY 4-7KT THROUGH 20Z THEN 24008KT 20Z
ONWARD. TSTM ACTIVITY NOW DEVELOPING AND MOVING INTO CENTRAL LEWIS
COUNTY AT 0330Z TO MOVE TO NEAR THE TERMINAL ABOUT 05Z AND CONTINUE
FOR ABOUT 2 HRS. STRATUS WILL DEVELOP INTO THE TERMINAL AROUND 13Z
AND AT THIS TIME WILL INDICATE BKN008 OVC060 13Z-17Z. ALBRECHT

&&

.MARINE...STRONG ONSHORE GRADIENTS ARE GIVING A GOOD MARINE PUSH TO
THE AREA THIS EVENING. GRADIENTS APPEAR TO BE STRONGLY RELATED TO
DIURNAL TEMPERATURE DIFFERENCES BETWEEN LAND AND SEA AND CONVECTION
MOVING NORTH THROUGH THE AREA. GRADIENTS WILL DECREASE RAPIDLY AFTER
09-12Z. AT THIS POINT WILL EXPAND THE SMALL CRAFT ADVISORY IN THE
CENTRAL STRAIT TO INCLUDE THE EAST ENTRANCE. OTHER WATERS WILL STAY
WELL BELOW SMALL CRAFT ADVISORY CRITERIA. THERE IS A RATHER HIGH
THREAT OF LIGHTNING FOR THE INLAND WATERS OVERNIGHT. FOG WILL BE
PATCHY LATE TONIGHT AND EARLY THU. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
PZ...SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA AND EAST
ENTRANCE.

WEATHER.GOV/SEATTLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
546 PM PDT WED JUL 02 2008

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

0500 PM     HAIL             1 SW NORTH BEND         47.49N 121.80W
07/02/2008  M0.25 INCH       KING               WA   TRAINED SPOTTER

0500 PM     HAIL             4 WNW NORTH BEND        47.52N 121.85W
07/02/2008  E0.50 INCH       KING               WA   TRAINED SPOTTER

            SPOTTER ESTIMATED HAIL TO BE LARGER THAN PEA...ABOUT DIME
            SIZED HAIL.

&&

DDAMICO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
639 PM PDT WED JUL 02 2008

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

0610 PM     HAIL             ENUMCLAW                47.20N 121.97W
07/02/2008  E0.50 INCH       KING               WA   PUBLIC

            DIME SIZE HAIL REPORTED WITH HEAVY DOWNPOURS. VISIBILITY
            REDUCED TO 1/4SM OR LESS.

&&

DDAMICO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
702 PM PDT WED JUL 02 2008

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

0640 PM     HAIL             3 SSW MAPLE VALLEY      47.36N 122.06W
07/02/2008  E0.50 INCH       KING               WA   TRAINED SPOTTER

            1/4 INCH TO 1/2 INCH SIZE HAIL.

&&

DDAMICO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
1014 PM PDT WED JUL 02 2008

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

0942 PM     LIGHTNING        4 WNW ENUMCLAW          47.23N 122.06W
07/02/2008                   KING               WA   TRAINED SPOTTER

            FREQUENT LIGHTNING AND HEAVY RAINFALL.

1000 PM     LIGHTNING        1 SW ENUMCLAW           47.19N 121.99W
07/02/2008                   KING               WA   TRAINED SPOTTER

            FREQUENT LIGHTNING. POWER OUT.

1011 PM     LIGHTNING        MAPLE VALLEY            47.40N 122.03W
07/02/2008                   KING               WA   TRAINED SPOTTER

            FREQUENT LIGHTNING. POWER WENT OUT FOR LESS THAN ONE
            MINUTE. KING-181.

&&

BUEHNER