Excessive Heat in Washington, July 27 to 30, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT MON JUL 27 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL PRODUCE HOT DRY CONDITIONS
ACROSS THE REGION THIS WEEK. TEMPERATURES WILL REMAIN MUCH ABOVE OUR
SEASONAL NORMALS AND NEAR RECORD VALUES. THIS HOT SPELL COULD LAST
INTO THURSDAY...WITH SLIGHTLY COOLER WEATHER EXPECTED OVER THE
WEEKEND.

&&

.SHORT TERM...A STRONG UPPER RIDGE REMAINS CENTERED JUST OFF THE
COAST WITH LIGHT NORTHERLY FLOW CONTINUING ALOFT OVER WESTERN
WASHINGTON. AT THE SURFACE A THERMALLY INDUCED TROUGH OF LOW
PRESSURE STRENGTHENED SOMEWHAT ALONG THE COAST OVERNIGHT. HIGH
TEMPERATURES TODAY WILL BE MOSTLY IN THE MID 80S TO MID 90S ACROSS
THE INTERIOR LOWLANDS AND ROUGHLY TEN DEGREES COOLER ALONG THE
COAST. A WEAK VORTICITY CENTER MOVING SOUTHWEST DOWN THE EAST SIDE
OF THE RIDGE IN SOUTHERN BRITISH COLUMBIA COULD PRODUCE ISOLATED
THUNDERSTORMS OVER THE NORTHERN CASCADES.

THE UPPER RIDGE WILL HARDLY MOVE FROM ITS POSITION OVER THE OFFSHORE
WATERS THROUGH WEDNESDAY. THE SURFACE THERMAL TROUGH WILL BE MORE OR
LESS ALONG THE COASTLINE EACH MORNING...SHIFT INTO THE INTERIOR EACH
AFTERNOON DUE TO THE DAYS HEATING...THEN RETREAT TO THE COAST AT
NIGHT. IT STILL LOOKS LIKE WEDNESDAY WILL BE THE WARMEST DAY OF THIS
UNUSUALLY LONG STRETCH OF HOT WEATHER...AS THE LOWER REACHES OF THE
ATMOSPHERE WARM A LITTLE EACH DAY AND THE FLOW BECOMES A BIT MORE
EASTERLY.

THE EXCESSIVE HEAT WARNING AND AIR STAGNATION ADVISORY ARE IN EFFECT
THROUGH THURSDAY...AND I DONT SEE ANY REASON THAT THINKING WILL NOT
REMAIN VALID.

MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING
ISSUANCE OF THE FORECAST DISCUSSION -- FORECASTING THE END OF THE
HOT STREAK STILL TRICKY AS THE MODELS STILL KEEP THE INVERTED SFC
TROUGH OVER THE REGION THROUGH FRI AND INTO THE WEEKEND. 500 MB
HEIGHTS HOWEVER DO FALL A BIT AS AN UPPER CLOSED LOW DEVELOPS OFF
THE CA COAST...WITH TROUGHING OVER THE CENTRAL US. HOWEVER STILL DO
NOT SEE A STRONG MARINE PUSH IN THE FORECAST MEANING WE COULD SEE A
FEW MORE DAYS IN THE MID 70S TO 80S. THIS COULD EASILY CHANGE
THOUGH. OTHER CONCERN OVER THE WEEKEND IS MORE T-STORMS OVER THE
MOUNTAINS AS THE UPPER LOW SPREADS MID LEVEL MOISTURE INTO THE
REGION. COULD POSE ADDITIONAL PROBLEMS WITH MORE WILD FIRES /SEE
FIRE WEATHER SECTION BELOW/. 33

&&

.AVIATION...A STRONG RIDGE OF HIGH PRES ALOFT OVER THE COASTAL
WATERS AND A DEPARTING UPPER LOW OVER IDAHO WILL GIVE LIGHT NORTH
WIND ALOFT. A THERMALLY INDUCED TROUGH OF LOW PRES ALONG THE OREGON
COAST WILL BUILD NORTH ONTO THE WASHINGTON COAST TODAY THROUGH TUE.

PATCHY FOG ALONG THE CENTRAL WA COAST WILL DISSIPATE BY 18Z...THEN
EXPECT CLEAR CONDITIONS ACROSS THE AREA. A WEAK UPPER DISTURBANCE
MOVING SOUTH OUT OF BRITISH COLUMBIA INTO THE NORTH WA CASCADES WILL
GIVE A FEW TOWERING CU THERE LATE THIS AFTERNOON AND EVENING...WITH
TSTM ACTIVITY MOST LIKELY EAST OF THE CASCADE CREST.

KSEA...LIGHT WIND BECOMING N 6-10 KT 20Z THEN BECOMING NE 3-5 KT
AFTER 03Z. CLEAR SKIES AND VISIBILITIES GREATER THAN 10 SM. ALBRECHT

&&

.MARINE...A THERMALLY INDUCED TROUGH OF LOW PRES ALONG THE OREGON
COAST WILL BUILD NORTH ONTO THE WASHINGTON COAST TODAY THROUGH TUE.
THE TROUGH WILL SHIFT INTO THE INLAND WATERS WED AND THU...THEN PUSH
EAST OF THE CASCADES FRI.

EXPECT NW WIND 10 TO 20 KT OVER THE OUTER COASTAL WATERS TO DOMINATE
WITH LIGHTER NW WIND NEAR-SHORE. DIURNALLY DRIVEN WINDS WITH W WIND
10 TO 20 KT DURING THE EVENING HOURS WILL DOMINATE IN THE STRAIT OF
JUAN DE FUCA WHILE MAINLY LIGHT NLY TO NWLY FLOW WILL DOMINATE IN
THE INLAND WATERS. ALBRECHT

&&

.FIRE WEATHER...THE PROLONGED SPELL OF HOT WEATHER FOR THIS UPCOMING
WEEK CONTINUES IN THE LATEST MODEL TRENDS. WITH ALL THE
CLOUD-TO-GROUND LIGHTNING STRIKES OF THE PAST FEW DAYS IT APPEARS
VERY LIKELY THAT NEW WILDFIRES WILL POP OUT AS THE DAYS GET WARMER
AND DRIER. WITH THE THERMAL TROUGH DEVELOPING OVER WESTERN
WASHINGTON BY MID WEEK...THE AIR MASS WILL BECOME CRITICALLY DRY AND
UNSTABLE AS WELL...ADDING THE POSSIBILITY OF RED FLAG CONDITIONS TO
THE WILDLAND FIRE ENVIRONMENT. PRANGE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...EXCESSIVE HEAT WARNING SEATTLE...TACOMA...BREMERTON...
     EVERETT AND VICINITY...EAST PUGET SOUND LOWLANDS THROUGH
     THURSDAY.
     HEAT ADVISORY SW INTERIOR...HOOD CANAL...LOWER
     CHEHALIS VALLEY THROUGH THURSDAY.
     AIR STAGNATION ADVISORY INTERIOR LOWLANDS EVERETT SOUTHWARD
     THROUGH THURSDAY.
PZ...NONE.

WWW.WEATHER.GOV/SEATTLE