Funnel Clouds in Washington, August 25, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 AM PDT MON AUG 25 2008

.SYNOPSIS...A TROUGH OF LOW PRESSURE ALOFT WILL MOVE ACROSS THE AREA
TODAY PRODUCING SCATTERED SHOWERS. SHOWERS WILL END THIS EVENING AS
THE TROUGH MOVES EAST. WEAK HIGH PRESSURE ON TUESDAY WILL RESULT IN
DRY WEATHER. ANOTHER FRONTAL SYSTEM WILL BRING MORE RAIN TO THE AREA
LATE TUESDAY NIGHT AND WEDNESDAY. WESTERLY FLOW WILL CONTINUE INTO
THE WEEKEND WITH A WEAK UPPER TROUGH DEVELOPING OVER THE NORTHWEST
FOR THE HOLIDAY WEEKEND.

&&

.SHORT TERM...STEADY PRECIPITATION PRODUCED BY THE FRONTAL SYSTEM
THAT MOVED ACROSS THE AREA SUNDAY AFTERNOON IS NOW MOVING EAST OF
THE CASCADE CREST. AN EASTWARD MOVING TROUGH OF LOW PRESSURE ALOFT
CURRENTLY CENTERED NEAR 130W WILL MOVE ACROSS THE AREA LATER THIS
AFTERNOON AND EVENING. THE COMBINATION OF SOME DECREASE IN STABILITY
WITH THE UPPER TROUGH...AND LOW LEVEL ONSHORE FLOW...WILL PRODUCE
SCATTERED SHOWERS TODAY. THE SHOWERS WILL BE MOST LIKELY IN THE
CASCADES. MESOSCALE MODELS INCLUDING THE NAM AND THE MM5 SHOW
SHOWERS IN THE LOWLANDS PRIMARILY ASSOCIATED WITH A WEAK PUGET SOUND
CONVERGENCE ZONE THAT IS EXPECTED TO SET UP FROM THE CENTRAL KITSAP
PENINSULA EAST ACROSS THE SEATTLE AREA AND INTO THE CENTRAL CASCADE
PASSES.

BOTH THE NAM AND THE MM5 KEEP SOME SHOWERS GOING INTO THIS EVENING
IN A LINGERING CONVERGENCE ZONE DESPITE THE DECREASE IN LOW LEVEL
ONSHORE FLOW AND BUILDING HEIGHTS ALOFT. WITH THE TYPICAL LINGERING
NATURE OF THESE CONVERGENCE ZONES...WILL GIVE INTO ISOLATED SHOWERS
THIS EVENING IN THE CENTRAL PUGET SOUND AREA AND IN THE CASCADES.

AFTER SOME MORNING CLOUDS...TUESDAY WILL BE A PARTLY SUNNY DAY.
MORNING CLOUDS...LINGERING LOW LEVEL MOISTURE FROM RECENT
RAINFALL...AND LOWER LATE AUG SOLAR ANGLES WILL INHIBIT A RAPID
WARMING TUE AFTERNOON...SO WILL GIVE INTO THE MODEL AND MOS GUIDANCE
AND KEEP MOST AFTERNOON TEMPERATURES ACROSS W WA IN THE 60S. A
QUICKER CLEARING COULD RESULT IN 2-4 DEG FURTHER WARMING THAN WILL
BE SHOWN IN THE UPCOMING FORECAST.

IT IS ALMOST AS FALL IS STARTING A COUPLE WEEKS EARLY AS A VERY LOW
AMPLITUDE RIDGE DEVELOPS AROUND 130W AND A STRONG 130-140 KT 300 MB
JET PLOWS INTO SOUTH CENTRAL VANCOUVER ISLAND LATE TUE NIGHT AND
WED. 00Z AND 06Z GLOBAL MODEL SOLUTIONS CONTINUE TO SHOW WHAT
APPEARS TO BE WARM ADVECTION PRECIPITATION MOVING ACROSS THE AREA.
WITH THE STRENGTH OF THE WESTERLY FLOW ALOFT...IT APPEARS THAT THE
HIGHEST POPS AND PRECIPITATION AMOUNTS WOULD OCCUR ON THE NORTH
COAST AND IN THE MOUNTAINS. THE PUGET SOUND LOWLANDS WILL BE MAINLY
SHADOWED BY THE OLYMPICS AND MOUNTAINS OF VANCOUVER ISLAND. ALBRECHT

.LONG TERM...A PROGRESSIVE RIDGE MOVES ACROSS THE AREA THU INTO
FRI...AND IS FOLLOWED BY A WEAK TROUGH AND RATHER STRONG ONSHORE
FLOW NEXT WEEKEND. THE ECMWF IS A BIT MORE PESSIMISTIC NEXT WEEKEND
THAN THE GFS...BRINGING SOME PRECIPITATION TO THE AREA. CURRENT
FORECASTS ARE DRY IN LINE WITH THE GFS. CONFIDENCE IS NOT STRONG
ENOUGH TO MAKE CHANGES AT THIS TIME. ALBRECHT

&&

.AVIATION...SHOWERS STILL LINGERING ACROSS THE INTERIOR AND CASCADES
THIS MORNING WITH LOW CLOUDS MAINLY AFFECTING THE INTERIOR TAF
SITES...WITH PATCHY FOG ALONG THE NORTH COAST. THE UPPER LEVEL
TROUGH OFFSHORE WILL MOVE INLAND AND PASS THROUGH THE AREA THIS
AFTERNOON WITH SCATTERED SHOWERS DEVELOPING. SHOWERS THIS EVENING
WILL MAINLY AFFECT THE PUGET SOUND AREA AS MODELS ARE HINTING AT A
PUGET SOUND CONVERGENCE ZONE. LOW CIGS AND VIS WILL BE FOUND IN THIS
AREA THROUGH EARLY TUE.

KSEA...LOW CLOUDS WILL HANG AROUND THROUGH THIS MORNING THEN
GRADUALLY LIFT BY THE AFTERNOON. INCREASING SHOWERS EXPECTED DURING
THE AFTERNOON AS THE UPPER TROUGH AXIS PASSES THROUGH. SHOWERS MAY
PERSIST INTO THE NIGHT AS PSCZ DEVELOPS. AS A RESULT...WINDS WILL BE
VARIABLE BUT NOT EXPECTING SPEEDS IN EXCESS OF 10 KT. 33

&&

.MARINE...ONSHORE FLOW WILL PERSIST ACROSS W WA TODAY AS HIGH PRES
BUILDS OFFSHORE. PRES GRADIENT THROUGH THE STRAIT OF JUAN DE FUCA
WILL PEAK LATE THIS AFTERNOON AND EARLY EVENING WITH WLY WINDS
RISING TO 20 TO 30 KT THROUGH THE CENTRAL AND EAST PORTIONS. DRY
WEATHER EXPECTED ON TUE WITH ONSHORE FLOW CONTINUING.

BY WED...NEXT SYSTEM IS SLATED TO SLIDE SOUTH THROUGH BC AND INTO W
WA. THIS SYSTEM WILL BRING INCREASING RAIN TO THE AREA AS WELL AS AN
INCREASE IN SOUTHERLY FLOW...WITH SMALL CRAFT WINDS POSSIBLE THROUGH
THE INTERIOR WATERS. SEAS WILL BUILD TO NEAR 10 FT BY THU WITH
POSSIBLE HAZARDOUS SEAS HEADLINES FOR THE COASTAL WATERS. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT OF JUAN DE FUCA
     AND ADMIRALTY INLET.