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Hawaiian Islands Synoptic Discussion and Guidance

600
FXHW60 PHFO 211330
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST THU AUG 21 2014

.SYNOPSIS...
TRADE WINDS WILL CONTINUE WITH MINOR CHANGES IN SPEED THROUGH THE
WEEKEND. A BATCH OF SHOWERS MOVING IN FROM THE EAST DAMPENED MAUI
AND HAWAII COUNTIES OVERNIGHT...AND WILL CROSS THE REST OF THE STATE
INTO FRIDAY. GENERALLY LIGHTER SHOWERS THEN WILL PREVAIL OVER LEE
SLOPES OF THE BIG ISLAND EACH AFTERNOON...AND WINDWARD SLOPES ACROSS
THE STATE ALL DAY.

&&

.DISCUSSION...
A MASS OF NIMBOSTRATUS PRESSED ASHORE ALONG EAST SLOPES OF THE BIG
ISLAND AND MAUI OVERNIGHT...DROPPING RAIN THAT BRIEFLY APPROACHED
RATES OF ONE INCH AN HOUR IN PLACES. HOWEVER...MOST GAUGES RECORDED
SIX-HOUR ACCUMULATION LESS THAN TWO INCHES. THIS FEATURE LIKELY
CONSISTED OF MOISTURE DRAWN UP FROM THE DEEP TROPICS BY THE
DISTURBANCES NEAR 140W EARLIER IN THE WEEK...BUT IT NO LONGER
SPAWNED MANY TOWERING CUMULI...AND APPEARED LESS ORGANIZED THAN IT
DID ON TUESDAY EVENING. GUIDANCE REALLY EMPHASIZED THE DEVELOPMENT
OF THIS SHOWER CLUSTER EARLY ON...BUT MORE RECENT SOLUTIONS
GENERALLY HAVE DOWNPLAYED ITS SIGNIFICANCE. IN ANY CASE...THIS
SHOWER CLUSTER WILL SPREAD ENHANCED RAINFALL FURTHER WEST ACROSS THE
STATE TODAY...TONIGHT...AND INTO FRIDAY. POPS AND ASSOCIATED GRIDS
HAVE BEEN ADJUSTED ACCORDINGLY.

OTHERWISE...GUIDANCE PREDICTED THE SUBTROPICAL RIDGE TO THE NORTH TO
REMAIN IN PLACE THROUGH THE FORECAST PERIOD...KEEPING THE TRADE
WINDS BLOWING OVER THE ISLANDS. RIDGING ALOFT WILL VARY IN STRENGTH
ALMOST RANDOMLY WITH TIME...AND THE SUBTROPICAL RIDGE WILL REACT
ACCORDINGLY...BUT THE OSCILLATIONS SHOULD REMAIN CLOSE TO
CLIMATOLOGY.

SIMILARLY...AFTER THE PATCH OF NIMBOSTRATUS MOVES WEST OF THE
STATE...PRECIPITATION WILL FOCUS OVER SLOPES WHERE OROGRAPHIC
FORCING IS STRONGEST. CHANGES FROM DAY TO DAY WILL AFFECT MAINLY THE
AMOUNT OF ACCUMULATION...AND LIKELY WILL BE SMALLER IN AMPLITUDE
THAN THE MODELS SUGGEST. A BLEND OF MODEL OUTPUT PREDICTED THAT
PRECIPITABLE WATER WILL PEAK FROM SUNDAY INTO MONDAY...AND SHOWER
ACTIVITY PROBABLY WILL FOLLOW THIS TREND AS WELL...IN ACCUMULATION
IF NOT IN AREAL COVERAGE.

&&

.AVIATION...
AN AREA OF ENHANCED TROPICAL MOISTURE MOVING ACROSS THE EASTERN
AND CENTRAL PARTS OF THE STATE WILL PRODUCE INCREASED SHOWERS AND
CLOUDS TODAY INTO TONIGHT. AIRMET SIERRA REMAINS IN EFFECT FOR THE
WINDWARD SECTIONS OF THE BIG ISLAND AND MAUI...WHERE MOUNTAIN
OBSCURATION AND PERIODS OF MVFR CONDITIONS ARE LIKELY. PHTO
TERMINAL WILL ALSO SEE PERIODS OF MVFR THROUGH THIS MORNING...WITH
BRIEF IFR POSSIBLE. FURTHER WEST...INCREASED WINDWARD SHOWERS WILL
AFFECT OAHU AND POSSIBLY KAUAI AS WELL LATER TODAY INTO TONIGHT...
AND AIRMET SIERRA MAY BECOME NECESSARY.

VFR CONDITIONS WILL PREVAIL ACROSS LEEWARD AREAS THROUGH THE
PERIOD...WITH BRIEF MVFR POSSIBLE IN PASSING SHOWERS.

&&

.MARINE...
WINDS LIKELY WILL PERSIST JUST BELOW THE THRESHOLD FOR A SMALL CRAFT
ADVISORY THROUGH THE REST OF THE WEEK.

A NEW S SWELL ARRIVING THIS MORNING LOOKS LIKE IT WILL PRODUCE SURF
JUST A BIT OVER THE CRITERION FOR A HIGH SURF ADVISORY OVER MOST
SOUTH-FACING SHORES INTO FRIDAY. A SERIES OF SOUTH SWELLS WILL
CONTINUE THROUGH EARLY NEXT WEEK...BUT SHOULD REMAIN BELOW ADVISORY
LEVELS. SHORT-PERIOD TRADE-WIND SWELL WILL CONTINUE ALONG
EAST-FACING SHORES. NO SIGNIFICANT NORTHWEST SWELL IS EXPECTED
THROUGH THE WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

DISCUSSION/MARINE...RYSHKO
AVIATION...JACOBSON



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