Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 030639
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
840 PM HST TUE SEP 2 2014

.SYNOPSIS...
LIGHT TO MODERATE TRADE WINDS WILL CONTINUE FOR MUCH OF THE UPCOMING
WEEK...AND LIKELY INTO THE WEEKEND. THE TRADE WINDS WILL FOCUS
CLOUDS AND SHOWERS OVER WINDWARD SLOPES AND COASTS...MAINLY DURING
NIGHTS AND MORNINGS. HOWEVER...THE WINDS WILL BE LIGHT ENOUGH TO
ALLOW AFTERNOON SEA BREEZES TO DRIVE CLOUD AND SHOWER FORMATION OVER
LEEWARD SLOPES AND COASTS EACH AFTERNOON...BEFORE SKIES GRADUALLY
CLEAR EACH EVENING.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL HIGH AND VERY DRY
CONDITIONS ALOFT OVER THE ISLANDS...WHILE AMPLE LOW LEVEL MOISTURE
IS CAPPED BELOW AN INVERSION THAT EXTENDS AS HIGH AS 10 THOUSAND
FEET...BASED ON AFTERNOON SOUNDINGS...AND THE FACT THAT LOW CLOUD
TOPS DID NOT EXTEND HIGHER THAN THE BIG ISLAND SUMMITS THIS
AFTERNOON. AT THE SURFACE...MODERATE HIGH PRESSURE IS LOCATED FAR TO
THE NNE...WITH AN ASSOCIATED RIDGE PASSING SEVERAL HUNDRED MILES TO
THE N. LOW PRESSURE...FORMER TC MARIE...IS FAR NE OF THE STATE AND
DRIFTING W. MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS AN AREA OF
HIGHER MOISTURE EXTENDING UP TO 200 MILES E OF THE ISLANDS...WITH
DRIER AIR FARTHER UPSTREAM.

THE PRESSURE GRADIENT ACROSS THE ISLANDS REMAINS ON THE LIGHT SIDE
DUE TO OUR SEPARATION FROM THE HIGH AND THE PRESENCE OF FORMER
MARIE...AND THIS GENERAL PATTERN WILL REMAIN IN PLACE FOR THE NEXT
SEVERAL DAYS...POSSIBLY THROUGH THE WEEKEND. THUS LIGHT TO MODERATE
TRADE WINDS ARE EXPECTED TO DELIVER CLOUDS AND A FEW SHOWERS TO
WINDWARD AREAS...MAINLY DURING NIGHTS AND MORNINGS. THE FOCUS FOR
CLOUDS AND SHOWERS WILL SHIFT TO LEEWARD SLOPES AND COASTS IN THE
AFTERNOON AND EARLY EVENING...AFTER WHICH GENERAL CLEARING CAN BE
EXPECTED OVERNIGHT.

AN AREA OF LOW CLOUDS IS CURRENTLY LOCATED NE OF MAUI AND THE BIG
ISLAND...MOVING W NEAR 10 MPH. THESE CLOUDS ARE AN INDICATOR OF THE
HIGHER MOISTURE JUST UPSTREAM OF THE STATE...AND WILL HELP TO
ENHANCE TRADE WIND SHOWERS MAINLY ACROSS WINDWARD SECTIONS TONIGHT
INTO WEDNESDAY. GFS FORECAST SOUNDINGS/ MOISTURE PLOTS SHOW DRIER
AND MORE STABLE CONDITIONS SPREADING ACROSS THE ISLANDS LATER
WEDNESDAY...WITH LOWER RAINFALL CHANCES EXPECTED.

BY THE END OF THE WEEK...BOTH GFS AND ECMWF CONTINUE TO SHOW A
TONGUE OF MOISTURE TRAILING FROM FORMER MARIE REACHING THE ISLANDS
FRIDAY AND LINGERING INTO SATURDAY. FORECAST SOUNDINGS SHOW
INCREASING MOISTURE DEPTH BACK TO NEAR 10KFT. WHILE EARLIER MODEL
RUNS SHOWED POTENTIAL FOR INCREASING TRADE WINDS THIS WEEKEND...
LATEST GUIDANCE IS TRENDING TOWARD A CONTINUATION OF LIGHTER WINDS
AS WE HEAD INTO NEXT WEEK...AS THE RIDGE ASSOCIATED WITH THE HIGH IS
SHUNTED SOUTHWARD BY A LARGE LOW DEVELOPING TO THE NW OF THE STATE.

&&

.AVIATION...
LIGHT TO MODERATE TRADES WILL CONTINUE OVER THE NEXT FEW DAYS. WINDS
SHOULD REMAIN LIGHT ENOUGH TO LIMIT THE NEED FOR A LOW LEVEL TURB
AIRMET. SHOWERS WILL FAVOR WINDWARD AND MAUKA AREAS OVERNIGHT AND IN
THE EARLY MORNING AS SKIES GRADUALLY CLEAR LEEWARD. AFTERNOON AND
EVENING CLOUDS AND SHOWERS ARE EXPECTED OVER LEEWARD SLOPES AND
COASTS AGAIN ON WEDNESDAY. MVFR CONDITIONS ARE LIKELY WITHIN THESE
SHOWERS. OTHERWISE...VFR WILL PREVAIL.

&&

.MARINE...
ELEVATED SURF WILL CONTINUE ALONG S FACING SHORES THIS WEEK.
FORERUNNERS OF A VERY LONG PERIOD S SWELL /20-25 SECONDS/ THAT
ARRIVED EARLY TODAY ARE EXPECTED TO BUILD FURTHER TONIGHT...ALTHOUGH
AT LOW HEIGHTS. THE LARGEST PART OF THE SWELL WILL MISS US TO THE
E...AS SEEN BY THE GREATER LONG-PERIOD ENERGY AT BUOY 51004 SE OF
THE ISLANDS AS OPPOSED TO THE MUCH LOWER ENERGY READINGS AT BUOY
51003 TO THE SE. WITH THE BULK OF THE SWELL ENERGY PASSING E OF THE
ISLANDS...THE CURRENT EXPECTATION IS THAT SURF WILL REMAIN JUST
BELOW ADVISORY LEVELS ALONG S FACING SHORES. WILL MONITOR BUOY
OBSERVATIONS FOR POTENTIAL LARGER THAN EXPECTED SWELLS.

FORECAST MODELS SHOW A LARGE LOW DEVELOPING WELL NW OF THE ISLANDS
THIS WEEK...WITH LATEST FORECASTS FROM THE OCEAN PREDICTION CENTER
ANTICIPATING STORM FORCE WINDS. THIS LOW WILL GENERATE A NW SWELL
THAT IS EXPECTED TO ARRIVE MONDAY AND PEAK ON TUESDAY. IT IS TOO
EARLY FOR SPECIFICS AS THE LOW HAS YET TO DEVELOP...BUT RESULTANT
SURF LOOKS TO REMAIN BELOW ADVISORY LEVEL HEIGHTS ALONG N AND W
FACING SHORES.

WINDS AND SEAS WILL REMAIN BELOW SMALL CRAFT ADVISORY THRESHOLDS FOR
AT LEAST THE NEXT SEVERAL DAYS.

&&

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

&&

$$

BIRCHARD




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