Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 300605
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE JUL 29 2014

.SYNOPSIS...
HIGH PRESSURE FAR NORTH OF THE STATE WILL KEEP TRADE WINDS BLOWING
THROUGH THE WEEKEND. DRY CONDITIONS INTO THURSDAY WILL YIELD TO A
VERY MOIST AIR MASS BRINGING INCREASING SHOWERS FROM THE SOUTHEAST
BY FRIDAY EVENING. CONDITIONS WILL DRY OUT AGAIN SLOWLY OVER THE
WEEKEND AS THE MOIST AIR MASS MOVES AWAY TOWARD THE WEST.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED
GENERALLY MODERATE TRADE WINDS...THOUGH AS USUAL SPEEDS WERE HIGHER
WHERE THE FLOW ACCELERATED AROUND HEADLANDS AND BETWEEN ISLANDS.
CLOUD COVER WAS LIMITED AND PRECIPITATION QUITE SPARSE. THE DRY AIR
MASS AND LOW INVERSION PREVAILING OVER MOST OF THE STATE LARGELY
EXPLAINED THIS DISTRIBUTION...BUT MOISTER AIR DRIFTING OVER KAUAI
FROM THE NORTHWEST FAILED TO SUPPORT MUCH RAINFALL EVEN THERE. THE
BROAD BUT SHALLOW MID-LEVEL TROUGH EXTENDING TOWARD KAUAI FROM THE
NORTHEAST FURTHER CONFOUNDED A SIMPLE EXPLANATION. IN ANY CASE...
NIGHTTIME LAND BREEZES LIKELY WILL HELP TO KEEP DEVELOPING SHOWERS
OFFSHORE DURING THE NEXT TWELVE HOURS.

GUIDANCE AGREED IN PREDICTING LITTLE CHANGE THROUGH THE NEXT COUPLE
OF DAYS...AND THEN ONLY GRADUAL CHANGE INTO THE WEEKEND. THE MAIN
TWO TRENDS WILL CONSIST OF A SIGNIFICANTLY MOISTER AIR MASS ARRIVING
FROM THE ACTIVE ITCZ BY THURSDAY OR FRIDAY...AND A MID-LEVEL RIDGE
BUILDING IN OVERHEAD FROM THE EAST THROUGH THE FIRST PART OF THE
WEEKEND. THE MOISTER AIR WILL PROVIDE MORE FUEL FOR SHOWERS...BUT
THE RIDGE ALOFT WILL PRODUCE SUBSIDENCE SUPPRESSING CLOUD
DEVELOPMENT. HOWEVER...INCREASED SUPPORT FOR THE SUBTROPICAL RIDGE
AT THE SURFACE ALSO WILL BRING SOMEWHAT FASTER WIND SPEEDS...
PROVIDING MORE OROGRAPHIC FORCING ALONG WINDWARD SLOPES. FINALLY...
IF ENOUGH OF THE MOIST AIR MASS ARRIVES BEFORE THE RIDGE ALOFT
REALLY STARTS TO BUILD...THEN THE MID-LEVEL TROUGH LINGERING TO THE
NORTH NORTHEAST MAY HAVE A BRIEF OPPORTUNITY TO TRIGGER CONVECTION
OVER THE STATE.

THIS WINDOW OF OPPORTUNITY APPEARED TO CENTER AROUND FRIDAY NIGHT.
ALTHOUGH LOCAL TOOLS RUN ON MODEL OUTPUT DID NOT PREDICT MUCH CHANCE
OF THUNDERSTORMS...EVENTS DRIVEN PRIMARILY BY LOW-LEVEL MOISTURE
WITH LITTLE SUPPORT FROM ALOFT DID NOT EXACTLY PLAY TO THE STRENGTHS
OF THOSE TOOLS. ALSO...TEMPERATURES ALOFT APPEARED TOO WARM TO FAVOR
DEEP CONVECTION. FOR NOW...MENTION OF LOCALLY HEAVY RAIN CREEPING
NORTHWEST FROM THE BIG ISLAND FROM FRIDAY EVENING INTO SATURDAY
SEEMED LIKE A PRUDENT COMPROMISE...THOUGH THE CHANCE OF THUNDER WILL
HAVE TO REMAIN A CONCERN DURING THAT TIME.

BY LATE IN THE WEEKEND...SOLUTIONS STARTED TO TREND TOWARD
CLIMATOLOGY AGAIN. WIND SPEEDS SHOULD DIMINISH SLIGHTLY...WHILE THE
MOIST AIR MASS WILL DEPART TOWARD THE WEST...ALLOWING DRIER AIR TO
ADVECT IN FROM THE NORTHEAST.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE MAIN HAWAIIAN ISLANDS
OVERNIGHT AND WEDNESDAY. PASSING SHOWERS AND ISOLATED MVFR CIGS WILL
BE POSSIBLE ON WINDWARD SLOPES. AIRMET TANGO REMAINS IN EFFECT FOR
LOW LEVEL TURBULENCE OVER AND IMMEDIATELY DOWNWIND OF THE HIGHER
TERRAIN...DUE TO MODERATE TO LOCALLY BREEZY TRADE WINDS.

&&

.MARINE...
A SMALL CRAFT ADVISORY REMAINED IN EFFECT FOR TYPICALLY WINDY WATERS
AROUND MAUI COUNTY AND THE BIG ISLAND. THESE CONDITIONS ARE EXPECTED
TO CONTINUE INTO THE WEEKEND.

A SERIES OF SMALL SOUTHERLY SWELLS SHOULD ARRIVE DURING THE NEXT FEW
DAYS...BUT A LARGER SOUTH SWELL MAY REACH THE STATE OVER THE
WEEKEND. TRADE WINDS WILL CONTINUE TO PRODUCE SHORT-PERIOD CHOPPY
SURF ALONG EAST-FACING SHORES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

&&

$$

AVIATION...JACOBSON
REST...RYSHKO





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