Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 042002
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1002 AM HST WED MAY 4 2016

.SYNOPSIS...
WET TRADE WIND CONDITIONS WILL PERSIST ACROSS MOST OF THE STATE
TODAY. A COLD FRONT APPROACHING THE REGION FROM THE NORTHWEST WILL
CAUSE OUR WINDS TO WEAKEN AND VEER FROM THE SOUTHEAST STARTING
LATER TODAY...AND CONTINUING THROUGH FRIDAY. THIS WILL ALLOW SEA
BREEZE CONVERGENCE TO TRIGGER CLOUD BUILD UPS AND LOCALLY HEAVY
AFTERNOON SHOWERS ACROSS ISLAND INTERIORS THURSDAY AND FRIDAY.
THE FRONT IS EXPECTED TO WEAKEN TO A SURFACE TROUGH AS IT REACHES
KAUAI FRIDAY. LOCALLY BREEZY NORTHEASTERLY TRADE WINDS WILL FILL
IN BEHIND THIS TROUGH AS IT MOVES ACROSS THE ISLAND CHAIN THIS
WEEKEND.

&&

.DISCUSSION...
A 1031 MB SURFACE HIGH LOCATED NEAR 36N142W HAS A RIDGE EXTENDING
WEST-SOUTHWEST TO A POINT ABOUT 450 MILES NORTH OF LIHUE. THE
TIGHT PRESSURE GRADIENT SOUTH OF THESE FEATURES IS MAINTAINING
LOCALLY BREEZY TRADE WINDS ACROSS THE STATE THIS MORNING. MID-
LEVEL RIDGING IN THE VICINITY OF THE ISLANDS IS PROVIDING
RELATIVELY STABLE ATMOSPHERIC CONDITIONS. HOWEVER...THE MORNING
SOUNDINGS FROM LIHUE AND HILO SHOW SLIGHTLY ELEVATED TRADE WIND
INVERSIONS OF 8 TO 9 THOUSAND FEET...WITH PRECIPITABLE WATER
VALUES OF 1.3 TO 1.5 INCHES. THE CURRENT SATELLITE IMAGERY AND
RADAR REFLECTIVITY DATA SHOW SCATTERED TO BROKEN LOW CLOUDS AND
SCATTERED SHOWERS ASSOCIATED WITH AN OLD FRONT IN THE VICINITY OF
THE ISLANDS. MOST OF THESE CLOUDS AND SHOWERS ARE AFFECTING THE
WINDWARD SIDES OF THE ISLANDS.

THIS AREA OF LOW CLOUDS AND SHOWERS IS EXPECTED TO SAG SLOWLY SOUTH
AND WEST TODAY. THE BREEZY TRADES WILL KEEP THE SHOWERS MOVING
ACROSS THE WINDWARD AND MAUKA AREAS TODAY. A FEW OF THE SHOWERS
WILL MOVE OVER TO LEEWARD SIDES OF THE SMALLER ISLANDS...ESPECIALLY
THIS MORNING.

THE TRADE WINDS ARE EXPECTED TO BEGIN TO WEAKEN FROM LATE THIS
AFTERNOON THROUGH TONIGHT AS A COLD FRONT...WHICH IS CURRENTLY
ABOUT 1050 MILES WEST NORTHWEST OF KAUAI...APPROACHES FROM THE
NORTHWEST. THIS WILL CAUSE THE SURFACE HIGH AND ITS ASSOCIATED
RIDGE TO WEAKEN...AND MAKE THE PRESSURE GRADIENT RELAX OVER THE
STATE. AS A RESULT...THE SURFACE WINDS WILL WEAKEN AND VEER OUT OF
THE SOUTHEAST FROM LATER TODAY THROUGH FRIDAY. THIS WEAKER
SOUTHEASTERLY WIND REGIME WILL ALLOW LOCAL AFTERNOON SEA BREEZE
AND NIGHTTIME LAND BREEZE CIRCULATIONS TO DEVELOP OVER LEEWARD AND
INTERIOR SECTIONS OF MOST ISLANDS. SOMEWHAT ENHANCED MOISTURE
VALUES AND SLIGHT ATMOSPHERIC INSTABILITY ARE ALSO EXPECTED ALONG
AND AHEAD OF THE FRONT. THE CONVERGENCE OF LOCAL SEA BREEZE
CIRCULATIONS OVER THE ISLANDS WILL PRODUCE CONVECTIVE AFTERNOON
CLOUD BUILD UPS AND SCATTERED SHOWERS. SOME OF THESE SHOWERS MAY
PRODUCE LOCALLY HEAVY RAINFALL ACROSS ISLAND INTERIORS...MAINLY
DURING THURSDAY AND FRIDAY AFTERNOONS.

THE FORECAST MODELS APPEAR TO AGREE THAT THE FRONT WILL WEAKEN TO
A SURFACE TROUGH AS IT REACHES KAUAI FRIDAY. AS THIS TROUGH MOVES
ACROSS THE ISLAND CHAIN...NORTHEASTERLY TRADES WILL SPREAD FROM
WEST TO EAST ACROSS THE STATE THIS WEEKEND. THESE NORTHEASTERLY
WINDS MAY BE LOCALLY BREEZY...ESPECIALLY OVER THE WESTERN END OF
THE STATE ON SATURDAY. THE WINDS WILL EVENTUALLY WEAKEN TO
MODERATE BREEZES FROM SUNDAY INTO NEXT WEEK. THE REMNANT MOISTURE
ASSOCIATED WITH THE FORMER FRONT/TROUGH WILL ALSO LIKELY PRODUCE
AN EXTENDED PERIOD OF WET TRADE WIND CONDITIONS FROM SUNDAY INTO
THE MIDDLE OF NEXT WEEK. BOTH THE LONG RANGE GFS AND ECMWF MODELS
INDICATE THE TRADES WILL FOCUS SHOWERS MAINLY OVER THE WINDWARD
SIDES OF THE ISLANDS FROM OAHU TO THE BIG ISLAND.

&&

.AVIATION...
A SURFACE RIDGE ABOUT 745 NM NORTH OF THE STATE WILL MAINTAIN AN
EASTERLY TRADE WIND FLOW TODAY. MID LEVEL RIDGING IS MAINTAINING AN
INVERSION IN THE 7000 TO 9000 FT RANGE...WHICH WILL KEEP THE
ATMOSPHERE SOMEWHAT STABLE...BUT A BAND OF MOISTURE MOVING ALONG THE
TRADE WIND FLOW WILL AFFECT WINDWARD AREAS THROUGH MUCH OF THE DAY.
THE BAND WILL SHIFT WEST...LIKELY AFFECTING KAUAI AND OAHU TONIGHT.

AIRMET SIERRA FOR MTN OBSC WAS EXPANDED FROM KAUAI TO COVER OAHU
MOLOKAI AND MAUI. THIS AIRMET WILL LIKELY BE EXTENDED INTO THE
AFTERNOON AND MAY NEED TO BE EXTENDED INTO TONIGHT FOR KAUAI AND
OAHU.

AIRMET TANGO FOR LOW LEVEL TURB REMAINS IN PLACE TO THE LEE OF ALL
ISLANDS. RADAR VAD WINDS ARE SHOWING PRIMARILY 20 KT OR LESS BELOW
THE INVERSION...BUT RECENT AIRCRAFT SOUNDINGS OUT OF HNL AND OGG
SHOWED SOME 25 KT WINDS JUST BELOW THE INVERSION. GIVEN THESE BORDER
LINE CONDITIONS...WE WILL LET AIRMET TANGO CONTINUE FOR NOW...BUT IN
ABSENCE OF OBSERVATIONS AND AN EXPECTATION OF WEAKENING WINDS...IT
MAY COME DOWN LATER TODAY.


&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ REMAINS POSTED FOR THE TYPICALLY
WINDY WATERS ADJACENT TO THE ISLANDS OF MAUI COUNTY AND THE BIG
ISLAND TODAY. THE WINDS ARE EXPECTED TO DECREASE AND VEER OUT OF
THE SOUTHEAST BY TONIGHT. THESE WEAKER WINDS WILL PERSIST THROUGH
FRIDAY. THE FORECAST MODELS CONTINUE TO SHOW INCREASING
NORTHEASTERLY TRADE WINDS SPREADING FROM WEST TO EAST OVER THE
AREA THIS WEEKEND. THESE WINDS MAY BRIEFLY REACH THE SCA CRITERIA
NEAR KAUAI ON SATURDAY.

A SMALL LONG PERIOD WEST-NORTHWEST SWELL HAS ARRIVED AT BUOY
51101 NORTHWEST OF KAUAI...WHILE FORERUNNERS FROM THIS SWELL ARE
SHOWING UP AT THE WAIMEA BAY AND BARBERS POINT BUOYS. THIS SWELL
WILL CONTINUE TO FILL IN ACROSS THE HAWAIIAN WATERS TODAY...AND
WILL PERSIST THROUGH FRIDAY. A REINFORCING NORTH-NORTHWEST SWELL
WILL ARRIVE SATURDAY AND CONTINUE INTO EARLY NEXT WEEK. SMALL
SOUTH SWELLS WILL MAINTAIN BACKGROUND SURF ALONG SOUTH FACING
SHORES THROUGH THURSDAY. A LONG PERIOD SOUTH SWELL WILL ARRIVE FRIDAY
AND PEAK LATE SATURDAY ABOVE THE SEASONAL SOUTH SWELL AVERAGE...
AND THEN SLOWLY SUBSIDE INTO EARLY NEXT WEEK. WHILE SURF WILL BE
ELEVATED...SURF HEIGHTS ARE EXPECTED TO REMAIN JUST BELOW THE HIGH
SURF ADVISORY CRITERIA ALONG SOUTH FACING SHORES THIS WEEKEND.
ALONG EAST FACING SHORES...THE BREEZY TRADE WINDS WILL CONTINUE TO
PRODUCE CHOPPY SURF TODAY...THEN TAPER OFF THROUGH THE END OF THE
WEEK AS THE WINDS WEAKEN AND VEER OUT OF THE SOUTHEAST.


&&

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


&&

$$

DISCUSSION/MARINE...FOSTER
AVIATION...WROE


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