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Hawaiian Islands Synoptic Discussion and Guidance

885
FXHW60 PHFO 222010
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1010 AM HST SAT NOV 22 2014

.SYNOPSIS...
DIMINISHING TRADE WINDS TODAY WILL LEAD TO LIGHT AND VARIABLE WINDS
LATE TONIGHT AND SUNDAY...WITH MOSTLY DRY WEATHER PREVAILING
STATEWIDE. A FRONT WILL MOVE DOWN THE ISLAND CHAIN ON MONDAY...
BRINGING A BRIEF BOUT OF SHOWERS. COOL AND DRY NORTHERLY WINDS WILL
SURGE BEHIND THE FRONT...WITH WINDS BECOMING NORTHEASTERLY ON
TUESDAY. BREEZY TRADE WINDS WILL PERSIST FOR THE REMAINDER OF THE
WEEK...DELIVERING BRIEF WINDWARD SHOWERS.

&&

.DISCUSSION...
CURRENTLY...LIGHT TO LOCALLY FRESH EAST-SOUTHEAST TRADE WINDS ARE
BEING SUPPORTED BY A SURFACE HIGH CENTERED ABOUT 1500 MILES NE OF
THE ISLANDS...AND ITS ASSOCIATED SURFACE RIDGE ABOUT 400 MILES N
OF OAHU. LATEST SATELLITE IMAGERY SHOWS A DEARTH OF LOW CLOUDS OVER
NEAR SHORE WINDWARD WATERS...WITH SCATTERED LOW CLOUDS OVER WINDWARD
SLOPES. THE ISLAND ATMOSPHERE IS VERY STABLE...STRONGLY CAPPED AT
ABOUT 6 KFT...AND VERY DRY ABOVE. A SPIKE IN MOISTURE IS NOTED NEAR
30 KFT IN THE PHTO SOUNDING DUE TO A NEARBY E-W ORIENTED TROUGH
ALOFT...WITH THIS HIGH LEVEL MOISTURE SUPPORTING THIN HIGH CLOUDS
NEAR AND OVER THE BIG ISLAND. OTHER THAN THE HIGH CLOUDS DIMMING
THE SUN OVER THE BIG ISLAND...A MOSTLY SUNNY AND DRY DAY IS
EXPECTED...AND THE SHORT TERM FORECAST WAS UPDATED A SHORT WHILE
AGO FOR SOME MINOR TWEAKS.

THE LEADING EDGE OF A STALLED FRONT LIES ABOUT 600 MILES NW OF
KAUAI...BUT IS EXPECTED TO BEGIN A STEADY MARCH TOWARD THE ISLANDS
LATER TODAY. THIS WILL OCCUR AS HIGH PRESSURE BUILDS TO THE NW OF
THE FRONT...AND A WAVE OF LOW PRESSURE DEVELOPS N OF THE ISLANDS...
WITH THE TWO COMBINING TO PUSH THE BOUNDARY TOWARD...AND EVENTUALLY
OVER...THE ISLANDS. LATEST GUIDANCE INDICATES THAT THE SURFACE
RIDGE WILL WEAKEN AND MOVE S AS THE FRONT APPROACHES TONIGHT AND
SUNDAY...WITH WINDS DIMINISHING TO THE POINT THAT THEY BECOME LIGHT
AND VARIABLE OVER THE ISLANDS. THIS SHOULD ALLOW LAND AND SEA
BREEZES TO PREVAIL LATE TONIGHT THROUGH MOST OF SUNDAY.

WINDS AHEAD OF THE FRONT WILL BECOME LIGHT WESTERLY MOST AREAS
SUNDAY NIGHT...WITH THE FRONT BRINGING A SHIFT TO STRONGER NORTHERLY
WINDS AS IT PASSES KAUAI EARLY MONDAY MORNING...OAHU BY
MID-MORNING...MAUI COUNTY IN THE AFTERNOON...AND THE BIG ISLAND BY
MONDAY NIGHT. THIS TIMING IS SUPPORTED BY THE MOST RECENT RUNS OF
BOTH THE GFS AND ECMWF. LIMITED MOISTURE AND INSTABILITY MEANS THAT
THIS FRONT WILL BE FAIRLY SHALLOW...WITH A RELATIVELY BRIEF PERIOD
OF SHOWERS AS IT PASSES...MAINLY OVER THE SMALLER ISLANDS. THE FRONT
WILL BE DISSIPATING AS IT NEARS THE BIG ISLAND...BUT STILL EXPECT A
GENERAL INCREASE IN SHOWERS...MAINLY ALONG NE FACING SLOPES MONDAY
NIGHT AND TUESDAY.

POST-FRONTAL NORTHERLY WINDS WILL USHER IN A COOL AND DRY AIR MASS
WITH LIMITED SHOWERS EXPECTED THEREAFTER...MAINLY LIGHT AND ALONG N
FACING SLOPES. DEW POINTS ARE EXPECTED TO DIP INTO THE UPPER 50/S BY
MONDAY NIGHT...REMAINING THERE INTO TUESDAY. THE HIGH SUPPORTING THE
COOL NORTHERLIES WILL EVENTUALLY MOVE INTO A POSITION N OF THE
ISLANDS TO SUPPORT A MORE TYPICAL ENE TRADE WIND BY WEDNESDAY...
WITH DEW POINTS GRADUALLY REBOUNDING INTO THE MID TO UPPER 60/S.
BREEZY TRADE WINDS ARE THEN EXPECTED TO CONTINUE THROUGH NEXT
WEEKEND. WITH A MID LEVEL RIDGE NEARBY...THE ISLAND ATMOSPHERE WILL
BE STABLE...WITH JUST A FEW BRIEF WINDWARD SHOWERS EXPECTED.

&&

.AVIATION...
WITH AN UPPER LEVEL TROUGH OVER THE STATE...DEFORMATION/VERTICAL
SHEAR GUIDANCE CONTINUES TO SUPPORT THE AIRMET FOR MODERATE
TURBULENCE. THE 12Z HILO SOUNDING SHOWED 60 KTS OF SPEED SHEAR
BETWEEN FL320 AND FL400...WHICH IS BORDERLINE FOR TURBULENCE...BUT
WITH STRONGER DIRECTIONAL SHEAR ALONG THE TROUGH JUST TO THE
NORTHWEST. EARLIER PILOT REPORTS INDICATED TURBULENCE OCCURRING AT
SLIGHTLY LOWER ALTITUDES. HAVE EXPANDED THE LAYER REFERENCED IN THE
AIRMET BASED ON THESE REPORTS...AND ALSO INCLUDED AREAS FROM OAHU TO
THE BIG ISLAND. CONDITIONS ARE EXPECTED TO IMPROVE THIS AFTERNOON AS
THE UPPER TROUGH AND JET WEAKEN AND MOVE EAST.

OTHERWISE PRIMARILY VFR CONDITIONS ARE EXPECTED. BRIEF MVFR CEILINGS
ARE POSSIBLE IN SHOWERS ACROSS EASTERN SECTIONS OF THE ISLANDS.
WEAKENING WINDS TONIGHT WILL ALLOW LAND BREEZES TO FORM ACROSS
LEEWARD SECTIONS...WHICH WILL HELP TO CLEAR OUT ANY LOW-LEVEL
CLOUDS THAT DEVELOP OVER THESE AREAS DURING THE DAY.

&&

.MARINE...
A SMALL CRAFT ADVISORY /SCA/ REMAINS POSTED FOR WINDIER MARINE ZONES
AROUND THE BIG ISLAND...WITH WINDS EXPECTED TO DIMINISH BELOW SCA
SPEEDS BY TONIGHT. INCREASING WINDS...SEAS AND LONG-PERIOD SWELL
LATE SUNDAY AND MONDAY WILL REQUIRE THE SCA FLAG TO BE HOISTED ONCE
AGAIN...THIS TIME FOR MOST MARINE ZONES...WITH SEAS REMAINING
ELEVATED THROUGH TUESDAY. BREEZY TRADE WINDS WILL REQUIRE THE SCA
FOR THE WINDIER MAUI AND BIG ISLAND ZONES FOR MUCH OF THE WEEK
HOWEVER.

A NEW LONG-PERIOD NNW SWELL WILL BUILD TONIGHT INTO SUNDAY...WITH
PEAK SURF HEIGHTS APPROACHING BUT EXPECTED TO REMAIN BELOW ADVISORY
LEVELS ALONG EXPOSED N AND W SHORES. THE SWELL WILL BE REINFORCED
ON MONDAY...WITH PEAK SURF HEIGHTS LIKELY REQUIRING AN ADVISORY FOR
EXPOSED N AND W SHORES THROUGH TUESDAY. BREEZY TRADE WINDS LATER IN
THE WEEK WILL PRODUCE SHORT-PERIOD CHOPPY SURF ALONG EAST-FACING
SHORES THAT IS EXPECTED TO REMAIN BELOW ADVISORY LEVELS.

&&

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

&&

$$

SYNOPSIS/DISCUSSION/MARINE...BIRCHARD
AVIATION...BRAVENDER



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