Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 200208
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
408 PM HST FRI DEC 19 2014

.SYNOPSIS...
TRADES WILL WEAKEN TO A LAND AND SEA BREEZE PATTERN THIS WEEKEND
AS A COLD FRONT AND UPPER TROUGH APPROACH FROM THE NORTHWEST.
NORTH TO NORTHEAST WINDS AND RAINFALL WILL INCREASE LATE SUNDAY
NIGHT WITH THE POTENTIAL FOR HEAVY RAIN AND THUNDERSTORMS MONDAY
EVENING INTO TUESDAY.

&&

.DISCUSSION...
SATELLITE IMAGERY SHOWS A POSITIVELY-TILTED UPPER LEVEL TROUGH
OVER HAWAII WITH A WEST TO EAST ORIENTED JET STREAM RUNNING JUST
SOUTH OF THE BIG ISLAND INDICATED BY A SWATH OF CIRRUS CLOUDS. AT
THE SURFACE...HIGH PRESSURE IS CENTERED TO THE EAST-NORTHEAST OF
THE STATE PRODUCING MODERATE EAST-SOUTHEAST WINDS...WITH THE
STRONGEST WINDS NEAR MAUI COUNTY AND THE BIG ISLAND. RADAR AND
WEATHER OBSERVATIONS ARE SHOWING TRACE AMOUNTS OF SHOWERS THIS
AFTERNOON.

THE SURFACE HIGH IS EXPECTED TO MOVE EAST...AWAY FROM
HAWAII...TONIGHT AND WILL RESULT IN PROGRESSIVELY LESS INFLUENCE
ON OUR WINDS. MEANWHILE...A STRONG COLD FRONT IS APPROXIMATELY
1300 MILES NORTHWEST OF KAUAI MOVING TOWARD US. THE FRONT IS
SUPPORTED BY AN UPPER TROUGH WHICH WILL STAY WITH IT AS APPROACHES
THE HAWAIIAN ISLANDS. WINDS WILL WEAKEN AND VEER SOUTHEASTERLY ON
SATURDAY AHEAD OF THIS SYSTEM...RESULTING IN A SHIFT TO MORE WARM
AND MUGGY CONDITIONS THIS WEEKEND. WINDS WILL LIKELY STAY TOO
EASTERLY NEAR THE BIG ISLAND TO BRING VOG UP TO THE SMALLER
ISLANDS. THE WEAKENING FLOW WILL ALSO ALLOW A SEA AND LAND BREEZE
PATTERN TO SET UP SATURDAY AND SUNDAY...RESULTING IN DAYTIME CLOUD
BUILD UPS OVER THE ISLANDS AND CLEARING OUT AT NIGHT. WEAK RIDGING
ALOFT AND GENERALLY DRY CONDITIONS OVER THE STATE THIS WEEKEND
WILL KEEP THE AFTERNOON SHOWER ACTIVITY ON THE LIGHT SIDE.

THE COLD FRONT WILL ARRIVE ON KAUAI LATE SUNDAY NIGHT / EARLY
MONDAY MORNING...FOLLOWED BY THE COLD POOL ALOFT FROM THE UPPER TROUGH
MONDAY EVENING. NORTHEAST SURFACE WINDS WILL DRASTICALLY STRENGTHEN ALONG
THE FRONTAL BOUNDARY BEGINNING ON KAUAI LATE SUNDAY NIGHT...SPREADING
EASTWARD DOWN THE ISLAND CHAIN THROUGH TUESDAY NIGHT.

WITH SIGNIFICANT SUPPORT FROM THE LOW / TROUGH ALOFT...THE FRONT
WILL HAVE THE OPPORTUNITY TO PRODUCE LOCALLY HEAVY RAIN AND
THUNDERSTORMS. THE GFS AND ECMWF MODELS ARE IN FAIR AGREEMENT
THROUGH TUESDAY MORNING...BUT BEGIN TO DIVERGE SOLUTIONS IN TERMS OF
PLACEMENT OF THIS UPPER LEVEL FEATURE.

BOTH MODELS BRING THE TROUGH AXIS ACROSS KAUAI MONDAY
NIGHT...WITH 500 MB TEMPERATURES NEAR -16 DEGREES CELSIUS FOR THE
GFS AND -17 DEGREES CELSIUS FOR THE ECMWF. THE GFS FORMS A CUT OFF
LOW AND BRINGS IT OVER THE BIG ISLAND ON TUESDAY AND TUESDAY
NIGHT...WHILE THE ECMWF LIFTS THE LOW TO THE NORTH OF THE STATE.
GIVEN THE STRONG DIVERGENCE IN SOLUTIONS...IT IS HARD TO SAY WITH
MUCH CERTAINTY RIGHT NOW WHERE THE IMPACTS OF INSTABILITY CAUSED
BY THE UPPER LOW WILL OCCUR.

FOR NOW...IT LOOKS LIKE THE NORTHWESTERN END OF THE STATE...NAMELY
KAUAI AND OAHU...WOULD HAVE THE BEST CHANCES FOR THUNDERSTORMS AND
HEAVY RAINFALL MONDAY NIGHT AND TUESDAY. THE STRONG VERTICAL WIND
SHEAR WILL IMPROVE THE VENTILATION OF ANY THUNDERSTORMS THAT DO
DEVELOP...POTENTIALLY ALLOWING SOME TO BECOME SEVERE...THOUGH
REMAINS A BIT EARLY TO CONSIDER A SEVERE THUNDERSTORM WATCH. IF
THE GFS SOLUTION HOLDS...AND THE UPPER LOW TRACKS OVER THE BIG
ISLAND...IT WOULD DRAW UP DEEP MOISTURE BRINGING THE POTENTIAL FOR SNOW
OVER THE SUMMITS OF MAUNA KEA AND MAUNA LOA MONDAY NIGHT THROUGH
TUESDAY.

AS MENTIONED...THERE STILL A BIT OF DISCREPANCY BETWEEN LONG RANGE
MODEL SOLUTIONS. WE WILL CONTINUE TO MONITOR FOR BETTER AGREEMENT
AND FORECAST CONFIDENCE DURING THE SECOND HALF OF NEXT WEEK.


&&

.AVIATION...
WITH A FRONT APPROACHING AND HIGH PRESSURE NORTH OF THE ISLANDS
BEING PUSHED EASTWARD...TRADES WILL BE REPLACED WITH A MORE
SOUTHEAST WIND FLOW INTO SATURDAY. BUT THE FRONT WILL BE WEAKENING
AS IT NEARS THE STATE THUS INSTEAD OF STRONG SE WINDS...MAY SEE MORE
LAND/SEA BREEZES FROM TIME TO TIME. THUS EXPECT MORE INTERIOR
CLOUDINESS DURING THE AFTERNOONS ALONG WITH CLEARING AT NIGHT WITH
SLIGHTLY LESS WINDWARD CLOUDINESS. BOTTOM LINE...EXPECT MOSTLY VFR
CONDITIONS TO PREVAIL INTO SATURDAY.


&&

.MARINE...
THE CURRENT NORTHWEST SWELL WILL CONTINUE TO LOWER THROUGH
SATURDAY NIGHT. A LARGER NORTHWEST SWELL IS EXPECTED TO ARRIVE ON
SUNDAY AND WILL LIKELY BUILD TO HIGH SURF WARNING LEVEL HEIGHTS
FOR NORTH AND WEST FACING SHORES BY LATE IN THE DAY. THIS SWELL
SHOULD HAVE ENOUGH WESTERLY COMPONENT TO IMPACT THE WEST SIDE OF
THE BIG ISLAND...LIKELY REACHING WARNING LEVELS FOR KONA SUNDAY
NIGHT THROUGH MONDAY NIGHT.

EAST-SOUTHEAST WINDS ARE EXPECTED TO TAPER OFF TONIGHT AS A COLD FRONT
APPROACHES THE STATE FROM THE NORTHWEST. THE FRONT WILL PUSH THE
SUBTROPICAL HIGH NORTHEAST OF THE STATE FURTHER AWAY TO THE EAST.
WINDS WILL REMAIN LIGHT ON SUNDAY AS THE NORTHWEST SWELL
BUILDS...BUT NORTH TO NORTHEAST WINDS WILL DRASTICALLY INCREASE
SUNDAY NIGHT INTO MONDAY AS THE SWELL PEAKS.

A SMALL CRAFT ADVISORY /SCA/ IS IN EFFECT THROUGH 6 AM SATURDAY
FOR OUR TYPICAL WINDY ZONES AROUND MAUI COUNTY AND THE BIG
ISLAND...EXCEPT FOR MAALEA BAY DUE TO THE EAST-SOUTHEAST WIND
DIRECTION. THE SCA WILL LIKELY BE DROPPED SATURDAY MORNING.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST SATURDAY FOR MAUI COUNTY
WINDWARD WATERS-PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND
WINDWARD WATERS-BIG ISLAND LEEWARD WATERS-BIG ISLAND SOUTHEAST
WATERS.


&&

$$

DISCUSSION/MARINE...FOSTER
AVIATION...FUJII



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