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Hawaiian Islands Synoptic Discussion and Guidance

267
FXHW60 PHFO 191959
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
959 AM 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 SUNDAY NIGHT
WITH THE POTENTIAL FOR HEAVY RAIN AND THUNDERSTORMS MONDAY INTO
TUESDAY. A GRADUAL RETURN TO TRADE WIND WEATHER IS EXPECTED LATE
NEXT WEEK.

&&

.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 TO LOCALLY FRESH TRADE WINDS...WITH
THE STRONGEST WINDS NEAR MAUI COUNTY AND THE BIG ISLAND. RADAR IS
SHOWING A FEW PASSING WINDWARD SHOWERS ACROSS OAHU AND MAUI COUNTY
THIS MORNING.

THE SURFACE HIGH IS EXPECTED TO MOVE EAST...AWAY FROM HAWAII...OVER
THE NEXT DAY AND WILL RESULT IN PROGRESSIVELY LESS INFLUENCE ON
OUR WINDS. MEANWHILE...A STRONG COLD FRONT IS APPROXIMATELY 1400
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 OVER THE WEEKEND. 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.

THE COLD FRONT WILL ARRIVE SUNDAY NIGHT...FOLLOWED BY THE COLD
POOL ALOFT FROM THE UPPER TROUGH ON MONDAY. NORTHEAST SURFACE
WINDS WILL DRASTICALLY STRENGTHEN ALONG THE FRONTAL BOUNDARY
BEGINNING ON KAUAI SUNDAY NIGHT AND SPREADING EASTWARD DOWN THE
ISLAND CHAIN THROUGH TUESDAY NIGHT. WITH SIGNIFICANT SUPPORT FROM
THE TROUGH ALOFT...THE FRONT WILL HAVE THE OPPORTUNITY TO PRODUCE
LOCALLY HEAVY RAIN AND THUNDERSTORMS.

THE GFS AND ECMWF MODELS ARE NOT CONSISTENT WITH THE AMOUNT OF
INSTABILITY THAT WILL BE PRESENT TO TRIGGER THUNDERSTORMS...LET
ALONE PINPOINTING LOCATIONS ON THE ISLANDS. BOTH MODELS BRING THE
TROUGH AXIS ACROSS KAUAI MONDAY NIGHT...WITH 500 MB TEMPERATURES
NEAR -14 DEGREES CELSIUS FOR THE GFS AND -17 DEGREES CELSIUS FOR
THE ECMWF. AFTER THIS POINT...THE GFS QUICKLY SHIFT THE TROUGH
EASTWARD WHILE THE ECMWF KEEP THE UPPER TROUGH IN THE VICINITY OF
KAUAI AND OAHU THROUGH WEDNESDAY MORNING. 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
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.

THE UPPER TROUGH AND DEEP MOISTURE ALSO INVITES THE POTENTIAL FOR
SNOW OVER THE SUMMITS OF MAUNA KEA AND MAUNA LOA MONDAY NIGHT
THROUGH TUESDAY.

FROM WEDNESDAY INTO FRIDAY OF NEXT WEEK...THE TROUGH ALOFT WILL LIFT
OUT TOWARD THE NORTHEAST...AND CONDITIONS WILL RETURN TOWARD
CLIMATOLOGY. AS MENTIONED...MODEL SOLUTIONS SHOW THIS PROCESS MAY
BE DRAWN OUT...SO THE RELATIVELY QUICK RETURN OF TRADE-WIND
WEATHER IN THE FORECAST MAY BE TOO OPTIMISTIC...AND SHOULD BE
REEVALUATED AS MORE RUNS BECOME AVAILABLE. TRADE-WIND WEATHER
SHOULD RETURN BY LATE 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 BEGINNING TODAY 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 THROUGH TONIGHT.

MODERATE AND GUSTY TRADE WINDS WILL START TO EASE TODAY THUS WILL BE
ABLE TO DROP THE AIRMET TANGO FOR LOW LEVEL TURB JUST OVER AND LEE
OF MOUNTAINS SOMETIME TODAY. UPPER JET STREAM NEAR AND JUST SOUTH
OF BIG ISLAND DROPPING SOUTHWARD SO WILL ALSO CEASE MENTION OF
POSSIBLE HIGH LEVEL TURB FROM THE TURB SECTION OF AIRMET IN THE NEAR
FUTURE AS WELL.

&&

.MARINE...
THE HIGH SURF ADVISORY FOR NORTH AND WEST FACING SHORES HAS BEEN
CANCELLED THIS MORNING AS OBSERVATIONS AND BUOY REPORTS SHOW BELOW
ADVISORY FACE HEIGHTS. THE NORTHWEST SWELL PEAKED OVERNIGHT AND IS
EXPECTED TO DECLINE TODAY AND SATURDAY...THOUGH CONDITIONS MAY STILL
REMAIN CLOSE TO ADVISORY LEVELS THROUGH THIS AFTERNOON.

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 AS
WELL.

EASTERLY WINDS ARE EXPECTED TO TAPER OFF TONIGHT AND SATURDAY AS A
COLD FRONT APPROACHES THE STATE FROM THE NORTHWEST AND FORCES THE
SUBTROPICAL HIGH FURTHER AWAY FROM HAWAII. 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 PM TODAY FOR THE EASTERN HALF OF THE STATE FOR WINDS AND
SEAS. WITH THE SWELL ON THE DECLINE TODAY...THE SCA WILL LIKELY BE
TRIMMED BACK TO SOME OF THE WINDY ZONES AROUND MAUI COUNTY AND THE
BIG ISLAND THROUGH SATURDAY MORNING...THEN DROPPED ALTOGETHER.


&&

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


&&

$$

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




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office