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Hawaiian Islands Synoptic Discussion and Guidance

917
FXHW60 PHFO 201330
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 AM HST SAT DEC 20 2014

.SYNOPSIS...
LIGHTER SOUTHEAST FLOW TODAY WILL ALLOW FOR LAND AND SEA BREEZES TO
DEVELOP AS A COLD FRONT AND UPPER TROUGH APPROACH FROM THE
NORTHWEST. SHOWERS WILL INCREASE STARTING LATE SUNDAY NIGHT WITH THE
POTENTIAL FOR HEAVY RAIN AND A FEW THUNDERSTORMS MONDAY AFTERNOON
THROUGH TUESDAY. STRONG NORTHERLY WINDS WILL THEN DEVELOP BY MID
WEEK.

&&

.DISCUSSION...
DRY WEATHER PATTERN CONTINUES THIS MORNING ACROSS THE ALOHA STATE AS
SFC HIGH PRESSURE NE OF THE ISLANDS MIGRATES E. A FEW SMALL SHOWERS
CAN BE OBSERVED UPSTREAM OF THE ISLANDS...WHICH WILL MAINLY AFFECT
WINDWARD AND MOUNTAIN AREAS AT TIMES. MEANWHILE...A WEAK
POSITIVELY-TILTED UPPER LVL TROUGH REMAINS OVER THE AREA WITH AN
ASSOCIATED JET STREAM PRODUCING A LINE OF CIRRUS CLOUDS JUST S OF
THE BIG ISLAND. THIS TROUGH IS ALSO KEEPING TIGHTER PRESSURE
GRADIENTS OVER THE BIG ISLAND AT SUMMIT LEVEL...PRODUCING STRONG
WINDS NEAR THE TOPS OF MAUNA LOA AND MAUNA KEA. THUS...A WIND
ADVISORY HAS BEEN ISSUED FOR THE BIG ISLAND SUMMITS UNTIL NOON
TODAY.

AS THE SFC HIGH MIGRATES E AND FURTHER AWAY FROM THE ISLAND
CHAIN...THE TRADES WILL BECOME LIGHT TODAY. MODELS ALSO SHOW A
STRONG SFC COLD FRONT APPROACHING THE ISLANDS ON SUN...WHICH SEEMS
TO HAVE GOOD UPPER LEVEL SUPPORT FROM A SHARP TROUGH ALSO MOVING
TOWARDS THE AREA. AS THIS TROUGH/FRONT COMPLEX MOVES CLOSER...THE
WINDS TODAY WILL BECOME SE AND GENERALLY LIGHT WITH WARMER AND HUMID
CONDITIONS EXPECTED THROUGH THIS EVENING. SOME VOG FROM THE BIG
ISLAND MAY REACH MAUI COUNTY TODAY AND SUN...BUT CHANCES ARE LOW AT
THIS TIME AS WINDS AROUND THE BIG ISLAND MAY REMAIN MORE EASTERLY.
THIS SYNOPTIC SETUP WILL ALLOW FOR SEA/LAND BREEZE CIRCULATIONS TO
DEVELOP TODAY AND PROBABLY AGAIN ON SUN...WITH CLOUDS AND SHOWERS
FORMING OVER LEEWARD AND INTERIOR AREAS EACH AFTERNOON AND EARLY
EVENING. BUT ANY SHOWERS THAT FORM WILL LIKELY REMAIN SHALLOW AND
PRODUCE LIGHT RAIN IN GENERAL AS A WEAK RIDGE ALOFT WILL DEVELOP
LATER TODAY AND REMAIN IN PLACE THROUGH SUN.

MODELS CONTINUE TO SHOW FAIR AGREEMENT REGARDING THE FORECAST
SOLUTION THROUGH EARLY TUE...DEPICTING THE COLD FRONT REACHING KAUAI
LATE SUN NIGHT AND INTO MON MORNING...AND A COLDER AIRMASS MOVING
ALOFT AS THE UPPER TROUGH ARRIVES LATE MON. EXPECT WIDESPREAD
SHOWERS...LOCALLY HEAVY RAIN AND POTENTIAL FOR THUNDERSTORM ACTIVITY
FROM MON AFTERNOON THROUGH TUE. THE TROUGH AXIS SEEMS TO MOVE ACROSS
KAUAI COUNTY SOMETIME LATE MON NIGHT...WITH 500 MB TEMPS COOLING
DOWN TO AROUND -15 TO -17 DEGREES CELSIUS. THUS...THE FORECAST WILL
CONTINUE TO CARRY SLIGHT CHANCES OF THUNDERSTORMS MON NIGHT AND
TUE...AND THE OVERALL COVERAGE HAS BEEN ADJUSTED TO INCLUDE VERY
MUCH THE ENTIRE AREA MON NIGHT.

DIFFERENCES REMAIN BETWEEN GLOBAL MODELS AS GFS DEVELOPS A CUTOFF
LOW AND MOVES IT OVER THE E HALF OF THE ISLAND CHAIN TUE AND TUE
NIGHT...WHILE ECMWF KEEPS THIS LOW N OF THE ISLANDS DURING THE SAME
TIME FRAME. THE FINAL OUTCOME OF THIS SCENARIO WILL CERTAINLY HAVE A
SIGNIFICANT IMPACT...SO FOR NOW WILL KEEP THE FORECAST CLOSER TO
ECMWF AND WILL ADJUST ACCORDINGLY.

BEST CHANCES FOR LOCALLY HEAVY RAIN AND THUNDERSTORMS RESIDE OVER
THE W HALF OF THE ISLAND CHAIN MON NIGHT AND TUE. THERE WILL BE
POTENTIAL FOR A FEW CELLS TO BECOME STRONG. ALSO...AS THE UPPER LOW
MOVES CLOSER TO THE BIG ISLAND...THE POTENTIAL FOR SNOW OVER THE
SUMMITS WILL INCREASE...MAINLY FROM MON NIGHT THROUGH TUE.

MODELS SHOW STRONG NE WINDS ESTABLISHING ACROSS THE AREA IN THE WAKE
OF THE FROPA...STARTING AROUND KAUAI COUNTY LATE SUN NIGHT...THEN
SPREADING DOWN THE REST OF THE ISLAND CHAIN BY TUE NIGHT AND
PREVAILING THROUGH 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 LIGHTER
SOUTHEAST WIND TODAY. MAY SEE LAND/SEA BREEZES FROM TIME TO TIME.
THUS EXPECT MORE INTERIOR CLOUDINESS DURING THE NEXT COUPLE OF
AFTERNOONS ALONG WITH CLEARING AT NIGHT. BOTTOM LINE...EXPECT
MOSTLY VFR CONDITIONS TO PREVAIL TODAY.


&&

.MARINE...
THE CURRENT NORTHWEST SWELL WILL CONTINUE TO LOWER THROUGH TONIGHT.
A LARGER NORTHWEST SWELL IS EXPECTED TO ARRIVE ON SUNDAY AND WILL
LIKELY BUILD TO HIGH SURF WARNING LEVELS 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.

THE EAST-SOUTHEAST WINDS ARE BECOMING LIGHT AS A COLD FRONT
APPROACHES THE ALOHA STATE FROM THE NORTHWEST. 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.

THE SMALL CRAFT ADVISORY /SCA/ FOR OUR TYPICAL WINDY ZONES IS NO
LONGER IN EFFECT.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY THROUGH NOON HST TODAY FOR THE BIG ISLAND SUMMITS.

&&

$$

REYNES







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