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Hawaiian Islands Synoptic Discussion and Guidance

912
FXHW60 PHFO 190614
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST THU DEC 18 2014

.SYNOPSIS...
MODERATE TRADE WINDS WILL KEEP MOSTLY LIGHT SHOWERS FOCUSED OVER
WINDWARD SLOPES INTO THE WEEKEND. FROM SUNDAY NIGHT INTO MONDAY...A
FRONT WILL SPREAD RAIN...COLD AIR...AND STRONG NORTHERLY WINDS
ACROSS THE STATE FROM THE NORTHWEST. UNSETTLED WEATHER WILL CONTINUE
THROUGH ABOUT TUESDAY...BEFORE RETURNING GRADUALLY TO TRADE-WIND
CONDITIONS BY THE END OF NEXT WEEK.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED
MOSTLY MODERATE TO STRONG TRADE WINDS. UNDER A FAIRLY TYPICAL
INVERSION...CLOUD COVER AND SHOWER ACTIVITY APPROXIMATED
CLIMATOLOGY...EXCEPT FOR ENHANCED STRATOCUMULUS AND STRATUS COVER
PULSING UP NORTHWEST OF OAHU UNDER THE INFLUENCE OF A PASSING
GRAVITY WAVE. RADAR DETECTED SCATTERED SHOWERS OVER THE COASTAL
WATERS...BUT ACCUMULATION OVER LAND REMAINED GENERALLY LIGHT...AND
LIMITED MAINLY TO WINDWARD SLOPES. A PASSING CLUMP OF CIRRUS OVER
MAUI COUNTY SHOULD DISSIPATE PROMPTLY...THOUGH MORE LAYERS MOVING
NORTH OUT OF THE ITCZ MAY SHADE THE STATE INTO FRIDAY. ALOFT...
STRONG AND BROAD RIDGING PREVAILED OVER THE ISLANDS...SUPPORTING THE
SUBTROPICAL RIDGE AT THE SURFACE TO THE NORTH.

GUIDANCE AGREED FAIRLY WELL THAT A LONG-WAVE MID-LEVEL TROUGH WILL
DIG TOWARD THE STATE FROM THE NORTHWEST THROUGH THE WEEKEND...
EFFECTIVELY DISPLACING THE RIDGE BY SUNDAY EVENING...AND THEN
SHARPENING TOWARD THE SOUTHWEST INTO TUESDAY. THIS DEVELOPMENT ALOFT
WILL SUPPORT THE APPROACH OF A FRONT AT THE SURFACE. THROUGH ABOUT
SUNDAY...THE SUBTROPICAL RIDGE WILL RECEDE TOWARD THE EAST AHEAD OF
THE FRONT...CAUSING SURFACE WINDS AROUND THE ISLANDS TO DIMINISH AND
BECOME MORE SOUTHEASTERLY. FROM SUNDAY NIGHT INTO NEXT WEEK...THE
FRONT WILL WORKS ITS WAY SOUTHEAST ACROSS THE STATE. WITH
SIGNIFICANT SUPPORT FROM THE DIGGING TROUGH ALOFT...THE FRONT WILL
HAVE THE OPPORTUNITY TO PRODUCE WIDESPREAD RAIN. THE PUSH OF COLD
AIR AT LOW LEVELS BEHIND THE FRONT ALSO WILL BRING STRONG NORTHERLY
WINDS.

PERHAPS MOST IMPORTANTLY...A VERY COLD POOL OF AIR ALOFT WILL
ACCOMPANY THE MID-LEVEL TROUGH. THIS COLD POOL WILL DESTABILIZE THE
ATMOSPHERE SIGNIFICANTLY...INTRODUCING A NON-TRIVIAL RISK OF
THUNDERSTORMS AROUND MONDAY...ESPECIALLY FROM MOLOKAI NORTHWEST.
STRONG VERTICAL WIND SHEAR WILL IMPROVE THE VENTILATION OF ANY
THUNDERSTORMS THAT DO DEVELOP...POTENTIALLY ALLOWING SOME TO BECOME
SEVERE. WHILE MODEL DIFFERENCES CAST SOME DOUBT ON THE LIKELIHOOD OF
SUCH AN OUTCOME THAT FAR OUT...POSSIBLE HIGH IMPACTS REQUIRED
CONTINUED VIGILANCE. AT THE VERY LEAST...IF CONSENSUS CONTINUES DOWN
THIS ROAD...THEN IT WILL BECOME PRUDENT SOON TO MENTION IN THE
OFFICIAL FORECAST A SLIGHT CHANCE OF THUNDERSTORMS AROUND MONDAY FOR
KAUAI AND OAHU.

FROM WEDNESDAY INTO FRIDAY OF NEXT WEEK...THE TROUGH ALOFT WILL LIFT
OUT TOWARD THE NORTHEAST...AND CONDITIONS WILL RETURN TOWARD
CLIMATOLOGY...THOUGH THIS PROCESS REMAINED SLIGHTLY SLOWER IN ECMWF
SOLUTIONS. MOST MODELS ALSO KEPT LOCAL SURFACE WINDS MORE NORTHERLY
THAN USUAL AS LATE AS FRIDAY. ON THE WHOLE THOUGH...TRADE-WIND
WEATHER WILL PREVAIL BY NEXT WEEKEND.

&&

.MARINE...
A COMBINATION OF INCREASING NORTHWEST SWELL AND STRONG WINDS
CONTINUED TO REQUIRE A SMALL CRAFT ADVISORY /SCA/ FOR NEARLY ALL
COASTAL WATERS THROUGH 6 PM HST FRIDAY. CONDITIONS SHOULD SUBSIDE
FRIDAY EVENING...BUT THEN ANOTHER LARGE NORTHWEST WILL PRODUCE SCA
CONDITIONS AGAIN BY SATURDAY NIGHT.

AN EVEN LARGER NW SWELL IS EXPECTED TO ARRIVE ON SUNDAY...LIKELY
PUSHING SURF HEIGHTS INTO WARNING LEVELS OVER BOTH NORTH- AND
WEST-FACING SHORES OF MOST ISLANDS. DEEP LOW PRESSURE FAR NORTH OF
THE STATE MAY CONTINUE TO PRODUCE SWELLS KEEPING SURF ELEVATED TO
NEAR THE ADVISORY LEVELS THROUGH THE SECOND HALF OF NEXT WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST FRIDAY FOR NIIHAU-KAUAI
WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH SHORE-OAHU
KOOLAU-MOLOKAI-MAUI WINDWARD WEST-WINDWARD HALEAKALA.

SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR ALL HAWAIIAN
WATERS.

&&

$$

RYSHKO



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