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Hawaiian Islands Synoptic Discussion and Guidance

682
FXHW60 PHFO 191331 AAA
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST FRI DEC 19 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 TRADE WINDS. GUIDANCE STRONGLY SUPPORTED A DECREASE
IN WIND SPEEDS THROUGH THE DAY...BUT A FEW OUTLYING OBSERVATIONS
SUGGESTED THAT THIS PROCESS MAY TAKE LONGER THAN CURRENTLY EXPECTED.
FOR NOW...WE WILL FOLLOW PRIOR THINKING.

THE INVERSION HEIGHT AND STRENGTH...CLOUD COVER...AND SHOWER
ACTIVITY STILL APPROXIMATED CLIMATOLOGY. RADAR DETECTED SLIGHTLY
FEWER SHOWERS OVER THE COASTAL WATERS...BUT OROGRAPHIC LIFTING
CONTINUED TO SQUEEZE OUT WIDESPREAD...IF GENERALLY LIGHT...
ACCUMULATION OVER RAIN GAUGES...DESPITE PERSISTENTLY LOW
PRECIPITABLE WATER. ALOFT...STRONG AND BROAD RIDGING KEPT CIRRUS
LAYERS FROM THE ITCZ SOUTH OF THE BIG ISLAND...BUT ONLY JUST.

GUIDANCE STILL 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 COLD 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 WORK 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...MAINLY FROM MOLOKAI NORTHWEST. A
SLIGHT CHANCE OF THUNDERSTORMS HAS BEEN INTRODUCED INTO THE FORECAST
FOLLOWING THE LOCAL TOOL RUN OFF OF MODEL CONSENSUS. STRONG VERTICAL
WIND SHEAR WILL IMPROVE THE VENTILATION OF ANY THUNDERSTORMS THAT DO
DEVELOP...POTENTIALLY ALLOWING SOME TO BECOME SEVERE. IT REMAINED
MUCH TOO EARLY TO CONSIDER A SEVERE THUNDERSTORM WATCH...BUT
POSSIBLE HIGH IMPACTS REQUIRED CONTINUED VIGILANCE WITH RESPECT TO
THIS MATTER.

THE COLD POOL ALSO WILL INTRODUCE THE POSSIBILITY OF WINTER WEATHER
OVER HIGHER ELEVATIONS OF MAUI AND THE BIG ISLAND. THIS FAR OUT...
MUCH REMAINED UNCERTAIN...BUT GENERALLY SPEAKING THE AVAILABLE
MOISTURE APPEARED LIMITED LARGELY TO BELOW THE SUMMITS THROUGH MOST
OF THE FORECAST PERIOD. WHERE BOTH COLD TEMPERATURES AND AVAILABLE
MOISTURE COINCIDED...A SLIGHT CHANCE OF LIGHT SNOW HAS BEEN INCLUDED
IN THE FORECAST AS A PLACE HOLDER. DEEPER MOISTURE...OR
THUNDERSTORMS DEVELOPING FURTHER SOUTH THAN EXPECTED...COULD
INCREASE THE RISK OF WINTER WEATHER OVER THE SUMMITS DRAMATICALLY.

FROM WEDNESDAY INTO FRIDAY OF NEXT WEEK...THE TROUGH ALOFT WILL LIFT
OUT TOWARD THE NORTHEAST...AND CONDITIONS WILL RETURN TOWARD
CLIMATOLOGY. RECENT SOLUTIONS TENDED TO DRAW OUT THIS PROCESS...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. ON THE WHOLE THOUGH...TRADE-WIND WEATHER SHOULD
RETURN BY NEXT WEEKEND.

&&

.AVIATION...
WINDWARD AND MAUKA AREAS WILL CONTINUE TO EXPERIENCE PASSING SHOWERS
WITH EMBEDDED ISOL MVFR CONDITIONS. HEAVIER CLOUD COVER AND SHOWERS
REMAIN OVER KAUAI AND BIG ISLAND THIS MORNING...THEREFORE AIRMET
SIERRA CONTINUES. ELSEWHERE...AIRMET SIERRA HAS BEEN CANCELLED.

MODERATE AND GUSTY TRADE WINDS WILL START TO EASE TODAY AS A RIDGE
BEGINS TO BUILD JUST NORTH OF THE ISLANDS. THIS WILL LIKELY ALLOW
FOR THE CANCELLATION OF AIRMET TANGO FOR LOW LEVEL TURB.

THERE IS ALSO MENTION FOR POSSIBLE UPPER LEVEL TURB BETWEEN FL280
AND FL420. A JET...WITH MAX WINDS TO 100 KT...SITS JUST SOUTH OF
HAWAII. AT THIS TIME HAVE NOT RECEIVED ANY PIREPS...THIS WILL BE
UPGRADED TO AN OFFICIAL AIRMET IF REPORTS ARE RECEIVED.

&&

.MARINE...
WITH WINDS EXPECTED TO DIMINISH TODAY...AND THE LARGE NORTHWEST
SWELL SLOWLY PASSING THE STATE...WATERS AROUND KAUAI AND MOST OF
OAHU SHOULD FALL BELOW THE CRITERIA FOR A SMALL CRAFT ADVISORY
/SCA/ THIS MORNING. ELSEWHERE...THE ADVISORY WILL REMAIN IN EFFECT
THROUGH 6 PM HST TODAY. CONDITIONS SHOULD SUBSIDE FURTHER THIS
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 THIS EVENING 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 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.

&&

$$

AVIATION...EATON
REST...RYSHKO



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