Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 221353
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
340 AM HST MON DEC 22 2014

.SYNOPSIS...
A WEAKENING COLD FRONT BOUNDARY WILL MOVE ACROSS OAHU AND STALL NEAR
MAUI TODAY. SHOWERS WILL AFFECT MOST OF THE SMALLER ISLANDS WITH
SLIGHT CHANCES OF THUNDERSTORMS THROUGH THIS AFTERNOON. BEST CHANCES
FOR RAIN REMAIN OVER MOUNTAIN AND WINDWARD AREAS...WITH LOCALIZED
HEAVY DOWNPOURS POSSIBLE. THEN THE ARRIVAL OF AN UPPER LEVEL TROUGH
WILL COMBINE WITH THE FORMATION OF A SFC LOW NEAR THE BIG ISLAND TO
BRING POTENTIAL FOR PERIODS OF HEAVY RAIN AND SCATTERED
THUNDERSTORMS. A FLASH FLOOD WATCH WILL BE IN EFFECT FROM TONIGHT
THROUGH WEDNESDAY. FOR THE SECOND HALF OF THE WEEK...THE FORECAST
WILL DEPEND ON THE FINAL OUTCOME OF THE AFOREMENTIONED LOW NEAR THE
BIG ISLAND.

&&

.DISCUSSION...
THERE HAS BEEN A SIGNIFICANT CHANGE IN FORECAST PHILOSOPHY FOR THE
NEXT 48 HOURS AS THE GLOBAL MODELS HAVE SHIFTED TO A MORE AGGRESSIVE
WEATHER SCENARIO.

AT 330 AM HST...DATA FROM THE DOPPLER RADARS AROUND HAWAII SHOWED
LINES OF SHOWERS WITH LOCALIZED DOWNPOURS MOVING ACROSS KAUAI AND
THE KAUAI CHANNEL WATERS...WHERE A WEAK COLD FRONT IS NOW LOCATED. A
BAND OF CLOUDS MARKING THE LEADING EDGE OF THE FRONT CAN BE OBSERVED
ON LATEST SATELLITE IR IMAGERY. SHOWERS ARE ALSO DEVELOPING ALONG
THE WINDWARD AND NORTH COASTLINE OF OAHU. ALTHOUGH THE OVERALL
MOVEMENT OF THE COLD FRONT BOUNDARY IS TOWARDS THE SE...INDIVIDUAL
CLOUD ELEMENTS INSIDE THE FRONTAL CLOUD BAND ARE STREAMING FROM NE
TO SW ACROSS KAUAI COUNTY...THE KAUAI CHANNEL AND OAHU.

AN UPPER LEVEL TROUGH IS ALSO NEARING THE ALOHA STATE FROM THE NW.
SHOWER COVERAGE WILL CONTINUE TO STEADILY INCREASE TODAY AS THE
FRONTAL BOUNDARY PUSHES THROUGH THE W HALF OF THE ISLAND CHAIN.
MODEL SOLUTIONS BRING THE DECAYING BOUNDARY THROUGH OAHU...AND
STALLING IT NEAR MAUI AROUND NOON...PRODUCING WIDESPREAD SHOWERS
OVER KAUAI...OAHU AND MAUI ALONG WITH SLIGHT CHANCES OF
THUNDERSTORMS. RAINFALL ACCUMULATIONS SHOULD REMAIN MODERATE
TODAY...ALTHOUGH LOCALIZED HEAVY DOWNPOURS CAN NOT BE RULED OUT.

FOR TONIGHT AND INTO TUE...THERE HAS BEEN A DRAMATIC SHIFT IN THE
GLOBAL MODELS. BOTH GFS AND ECMWF HAVE REACHED A REMARKABLE
AGREEMENT IN A SHORT PERIOD OF TIME...WITH THE LATEST SOLUTIONS
DEPICTING THE DEVELOPMENT OF A SFC TROUGH/LOW STRUCTURE OVER THE
ISLANDS. THIS FEATURE WILL BE SUPPORTED BY THE UPPER TROUGH...WHICH
SEEMS TO PUSH ITS AXIS RIGHT ALONG THE ISLAND CHAIN IN THE MOST
RECENT MODEL RUNS. THIS NEW SYNOPTIC SETUP WILL TAP INTO THE
ABUNDANT TROPICAL MOISTURE FAR S OF THE ISLANDS AND BRING A MUCH
MORE VERTICALLY SATURATED PROFILE ACROSS THE ALOHA STATE THAN
PREVIOUSLY ANTICIPATED. THE CURRENT MODEL DEPICTION OF THE LOW
RESEMBLES A KONA LOW FORMING JUST TO THE WNW OF THE BIG ISLAND.
NEEDLESS TO SAY...WITH THIS NEW DEVELOPMENT...THE WHOLE FORECAST HAS
BEEN MODIFIED TO REFLECT A MUCH MORE WETTER AND AGGRESSIVE SCENARIO
FOR THE NEXT 48 HOURS AS THE MOISTURE S OF THE ISLANDS IS ADVECTED
ACROSS THE STATE BY THE LOW. WIDESPREAD SHOWERS...PERIODS OF HEAVY
RAIN AND SCATTERED THUNDERSTORMS WILL BRING POTENTIAL FOR FLASH
FLOODING...STARTING TONIGHT. HIGHEST CHANCES OF RAIN RESIDE OVER
MOUNTAIN AND WINDWARD AREAS. A FLASH FLOOD WATCH WILL BE IN EFFECT
STARTING TONIGHT AND CONTINUING THROUGH WEDNESDAY AFTERNOON FOR ALL
THE ISLANDS.

IN ADDITION...TEMPS AROUND 500 MB WILL DROP TO AROUND -12 TO -16
CELSIUS AND DEEPER MOISTURE WILL MOVE OVER THE BIG ISLAND...WITH
SNOW VERY LIKELY OVER THE MAUNA LOA AND MAUNA KEA SUMMITS TONIGHT
AND AGAIN ON TUE NIGHT. SNOW ACCUMULATIONS OF AROUND 2 INCHES ARE
POSSIBLE...EVEN HIGHER WITH THUNDERSTORMS. A WINTER STORM WATCH HAS
BEEN HAS BEEN ISSUED FOR THE BIG ISLAND SUMMITS STARTING THIS
EVENING AND CONTINUING THROUGH WED.

WINDS WILL BE MOSTLY VARIABLE THROUGH MON MORNING...THEN BECOMING
STRONGER AND VEERING TO THE N AND NW TODAY OVER THE W EDGE OF THE
ISLAND CHAIN BEHIND THE FROPA. HOWEVER...IT HAS BECOME RATHER
DIFFICULT DETERMINING THE FINAL OUTCOME FOR THE WINDS OVER THE E
HALF OF THE ALOHA STATE AS THE OVERALL LOCATION...STRUCTURE AND
INTENSITY OF THE ADVERTISED LOW/TROUGH COMPLEX ON TUE AND BEYOND
REMAIN HIGHLY UNCERTAIN AT THIS TIME. FOR NOW...WILL KEEP A BLEND
OF MODEL SOLUTIONS WITH WEAKER SE WINDS AROUND THE BIG ISLAND AND
MAUI FREQUENTLY SHIFTING DIRECTION THROUGH THE NEXT COUPLE OF DAYS.

THE UNCERTAINTY IN THE FORECAST MODELS EXTENDS INTO THE SECOND HALF
OF THE WEEK...FOR WHICH THE PREVIOUS FORECAST TREND HAS BEEN FOR THE
MOST PART KEPT IN PLACE WITH NE WINDS AND A DRYING TREND GRADUALLY
RETURNING ACROSS THE AREA. SURELY THE LONG RANGE FORECAST WILL BE
ADJUSTED...DEPENDING ON THE FINAL OUTCOME OF THE AFOREMENTIONED
LOW/TROUGH COMPLEX. STAY TUNED.


&&

.AVIATION...
CLOUDS AND SHOWERS ASSOCIATED WITH A WEAKENING AND STALLING COLD
FRONT ARE AFFECTING KAUAI AND TO A LESSER EXTENT OAHU...WHERE
AIRMET SIERRA IS POSTED FOR MOUNTAIN OBSCURATION. AIRMET TANGO IS
POSTED FOR LOW LEVEL TURB OVER KAUAI...DUE TO GUSTY N WINDS THAT
WILL CONTINUE AT LEAST INTO TONIGHT. WIDESPREAD MVFR VIS/CIG IN SHRA
CAN BE EXPECTED IN ASSOCIATION WITH THE FRONTAL CLOUD BAND...WITH
THE POSSIBILITY OF IFR VIS/CIG IN AREAS OF HEAVIER SHOWERS. VFR
CONDITIONS WILL PREVAIL OVER MAUI AND THE BIG ISLAND INTO TONIGHT...
WITH THE FRONTAL CLOUD BAND EXPECTED TO BE RELATIVELY SLOW IN ITS
SOUTHEASTWARD PROGRESSION ACROSS THE ISLAND CHAIN. LATER TODAY INTO
TUESDAY HOWEVER...A DEVELOPING LOW ALOFT WILL BRING THE STATEWIDE
POSSIBILITY OF THUNDERSTORMS AND THEIR ASSOCIATED AVIATION HAZARDS...
INCLUDING ICING...TURBULENCE...WIND SHEAR AND IFR CONDITIONS.

&&

.MARINE...
A LARGE NORTHWEST SWELL IS KEEPING WARNING LEVEL SURF OVER NORTH AND
WEST FACING SHORES OF THE SMALLER ISLANDS AND WEST FACING SHORES OF
THE BIG ISLAND UNTIL 6 AM HST TUESDAY. THIS SWELL WILL ALSO GENERATE
SMALL CRAFT ADVISORY CONDITIONS OVER MOST COASTAL WATERS DUE TO
HAZARDOUS SEAS.

STRONG NORTH TO NORTHEAST WINDS ARE EXPECTED TO MOVE OVER THE AREA
STARTING LATER TODAY OVER THE NORTHERN WATERS. BUT IT IS NOT CLEAR
IF SMALL CRAFT ADVISORY CONDITIONS WILL BE EXPERIENCED AGAIN BEYOND
TUESDAY MORNING. THEREFORE...THE SMALL CRAFT ADVISORY WILL BE
ALLOWED
TO EXPIRE AT 6 AM HST TUESDAY AND REISSUED LATER ON IF CONDITIONS
WARRANT IT.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM THIS EVENING THROUGH WEDNESDAY AFTERNOON
FOR ALL HAWAII ISLANDS.

HIGH SURF WARNING UNTIL 6 AM HST TUESDAY FOR NIIHAU-KAUAI
WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH SHORE-OAHU
KOOLAU-OLOMANA-MOLOKAI-MAUI WINDWARD WEST-MAUI CENTRAL VALLEY-
WINDWARD HALEAKALA-KONA-KOHALA.

WINTER STORM WATCH FROM THIS EVENING THROUGH WEDNESDAY AFTERNOON
FOR BIG ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR ALL HAWAIIAN
WATERS EXCEPT MAALAEA BAY-

&&

$$

DISCUSSION/MARINE...REYNES
AVIATION...BIRCHARD





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