Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 310650
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST FRI JAN 30 2015

.SYNOPSIS...
LOW CLOUDS AND ISOLATED SHOWERS ASSOCIATED WITH THE REMNANTS OF A
FORMER FRONTAL SYSTEM LINGER OVER PARTS OF THE ISLAND CHAIN TONIGHT.
TRADE WINDS ARE EXPECTED TO RETURN TO THE ALOHA STATE LATER THIS
WEEKEND AS A SURFACE RIDGE BUILDS NORTH OF THE AREA. THE TRADES WILL
CONTINUE INTO EARLY NEXT WEEK...WITH LOW CLOUDS AND SHOWERS MAINLY
OVER WINDWARD AND MAUKA AREAS. STARTING TUESDAY...THE CIRCULATION
AROUND A BROAD SURFACE LOW NORTH OF THE ISLANDS WILL BRING AN
EXTENDED PERIOD OF GUSTY WESTERLY WINDS AND WET WEATHER TO THE
STATE. THESE UNSETTLED WEATHER CONDITIONS MAY CONTINUE THROUGH THE
END OF NEXT WEEK.

&&

.DISCUSSION...
THE TAIL END OF A WEAKENING NORTH NORTHEAST TO SOUTH SOUTHWEST
ORIENTED FRONTAL SYSTEM IS EVIDENT IN SATELLITE IMAGERY JUST
NORTHEAST OF THE BIG ISLAND. AS A RESULT...THERE ARE LINGERING LOW
CLOUDS AND ISOLATED SHOWERS OVER PARTS OF THE STATE...ESPECIALLY IN
THE VICINITY OF THE WINDWARD AND SOUTHEASTERN BIG ISLAND. RADAR
REFLECTIVITY DATA APPEARS TO INDICATE THE FEW SHOWERS THAT ARE
FALLING ARE RELATIVELY LIGHT.

IN ADDITION TO THE FORMER FRONT...A WEAK SURFACE RIDGE IS NOSING
INTO THE ISLANDS FROM THE WEST NORTHWEST. THE CLOSE PROXIMITY OF
THESE SYNOPTIC FEATURES IS KEEPING WINDS ACROSS THE ISLANDS LIGHT
AND VARIABLE TONIGHT. IN THE UPPER LEVELS ABOVE THE STATE...WATER
VAPOR IMAGERY AND THE AFTERNOON BALLOON SOUNDINGS FROM HILO AND
LIHUE INDICATE THE FLOW ALOFT IS NEARLY ZONAL. THE SOUNDINGS ALSO
SHOW RELATIVELY STABLE ATMOSPHERIC CONDITIONS REMAIN ACROSS THE
REGION.

THE FORECAST MODELS INDICATE THE SURFACE FRONT WILL CONTINUE TO
DISSIPATE NORTHEAST OF THE BIG ISLAND. THEY ALSO SHOW A SURFACE
RIDGE WILL BUILD NORTH OF THE AREA SATURDAY. AS THIS OCCURS...THE
PRESSURE GRADIENT WILL INCREASE ACROSS THE ISLANDS...WHICH WILL
BRING WEAK TRADES LATER THIS WEEKEND. RELATIVELY DRY AIR WILL ALSO
BE OVER THE ISLANDS...AND THE TRADES WILL CARRY SOME LOW CLOUDS AND
LIGHT SHOWERS MAINLY INTO WINDWARD AND MAUKA SECTIONS OF THE
ISLANDS.

THE FORECAST GUIDANCE SHOWS THE BRIEF PERIOD OF TRADES WILL CONTINUE
INTO EARLY NEXT WEEK. HOWEVER...THE FORECAST BECOMES CHALLENGING
AFTER MONDAY. BY TUESDAY...ANOTHER FRONT WILL PUSH DOWN ACROSS THE
ISLANDS FROM THE NORTHWEST. THE GFS AND ECMWF MODELS DIFFER ON THE
EXACT PLACEMENT OF THE PARENT SURFACE LOW ASSOCIATED WITH THIS
FRONT. HOWEVER...BOTH AGREE THAT WESTERLY SURFACE WINDS WILL
DOMINATE OUR WEATHER BY EARLY TUESDAY AFTER THE FRONT PASSES
THROUGH. BOTH MODELS ALSO INDICATE CLOUDY AND WET WEATHER WILL
DEVELOP...ESPECIALLY OVER THE SMALLER ISLANDS. THE GFS MODEL BRINGS
ANOTHER WELL-DEFINED FRONT ACROSS THE ISLANDS WEDNESDAY NIGHT AND
FRIDAY...WHILE THE ECMWF MODEL HAS LESS-DEFINED FEATURES MOVING NEAR
OR NORTH OF THE ISLANDS. BOTH MODELS APPEAR TO SHOW THE WET WEATHER
CONTINUING THROUGH THE END OF NEXT WEEK. HOWEVER...THE GFS SHOWS AN
EXTENDED PERIOD OF UNUSUALLY STRONG WESTERLY WINDS ACROSS THE STATE
AS THE BROAD SURFACE LOW SPINS FAR NORTH OF THE AREA. THE ECMWF
MODEL SHOWS SOMEWHAT WEAKER WESTERLY WINDS...SO WE WILL NEED TO
MONITOR THE FUTURE CHANGES IN THE MODEL OUTPUT BEFORE WE CAN BE
CERTAIN THAT THEY HAVE CONVERGED ON A REALISTIC SOLUTION.

WITH SOME UNCERTAINTY REMAINING IN THE MODEL SOLUTIONS...THE CURRENT
FORECAST IS A BLEND OF THE MODELS. HOWEVER... DURING TIME PERIODS
WITH SIGNIFICANT DIFFERENCES IN THE GUIDANCE...THE FORECAST HAS BEEN
TRENDED TOWARDS THE GFS SOLUTION. FURTHER REFINEMENTS WILL BE NEEDED
AS ADDITIONAL MODEL RUNS ARE EXAMINED.

FINALLY...ASSUMING THERE IS AN EXTENDED PERIOD OF WESTERLY WINDS...
THIS WIND DIRECTION IS VERY UNUSUAL IN THE ISLANDS FOR MORE THAN A
DAY OR TWO. NOTE THAT WINDS FROM THE WEST MAY PRODUCE LOCAL
ACCELERATIONS AND STRONG GUSTS DUE TO DOWNSLOPING ON THE DOWNWIND
SIDES OF HIGHER TERRAIN...SUCH AS THE EAST AND NORTHEAST FACING
SLOPES OF THE WAIANAE AND KOOLAU MOUNTAIN RANGES ON OAHU. SHOULD
THESE UNUSUALLY GUSTY CONDITIONS OCCUR OVER A PERIOD OF DAYS...THERE
MAY BE POTENTIAL FOR SOME STRUCTURES AND VEGETATION TO BE ADVERSELY
IMPACTED. LOCALIZED POWER OUTAGES ARE ALSO A POSSIBILITY...
ESPECIALLY IF THE WINDS ARE ACCOMPANIED BY HEAVY RAIN. ANYBODY
LIVING IN THESE POTENTIALLY VULNERABLE LOCATIONS SHOULD MONITOR
FUTURE FORECAST UPDATES FOR THE POTENTIAL OF SEVERE OR DAMAGING
LOCAL WIND EFFECTS.

&&

.AVIATION...
WE EXPECT PREDOMINANTLY VFR CONDITIONS AT THE TERMINALS TONIGHT.
LINGERING INTERMITTENT MVFR CONDITIONS ARE POSSIBLE ALONG WINDWARD
SLOPES AS LOW CLOUDS CLING TO THE MOUNTAINS AND EXTEND OFFSHORE TO
THE NORTHEAST. NO AIRMETS ARE CURRENTLY IN EFFECT AND NONE ARE
ANTICIPATED AT THIS TIME.

&&

.MARINE...
THE CURRENT NORTHWEST SWELL CONTINUES TO PRODUCE ELEVATED SURF ALONG
MOST NORTH AND SOME WEST FACING SHORES OF THE SMALLER HAWAIIAN
ISLANDS EARLY THIS EVENING. THE MOST RECENT OBSERVATIONS FROM
NATIONAL DATA BUOY CENTER BUOY NUMBER 51101 LOCATED NORTHWEST OF
KAUAI INDICATES THE SWELL HEIGHT IS 12 TO 13 FEET WITH A PERIOD OF
12 TO 13 SECONDS. CLOSER TO THE ISLANDS...THE WAIMEA BUOY READINGS
WERE 8 TO 9 FEET WITH A PERIOD OF 12 TO 13 SECONDS. A SLIGHTLY
LARGER REINFORCING NORTH NORTHWEST SWELL IS EXPECTED TO REACH BUOY
51101 SHORTLY AFTER MIDNIGHT HST SATURDAY MORNING. THIS LARGER SWELL
WILL REACH THE SMALLER HAWAIIAN ISLANDS DURING THE DAY SATURDAY...
WHICH WILL MAINTAIN ELEVATED SURF INTO THE WEEKEND. NOTE THAT IF THE
SWELL IS AS LARGE AS EXPECTED...WEST FACING SHORES OF OAHU AND
MOLOKAI MAY NEED TO BE ADDED TO THE HIGH SURF ADVISORY ON SATURDAY.
THIS NORTH NORTHWEST SWELL WILL GRADUALLY FADE BY LATE SUNDAY.

IN ADDITION TO HIGH SURF ACROSS PARTS OF THE STATE...THE ELEVATED
SWELL HAS INCREASED SEAS ACROSS PORTIONS OF THE HAWAIIAN COASTAL
WATERS TO THE SMALL CRAFT ADVISORY /SCA/ THRESHOLD THROUGH SATURDAY
NIGHT. THIS HAS REQUIRED THE ISSUANCE OF A SCA FOR ELEVATED SEAS FOR
THE WATERS SURROUNDING KAUAI...THE OAHU WINDWARD WATERS...THE KAIWI
CHANNEL AND THE MAUI COUNTY WINDWARD WATERS. NOTE THAT WINDS ARE
CURRENTLY FORECAST TO REMAIN BELOW THE SCA CRITERIA THROUGH EARLY
NEXT WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST SUNDAY FOR NORTH AND WEST FACING
SHORES OF NIIHAU AND KAUAI...AND FOR NORTH FACING SHORES OF OAHU...
MOLOKAI AND MAUI.

SMALL CRAFT ADVISORY UNTIL 6 AM HST SUNDAY FOR THE WATERS
SURROUNDING KAUAI...THE OAHU WINDWARD WATERS...THE KAIWI CHANNEL AND
THE MAUI COUNTY WINDWARD WATERS.

&&

$$

DISCUSSION/MARINE...HOUSTON
AVIATION...POWELL





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