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Hawaiian Islands Synoptic Discussion and Guidance

465
FXHW60 PHFO 311433 CCA
AFDHFO

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SAT JAN 31 2015

.SYNOPSIS...
TRADE WINDS ARE GRADUALLY RETURNING TO THE ALOHA STATE THIS WEEKEND
AS A SURFACE RIDGE BUILDS NORTH OF THE AREA. LOW CLOUDS AND SHOWERS
WILL FAVOR WINDWARD AND MAUKA SECTIONS...BUT ISOLATED SHOWERS MAY
DEVELOP OVER LEEWARD SECTIONS EACH AFTERNOON. STARTING LATE
MONDAY...A FRONT APPROACHING THE ISLANDS FROM THE NORTHWEST WILL
CAUSE GUSTY SOUTHWESTERLY WINDS AND WET WEATHER TO SPREAD ACROSS THE
ISLAND CHAIN. AFTER THE FRONT PASSES...THE CIRCULATION AROUND A
BROAD SURFACE LOW NORTH OF THE ISLANDS WILL MAINTAIN AN EXTENDED
PERIOD OF GUSTY WESTERLY WINDS AND WET WEATHER ACROSS MOST OF 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 THE VICINITY OF THE BIG ISLAND
ACCORDING TO SATELLITE IMAGERY. IN ADDITION TO THE FORMER FRONT...A
WEAK SURFACE RIDGE IS JUST NORTH OF THE ISLANDS. THE CLOSE PROXIMITY
OF THESE SYNOPTIC FEATURES IS KEEPING WINDS ACROSS THE ISLANDS LIGHT
AND VARIABLE EARLY THIS MORNING. IN THE UPPER LEVELS ABOVE THE
STATE...WATER VAPOR IMAGERY AND THE EARLY MORNING BALLOON SOUNDINGS
FROM HILO AND LIHUE INDICATE THE UPPER LEVEL FLOW IS NEARLY ZONAL.
HOWEVER...A MID-LEVEL TROUGH LOCATED WEST OF KAUAI IS PROPAGATING
SLOWLY TOWARD THE ISLANDS. THIS FEATURE IS PRODUCING SOME DYNAMIC
LIFT ACROSS THE REGION...SO THE COVERAGE OF LOW CLOUDS AND SHOWERS
HAS INCREASED DURING THE PAST FEW HOURS ACROSS MANY PARTS OF THE
ISLAND CHAIN COMPARED WITH EARLIER TONIGHT. THE ATMOSPHERE IS RATHER
DRY NEAR KAUAI ACCORDING TO THE EARLY MORNING SOUNDING...WITH
HIGHER MOISTURE VALUES OVER THE EASTERN ISLANDS. AS A RESULT...THERE
ARE LOW CLOUDS AND ISOLATED SHOWERS MAINLY OVER THE EASTERN HALF OF
THE STATE...ESPECIALLY IN THE VICINITY OF THE WINDWARD AND
SOUTHEASTERN BIG ISLAND. RADAR REFLECTIVITY RETURNS APPEAR TO
INDICATE THE SHOWERS ARE RELATIVELY LIGHT.

THE FORECAST MODELS INDICATE THE SURFACE FRONT WILL CONTINUE TO
DISSIPATE NORTHEAST OF THE BIG ISLAND. THEY ALSO SHOW THE SURFACE
RIDGE WILL CONTINUE TO BUILD NORTH OF THE AREA THIS WEEKEND. AS THIS
OCCURS...THE PRESSURE GRADIENT WILL INCREASE ACROSS THE ISLANDS...
WHICH WILL ALLOW WEAK TRADES TO DEVELOP 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. SOME SHOWERS MAY ALSO DEVELOP OVER LEEWARD SECTIONS
DURING THE AFTERNOON HOURS.

THE FORECAST GUIDANCE SHOWS THE BRIEF PERIOD OF TRADES WILL CONTINUE
INTO MONDAY. BY LATE MONDAY...A FRONT WILL APPROACH KAUAI FROM THE
NORTHWEST. THIS FRONT WILL NOT ONLY BRING WET WEATHER TO THE
ISLANDS...BUT IT WILL INCREASE SOUTHWEST WINDS ACROSS THE STATE.
THESE STRONG SOUTHWESTERLY WINDS AHEAD OF THE FRONT MAY PRODUCE
LOCALLY GUSTY DOWNSLOPE WINDS OVER NORTHEAST AND EAST FACING SLOPES
OF SOME OF THE SMALLER ISLANDS THROUGH TUESDAY.

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 AFTER THE FRONT
PASSES THROUGH LATE TUESDAY. BOTH MODELS ALSO INDICATE CLOUDY AND
WET WEATHER WILL CONTINUE...ESPECIALLY OVER THE SMALLER ISLANDS. THE
MODELS APPEAR TO BRING 1 OR 2 ADDITIONAL FRONTS DOWN ACROSS THE
ISLANDS FROM WEDNESDAY NIGHT THROUGH EARLY FRIDAY. THE GFS MODEL
SHOWS AN EXTENDED PERIOD OF UNUSUALLY STRONG WESTERLY WINDS ACROSS
THE STATE AS THE BROAD SURFACE LOW SPINS 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 BEYOND TUESDAY 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 BECOME
AVAILABLE.

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 STRONG GUSTY WINDS ARE ACCOMPANIED BY HEAVY RAIN
THAT ADDS ADDITIONAL STRESS TO WEAK TREE LIMBS. ANYBODY LIVING IN
THESE POTENTIALLY VULNERABLE LOCATIONS SHOULD MONITOR FUTURE
FORECAST UPDATES FOR THE POTENTIAL OF DAMAGING LOCAL WIND EFFECTS
DEVELOPING NEXT WEEK.

&&

.AVIATION...
WHILE VFR CONDITIONS ARE EXPECTED AT MOST OF THE TERMINALS...
INTERMITTENT MVFR CONDITIONS ARE OCCURRING ACROSS NORTHEAST-FACING
SLOPES OF THE EASTERN ISLANDS THIS MORNING. CLOUD COVER IS
SUFFICIENT TO WARRANT ISSUANCE OF AIRMET SIERRA FOR MOUNTAIN
OBSCURATION ACROSS ALL BIG ISLAND SLOPES ABOVE 3000 FEET AND BELOW
7000 FEET. THIS CLOUD COVER IS EXPECTED TO LAST THROUGH THE MORNING.

&&

.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 MORNING. THE MOST RECENT OBSERVATIONS FROM
NATIONAL DATA BUOY CENTER BUOY NUMBER 51101 LOCATED NORTHWEST OF
KAUAI INDICATES THE SWELL HEIGHT IS 11 TO 12 FEET WITH A PERIOD OF
AROUND 14 SECONDS. CLOSER TO THE ISLANDS...THE WAIMEA BUOY READINGS
WERE 8 TO 9 FEET WITH A PERIOD OF AROUND 13 SECONDS. WEST FACING
SHORES OF OAHU AND MOLOKAI HAVE BEEN ADDED TO THE HIGH SURF
ADVISORY. THE HIGH SURF ADVISORY MAY NEED TO BE EXTENDED BEYOND 6 AM
HST SUNDAY MORNING IF BUOY READINGS SHOW THE SWELL REMAINS ELEVATED
LATER TONIGHT. THIS NORTH NORTHWEST SWELL SHOULD START TO GRADUALLY
SUBSIDE STARTING SUNDAY AFTERNOON.

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 TONIGHT.
THEREFORE...A SCA REMAINS IN EFFECT DUE TO ELEVATED SEAS FOR THE
WATERS SURROUNDING KAUAI...THE OAHU WINDWARD WATERS...THE KAIWI
CHANNEL AND THE MAUI COUNTY WINDWARD WATERS.

WINDS ARE CURRENTLY FORECAST TO REMAIN BELOW THE SCA CRITERIA THIS
WEEKEND. HOWEVER...AS SOUTHWEST WINDS AHEAD OF THE FRONT BEGIN TO
INCREASE...WINDS ARE FORECAST TO REACH THE SCA CRITERIA ACROSS PARTS
OF THE WESTERN COASTAL WATERS STARTING LATE MONDAY. STRONG SOUTHWEST
TO WEST WINDS MAY CONTINUE ACROSS MOST OF THE COASTAL WATERS FROM
TUESDAY THROUGH THE END OF THE NEW WORK WEEK. IN ADDITION TO THE
WINDS...SEAS WILL ALSO BECOME ELEVATED AGAIN NEXT WEEK...ESPECIALLY
STARTING LATE WEDNESDAY.

&&

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