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Hawaiian Islands Synoptic Discussion and Guidance

911
FXHW60 PHFO 271954
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST TUE JAN 27 2015

.SYNOPSIS...
WEAKENING TRADE WINDS WILL TURN SOUTHEASTERLY ON WEDNESDAY...AS A
WEAK FRONT APPROACHES FROM THE WEST. THIS FRONT WILL BRING SOME
SHOWERS AND LIGHT WINDS TO THE ISLANDS THURSDAY AND FRIDAY...THEN
LIGHT NORTHEASTERLY TRADES WILL FILL BACK ON OVER THE WEEKEND. A
STRONGER FRONT MAY APPROACH THE ISLANDS EARLY NEXT WEEK.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE TO THE SOUTH AND EAST
OF THE STATE. A SHORTWAVE TROUGH IS LOCATED WEST OF THE DATELINE...
WHICH WILL COME INTO PLAY LATER IN THE WEEK. AT THE SURFACE...THE
RIDGE IS LOCATED A COUPLE HUNDRED MILES NORTH OF KAUAI...WITH A COLD
FRONT OVER 600 MILES TO THE NORTHWEST. 12Z RAOBS FROM HILO AND LIHUE
SHOW INVERSIONS OF 4-6KFT AND PRECIPITABLE WATERS OF 0.8-1.0 INCHES.
EARLY MORNING MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS SIMILAR
VALUES UPSTREAM TO THE NORTHEAST AND EAST...WITH A NARROW BAND OF
SLIGHTLY HIGHER PW VALUES POOLED ALONG THE FRONT TO THE NORTHWEST.
BANDS OF HIGHER MOISTURE ARE ALSO LOCATED FAR SOUTHEAST OF THE BIG
ISLAND.

EVEN THOUGH SURFACE PRESSURES ACROSS THE ISLANDS HAVE RISEN AROUND
1MB IN THE PAST 24 HOURS...THE PRESSURE DIFFERENCE BETWEEN BUOY
51101 AND 51003 HAS DROPPED ABOUT 2MB OVER THE SAME PERIOD. THE
SURFACE RIDGE IS WEAKENING AS IT SHIFTS SOUTHWARD OVER THE STATE...
LEADING TO A WEAKENING GRADIENT AND DECREASE IN WIND SPEEDS. TRADES
WILL WEAKEN AND VEER SOUTHEAST ON WEDNESDAY AS THE RIDGE MOVES SOUTH
ACROSS THE STATE. THE AIRMASS IS INITIALLY DRY AND STABLE...WHICH
WILL CONTINUE THROUGH WEDNESDAY. EVEN WITH ONSHORE FLOW WITH SEA
BREEZES DURING THE AFTERNOON...WHICH WILL BE MORE WIDESPREAD ACROSS
THE WESTERN HALF OF THE STATE...THE STABLE ATMOSPHERE WILL LIMIT
RAINFALL AMOUNTS FROM AFTERNOON SHOWERS.

THE FRONT CURRENTLY NORTHWEST OF THE STATE IS EXPECTED TO WEAKEN AS
IT APPROACHES THE ISLANDS WEDNESDAY NIGHT. THE SHORTWAVE NEAR THE
DATELINE IS EXPECTED TO AMPLIFY AND MAY EITHER LEAD TO A SECONDARY
FRONT FORMING BEHIND THE FIRST...AS INDICATED IN THE 12Z GFS AND
ECMWF...OR JUST A REINTENSIFICATION OF THE ORIGINAL FRONT. EITHER
WAY...THIS FEATURE WILL GRADUALLY PROPEL THE FRONT SOUTHEAST ACROSS
THE ISLANDS THROUGH FRIDAY. THERE IS LIMITED UPPER LEVEL SUPPORT
ASSOCIATE WITH THE FRONT...AND MOISTURE WILL BE MARGINAL. FORECAST
SOUNDINGS AND CROSS SECTIONS SHOW INVERSIONS RAISING/MOISTURE DEPTH
INCREASING SLIGHTLY WITH THE FRONT...ALTHOUGH RAINFALL AMOUNTS ARE
STILL EXPECTED TO BE RELATIVELY LIGHT.

BOTH THE GFS AND ECMWF SHOW THE SURFACE BOUNDARY STALLING NEAR THE
BIG ISLAND LATE FRIDAY/SATURDAY...WITH SOME ENHANCEMENT FROM THE
SOUTHEAST. WITH THE LOW-LEVEL FLOW TURNING SOUTHEASTERLY AS THE
RIDGE SHIFTS EASTWARD...THE EXISTING CLOUD BANDS ASSOCIATED WITH OLD
FRONTS CURRENTLY SOUTHEAST OF THE BIG ISLAND WILL BE PULLED
NORTHWESTWARD TOWARD THE STATE. THE 12Z MODELS SHOW THESE FEATURES
ENHANCING THE MOISTURE AND LOW-LEVEL VORTICITY ASSOCIATED WITH THE
OLD FRONT...AND LINGERING THIS BAND NEAR THE BIG ISLAND THROUGH THE
WEEKEND. THIS FEATURE IS EXPECTED TO LIFT NORTHWARD EARLY NEXT WEEK
AS THE NEXT SYSTEM APPROACHES FROM THE WEST...BUT IT MAY STILL
ENHANCE MAINLY WINDWARD SHOWERS ACROSS THE BIG ISLAND AND MAUI.

EARLY NEXT WEEK A STRONGER FRONT MAY APPROACH THE ISLANDS...BUT
THERE REMAINS A FAIR AMOUNT OF UNCERTAINTY WITH THIS SYSTEM. THE 12Z
GFS NOW DEVELOPS A LOW MUCH FARTHER SOUTH...TRACKING ALONG
30N...THAT PROPELS A FRONT SOUTHWARD ACROSS THE ISLANDS. EARLIER
RUNS HAD VARYING DEGREES OF FEEDBACK THAT AFFECTED TIMING AND
PLACEMENT AS WELL...BUT STILL FEATURED THE PARENT LOW MUCH FARTHER
NORTH. THE 12Z ECMWF HAS A WEAKER BOUNDARY THAT COMES THROUGH WITH A
SIMILAR TIMING...BUT WITH THE SURFACE WAVE TRACKING CLOSER TO 40N
THAN TO 30N. ONLY ONE GFS ENSEMBLE MEMBER IS CLOSE TO THE STRENGTH
OF THE OPERATIONAL RUN...WITH MOST MUCH WEAKER. THE CURRENT FORECAST
FEATURES WINDS THAT ARE MUCH WEAKER THAN THE OPERATIONAL GFS...WHICH
SEEMS REASONABLE GIVEN ITS OUTLIER STATUS.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE STATE TODAY. WINDWARD
SECTIONS OF ALL ISLANDS MAY SEE ISOLATED MVFR CEILINGS AT TIMES
THROUGHOUT THE DAY...LESS SO AROUND OAHU...BUT OCCASIONALLY OVER
LANAI AT MIDDAY INTO THE AFTERNOON.

WINDS LIGHTENING UP AS A FRONT APPROACHING FROM THE WEST WILL
GRADUALLY WEAKEN AND PUSH THE HIGH PRESSURE RIDGE JUST NORTH OF THE
STATE EASTWARD. LOOKING FOR LIGHT TRADES TODAY WITH SOME SEA BREEZES
TAKING OVER AT TIMES AND LIGHT AND VARIABLE WINDS AT NIGHT WITH SOME
LAND BREEZES PREVAILING IN SOME AREAS.

&&

.MARINE...
LONG PERIOD FORERUNNERS OF THE NEXT NORTHWEST SWELL PICKED UP AT
BUOY 51101 EARLY THIS MORNING...WITH OVERALL SWELL HEIGHT SHOWING
ONLY A GRADUAL RISE. THIS SWELL IS EXPECTED TO BRING SURF TO
ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES. WE WILL HOLD OFF
TO SEE HOW THE SWELL BUILDS AT THE BUOY BEFORE ISSUING THE ADVISORY
FOR THE STATE...WHICH WOULD BE NEEDED AFTER MIDNIGHT GIVEN THE
WAVEWATCH III TIMING. THIS SWELL IS EXPECTED TO PEAK
WEDNESDAY/WEDNESDAY NIGHT...THEN DIMINISH ON THURSDAY. WITH THE
INCOMING SWELL...COMBINED SEAS MAY ALSO FLIRT WITH THE 10 FOOT
SMALL CRAFT ADVISORY LEVEL FOR SOME AREAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...FUJII



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