Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 210156
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
356 PM HST MON APR 20 2015

.SYNOPSIS...
HIGH PRESSURE PASSING NORTH OF THE STATE WILL BRING STRONG TRADE
WINDS THROUGH TUESDAY. THE WINDS WILL FOCUS CLOUDS AND SHOWERS
OVER WINDWARD SLOPES AND COASTS...BUT SOME SHOWERS WILL SPREAD
LEEWARD OF THE SMALLER ISLANDS. SHOWERS WILL BE MOST ACTIVE DURING
THE NIGHTS AND MORNINGS. BY THE MIDDLE OF THE WEEK...A LOW DEVELOPING
NORTHWEST OF THE ISLANDS WILL WEAKEN WINDS AND VEER THEM OUT OF
THE SOUTHEAST.

&&

.DISCUSSION...
VISIBLE SATELLITE AND RADAR SHOW SCATTERED PASSING TRADE WIND SHOWERS
ACROSS HAWAII THIS AFTERNOON. TRADE WINDS WILL CONTINUE TO BE
STRONG...DRIVEN BY AN EASTWARD TRAVELING HIGH PRESSURE SYSTEM
NORTH OF THE STATE. THE BREEZY CONDITIONS ARE THE RESULT OF THE
PRESSURE DIFFERENCE BETWEEN THE HIGH AND EQUATORIAL LOW PRESSURE
SOUTH OF THE AREA. AFTERNOON SOUNDINGS SHOW A STABLE ATMOSPHERE
ALOFT WITH SUBSIDENCE INVERSIONS AT AROUND 8000 FEET WITH PRECIPATBLE
WATER VALUES AT BOTH LOCATIONS ARE BELOW NORMAL.

ISLAND TOPOGRAPHY...COUPLED WITH THE STRONG TRADE WINDS...IS
RESULTING IN SOME PASSING SHOWERS ACROSS HAWAII LATE THIS
AFTERNOON. WINDWARD AREAS ARE SEEING MOST OF THE SHOWERS...BUT
WINDS ARE CARRYING OCCASIONAL CLOUDS AND SHOWERS ACROSS TO THE
LEEWARD AREAS AS WELL. EXPECT THIS PATTERN TO CONTINUE AT LEAST
THROUGH TUESDAY NIGHT...WITH MORE SHOWERS DURING THE NIGHTTIME
HOURS. THE ATMOSPHERE WILL REMAIN STABLE...THUS SHOWERS WILL BE
LIGHT AND FAST MOVING.

BOTH GFS AND ECMWF SHOW AN UPPER / SURFACE LOW DEVELOPING NORTHWEST OF
HAWAII ON TUESDAY NIGHT AND WEDNESDAY. THIS SYSTEM IS EXPECTED TO
LINGER TO THE NORTHWEST OF THE STATE THROUGH THE WEEKEND...WITH A
SIGNIFICANT FRONT / CONVERGENCE BAND FEATURE EXTENDING SOUTHWARD
FROM THE LOW. MODEL SOLUTIONS BRING A SLIGHT INCREASE IN INSTABILITY
AND MOISTURE AHEAD OF THE BAND OVER THE WESTERN PORTION OF THE
ISLAND CHAIN.

MODELS SHOWS THIS FEATURE LINGERING WEST OF HAWAII FROM WEDNESDAY INTO
THE WEEKEND...AND COULD PRODUCE SOME SHOWERS OVER THE WESTERN PORTION
OF THE STATE LATER THIS WEEK. THE LOW IS EXPECTED TO BE CLOSE AND
STRONG ENOUGH TO SHIFT WINDS OUT OF THE SOUTHEAST FROM WEDNESDAY
THROUGH THE WEEKEND TO BRING VOG AND HUMID CONDTIONS OVER THE
STATE. CLOUDS AND SHOWERS MOVING IN WITH THE SOUTHEAST FLOW WILL
FOCUS OVER WINDWARD AND SOUTHEASTERN FACING SLOPES...AND SHOULD BE
A LITTLE BIT WETTER THAN TODAY AND TUESDAY. PLUME CLOUDS AND
SHOWER BANDS MAY SET UP UNDER THESE CONDITIONS. IT IS NOT LIKELY
TO BE UNSTABLE ENOUGH TO PRODUCE HEAVY RAIN...BUT SHOWERS COULD
ANCHOR OVER THE WINDWARD AND SOUTHEAST FACING SLOPES BRINGING MORE
PERSISTENT RAIN.

STILL TOO EARLY TO CALL FOR WEATHER THIS WEEKEND AS THE SYSTEM
WILL REMAIN JUST WEST OF THE STATE WITH GFS AND ECMWF DIVERGING
SOLUTIONS. REGARDLESS...BOTH SOLUTIONS HINT A FURTHER INCREASE IN
MOISTURE OVER NIIHAU AND KAUAI. THIS WILL LEAD TO A POSSIBLE MORE
WIDESPREAD SHOWERS FOR THOSE ISLANDS FOR THE WEEKEND.


&&

.AVIATION...
STRONG SURFACE HIGH PRESSURE NORTH OF THE STATE WILL SUSTAIN BREEZY
EASTERLY TRADE WINDS THROUGH TONIGHT. OCCASIONAL MVFR CIGS WILL
LIKELY IMPACT WINDWARD SECTIONS THROUGH MID MORNING TUESDAY.
ELSEWHERE VFR CONDITIONS ARE EXPECTED TO PREVAIL.

AS THE HIGH WILL SHIFTS EAST TUESDAY THE WINDS ACROSS THE ISLANDS
WILL BECOME MORE SOUTHEASTERLY AND WEAKER...WITH THE BIG ISLAND
ACTING AS A SHIELD FOR THE OTHER ISLANDS. THUS THE LOW LEVEL
TURBULENCE AIRMET WILL NOT BE REQUIRED AT THAT POINT IN TIME.


&&

.MARINE...
STRONG WINDS WILL AFFECT ALL HAWAIIAN COASTAL WATERS THROUGH
TUESDAY. A GALE WARNING IS IN EFFECT THROUGH 6 AM TUESDAY FOR THE
ALENUIHAHA CHANNEL...WITH SMALL CRAFT ADVISORIES /SCA/ IN EFFECT
FOR THE REST OF THE COASTAL WATERS. THE SCA WILL LIKELY CONTINUE
FOR ALL WATERS ON TUESDAY...AND BEYOND FOR SELECT ZONES.

WINDS ARE EXPECTED TO WEAKEN A BIT AND VEER SOUTHEASTERLY OVER THE
ISLANDS TUESDAY NIGHT. THE SOUTHEAST FLOW WILL CONTINUE THROUGH
THE REST OF THE WEEK INTO THE WEEKEND.

THE STRONG TRADE WINDS WILL CONTINUE TO BRING ADVISORY LEVEL ROUGH
AND CHOPPY SURF ALONG EAST FACING SHORES FOR THE NEXT COUPLE OF
DAYS. THEREFORE A HIGH SURF ADVISORY REMAINS IN EFFECT AT LEAST
THROUGH TUESDAY NIGHT AND MAY NEED TO BE EXTENDED IF WINDS ARE
SLOW TO WEAKEN.

A LONG PERIOD SOUTH SWELL WILL SLOWLY BUILD TODAY...AND PEAK AROUND
TUESDAY...BRINGING NEAR ADVISORY LEVEL SURF ALONG SOUTH FACING
SHORES...THEN SLOWLY DIMINISH THROUGH THE REST OF THE WEEK.
ANOTHER SMALL NORTHWEST SWELL IS EXPECTED ON TUESDAY...WITH PEAK SURF
HEIGHTS WELL BELOW ADVISORY LEVELS.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST WEDNESDAY FOR EAST FACING SHORES
OF KAUAI...OAHU...MOLOKAI...MAUI AND BIG ISLAND.

GALE WARNING UNTIL 6 AM HST TUESDAY FOR ALENUIHAHA CHANNEL.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR REST OF HAWAIIAN
COASTAL WATERS.


&&

$$

DISCUSSION/MARINE...FOSTER
AVAITION...FUJII




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