Heavy Rain in Hawaii, April 5, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1010 AM HST SUN APR 5 2015

.SYNOPSIS...
LOCALLY BREEZY TRADE WINDS WILL PREVAIL THROUGH THE UPCOMING WEEK...
DELIVERING PASSING CLOUDS AND SHOWERS...MAINLY TO WINDWARD SLOPES
AND COASTS. SHOWERS WILL BE MOST ACTIVE DURING THE NIGHT AND MORNING
HOURS...AND WILL OCCASIONALLY SPREAD LEEWARD ON THE SMALLER ISLANDS.

&&

.DISCUSSION...
FRESH TO LOCALLY STRONG TRADE WINDS OVER THE ISLANDS ARE BEING
SUPPORTED BY A 1032 MB SURFACE HIGH CENTERED ABOUT 1500 MILES TO THE
NNE. LATEST SATELLITE IMAGES SHOW LITTLE IN THE WAY OF SHOWERY LOW
CLOUDS IMMEDIATELY UPSTREAM OF THE ISLANDS...AND RADAR DEPICTS FEWER
WINDWARD SHOWERS THAN WHAT WAS OBSERVED EARLIER THIS MORNING.
MEANWHILE...WINDWARD SLOPES AND COASTS OF MOST ISLANDS WERE
BLANKETED UNDER MOSTLY CLOUDY SKIES...AS OROGRAPHIC LIFTING OF THE
TRADE WINDS IS LEADING TO PERSISTENT TERRAIN-ANCHORED CLOUDINESS.
WINDWARD BIG ISLAND IS A BIT OF AN OUTLIER...AS SCATTERED LOW CLOUDS
ARE ALLOWING FOR MOSTLY SUNNY SKIES AFTER OVERNIGHT SHOWERS DROPPED
BETWEEN ONE-HALF AND ONE INCH OF RAINFALL. SOUNDINGS INDICATE THE
INVERSION HEIGHT IS BETWEEN 8 AND 9 KFT...WITH PWAT VALUES A LITTLE
HIGHER THAN NORMAL FOR APRIL. THE EXPECTATION FOR THE REMAINDER OF
TODAY IS THAT WINDWARD SLOPES WILL REMAIN PARTLY TO MOSTLY CLOUDY
WITH A FEW SHOWERS...WITH ONE POTENTIAL SHOWER SOURCE BEING AN AREA
OF CUMULUS CURRENTLY ABOUT 50 TO 100 MILES E OF WINDWARD MAUI AND
BIG ISLAND.

TRADE WIND WEATHER WILL PREVAIL FOR THE UPCOMING WEEK...WITH FAIRLY
STRONG SPEEDS EXPECTED. INITIALLY THE TRADES WILL BE SUPPORTED BY
THE HIGH CENTERED TO THE NNE...WHICH WILL BE WEAKENING AS IT MOVES
SE THROUGH TUESDAY. BY MID-WEEK THIS HIGH WILL BE DISSIPATING OFF
THE U.S. WEST COAST...AS A NEW RELATIVELY STRONG HIGH BUILDS FAR NW
OF THE ISLANDS. A LOCAL MIN IN WIND SPEEDS IS EXPECTED AROUND MONDAY
BEFORE SPEEDS INCREASE AGAIN...BUT THESE FLUCTUATIONS MAY NOT BE ALL
THAT NOTICEABLE TO THOSE NOT ENGAGED IN WIND-SENSITIVE ACTIVITIES.

MODELS SHOW WEAK MID-LEVEL TROUGHING EXTENDING OVER THE AREA FROM
THE NE FOR MOST OF THE WEEK...WHICH WILL LIKELY KEEP THE INVERSION
AT ITS SOMEWHAT ELEVATED HEIGHT. LOOKING FOR MOISTURE TRENDS IN THE
MODELS YIELDS INCONCLUSIVE GUIDANCE OVERALL. HOWEVER THERE IS SOME
CONSENSUS IN THE NOTION OF A DRIER SLOT SLIDING OVER THE ISLANDS ON
MONDAY...WITH TRADE SHOWERS INCREASING MONDAY NIGHT AND TUESDAY.
GFS DEPICTS NON-DESCRIPT AREAS OF LOW CLOUDS CURRENTLY BETWEEN 145W
AND 150W ALONG 25-28N AS THE SOURCE OF THIS MOISTURE. LATE IN THE
WEEK FORECAST MODELS ARE DEPICTING AN E-W ORIENTED CLOUD/SHOWER
BAND...ASSOCIATED WITH A DISSIPATED FRONTAL BOUNDARY...SLIDING S
TOWARD THE ISLANDS. GFS INDICATES THIS FEATURE MAY BRING SUBSIDENCE
AND FEWER SHOWERS AS IT APPROACHES THURSDAY NIGHT AND FRIDAY...AND
INCREASED SHOWERS AS IT MOVES OVER THE ISLANDS NEXT WEEKEND. THIS
SOLUTION IS NOT SHARED BY THE LATEST ECMWF GUIDANCE...WHICH KEEPS
THIS MOISTURE N OF THE ISLANDS...AND INSTEAD ADVECTS A DIFFERENT
AREA OF MOISTURE NW OVER THE BIG ISLAND.

IN SUMMATION...SEASONABLE TRADE WIND WEATHER WILL PREVAIL THROUGH
THE FORECAST PERIOD...BRINGING PERIODS OF CLOUDS AND SHOWERS...
MAINLY TO WINDWARD SLOPES AND COASTS. SPEEDS WILL BE STRONG ENOUGH
TO SPREAD SHOWERS LEEWARD OCCASIONALLY.

&&

.AVIATION...
THE PREVALENT TRADE WINDS WILL CONTINUE ACROSS THE ISLANDS INTO
MONDAY...THUS BOUTS OF MVFR CEILINGS CAN BE EXPECTED MAINLY ALONG
THE WINDWARD SLOPES AND MOUNTAIN AREAS ESPECIALLY ON KAUAI. THE
BREEZY CONDITIONS WILL JUSTIFY THE CONTINUATION OF AIRMET TANGO
FOR MODERATE LOW LEVEL TURBULENCE FOR AWHILE FOR AREAS OVER AND
SOUTHWEST THROUGH WEST OF THE MOUNTAINS.

&&

.MARINE...
FRESH TO STRONG TRADE WINDS NEAR AND UPSTREAM OF THE ISLANDS HAVE
LED TO THE DEVELOPMENT OF SEAS THAT ARE NEARLY FULLY-DEVELOPED...
AS SUSTAINED 22 KT WINDS CAN BE EXPECTED TO PRODUCE MAX SEAS NEAR 12
FT...AND RECENT OBSERVATIONS FROM WINDWARD BUOYS INDICATE PEAK SEAS
NEAR 11 FT. THESE OBSERVATIONS LED TO AN EARLIER EXPANSION OF THE
EXISTING SMALL CRAFT ADVISORY /SCA/...WHICH IS EFFECT THROUGH
TODAY FOR ALL HAWAIIAN WATERS. ALTHOUGH LOCAL WINDS WILL EASE ONLY
SLIGHTLY MONDAY... WEAKENING WINDS WITHIN THE UPSTREAM FETCH SHOULD
LEAD TO SLOWLY DIMINISHING SEA HEIGHTS. THE EXPECTATION IS THAT THE
SCA CAN BE SCALED BACK IN AREA BY TONIGHT.

THE SHORT-PERIOD SEAS AND EMBEDDED SLIGHTLY LONGER-PERIOD E SWELL
ARE PRODUCING ROUGH AND HIGH SURF ALONG E FACING SHORES...WHERE A
HIGH SURF ADVISORY IS IN EFFECT THROUGH THIS EVENING. WILL SEE HOW
THINGS PLAY OUT TODAY BEFORE MAKING A DETERMINATION IF AN EXTENSION
IN TIME IS NECESSARY...BUT A DOWNTREND IN SURF HEIGHTS IS EXPECTED
THROUGH MONDAY. OTHERWISE...A SMALL WNW SWELL TODAY WILL LINGER FOR
A COUPLE OF DAYS...WITH A SMALL NW SWELL POSSIBLE AROUND MID-WEEK.
SMALL SWELLS ARRIVING FROM THE S HEMISPHERE WILL BRING SMALL SURF TO
S FACING SHORES THROUGH MOST OF THE WEEK.

&&

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

SMALL CRAFT ADVISORY UNTIL 6 PM HST MONDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THIS EVENING FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-
KAUAI CHANNEL-OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI
CHANNEL-MAUI COUNTY WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS-
BIG ISLAND WINDWARD WATERS.

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...FUJII