Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 260612
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT APR 25 2015

.SYNOPSIS...
A SLOW-MOVING FRONT WILL SPREAD MODEST RAIN FROM KAUAI TO THE BIG
ISLAND THROUGH MONDAY NIGHT. LIGHT SOUTHEAST WINDS AHEAD OF THE
FRONT WILL TURN DIRECTLY INTO BREEZY TRADE WINDS BEHIND IT. THE
TRADE WINDS WILL CONTINUE TO BRING ACTIVE SHOWERS TO WINDWARD SLOPES
THROUGH MOST OF THE COMING WEEK.

&&

.DISCUSSION...
A SLOW-MOVING FRONTAL CLOUD BAND MOVED FROM EAST TO WEST ACROSS
KAUAI...WHILE LIMITED AFTERNOON CONVECTION PREVAILED IN THE LIGHT
SOUTHEAST FLOW OVER THE REST OF THE STATE. ALTHOUGH CONVERGENCE
BETWEEN SOUTHEAST AND MORE SOUTHERLY OR SOUTHWESTERLY WINDS OFTEN
SUPPORTS SHOWER DEVELOPMENT AS A FRONT APPROACHES THE STATE...IN
THIS CASE THE PRE-FRONTAL AIR MASS PROVIDED RELATIVELY LITTLE WATER
VAPOR. ALOFT...A WELL-DEFINED LOW TO THE NORTH OF THE STATE MIRRORED
THE SURFACE LOW TO WHICH THE FRONT WAS ATTACHED...BUT MOST OF THE
DYNAMIC SUPPORT WAS FOCUSED WELL NORTH OF 25N.

THOUGH OCCASIONAL TOWERING CUMULI ROSE THROUGH THE SURROUNDING
LAYERS OF THE CLOUD BAND EVEN SOUTH OF 20N...NEAR THE ISLANDS THE
MAIN STRENGTH OF THIS FEATURE WAS CONCENTRATED IN A THIN AND ROPY
LINE OF SHOWERS. DESPITE TRAINING FOR HOURS THIS AFTERNOON OVER
KAUAI...THE BAND DROPPED ONLY MODEST PRECIPITATION...AND PROBABLY
WILL DO NO MORE OVER OAHU ON SUNDAY. CONVECTIVE SHOWERS ALONG THE
FRONT...OR FROM CONVERGENCE IN THE PRE-FRONTAL SECTOR...LIKELY WILL
BE THE MAIN CONCERN. IN ANY CASE...THE SHOWER BAND SHOULD REACH OAHU
BY SUNDAY AFTERNOON...MAUI COUNTY BY SUNDAY EVENING...AND THEN THE
BIG ISLAND BY MONDAY EVENING. SLOWLY DISSIPATING CLOUD LAYERS WILL
OBSCURE THE SKY AND DROP A FEW SPRINKLES ON BOTH SIDES OF THE MAIN
RAIN BAND.

GUIDANCE AGREED WELL IN PREDICTING THE SUPPORTING MID-LEVEL LOW TO
LIFT OUT RAPIDLY TOWARD THE NORTHEAST THROUGH THE NEXT 24 HOURS...
LEAVING ONLY A SHALLOW TROUGH TO SUPPORT THE WEAKENING FRONT. SOLID
RIDGING ALOFT BUILDING IN FROM THE NORTHWEST WILL SUPPORT THE RAPID
DEVELOPMENT OF THE SURFACE HIGH BEHIND THE FRONT...BRINGING MODERATE
TO STRONG WINDS SHORTLY AFTER FRONTAL PASSAGE. THESE WINDS WILL HAVE
MORE OF AN EASTERLY COMPONENT THAN IS USUAL IN SUCH CASES...SO ONE
COULD SAY SIMPLY THAT MODERATE TO STRONG TRADE WINDS WILL RETURN TO
NEARLY ALL OF THE STATE BY MONDAY EVENING...AND THEN PERSIST THROUGH
MOST OF THE COMING WEEK.

AS THE FRONTAL CLOUD BAND FALLS APART...IT WILL SHED ITS LOAD OF
WATER VAPOR ONTO THE TRADES. THE WINDS IN TURN WILL CARRY THIS MOIST
AIR MASS OVER THE STATE AGAIN...PRODUCING CONTINUED RAINFALL OVER
WINDWARD SLOPES. THUS...CONDITIONS WILL REMAIN ON THE WET SIDE OF
USUAL THROUGH THE FORECAST PERIOD.

&&

.MARINE...
SURF ALONG SOUTH- AND EAST-FACING SHORES WILL SUBSIDE THROUGH THE
REST OF THE WEEKEND AS THE WINDS AND SOUTH-SOUTHWEST SWELL TAPER
OFF. A LARGER NORTH-NORTHWEST SWELL SHOULD ARRIVE SUNDAY NIGHT...AND
THEN BECOME MORE NORTHERLY BY MONDAY NIGHT. THIS SWELL LIKELY WILL
PRODUCE SURF ABOVE THE ADVISORY THRESHOLD ALONG NORTH-FACING SHORES
BY MONDAY...AND ALONG EAST-FACING SHORES BY LATE MONDAY OR TUESDAY
AS THE SWELL DIRECTION BECOMES MORE NORTHERLY.

SOUTH TO SOUTHEAST WIND SPEEDS SHOULD REMAIN BELOW THE THRESHOLD
FOR A SMALL CRAFT ADVISORY THROUGH SUNDAY. HOWEVER...STRONG
NORTHEAST WINDS BEHIND THE FRONT WILL SPREAD SCA CONDITIONS
SOUTHEAST ALONG THE ISLAND CHAIN FROM KAUAI TO THE BIG ISLAND FROM
SUNDAY EVENING THROUGH MONDAY NIGHT.

&&

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

&&

$$

RYSHKO




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