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WindNNW 19 mph
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Hawaiian Islands Synoptic Discussion and Guidance

797
FXHW60 PHFO 270602
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SUN APR 26 2015

.SYNOPSIS...
A SLOW-MOVING FRONT WILL SPREAD MODEST RAIN FROM THE KAIWI CHANNEL
TO THE BIG ISLAND THROUGH MONDAY NIGHT. LIGHT SOUTHERLY WINDS AHEAD
OF THE FRONT WILL TURN INTO BREEZY TRADE WINDS BEHIND IT. ACTIVE
SHOWERS WILL CONTINUE OVER WINDWARD SLOPES THROUGH MUCH OF THE
WEEK...BUT LEE AREAS SHOULD START TO DRY OUT AFTER MONDAY.

&&

.DISCUSSION...
A SLOW-MOVING FRONTAL CLOUD BAND MOVED FROM EAST TO WEST OFF OF OAHU
INTO THE KAIWI CHANNEL...WHILE RAIN TAPERED OFF OVER OAHU ITSELF.
CONVERGENCE BETWEEN SOUTHEAST AND MORE SOUTHERLY OR SOUTHWESTERLY
WINDS SUPPORTED LIMITED SHOWER DEVELOPMENT AHEAD OF THE FRONT...
ESPECIALLY IN COMBINATION WITH AFTERNOON CONVECTION OVER MAUI AND
THE BIG ISLAND. THIS CONVECTION WAS FAIRLY MINOR OVERALL...DESPITE A
FEW HEAVIER SHOWERS...SUCH AS OVER WAIMANALO. A DEEPLY STACKED LOW
WELL TO THE NORTH NORTHEAST OF THE STATE ACCELERATED AWAY...
STRETCHING OUT...FURTHER SLOWING DOWN...AND WEAKENING DYNAMIC
SUPPORT FOR THE FRONT.

DESPITE TRAINING FOR HOURS ACROSS OAHU TODAY...THE THIN AND ROPY
FRONTAL SHOWER BAND DROPPED ONLY MODEST PRECIPITATION IN MOST
PLACES...AND PROBABLY WILL DO NO MORE OVER MAUI COUNTY LATER
TONIGHT. CONVECTIVE SHOWERS FORMING UNDER THE INFLUENCE OF
CONVERGENCE IN THE PRE-FRONTAL SECTOR LIKELY WILL REMAIN THE MAIN
CONCERN FOR THE FORECAST IN THE SHORT RUN. IN ANY CASE...THE SHOWER
BAND SHOULD REACH MAUI COUNTY LATER THIS 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. FURTHER TO THE
NORTHWEST...RELATIVELY CLEAR CONDITIONS IN THE IMMEDIATE AFTERMATH
OF THE FRONT WILL YIELD TO COLD-AIR CUMULI...AND THEN A NEARLY SOLID
STRATUS OVERCAST...AT LEAST UNTIL THE AIR MASS MODIFIES.

GUIDANCE AGREED WELL IN PREDICTING THE SHALLOW TROUGH ALOFT
CURRENTLY SUPPORTING THE WEAKENING FRONT TO SHIFT SOUTHEAST AND
WEAKEN FURTHER THROUGH WEDNESDAY. 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 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...MAINTAINING CONTINUED RAINFALL OVER
WINDWARD SLOPES. LEE AREAS SHOULD EXPERIENCE SOME DRYING BY
MID-WEEK...BUT A FEW SHOWERS BLOOMING WINDWARD WILL CONTINUE TO
DRIFT LEEWARD BEFORE DISSIPATING. THUS...CONDITIONS WILL REMAIN ON
THE WET SIDE OF USUAL THROUGH THE FORECAST PERIOD.

&&

.MARINE...
A NEW NORTHERLY SWELL EXPECTED TO ARRIVE TONIGHT AND MONDAY LIKELY
WILL BUILD SURF HEIGHTS TO THE THRESHOLDS FOR A HIGH SURF ADVISORY
ALONG NORTH- AND EAST-FACING SHORES. THIS SWELL WILL CONTINUE TO
ARRIVE INTO TUESDAY...AND THEN WILL LOWER GRADUALLY ON WEDNESDAY.
STRENGTHENING TRADE WINDS WILL KEEP SHORT-PERIOD CHOPPY SURF
ELEVATED ALONG EAST-FACING SHORES THROUGH MOST OF THE WEEK.

STRONG HIGH PRESSURE BUILDING TO THE NORTH WILL INCREASE TRADE WINDS
OVER THE AREA THROUGH MID-WEEK. WINDS AND SEAS WILL EXCEED
THRESHOLDS FOR A SMALL CRAFT ADVISORY ACROSS MOST OF THE AREA BY
AROUND MONDAY MORNING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM 6 AM MONDAY TO 6 PM HST TUESDAY FOR
NIIHAU-KAUAI WINDWARD-KAUAI LEEWARD-OAHU NORTH SHORE-OAHU KOOLAU-
OLOMANA-MOLOKAI-MAUI WINDWARD WEST-MAUI CENTRAL VALLEY-WINDWARD
HALEAKALA.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI CHANNEL-MAUI
COUNTY WINDWARD WATERS-MAALAEA BAY-PAILOLO CHANNEL-ALENUIHAHA
CHANNEL.

&&

$$

RYSHKO



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office