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Hawaiian Islands Synoptic Discussion and Guidance

301
FXHW60 PHFO 290708
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT MAR 28 2015

.SYNOPSIS...
LIGHT TRADE WINDS WILL DELIVER A FEW WINDWARD SHOWERS OVERNIGHT...
WITH LAND BREEZES DEVELOPING OVER LEEWARD AREAS. TRADE WINDS WILL
INCREASE SUNDAY AND MONDAY...BRINGING A SLIGHT INCREASE IN WINDWARD
SHOWERS...MAINLY ON KAUAI AND OAHU. A UPPER LEVEL DISTURBANCE
PASSING NORTH OF THE ISLANDS ON TUESDAY WILL RESULT IN WEAKENING THE
TRADE WINDS AGAIN...AND MAY BRIEFLY ENHANCE SHOWERS.

&&

.DISCUSSION...
RATHER LIGHT TRADE WINDS PREVAIL ACROSS THE ISLANDS THIS EVENING AS
THE LOW LEVEL PRESSURE GRADIENT SLACKS ACROSS THE REGION DUE TO THE
PRESENT OF A FRONT ABOUT 400 MILES NORTHWEST OF KAUAI. A LINE OF
MOISTURE EMBEDDED IN THE LIGHT TRADE WIND FLOW HAS REACHED WINDWARD
OAHU IN THE PAST HOUR...BRINGING SOME LOCALLY HEAVIER SHOWERS TO
THAT AREA. RAIN GAGE AT WAIMANALO REPORTED OVER HALF AN INCH OF RAIN
IN A 15 MINUTES PERIOD. THIS LINE OF RAIN IS MOVING SLOWLY WEST AT
THIS TIME...AND MAY CONTINUE TO BRING LOCALLY HEAVY RAIN TO OAHU
WINDWARD AREAS THROUGH THE NEXT COUPLE OF HOURS. ELSEWHERE RATHER
FAIR WEATHER PREVAILS...WITH SOME SHOWERS AFFECTING WINDWARD AREAS
OF MAUI AS WELL AS THE BIG ISLAND. UNDER THE LIGHT WINDS...LAND
BREEZES WILL DEVELOP OVER THE ISLANDS AND MAY BRING SOME CLEARING
ACROSS THE AREA. THEREFORE EXPECT IMPROVING CONDITIONS OVER
OAHU...ESPECIALLY WINDWARD AREAS OVERNIGHT...WHILE RATHER FAIR
WEATHER WILL PREVAIL FOR THE REST OF THE ISLANDS.

THE BULK OF THE FRONT FAR NORTHWEST OF THE ISLANDS WILL PASS EAST
THROUGH SUNDAY...NOT REACHING THE STATE...WHILE A SURFACE HIGH
BEHIND THIS FRONT WILL FOLLOW IN FROM THE WEST. THIS WILL RESULT IN
THE TIGHTENING OF THE LOW LEVEL PRESSURE GRADIENT OVER THE ISLAND
CHAIN...ESPECIALLY ALONG THE WESTERN END. AS SUCH...STRONGER TRADE
WINDS ARE EXPECTED TO RETURN THE AREA LATER SUNDAY AND MONDAY. BULK
OF THE MOISTURE ASSOCIATED WITH THE FRONT WILL REMAIN NORTH OF THE
STATE...THOUGH FORECAST MODELS INDICATE THERE WILL BE MOISTURE FROM
THE EAST SPREADING ACROSS THE ISLANDS UNDER THE TRADE WIND FLOW.
THEREFORE...AN INCREASE IN TRADE SHOWERS IS EXPECTED AND THESE
SHOWERS WILL AFFECT MAINLY WINDWARD AREAS. LATEST FORECAST MODELS
STILL INDICATE THE ATMOSPHERE WILL REMAIN RELATIVELY STABLE SO
THESE SHOWERS ARE NOT EXPECTED TO BE TOO HEAVY.

ON TUESDAY...A PROGRESSIVE MID-LEVEL SHORT WAVE TROUGH IS FORECAST
TO PASS THE STATE. ITS PRESENT IN THE ISLAND VICINITY WILL INDUCE
THE DEVELOPMENT OF ANOTHER SURFACE TROUGH. AS THE SURFACE TROUGH
DEVELOPS...WINDS WILL ONCE AGAIN WEAKEN ACROSS THE ISLANDS. IN
FACT...WINDS MAY BECOME LIGHT AND VARIABLE ENOUGH ALLOWING LAND AND
SEA BREEZES TO DEVELOP. THE BULK OF THE INSTABILITY ASSOCIATED WITH
THE PASSING SHORT WAVE WILL REMAIN NORTH OF THE AREA...THOUGH SHALL
NOT BE SURPRISED IF DEEPER CONVECTION DEVELOPS OVER THE ISLANDS
UNDER THE CONVERGING SEA BREEZES IN THE AFTERNOON. THIS MID-LEVEL
TROUGH WILL TRACK SWIFTLY EAST WEDNESDAY. ANOTHER MID-LEVEL SHORT
WAVE TROUGH WILL PASS OVER THE ISLANDS LATER PART OF NEXT WEEK...AND
THIS MAY LEAD TO ANOTHER SURFACE TROUGH DEVELOPING IN THE ISLAND
VICINITY. LOOKS LIKE LIGHTER WINDS MAY LINGER IN THE ISLAND VICINITY
FOR THE LATER PART OF NEXT WEEK...WITH CONVECTIVE WEATHER AND LAND
AND SEA BREEZES POSSIBLY CONTINUING. THERE HAVE BEEN SOME
DISAGREEMENTS BETWEEN THE MODELS FOR THAT PERIOD. THUS FORECAST
UNCERTAINTY REMAINS SOMEWHAT HIGH. SHALL AWAIT NEWER MODEL RUNS
BEFORE ADJUSTING THE FORECAST FOR THE LATER PART OF NEXT WEEK TO
THIS SCENARIO.

&&

.AVIATION...
CLOUDS AND SHOWERS WILL REMAIN FOCUSED OVER WINDWARD SECTIONS OF THE
STATE OVERNIGHT...AS LIGHT TO MODERATE TRADE WINDS PREVAIL. AIRMET
SIERRA FOR MOUNTAIN OBSCURATION REMAINS IN EFFECT FOR WINDWARD
SECTIONS OF OAHU AND MAUI...WHERE TEMPO MVFR CONDITIONS WILL PERSIST.
OTHER SECTIONS OF OAHU MAY SEE SOME BRIEF PERIODS OF MVFR THIS
EVENING AS SHOWERS SPILL OVER THE MOUNTAINS. THE TRADES SHOULD BE
LIGHT ENOUGH TO ALLOW LAND BREEZES TO DEVELOP ACROSS MANY LEEWARD
AREAS LATE TONIGHT...WHICH SHOULD RESULT IN CLEARING CONDITIONS.

GENTLE TO MODERATE TRADES WILL CONTINUE ON SUNDAY...WITH LOCALIZED
SEA BREEZES DEVELOPING OVER SOME LEEWARD AREAS. VFR WILL PREVAIL...
WITH PERIODS OF MVFR REMAINING POSSIBLE IN CLOUDS AND SHOWERS OVER
WINDWARD SLOPES.

&&

.MARINE...
A LONG-PERIOD WNW SWELL IS STILL EXPECTED TO ARRIVE
OVERNIGHT...PEAKING LATE SUNDAY...WITH SURF HEIGHTS NEARING
ADVISORY LEVELS ALONG EXPOSED N AND W SHORES. THIS SWELL WILL
GRADUALLY DIMINISH THROUGH TUESDAY. A SMALL...SHORT PERIOD NORTH
SWELL WILL BRIEFLY APPEAR WEDNESDAY AND THURSDAY.

WINDS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT CONDITIONS FOR THE
COASTAL WATERS AT LEAST THROUGH EARLY PART OF NEXT WEEK. BUT THEN
THE PENDING WNW SWELL MAY BE BIG ENOUGH TO LIFT THE COMBINED SEAS
CLOSE TO THE 10 FT THRESHOLD FOR SMALL CRAFT ADVISORY ISSUANCE OVER
EXPOSED COASTAL WATERS ON SUNDAY. LATEST READINGS FROM THE BUOY FAR
NORTHWEST OF THE ISLANDS ARE STILL NOT SHOWING MUCH.

&&

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

&&

$$

AVIATION...JACOBSON
DISCUSSION/MARINE...HUI




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