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Hawaiian Islands Synoptic Discussion and Guidance

949
FXHW60 PHFO 210626
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST THU NOV 20 2014

.SYNOPSIS...
MODERATE TRADE WINDS WILL DIMINISH INTO THE WEEKEND. MOST SHOWERS
WILL BE LIMITED TO WINDWARD SLOPES...ESPECIALLY OVER THE BIG ISLAND.
A FRONT WILL BRING MODEST RAIN AND STRONGER...NORTHERLY WINDS TO
MOST OF THE STATE FROM SUNDAY NIGHT INTO TUESDAY. THE WINDS THEN
WILL WEAKEN AND BECOME NORTHEASTERLY AGAIN AS THE COOL AND DRY AIR
MASS MOISTENS GRADUALLY INTO FRIDAY.

&&

.DISCUSSION...
OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED MODERATE TO
LOCALLY STRONG TRADE WINDS BLOWING. ALTHOUGH A FAIRLY STRONG RIDGE
PREVAILED ALOFT TO THE NORTHWEST THROUGH NORTH OF THE STATE...A
BROAD BUT SHALLOW TROUGH PERSISTED ALONG THE SOUTH EDGE OF THE
RIDGE. THUS...WHILE SOUNDING DATA SHOWED A SOLID INVERSION OVER
KAUAI...THIS INVERSION WAS UNUSUALLY HIGH...AND THE HILO PROFILE
WAS FAR LESS STABLE OVERALL. SATELLITE ESTIMATES AND IN-SITU
OBSERVATIONS ALSO DEPICTED A FAIRLY SHARP GRADIENT IN PRECIPITABLE
WATER FROM RELATIVELY DRY CONDITIONS OVER THE NORTHWEST END OF THE
STATE...TO A MUCH MOISTER AIR MASS TO THE SOUTHEAST.

AFTERNOON CONVECTION REFLECTED THIS DISPARITY...BUT DRIER CONDITIONS
PREVAILED OVER THE ISLANDS BY THE EARLY PART OF THE EVENING.
STILL...THE MOISTER AND LESS STABLE CONDITIONS AROUND THE BIG ISLAND
SHOULD SUPPORT GREATER WINDWARD SHOWER ACTIVITY FROM LATER TONIGHT
INTO FRIDAY.

BY THE MIDDLE OF THE FORECAST PERIOD...GUIDANCE LARGELY AGREED THAT
A DEEPENING LONG-WAVE TROUGH ALOFT WILL DISPLACE THE MID-LEVEL RIDGE
TO WELL NORTHEAST OF THE STATE. THIS DEVELOPMENT IN TURN WILL
SUPPORT THE APPROACH AT THE SURFACE OF THE FRONT CURRENTLY LOCATED
NEAR 30N. AS THE SURFACE RIDGE RETREATS TO THE SOUTHEAST...THE TRADE
WINDS WILL DIMINISH LOCALLY...BECOMING SOUTHEASTERLY IN PLACES BY
SUNDAY.

ALTHOUGH THE FRONT ITSELF SHOULD REACH KAUAI BY SUNDAY NIGHT...
CURRENT SOLUTIONS PREDICT THE SUPPORTING MID-LEVEL TROUGH TO FOCUS
TO THE EAST OF THE STATE. THUS...MOST DEVELOPMENT ALONG THE FRONT
SHOULD REMAIN NORTHEAST OF THE ISLANDS...LEAVING ONLY A FAIRLY
SHALLOW BOUNDARY TO MOVE DOWN THE STATE FROM SUNDAY NIGHT INTO
TUESDAY. SOME RAIN LIKELY WILL ACCOMPANY THE FRONT...MAINLY OVER THE
SMALLER ISLANDS. THE BIG ISLAND MORE LIKELY WILL EXPERIENCE A LESS
ORGANIZED INCREASE IN SHOWERS FROM THE FRONTAL REMNANTS MOVING
ASHORE ON TUESDAY AND WEDNESDAY. MODERATE TO STRONG NORTHERLY WINDS
IN THE COOLER AND DRIER AIR MASS BEHIND THE FRONT MAY BE MORE
SIGNIFICANT THAN THE FRONT ITSELF.

FROM WEDNESDAY INTO THE LATTER HALF OF THE WEEK...THE SUBTROPICAL
RIDGE WILL REESTABLISH ITSELF NORTH OF THE STATE GRADUALLY...
RETURNING THE WINDS TO A MORE FAMILIAR NORTHEASTERLY DIRECTION...
THOUGH WIND SPEEDS WILL DROP OFF SIGNIFICANTLY UNTIL THE RIDGE
SHIFTS FAR ENOUGH NORTH. THE COOL AND DRY AIR MASS WILL MODIFY IN
PLACE...ALLOWING WINDWARD SHOWERS TO INCREASE WITH TIME.

&&

.MARINE...
WIND SPEEDS ARE EXPECTED TO REMAIN ABOVE THE THRESHOLD FOR A SMALL
CRAFT ADVISORY /SCA/ AROUND MAUI AND HAWAII COUNTIES INTO FRIDAY
NIGHT. AFTER A BRIEF LULL..SPEEDS LIKELY WILL INCREASE AGAIN BEHIND
THE FRONT FROM SUNDAY INTO MONDAY...PROBABLY REQUIRING AN SCA AROUND
MUCH OF THE STATE BY EARLY NEXT WEEK.

NORTHWEST SWELLS WILL CONTINUE TO PRODUCE MODEST SURF THROUGH
SATURDAY NIGHT. A SLIGHTLY LARGER SWELL IS EXPECTED TO ARRIVE BY
SUNDAY...POSSIBLY BUILDING SURF TO NEAR THE ADVISORY LEVEL ALONG
NORTH-FACING AND WEST-FACING SHORES OF THE SMALLER ISLANDS THROUGH
MONDAY. THIS SWELL ALSO MAY CONTRIBUTE TO THE NEED FOR AN SCA OVER
THE COASTAL WATERS.

ROUGH SURF FROM TRADE-WIND WAVES ALONG EAST-FACING SHORES SHOULD
REMAIN BELOW THE THRESHOLD FOR AN SCA THROUGH TONIGHT...AND WILL
DECLINE FURTHER AS THE TRADE WINDS DIMINISH.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL MIDNIGHT HST FRIDAY NIGHT FOR MAALAEA
BAY-PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-
BIG ISLAND SOUTHEAST WATERS.

&&

$$

RYSHKO



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