Small Craft Advisory 41, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1002 AM HST THU AUG 20 2015

.SYNOPSIS...
SOUTHEAST FLOW AND A PERSISTENT VERY MOIST AIRMASS OVER THE STATE
WILL KEEP THE THREAT FOR A FEW LOCALLY HEAVY SHOWERS THIS AFTERNOON.
THE AIRMASS WILL DRY OUT A BIT AS MODERATE TO FRESH TRADE WINDS
BECOME REESTABLISHED OVER THE ISLANDS ON FRIDAY AND SATURDAY...
BETWEEN HIGH PRESSURE OVER THE FAR NORTHEAST PACIFIC AND A
DEVELOPING TROPICAL CYCLONE TO OUR SOUTHEAST. MOISTURE ON THE
PERIPHERY OF THE DEVELOPING TROPICAL CYCLONE MAY START TO BRING
INCREASING SHOWERS AND HUMIDITY THIS WEEKEND. THE FORECAST FOR EARLY
NEXT WEEK IS HIGHLY DEPENDENT ON THE EVENTUAL TRACK AND INTENSITY OF
THIS DEVELOPING TROPICAL CYCLONE.

&&

.DISCUSSION...
TWO OF THE THREE SYNOPTIC FEATURES THAT HAVE BEEN RESPONSIBLE FOR
THE RECENT LOCALIZED DOWNPOURS OVER THE ISLANDS ARE MOVING AWAY...
BUT ONE INGREDIENT REMAINS. THE UPPER LOW AND SURFACE TROUGH FAR TO
OUR NW WILL CONTINUE TO HAVE LESS AND LESS OF AN INFLUENCE ON OUR
WEATHER...BUT SE FLOW ON THE FLANK OF HIGH PRESSURE OVER THE N
PACIFIC IS MAINTAINING A VERY MOIST AIRMASS OVER THE MAIN HAWAIIAN
ISLANDS...WITH PW RANGING FROM 1.8-2 INCHES ACROSS THE STATE. THE
NARROW RIBBON OF DRIER AIR AND SUBSIDENCE THAT KEPT OAHU QUIETER
YESTERDAY APPEARS TO HAVE FILLED IN. ALTHOUGH THE TRADES WILL TRY TO
BUILD IN LATER TODAY...THAT PROBABLY WON/T HAPPEN FAST ENOUGH TO
PREVENT A FEW AFTERNOON DOWNPOURS ONCE AGAIN. HAVE ADDED A MENTION
OF LOCALLY HEAVY SHOWERS TO THE GRIDS FOR SELECTED LOCATIONS THIS
AFTERNOON...BUT GENERALLY NOT AS ACTIVE AS IT WAS EARLIER THIS WEEK.

THE TRADES SHOULD FIRM UP A BIT ON FRIDAY AS THE SURFACE TROUGH
MOVES FARTHER AWAY AND THE LOW LEVEL FLOW BACKS FROM SE TO E.
SOMEWHAT DRIER AIR BEGINS TO WORK IN AS WELL ON FRIDAY...PROVIDING
A BIT OF RESPITE FROM THE RELENTLESS HUMIDITY AND HIGH DEWPOINTS
WE/VE HAD LATELY.

BEYOND FRIDAY OUR SENSIBLE WEATHER BECOMES INCREASINGLY DEPENDENT ON
THE TRACK AND INTENSITY OF THE DEVELOPING TROPICAL CYCLONE TO OUR
SE. THE MODELS ARE INDICATING A SURGE OF DEEP MOISTURE PUSHING BACK
UP THE CHAIN FROM SE TO NW ON THE PERIPHERY OF THE TC STARTING
FRIDAY NIGHT WHICH WILL IS LIKELY TO BRING BANDS OF SHOWERS BACK IN
OVER THE STATE. THE PRESSURE GRADIENT WILL ALSO TIGHTEN AS THE
SYSTEM INITIALLY PASSES S OF THE ISLANDS...SO THE TRADE WINDS WILL
INCREASE FURTHER POSSIBLY TO FRESH TO STRONG LEVELS.

OUR LOCAL FORECAST EARLY NEXT WEEK WILL BE HEAVILY BASED ON THE
SIX-HOURLY UPDATES TO THE OFFICIAL CPHC FORECAST TRACK AND INTENSITY
OF THIS SYSTEM. IT IS VERY IMPORTANT TO CONSIDER THAT FORECASTS
AT LONGER RANGES HAVE A LARGE MARGIN OF ERROR....WITH AN AVERAGE OF
NEARLY 300 MILES AT 5 DAYS...AND ROUGHLY 200 MILES AT 4 DAYS.
DO NOT GET HUNG UP ON ANY PRECISE TRACK...ALWAYS CONSIDER ALTERNATE
POSSIBILITIES USING THE ERROR CONE AND WIND SPEED PROBABILITIES AND
PREPARE AS NEEDED.

&&

.AVIATION...
SOUTHERLY FLOW AND DEVELOPING LOCAL SEA BREEZES WILL CONTINUE TO
BRING SHOWERS OVER THE ISLANDS INTO THE AFTERNOON. SOME SHOWERS
HAVE THE POTENTIAL TO BE LOCALLY HEAVY AND CAN CARRY BRIEF PERIODS
OF MVFR TO ISOL IFR. TRADE WINDS WILL CONTINUE TO GRADUALLY RETURN
OVER THE ISLANDS FROM EAST TO WEST. THE DEVELOPING TRADE WINDS WILL
HELP TO PUSH THE ENHANCED MOISTURE OVER THE STATE TO THE WEST.

CURRENTLY...THERE ARE NO AIRMETS IN EFFECT. HOWEVER AIRMETS FOR
MOUNTAIN OBSCURATIONS CAN NOT BE RULED OUT LATER TODAY AS CLOUDS
AND SHOWERS DEVELOP ALONG THE SEA BREEZE CONVERGENCE.

&&

.MARINE...
INCREASING TRADE WINDS MAY BRING A RETURN OF SMALL CRAFT ADVISORY
CONDITIONS IN THE TYPICALLY WINDY WATERS AROUND MAUI COUNTY AND THE
BIG ISLAND AS EARLY AS FRIDAY.

SEE DISCUSSION SECTION WHICH INCLUDES POTENTIAL MARINE IMPACTS TO
THE STATE EARLY NEXT WEEK. FORECASTS OF WEATHER...WIND...SWELL AND
SURF WILL BE HIGHLY DEPENDENT ON FUTURE FORECASTS FROM CPHC AND
MAJOR ADJUSTMENTS WILL LIKELY BE NEEDED.

IN THE MEANTIME...SURF IS EXPECTED TO REMAIN BELOW ADVISORY LEVELS
ALONG ALL SHORES AT LEAST THROUGH EARLY WEEKEND. SMALL SWELLS FROM
THE SOUTH AND WEST ARE EXPECTED THE NEXT COUPLE OF DAYS...WITH THE
POTENTIAL FOR A SLIGHTLY LARGER W SWELL FROM THE DISTANT BUT LARGE
TYPHOON ATSANI NEXT WEEK.

&&

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

&&

DISCUSSION/MARINE...R BALLARD
AVIATION...EATON