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Hawaiian Islands Synoptic Discussion and Guidance

646
FXHW60 PHFO 210205
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
405 PM HST SAT SEP 20 2014

.SYNOPSIS...
LIGHT WIND FLOW WILL RESULT IN LOCAL NIGHTTIME LAND AND DAYTIME
SEA BREEZE CIRCULATIONS THROUGH THE REST OF THE WEEKEND. IN THIS
REGIME...EXPECT MOSTLY CLEAR NIGHTS AND MORNINGS...WITH CLOUD
BUILD UPS AND ISOLATED SHOWERS DEVELOPING OVER LEEWARD AND
INTERIOR SECTIONS EACH AFTERNOON. LIGHT TRADES EXPECTED MONDAY
AND TUESDAY...WITH BRIEF PASSING SHOWERS FAVORING WINDWARD AND
MAUKA AREAS THROUGH MID-WEEK. THE TRADE WINDS MAY BECOME WETTER
FROM THURSDAY INTO NEXT WEEKEND AS MOISTURE ASSOCIATED WITH A
WEAKENING FRONT PUSHES DOWN OVER THE ISLANDS.


&&

.DISCUSSION...
THIS AFTERNOON/S VISIBLE SATELLITE AND RADAR OVER HAWAII SHOWS
MOSTLY CLOUDY SKIES WITH SCATTERED SHOWERS. THESE SHOWERS ARE
BEING INDUCED BY THE HIGH MOISTURE LEVELS OVER THE STATE...AND
THE SEA BREEZE CONVERGENCE OVER THE ISLANDS. DEW POINTS...DIRECTLY
RELATED TO THE MOISTURE CONTENT IN THE AIR...IS RUNNING BETWEEN 70
AND 74 DEGREES THIS AFTERNOON...WHILE TEMPERATURES RANGE FROM 86
TO 91. MEANWHILE...VERY LIGHT FLOW ACROSS HAWAII IS KEEPING THIS
MOISTURE IN PLACE. WE CAN EXPECT THESE MUGGY CONDITIONS TO
CONTINUE OVER THE NEXT SEVERAL DAYS.

THE LIGHT WINDS ACROSS HAWAII ARE THE RESULT OF TWO FEATURES...A
WEAK SURFACE TROUGH OVER THE KAIWI CHANNEL AND A WEAK SURFACE
FRONT LOCATED ABOUT 400 MILES NORTH OF LIHUE. BOTH THE FRONT AND
SURFACE TROUGH ARE DISRUPTING THE PRESSURE GRADIENT ACROSS THE
REGION RESULTING IN WEAK LOW-LEVEL FLOW. THIS IS ALLOWING AFTERNOON
SEA BREEZES AT THIS TIME...AND NIGHTTIME LAND BREEZES EXPECTED TO
KICK IN AFTER SUNSET. THE LAND BREEZES WILL ALLOW THE SHOWERS OVER
THE ISLANDS TO DISSIPATE AND CLEAR OUT MUCH OF THE SKY LATER
TONIGHT...AT LEAST FOR THE SMALLER ISLANDS.

A WEAK TROPICAL DISTURBANCE IS EVIDENT IN SATELLITE IMAGERY JUST
SOUTH OF THE BIG ISLAND. ALTHOUGH THIS FEATURE IS NOT HAVING A
SIGNIFICANT IMPACT THE LOCAL WINDS...THIS SYSTEM IS PROVIDING
A SOURCE OF DEEP TROPICAL MOISTURE THAT HAS BEEN ADVECTING UP OVER
PARTS THE BIG ISLAND SINCE FRIDAY. THE OUTER FRINGES OF THIS
MOISTURE CONTINUES TO STREAM UP OVER PARTS OF THE BIG ISLAND...AND
WILL LIKELY KEEP CLOUD COVER AND SHOWERS OVER THE WINDWARD AND
SOUTHEAST BIG ISLAND TONIGHT. THIS MOISTURE WILL SLOWLY TRACK WEST
OVER THE NEXT DAY OR TWO.

A WEAK MID-LEVEL TROUGH PASSING NORTH OF THE ISLANDS WILL
EVENTUALLY BE REPLACED BY MID-LEVEL RIDGING STARTING SOME TIME ON
SUNDAY. THE MENTIONED FRONT WILL STALL NORTH OF THE AREA SOMETIME
ON SUNDAY. AS A RESULT...THE PRESSURE GRADIENT OVER THE ISLANDS
WILL REMAIN WEAK THROUGH THIS WEEKEND MAINTAINING THE LOCAL LAND
AND SEA BREEZE CIRCULATION PATTERN THROUGH SUNDAY NIGHT.
THEREFORE...EXPECT GENERALLY CLEAR SKIES AT NIGHT AND MORNING...FOLLOWED
BY CLOUD BUILD UPS AND ISOLATED SHOWERS DEVELOPING BY LATE MORNING
AND LINGERING INTO THE EARLY EVENING HOURS. A SLIGHT CHANCE OF
THUNDERSTORMS HAS BEEN ADDED TO THE FORECAST FOR THE INTERIOR BIG
ISLAND ON SUNDAY AFTERNOON GIVEN THE CLOSE PROXIMITY OF THE DEEP
TROPICAL MOISTURE.

A MODERATELY STRONG SURFACE HIGH PASSING FAR NORTH OF THE ISLANDS
WILL INCREASE THE PRESSURE GRADIENT OVER THE AREA MONDAY...ALLOWING
THE TRADE WIND FLOW TO STRENGTHEN...BUT ONLY SLIGHTLY. THE
FORECAST MODELS SHOW A NEW FRONT MOVING DOWN TOWARD THE AREA
AROUND THURSDAY. THIS FEATURE APPEARS TO BE DROPPING DOWN TOWARD
THE ISLAND CHAIN AS AN EAST TO WEST ORIENTED BAND OF CLOUDS AND
SHOWERS. AS A RESULT...IT APPEARS THIS FEATURE WILL IMPACT THE
ISLANDS FROM THURSDAY INTO FRIDAY. MOST OF THE MOISTURE WILL
LIKELY BE FOCUSED OVER WINDWARD SECTIONS OF THE STATE...AND IT
WILL POSSIBLY LINGER INTO NEXT WEEKEND. THE SLIGHTLY STRONGER LOW-
LEVEL TRADE WIND FLOW MAY TRANSPORT A FEW OF THESE SHOWERS OVER TO
LEEWARD AREAS ON THE SMALLER ISLANDS...ESPECIALLY NIGHTS AND
MORNINGS.

NOTE THAT THE LIGHT WINDS WILL ALLOW VOG AND SMOKE FROM THE BIG
ISLAND/S ACTIVE VOLCANO/LAVA FLOWS TO POOL NEAR THE SURFACE OVER
PARTS OF THE BIG ISLAND DURING THE OVERNIGHT AND EARLY MORNING HOURS
THIS WEEKEND...BEFORE CLEARING BY MID-MORNING. STRONGER WINDS NEXT
WEEK WILL LIKELY PREVENT A STRONG GROUND-BASED INVERSION FROM
DEVELOPING...AND THE POOLING OF PARTICULATES NEAR THE SURFACE SHOULD
BE LESS PRONOUNCED.

LOCAL SEA SURFACE TEMPERATURES /SSTS/ ARE ANOMALOUSLY WARM FOR
MID-SEPTEMBER...WHICH IS CONTRIBUTING TO THE ONGOING MUGGY
CONDITIONS. SURFACE DEW POINTS REMAIN IN THE UPPER 60/S TO LOWER
70/S THIS MORNING. THE SSTS ARE NOT EXPECTED TO CHANGE SIGNIFICANTLY
IN THE NEAR FUTURE. WITH THE RETURN OF THE TRADE WINDS NEXT WEEK...
AIR TEMPERATURES MAY COOL SLIGHTLY. HOWEVER...THE DEW POINTS WILL
LIKELY REMAIN RELATIVELY HIGH DUE TO THE ADDITION OF MOISTURE FROM
THE ADJACENT WARM OCEAN WATERS.

&&

.AVIATION...
ISOLATED THUNDERSTORMS WITH CLOUD TOPS UP TO AROUND 35000 FEET HAVE
BEEN DETECTED OVER THE BIG ISLAND SLOPES. LOCALLY HEAVY SHOWERS
HAVE ALSO FORMED OVER THE SMALLER ISLANDS AS WELL. THIS IS THE
RESULT OF DAYTIME HEATING AND A MOIST UNSTABLE AIRMASS OVER THE
AREA. EXPECT THE INTENSITY AND COVERAGE OF THE SHOWERS TO GRADUALLY
TAPER OFF THIS EVENING AND ON THROUGH THE NIGHT AS THE LAND MASS
COOLS. DEEP CONVECTION WILL LIKELY PERSIST OVERNIGHT OVER THE WATERS
SOUTH OF THE BIG ISLAND DUE TO A WEAK LOW LEVEL DISTURBANCE PASSING
SOUTH OF THE STATE. VFR CONDITIONS PREVAIL AT THE TAF SITES AND THE
CONDITIONS ARE EXPECTED TO PREVAIL THROUGH THE NIGHT THOUGH THERE MAY
BE SOME BRIEF MVFR THROUGH EARLY THIS EVENING IN SHRA.


&&

.MARINE...
THE FORECAST FOR WINDS AND SEAS REMAIN BELOW SMALL CRAFT ADVISORY
/SCA/ CRITERIA INTO EARLY NEXT WEEK. THERE IS A POSSIBILITY TRADE
WIND SPEEDS MAY REACH THE SCA THRESHOLD ACROSS THE WINDIER MARINE
ZONES ADJACENT TO THE ISLANDS OF MAUI AND HAWAII COUNTIES BY
MID-WEEK.

SURF WILL LIKELY REMAIN BELOW THE HIGH SURF ADVISORY CRITERIA ALONG
ALL SHORES OF ALL ISLANDS THROUGH NEXT FRIDAY. HOWEVER...SMALL
SWELLS FROM THE NORTHWEST...NORTHEAST...EAST AND SOUTH ARE EXPECTED
TO PROVIDE SMALL BACKGROUND SURF ALONG MOST SHORELINES OF THE STATE
THROUGH THIS WEEKEND. NEW NORTH AND NORTHWEST SWELLS WILL ALSO
AFFECT MOST ISLANDS ON WEDNESDAY AND THURSDAY...WITH A NORTH
NORTHEAST SWELL POSSIBLE THURSDAY AND FRIDAY. TINY LONG-PERIOD
SWELLS FROM THE SOUTH WILL MAINTAIN SURF HEIGHTS NEAR OR BELOW
SEASONAL AVERAGES ALONG SOUTH FACING SHORES NEXT WEEK.


&&

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

&&

$$

DISCUSSION/MARINE...FOSTER
AVIATION...BURKE


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