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

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Hawaiian Islands Synoptic Discussion and Guidance

555
FXHW60 PHFO 010634
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST MON AUG 31 2015

.SYNOPSIS...
VERY HUMID CONDITIONS WITH SPOTTY HEAVY SHOWERS AND THUNDERSTORMS...
MAINLY EACH AFTERNOON...WILL CONTINUE THROUGH MOST OF THE WEEK. A
TROUGH IN THE MIDDLE ATMOSPHERE MAY ORGANIZE THE SHOWERS ENOUGH
AROUND THURSDAY TO POSE THE RISK OF FLOODING. THE EXTENDED FORECAST
WILL DEPEND HEAVILY ON THE TRACK OF TROPICAL CYCLONE JIMENA.

&&

.DISCUSSION...
SHOWERS EMBEDDED IN THE VERY MOIST AIR MASS TO THE SOUTHWEST OF
TROPICAL CYCLONE IGNACIO MOVED ASHORE AND INTENSIFIED ALONG NORTH
SHORES OF THE MAIN HAWAIIAN ISLANDS. SEA BREEZES ENCOURAGED BY THE
LIGHT PREVAILING FLOW ALSO SUPPORTED INDEPENDENT SHOWER DEVELOPMENT
OVER HIGHER TERRAIN INLAND. THIS ACTIVITY WAS STRONGEST OVER
KAUAI...WHERE THE HANALEI RIVER RAN HIGH AFTER CONSIDERABLE RAINFALL
THROUGH THE DAY. THUS FAR FLOODING HAD BEEN AVERTED.

IGNACIO WILL CUT OFF THE TRADE WINDS ENTIRELY AS IT PASSES NORTHEAST
OF THE ISLANDS THROUGH TUESDAY NIGHT. IN PRINCIPLE...SOUTHWEST WINDS
WILL PREVAIL ONCE THE STORM MOVES NORTH OF A GIVEN ISLAND...BUT THE
CURRENT FORECAST TRACK MEANT THAT THESE WINDS WILL BE VERY WEAK. IN
PRACTICE...DIURNAL CIRCULATIONS WILL BE MOST IMPORTANT...THOUGH MANY
PEOPLE MAY FIND CONDITIONS UNCOMFORTABLY STAGNANT AND HUMID. SPOTTY
HEAVY SHOWERS AND PERHAPS A THUNDERSTORM WILL COMPLETE THE SCENARIO
BY TUESDAY AFTERNOON.

FROM WEDNESDAY INTO THE WEEKEND...PREVAILING WINDS WILL BECOME MORE
SOUTHERLY...BUT STILL LIGHT ENOUGH SO THAT DIURNAL EFFECTS WILL
REMAIN MORE IMPORTANT. SIMILARLY...ADVECTION FROM THE SOUTH WILL DO
LITTLE TO DRY THE AIR MASS. THE HEAT INDEX WILL APPROACH THE
THRESHOLD FOR AN EXCESSIVE HEAT ADVISORY IN SOME AREAS...BUT FOR NOW
IT LOOKED LIKE THIS OUTCOME WILL BE AVOIDED.

A MID-LEVEL TROUGH TRAILING SOUTH FROM IGNACIO WILL LINGER OVER THE
STATE THROUGH THIS TIME PERIOD. A SUBSTANTIAL FRACTION OF SOLUTIONS
PREDICTED A SURFACE CONVERGENCE ZONE TO DEVELOP UNDER OR NEAR THIS
FEATURE...FOCUSING CONVECTION IN THE VERY MOIST AIR MASS. WITH
LITTLE PREVAILING WIND THROUGH MUCH OF THE ATMOSPHERE...RESULTING
SHOWERS OR THUNDERSTORMS MAY MOVE VERY SLOWLY. CONSIDERING THE
UNRELENTINGLY HIGH PRECIPITABLE WATER TOTALS...RESULTING SHOWERS
MAY BE VERY HEAVY AS WELL. THIS SITUATION PROBABLY REPRESENTED THE
GREATEST THREAT THROUGH THE FORECAST PERIOD...OUTSIDE OF THE DIRECT
EFFECTS OF TROPICAL CYCLONES IN ANY CASE. IT MAY BE PRUDENT TO
CONSIDER THE NEED FOR A FLASH FLOOD WATCH IF THIS SCENARIO REMAINS
PLAUSIBLE.

DRIER EASTERLY FLOW MAY DEVELOP BY SUNDAY...BUT THE EXTENDED
FORECAST WILL DEPEND HEAVILY ON THE TRACK OF TROPICAL CYCLONE
JIMENA. THE CURRENT FORECAST TRACK PREDICTED THIS STORM TO MEANDER
FAR EAST TO NORTHEAST OF THE STATE.

&&

.AVIATION...
SEE TC SIGMET SERIES TANGO FOR TROPICAL CYCLONE IGNACIO.

THE PASSING OF IGNACIO A FEW HUNDRED MILES NORTH OF THE STATE WILL
GREATLY INFLUENCE THE WIND. WEST OF THE MAIN STORM
CIRCULATION...EXPECT MODERATE N TO NE WINDS. WINDS SOUTH OF THE
STORM WILL BACK MORE WESTERLY...AND EVENTUALLY TURN MORE SOUTHERLY
AND WEAKEN AS THE CENTER OF IGNACIO MOVES WEST.

SHOWER ACTIVITY WILL CONTINUE TO FAVOR NORTHEAST FACING SLOPES AND
COASTS OF THE SMALLER ISLANDS. ON THE BIG ISLAND....PARTIAL CLEARING
TONIGHT WILL GIVE WAY ON TUESDAY TO AFTERNOON AND EARLY EVENING
SHOWERS OVER INTERIOR AND LEEWARDS AREAS. ISOLATED MVFR CONDITIONS
ARE TO BE EXPECTED IN SHOWERS OTHERWISE VFR CONDITIONS SHOULD
PREVAIL THROUGH TUESDAY.

THE AIRMASS OVER THE STATE REMAINS MOIST AND UNSTABLE. ISOLATED
THUNDERSTORMS WITH HEAVY DOWNPOURS ARE POSSIBLE OVER THE NE HALF OF
THE COASTAL WATERS AND LAND AREAS TONIGHT. SLIGHT CHANCE OF
THUNDERSTORMS OVER INTERIOR BIG ISLAND TUESDAY AFTERNOON.

&&

.MARINE...
LARGE SWELLS FROM HURRICANE IGNACIO WILL AFFECT THE ISLANDS DURING
THE NEXT COUPLE OF DAYS...WITH REINFORCING SWELLS FROM DISTANT
HURRICANE JIMENA COMING IN DURING THE REST OF THE WEEK. THE LARGEST
SWELLS WILL REMAIN AIMED AT THE BIG ISLAND AND EAST MAUI INTO
TONIGHT...WHERE A HIGH SURF WARNING REMAINED IN EFFECT. THE OTHER
ISLANDS MOSTLY HAD BEEN BLOCKED FROM THE EAST-SOUTHEAST SWELL OF
IGNACIO...BUT THE SYSTEM MOVED INTO THE SWELL WINDOW OF THESE
ISLANDS LAST NIGHT. AS A RESULT...EXPECT BUILDING SURF FOR THESE
ISLANDS INTO TUESDAY...AND THE HIGH SURF WARNING HAS BEEN EXPANDED
TO COVER EAST-FACING SHORES OF MOST ISLANDS. EVEN WITH SWELL ENERGY
FROM IGNACIO FADING ON WEDNESDAY...EAST SHORE SURF WILL REMAIN
ELEVATED...POSSIBLY TO WARNING LEVELS...THROUGH AT LEAST THE
WEEKEND DUE TO EASTERLY SWELL FROM HURRICANE JIMENA.

A SMALL CRAFT ADVISORY /SCA/ ALSO REMAINED IN EFFECT THROUGH TUESDAY
NIGHT FOR WATERS EXPOSED TO EASTERLY SWELL FROM HURRICANE IGNACIO.
THE SCA MAY NEED TO BE EXTENDED FURTHER IF SEAS REMAIN ELEVATED.
WINDS ARE EXPECTED TO REMAIN BELOW THE SCA THRESHOLD THIS WEEK.

LONG-PERIOD SWELLS FROM THE SOUTH-SOUTHWEST MAY PRODUCE
ADVISORY-LEVEL SURF ALONG SOUTH-FACING SHORES THIS WEEKEND.

PLEASE REFER TO THE LATEST COLLABORATIVE NEARSHORE SWELL AND WIND
FORECAST /SRDHFO/ FOR MORE DETAILS ON THE SWELLS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING UNTIL 6 AM HST WEDNESDAY FOR KAUAI WINDWARD-
OAHU KOOLAU-OLOMANA-MOLOKAI-MAUI WINDWARD WEST-MAUI CENTRAL
VALLEY-WINDWARD HALEAKALA-SOUTH BIG ISLAND-BIG ISLAND NORTH AND
EAST.

SMALL CRAFT ADVISORY UNTIL 6 AM HST WEDNESDAY FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI CHANNEL-OAHU
WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND WINDWARD WATERS-
BIG ISLAND SOUTHEAST WATERS.

&&

$$

PUBLIC AND MARINE...RYSHKO
AVIATION...BEDAL




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