Heavy Rain in Hawaii, December 30, 2004

HAWAIIAN ISLANDS FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
430 PM HST THU DEC 30 2004

.SYNOPSIS...
A STRONG LOW PRESSURE CENTER MOVING TOWARD THE ISLANDS FROM THE WEST
WILL PRODUCE WINDY CONDITIONS AND PERIODS OF RAIN TO THE ISLANDS
THROUGH EARLY NEW YEARS DAY. THE HIGHEST THREAT OF HEAVY SHOWERS AND
ISOLATED THUNDERSTORMS WILL BE FOCUSED ON KAUAI FRIDAY THROUGH
FRIDAY NIGHT...AND FRIDAY NIGHT INTO SATURDAY FOR OAHU. SHOWERS ARE
EXPECTED REACH MAUI COUNTY AND THE BIG ISLAND SATURDAY THROUGH
SUNDAY...BUT WILL NOT BE AS INTENSE AS THOSE IMPACTING KAUAI AND
OAHU.

&&

.DISCUSSION...
THE STRONG LOW PRESSURE SYSTEM WE ARE WATCHING IS CURRENTLY LOCATED
7505 MILES WEST OF KAUAI AND MOVING TOWARD THE SOUTH AT 20 TO 25
MPH. OF IMMEDIATE CONCERN IS THE BANDS OF HEAVY SHOWERS AND
ISOLATED THUNDERSTORMS ON THE EAST SIDE OF THE SYSTEM.THE EASTWARD
PROGRESSION OF THE CLOSEST BAD HAS SLOWED CONSIDERABLY SINCE THIS
MORNING...BUT IS STILL EXPECTED TO REACH KAUAI THIS EVENING. THE
BAND WILL LIKELY NOT REACH OAHU AS WAS EARLIER PROJECTED...BUT THE
ATMOSPHERE REMAIN SUFFICIENTLY MOIST AND UNSTABLE FOR SHOWER
DEVELOPMENT.

WATER VAPOR IMAGERY LAST NIGHT SHOWED AN APPARENT DISCONNECT
BETWEEN THE SYSTEM AND THE VAST AMOUNTS OF TROPICAL MOISTURE
BETWEEN 5N TO 10N LATITUDE. WATER VAPOR IMAGERY CURRENTLY INDICATES
THAT THE SYSTEM HAS SINCE BECOME SUFFICIENTLY STRONG AND FAR SOUTH
ENOUGH TO BEGIN TAPPING INTO THIS MOISTURE SOURCE. SATELLITE DERIVED
UPPER WIND FIELDS ALSO SHOW GOOD DIFLUENCE ON THE EAST SIDE OF THE
LOW...WHICH WILL WILL ENHANCE THE ACTIVE LOW LEVEL CONVERGENCE AT
THE LOWER LEVELS.

GLOBAL MODELS REMAIN IN GOOD AGREEMENT WITH THE LOCATION AND
MOVEMENT OF THIS LOW FOR THE NEXT COUPLE OF DAYS. THE LOW IS
EXPECTED TO CONTINUE ITS SOUTHWARD MOVEMENT OVER THE NEXT 12
HOURS TO A POINT NEAR 20N 170W...THEN DRIFT SLOWLY TOWARD THE EAST
NORTHEAST. IT IS DURING THIS EAST NORTHEAST DRIFT WHEN THE THREAT
FOR FLASH FLOODING IS HIGHEST ON NIIHAU KAUAI AND OAHU. MODEL PRECIP
FIELDS DO NOT HAVE A GOOD HANDLE ON THE RAIN ASSOCIATED WITH THE LOW
LEVEL CONVERGENCE...THUS APPEAR TO HAVE A SLIGHT WESTWARD BIAS.
TAKING THIS INTO ACCOUNT...HAVE INCREASED THE CLOUDS AND SHOWERS
ACROSS KAUAI TONIGHT AND OAHU TOMORROW. THERE IS A FAIRLY SHARP
GRADIENT BETWEEN THE ACTIVE AND NON ACTIVE SHOWERS AREAS...WHICH
WILL MAKE THE FORECAST FOR MAUI COUNTY A BIT MORE TRICKY. THE
CURRENT FORECAST REASONING IS FOR SHOWERS TO BEGIN PICKING UP FRIDAY
NIGHT AND FILL IN FURTHER SATURDAY INTO SUNDAY. THE BIG ISLAND WILL
EXPERIENCE THE INCREASED SHOWERS SATURDAY THROUGH SUNDAY.

WINDS WILL ALSO LIKELY BE A PROBLEM WITH THIS STORM. WINDS OVER
KAUAI ARE EXPECTED TO GRADUALLY INCREASE TONIGHT AND
TOMORROW...REACHING 20 TO 30 MPH...AND POSSIBLY STRONGER...WITH
HIGHER GUSTS IN WIND CHANNELLED AREAS AND DOWNWIND OF HILLS AND
MOUNTAINS. OAHU WILL START EXPERIENCING THE STRONGER WINDS LATE
FRIDAY INTO FRIDAY NIGHT. A HIGH WIND WATCH REMAINS IN EFFECT FOR
THESE ISLANDS THROUGH NEW YEARS DAY. WARNINGS WILL BE ISSUED AS
NECESSARY...AS THIS SITUATION DEVELOPS. IT DOES NOT APPEAR THAT THE
STRONG WINDS WILL REACH MAUI COUNTY AND THE BIG ISLAND.

SO THE SHORT STORY IS...VIGOROUS LOW PRESSURE WEST OF THE ISLANDS
WILL PRODUCE STRONG AND GUSTY WINDS...ALONG WITH PERIODS OF HEAVY
SHOWERS AND ISOLATED THUNDERSTORMS THE WINDS... SHOWERS AND
THUNDERSTORMS WILL BE FOCUSED OVER NIIHAU AND KAUAI STARTING
TONIGHT...AND OAHU TOMORROW...LASTING THROUGH NEW YEARS DAY. THE
THREAT FOR FLASH FLOODING IS HIGHEST ON THESE ISLANDS. THE REMAINING
ISLANDS WILL EXPERIENCE THE INCREASED SHOWERS SATURDAY THROUGH
SUNDAY...BUT RAINFALL IS NOT EXPECTED TO BE AS INTENSE.

&&

.LONG TERM...
GIVEN THE RECENT MODEL GUIDANCE...THE
FRONTAL SQUALL LINE WILL WEAKEN AND MOVE EASTWARD RATHER RAPIDLY AS
IT PASSES OAHU LATE ON NEW YEARS DAY. THIS WILL LIKELY SPARE THE
OTHER ISLANDS OF MAUI COUNTY AND THE BIG ISLAND FROM THE HEAVIEST OF
THE RAINFALL AND STRONGEST WINDS THAT MIGHT HAVE AFFECTED THEM GIVEN
EARLIER SCENARIOS. WITH THE MAIN LOW WRAPPING UP WELL NORTH OF THE
ISLANDS NEAR 37N 156W LATE SUNDAY...THE COLD FRONT WILL HAVE SWEPT
THROUGH MOST OF THE STATE AND BE NEAR THE BIG ISLAND AT THAT TIME.
WITH ALL THE ENERGY PULLED NORTHWARD INTO THE MAIN LOW...EXPECT ONLY
LIGHT TO MODERATE NORTHERLY WINDS OVER THE STATE BEHIND THE FRONT.
WITH HIGH PRESSURE RIDGING FOLLOWING THE FRONT JUST NORTH OF THE
ISLANDS... LIGHT TO MODERATE NORTH TO NORTHEAST WINDS ARE EXPECTED
TO CONTINUE OVER THE STATE WELL INTO THE FIRST WEEK OF THE NEW YEAR.
THIS WILL ALSO MEAN GENERALLY DRY AND SUNNY DAYS...ALBEIT CHILLY
NIGHTS...FOR ALL ISLANDS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH KAUAI AND OAHU THROUGH NEW YEARS DAY.
FLASH FLOOD WATCH NIIHAU KAUAI AND OAHU THROUGH NEW YEARS DAY.

&&

TANABE