Heavy Rain in Hawaii, January 10 to 11, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT JAN 10 2009

.SYNOPSIS...
A FAST-MOVING COLD FRONT WILL BRING LOCALLY HEAVY SHOWERS AND
POSSIBLE THUNDERSTORMS TO KAUAI LATE TONIGHT...AND THEN PUSH DOWN TO
THE BIG ISLAND BY MONDAY MORNING. CONDITIONS WILL DRY OUT THROUGH
MOST OF NEXT WEEK...BUT ANOTHER FRONT MAY BRING MORE RAIN TO THE
ISLANDS BY NEXT FRIDAY.

&&

.DISCUSSION...
A RATHER SUBSTANTIAL FRONTAL CLOUD BAND APPROACHED KAUAI FROM THE
NORTHWEST...SUPPORTED BY A SHARP BUT NARROW TROUGH ALOFT. LIGHT
SOUTHEAST FLOW PREVAILED OVER THE THE STATE IN THE MEANTIME...
PUSHING A FEW SHOWERS ONTO SOUTHEAST SLOPES. THESE PREFRONTAL
SHOWERS WERE HEAVIEST WHERE A PLUME FROM ONE ISLAND MOVED ASHORE
ONTO ANOTHER...BUT GENERALLY RESEMBLED THE TYPES OF SHOWERS
ACCOMPANYING WET TRADES. RADAR DATA ALREADY SHOWED A WALL OF ECHOES
OFFSHORE TO THE WEST OF NIIHAU...BUT IT WAS NOT CLEAR WHAT FRACTION
OF THE RADAR RETURNS REPRESENTED ACTUAL PRECIPITATION REACHING THE
SURFACE...AND HOW MUCH WAS ASSOCIATED WITH WET ICE REMAINING AT
CLOUD HEIGHT.

GUIDANCE AGREES IN PREDICTING THE FRONTAL CLOUD BAND TO REACH KAUAI
SHORTLY BEFORE MIDNIGHT TONIGHT...AND OAHU AROUND DAYBREAK ON
SUNDAY. ALTHOUGH THIS VIGOROUS SYSTEM IS LIKELY TO PRODUCE PERIODS
OF HEAVY RAIN...IT SHOULD MOVE THROUGH QUICKLY ENOUGH THAT
WIDESPREAD FLOODING IS NOT LIKELY. HOWEVER...INDIVIDUAL SMALL
WATERSHEDS MAY BE OVERLOADED BY LOCAL DOWNPOURS. THE COLD POOL
ASSOCIATED WITH THE TROUGH ALOFT CERTAINLY COULD SUPPORT ISOLATED
THUNDERSTORMS AS WELL.

VIGOROUS AS THIS SYSTEM MAY BE...IT DOES NOT EXTEND MUCH FURTHER
SOUTH THAN 20N...AND SO IT CAN BE EXPECTED TO WEAKEN RAPIDLY UP TO
THE LATITUDE OF THE ISLANDS ONCE THE FAST-MOVING MID-LEVEL TROUGH
STARTS TO PULL AWAY TOWARD THE NORTHEAST ON SUNDAY. THUS...THOUGH
SHOWERS LIKELY WILL AFFECT OAHU MOST OF THE DAY...OVERALL RAINFALL
THERE SHOULD BE LESS THAN ON KAUAI TONIGHT...WHILE MAUI COUNTY AND
THE BIG ISLAND WILL EXPERIENCE ONLY THE REMNANTS OF THIS SYSTEM FROM
SUNDAY NIGHT THROUGH MONDAY. EVEN SO...SNOW OR ICE MAY ACCUMULATE ON
THE HIGH MOUNTAINS OF THE BIG ISLAND FROM SUNDAY AFTERNOON THROUGH
MONDAY MORNING.

A WEAK HIGH MOVING EASTWARD BEHIND THE FRONT WILL BRING TRADE WINDS
TO THE STATE ON MONDAY AND MONDAY EVENING. HOWEVER...THE SUBTROPICAL
RIDGE WILL PUSH DOWN OVER THE ISLANDS ON TUESDAY...BRINGING LIGHT
AND VARIABLE WINDS. THE RIDGE WILL REMAIN SOUTH OF THE ISLANDS
THROUGH THE REST OF NEXT WEEK SO TRADE WINDS WILL BE ABSENT. THE
WEATHER WILL REMAIN MOSTLY DRY THROUGH THURSDAY. ANOTHER FRONTAL
BAND MAY BRING MORE RAIN TO THE ISLANDS BY THURSDAY NIGHT OR FRIDAY.

&&

.MARINE...
A SWELL PRODUCED BY A LOW NORTHWEST OF THE ISLANDS WILL BUILD
OVERNIGHT. THIS SWELL WILL RAISE ADVISORY-LEVEL SURF ALONG SHORES
FACING NORTH AND WEST ON SUNDAY. COMBINED SEAS WILL EXCEED THE
THRESHOLD FOR A SMALL CRAFT ADVISORY IN WATERS NORTHWEST OF THE BIG
ISLAND.

ANOTHER LOW IS DEEPENING RAPIDLY EAST OF JAPAN. SWELL FROM THAT LOW
WILL REACH THE ISLANDS TUESDAY NIGHT AND LIKELY WILL PRODUCE LARGE
ENOUGH SURF ALONG SHORES FACING NORTH AND WEST TO REQUIRE A WARNING.
ANOTHER LARGE NORTHWEST SWELL ARRIVING FRIDAY MAY PRODUCE MORE
WARNING-LEVEL SURF.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM 4 AM SUNDAY TO 6 PM HST TUESDAY FOR
NIIHAU-KAUAI WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH
SHORE-OAHU KOOLAU-WAIANAE MOUNTAINS-MOLOKAI-MAUI WINDWARD WEST-
MAUI CENTRAL VALLEY-WINDWARD HALEAKALA.

WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING
FOR BIG ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST SUNDAY FOR BIG ISLAND
WINDWARD WATERS-BIG ISLAND LEEWARD WATERS-BIG ISLAND SOUTHEAST
WATERS.

SMALL CRAFT ADVISORY FROM 6 AM SUNDAY TO 6 PM HST TUESDAY FOR
KAUAI NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD
WATERS-KAUAI CHANNEL-OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-
KAIWI CHANNEL.

&&

RYSHKO