Flash Flood and Heavy Rain in Hawaii, January 16, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SUN JAN 16 2011

.SYNOPSIS...
A COLD FRONT WEST OF THE AREA WILL REACH KAUAI BY MID-DAY TODAY. THE
FRONT WILL PRODUCE NUMEROUS SHOWERS...ISOLATED THUNDERSTORMS AND A
TEMPORARY BOOST IN WINDS AS IT MOVES DOWN THE ISLAND CHAIN THROUGH
MONDAY. EASTERLY TRADE FLOW WILL STRENGTHEN GRADUALLY MID-WEEK AND
PERSIST INTO THE FOLLOWING WEEKEND AS HIGH PRESSURE BUILDS FAR TO
THE NORTHEAST. A VERY LARGE WEST NORTHWEST SWELL WILL PRODUCE LARGER
THAN NORMAL SURF AND COASTAL FLOODING FOR MANY NORTH AND WEST FACING
AREAS THROUGH MONDAY. WET WEATHER IS LIKELY AGAIN DURING THE SECOND
HALF OF THE WORK WEEK AS A SURFACE TROUGH IMPACTS THE STATE.

&&

.DISCUSSION...
ISOLATED LIGHT SHOWERS TRAVELLING IN THE LIGHT TRADE FLOW CAN BE
SEEN ON CURRENT RADAR IMAGERY...ALONG WITH A NORTH-SOUTH LINE OF
HEAVY SHOWERS/THUNDERSTORMS SOUTHWEST OF KAUAI. THIS LINE IS LIKELY
THE RESULT OF SOME PRE-FRONTAL CONVERGENCE. DUE TO THIS
FEATURE...HAVE ADDED A CHANCE OF THUNDERSTORMS TO THE WEATHER GRIDS
FOR THE MORNING PERIOD OVER KAUAI AND NIIHAU.

A COLD FRONT...CURRENTLY ABOUT 120 MILES NORTHWEST OF WESTERN
KAUAI...WILL BRING NUMEROUS SHOWERS TO THE STATE AS IT MOVES DOWN
THE ISLAND CHAIN. IT IS EXPECTED TO ARRIVE OVER KAUAI BY MID-DAY
TODAY...MAKE IT TO OAHU AND MAUI COUNTY BY TONIGHT...AND THEN
WEAKEN AS IT ARRIVES TO THE BIG ISLAND EARLY MONDAY. AN UPPER TROUGH
DIGGING DOWN OVER THE ISLANDS AS THE FRONT MOVES THROUGH WILL
DESTABILIZE THE ATMOSPHERE...SO ISOLATED THUNDERSTORMS ARE LIKELY
WITH THE FRONTAL PASSAGE. ALSO...THE POSSIBILITY OF HEAVY SHOWERS
WITH THIS FRONT HAVE BEEN ADDED TO THE WEATHER GRIDS DUE TO THE
INSTABILITY. ALTHOUGH GROUNDS AROUND THE STATE ARE ALREADY SATURATED
FROM RECENT HEAVY RAIN EVENTS...THIS PARTICULAR EVENT IS NOT
EXPECTED TO BE A BIG THREAT FOR FLASH FLOODING SINCE THE FRONT IS
NOT EXPECTED TO BE SLOW-MOVING. THUS...PROLONGED HEAVY RAINFALL OVER
ONE AREA IS NOT LIKELY.

A 1027 MB SURFACE HIGH ABOUT 1730 MILES NORTHEAST OF OAHU IS
BRINGING A GENTLE TO MODERATE EASTERLY FLOW TO THE STATE THIS
MORNING. WINDS WILL BECOME LIGHT AND VARIABLE AHEAD OF THE
APPROACHING COLD FRONT TOMORROW...AND THEN BECOME BRIEFLY STRONG AND
GUSTY FROM THE NORTHERLY DIRECTION WITH THE FRONTAL PASSAGE. WINDS
WILL DECREASE ONCE AGAIN AFTER THE FRONT EXITS THE STATE BEFORE
MODERATE TRADES RETURN ON WEDNESDAY AS A HIGH BUILDS FAR NORTHEAST
OF THE REGION. TRADES WILL THEN BECOME MODERATE TO FRESH THURSDAY
THROUGH THE BEGINNING OF THE WEEKEND AS THE HIGH REMAINS FAR
NORTHEAST OF THE ISLANDS.

AFTER THE COLD FRONT EXITS THE STATE MONDAY AFTERNOON...DRY AIR
BEHIND THE FRONT WILL RESULT IN NOT MUCH IN THE WAY OF SHOWER
ACTIVITY MOST OF MONDAY THROUGH TUESDAY MORNING. HOWEVER...WEATHER
MAY GET INTERESTING LATER TUESDAY AS MOST MODELS ARE SHOWING A
CUT-OFF UPPER LOW DEVELOPING NEAR THE ISLANDS. ALTHOUGH THE 00Z GFS
RUN HAD BACKED OFF ON THE LOW MOVING OVER THE STATE...THE 06Z RUN IS
IN AGREEMENT WITH THE OTHER MODELS IN BRINGING THE LOW OVER THE
ISLANDS STARTING LATE TUESDAY. BOTH THE GFS AND ECMWF SHOW MOISTURE
INCREASING BY MID-WEEK...SO IF THE LOW DOES MOVE OVER THE
STATE...THERE WILL BE THE POTENTIAL FOR HEAVY SHOWERS AND
THUNDERSTORMS. THERE IS LOW CONFIDENCE IN THIS POSSIBILITY AT THE
MOMENT DUE TO MODEL INCONSISTENCY...SO MAJOR CHANGES TO THE GRIDS
HAVE NOT BEEN MADE WITH THIS FEATURE AS OF YET.

HOWEVER...MODELS SEEM TO BE CONSISTENTLY AGREEING THAT AN INVERTED
SURFACE TROUGH WILL DEVELOP FAR TO THE EAST OF THE BIG ISLAND LATE
TUESDAY AND MOVE OVER THE EASTERN PART OF THE STATE LATE WEDNESDAY
THROUGH FRIDAY MORNING. THE TROUGH WILL DRAW UP MOISTURE FROM THE
ITCZ...AND WITH UPPER LEVEL HEIGHT FALLS ACCOMPANYING THIS
FEATURE...THERE WOULD BE SOME POTENTIAL FOR ACTIVE WEATHER FOR MAUI
COUNTY AND THE BIG ISLAND. WITH TRADE WINDS PREVAILING DURING THIS
TIME...SHOWERS WOULD FOCUS OVER WINDWARD AND MOUNTAIN AREAS. HAVE
ENOUGH CONFIDENCE TO INCREASE POPS OVER WINDWARD LAND ZONES AND
COASTAL WATERS WITH THIS FEATURE. HEAVY SHOWERS AND THUNDERSTORMS
ALSO APPEAR POSSIBLE WITH THIS FEATURE...BUT HAVE HELD OFF ON
INSERTING THEM INTO THE GRIDS UNTIL WE SEE A FEW MORE MODEL RUNS
THAT WILL INCREASE OUR CONFIDENCE IN THIS OUTCOME FURTHER.

&&

.MARINE...
A VERY LARGE WEST NORTHWEST SWELL WILL CONTINUE TO IMPACT NORTH AND
WEST FACING SHORES OF THE ISLANDS TODAY. BUOY 51101 IS CURRENTLY
AROUND 17-18 FEET WITH PERIODS AROUND 18-19 SECONDS. MEANWHILE...THE
WAIMEA BAY BUOY SHOWS THAT THE SWELL HAS ARRIVED THERE...WITH SWELL
HEIGHTS/PERIODS CURRENTLY AT 13 FEET/12 SECONDS. THE WESTERLY
DIRECTION OF THIS GIANT SWELL IS UNCOMMON...AND COASTAL
FLOODING/DAMAGE IS POSSIBLE...PARTICULARLY FOR THE KONA COAST OF THE
BIG ISLAND. A HIGH SURF WARNING IS CURRENTLY IN EFFECT FOR NORTH AND
WEST FACING SHORES OF KAUAI COUNTY AND OAHU UNTIL NOON MONDAY. A
WARNING FOR THE WEST FACING SHORES OF THE BIG ISLAND ALSO GOES INTO
EFFECT STARTING 6 AM TODAY AND LASTING THROUGH 6 PM MONDAY.
HOWEVER...NORTH AND WEST FACING SHORES OF MAUI COUNTY ARE ONLY UNDER
A HIGH SURF ADVISORY BECAUSE THE WESTERN HAWAIIAN ISLANDS ARE
EXPECTED TO BLOCK SOME OF THE SWELLS ENERGY DUE TO THE WEST
NORTHWEST DIRECTION OF THE SWELL. PLEASE SEE THE HIGH SURF
PRODUCT...CFWHFO...FOR MORE DETAILS ON THE SWELL AND POTENTIAL
IMPACTS FROM IT.

A SMALL CRAFT ADVISORY IS ALSO IN EFFECT DUE TO THE HIGH SEAS
CREATED BY THE CURRENT SWELL. CURRENTLY...JUST THE COASTAL WATERS
AROUND KAUAI AND OAHU ARE IN THE ADVISORY. THE REST OF THE COASTAL
WATERS...EXCLUDING MAALAEA BAY DUE TO THE SWELLS ENERGY GETTING
BLOCKED...WILL JOIN THE ADVISORY STARTING 6 AM TODAY. ALSO...THE
FRONT THAT WILL MOVE DOWN THE ISLAND CHAIN SUNDAY AFTERNOON THROUGH
EARLY MONDAY WILL BRING STRONG AND GUSTY WINDS WITH IT FOR A SHORT
PERIOD OF TIME...ADDING TO THE HAZARDOUS SEA CONDITIONS.
ADDITIONALLY...THE SWELL MAY CAUSE HAZARDOUS BREAKING WAVES AND
SURGES FOR WEST FACING HARBORS...SO A MARINE WEATHER STATEMENT
REMAINS POSTED.

THE SWELL IS EXPECTED TO START LOWERING MONDAY NIGHT...ALTHOUGH SURF
WILL STILL REMAIN WELL ABOVE ADVISORY LEVELS FOR MOST NORTH AND WEST
SHORES.

A LOW PRESSURE SYSTEM IN THE NORTHWEST PACIFIC WILL STRENGTHEN
SIGNIFICANTLY STARTING THIS AFTERNOON...CREATING ANOTHER VERY
LARGE WEST NORTHWEST SWELL THAT WILL ARRIVE LATE TUESDAY NIGHT. AS A
RESULT...SURF WILL ONCE AGAIN GO ABOVE WARNING LEVELS FOR MOST NORTH
AND WEST FACING SHORES WEDNESDAY AND THURSDAY. THE SWELL WILL ALSO
CREATE HAZARDOUS SEAS...SO ANOTHER SMALL CRAFT ADVISORY ISSUANCE
WILL BE NECESSARY AT THAT TIME FOR THE MAJORITY OF THE COASTAL
WATERS.

&&

.AVIATION...
A COLD FRONT MOVING TO THE SOUTHEAST WILL BE ACCOMPANIED BY MVFR
CEILINGS AND VISIBILITIES WITH SHOWERS AND ISOLATED THUNDERSTORMS.
THE FRONT SHOULD REACH KAUAI SUNDAY AFTERNOON AND OAHU SUNDAY
EVENING. MOUNTAIN OBSCURATION IS LIKELY WITH THE FRONT.

HIGH LEVEL MODERATE OCCASIONAL SEVERE CLEAR AIR TURBULENCE IS
FORECAST BELOW FL440 FROM KAUAI NORTHWESTWARD AT 00Z...AND NORTH OF
15N BETWEEN 157W AND 166W AT 06Z. THE TURBULENCE IS ASSOCIATED WITH
A SHARP NORTH/SOUTH TROUGH ALOFT DIGGING SOUTHWARD ALONG 160W.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING UNTIL 12 PM HST MONDAY FOR NORTH AND WEST FACING
SHORES OF NIIHAU...KAUAI AND OAHU.

HIGH SURF WARNING FROM 6 AM HST SUNDAY TO 6 PM HST MONDAY FOR WEST
FACING SHORES OF THE BIG ISLAND.

HIGH SURF ADVISORY UNTIL 12 PM HST MONDAY FOR NORTH AND WEST FACING
SHORES OF MOLOKAI...LANAI...MAUI AND KAHOOLAWE.

SMALL CRAFT ADVISORY UNTIL 6 PM HST MONDAY FOR KAUAI NORTHWEST
WATERS...KAUAI WINDWARD WATERS...KAUAI LEEWARD WATERS...KAUAI
CHANNEL...OAHU WINDWARD WATERS AND OAHU LEEWARD WATERS.

SMALL CRAFT ADVISORY FROM 6 AM SUNDAY TO 6 PM HST MONDAY FOR KAIWI
CHANNEL...MAUI COUNTY WINDWARD WATERS...MAUI COUNTY LEEWARD
WATERS...PAILOLO CHANNEL...ALENUIHAHA CHANNEL...BIG ISLAND WINDWARD
WATERS...BIG ISLAND LEEWARD WATERS AND BIG ISLAND SOUTHEAST WATERS.

&&

DISCUSSION/MARINE...HARRISON
AVIATION...CRAIG