High Surf in Hawaii, December 19 to 21, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
526 AM HST SAT DEC 19 2009

.UPDATE...
AN UPDATED FORECAST HAS BEEN ISSUED TO KEEP ISOLATED THUNDERSTORMS
IN THE FORECAST FOR WINDWARD BIG ISLAND AND ADJACENT WATERS THROUGH
MID MORNING. THE SHORT WAVE ALOFT TRIGGERING THE THUNDERSTORMS HAS
NOT CLEARED AS QUICKLY AS ANTICIPATED.

&&

.SYNOPSIS...
A DRY NORTHEASTERLY TRADE WIND FLOW WILL PERSIST ACROSS MOST OF THE
STATE THROUGH THE WEEKEND...WHILE WINDWARD BIG ISLAND MAY EXPERIENCE
ENHANCED RAINFALL DUE TO MOISTURE LEFT BEHIND BY A DISSIPATED FRONT.
WINDS WILL DECREASE DURING THE FIRST HALF OF NEXT WEEK AS A
WEAKENING FRONT APPROACHES FROM THE NORTHWEST. LITTLE RAINFALL IS
EXPECTED FROM THIS FRONT. ANOTHER FRONT MAY IMPACT THE STATE LATE IN
THE WEEK.

&&

.PREVIOUS DISCUSSION 400 AM...
A LARGE CONTRAST IN WEATHER CONDITIONS HAS BEEN OCCURRING OVERNIGHT
ACROSS THE ISLAND CHAIN. MOST OF THE SMALLER ISLANDS ARE UNDER A
DRY AND STABLE POST-FRONTAL AIR MASS...WHILE WINDWARD BIG ISLAND AND
ITS ADJACENT WATERS HAVE BEEN EXPERIENCING LOCALIZED HEAVY SHOWERS
AND ISOLATED THUNDERSTORMS. THESE UNSTABLE CONDITIONS HAVE BEEN
TRIGGERED BY A SHORT WAVE ALOFT PASSING OVER LINGERING MOISTURE
POOLED OVER AND EAST OF THE BIG ISLAND LEFT BEHIND FROM A DISSIPATED
FRONT. THE RESULT HAS BEEN WIDESPREAD RAINFALL UP TO 0.75 OF AN INCH
WITH ISOLATED LOCALES RECEIVING OVER 2 INCHES. THUNDERSTORM ACTIVITY
IS WINDING DOWN AND IS EXPECTED TO CEASE DURING THE NEXT FEW HOURS.
SOME OF THIS MOISTURE HAS SPILLED OVER TO WINDWARD EAST MAUI...BUT
THE REST OF THE SMALLER ISLANDS ARE UNDER A DRY AND STABLE
NORTHEASTERLY FLOW THAT HAS PRODUCED ONLY A COUPLE OF HUNDREDTHS OF
RAINFALL OVERNIGHT AT A FEW WINDWARD LOCATIONS.

THE 12Z SOUNDINGS TELL THE STORY OF THE CONTRAST ACROSS THE ISLAND
CHAIN. AT LIHUE...THE INVERSION BASE IS JUST ABOVE 5000 FT...AND
PRECIPITABLE WATER...PW...VALUES ARE MUCH BELOW NORMAL AT 0.69 OF AN
INCH...WHILE ISLAND OBSERVATION SITES ARE REGISTERING LOW DEW POINTS
IN THE MID 50S. THESE ARE ALL SIGNS OF A DRY AND STABLE AIR MASS.
THE 12Z HILO SOUNDING SHOWS A MOIST LAYER ABOVE 10000 FT WITH A VERY
COLD 500 MB TEMP OF -16C.

ON THE BIG ISLAND...THUNDERSTORMS ARE EXPECTED TO CEASE BY MID
MORNING...BUT ANOTHER ROUND OF UNSETTLED WEATHER IS POSSIBLE LATE
TONIGHT AND EARLY SUNDAY MORNING AS ENHANCED LOW LEVEL MOISTURE
LINGERS. A ZONAL...OR NEARLY WEST TO EAST TRAVELLING...JET STREAM
BETWEEN 30N AND 35N FLOWS ACROSS THE WEST PACIFIC THEN SPLITS IN THE
CENTRAL PACIFIC...WITH MOST OF THE ENERGY MOVING OFF TO THE
NORTHEAST AND A PORTION CARVING A WEAK UPPER LEVEL TROUGH OUT JUST
EAST OF THE STATE. THE OVERNIGHT SHORTWAVE ALOFT PASSED OVER THE BIG
ISLAND IN THIS SPLIT FLOW...AND ANOTHER SHORTWAVE...NOW LOCATED
SEVERAL HUNDRED MILES NORTHWEST OF THE STATE WILL DIVE DOWN LATE
TONIGHT...POSSIBLY TRIGGERING THUNDERSTORMS INTO EARLY WEDNESDAY
MORNING...WITH THE GREATEST THREAT OVER WINDWARD AREAS.

DRY CONDITIONS ARE IN STORE FOR THE REST OF THE STATE THROUGH
THROUGH THE WEEKEND. THE ABNORMALLY DRY AIRMASS WILL ONLY MOISTEN
SLIGHTLY...WITH PW VALUES EXPECTED TO REMAIN UNDER AN INCH...STILL
BELOW DECEMBER NORMS. WITH LITTLE AVAILABLE MOISTURE...TONIGHT/S
SHORTWAVE WILL BE UNABLE TO AFFECT THE WESTERN PORTION OF THE STATE.
VERY LIGHT RAINFALL WILL BE CONFINED TO MAUKA PORTIONS OF WINDWARD
AREAS...WHILE LEEWARD AREAS REMAIN DRY. THE ONLY EXCEPTION WILL BE
WINDWARD EAST MAUI...WHERE MODEST RAINFALL MAY OCCUR AS SOME OF THE
MOISTURE POOLED OVER THE BIG ISLAND SPILLS WESTWARD.

LIGHTER WINDS AND DRY AND STABLE CONDITIONS ARE EXPECTED TO START
THE NEW WORK WEEK. A WEAK FRONT MAY BRING SOME LIGHT SHOWERS TO
KAUAI AND OAHU TUESDAY NIGHT AND WEDNESDAY...BUT ANY RAINFALL WOULD
BE LIGHT. THE FRONT WILL FORCE THE SUBTROPICAL RIDGE DOWN NEAR THE
ISLANDS...LEADING TO LIGHT WINDS AND A RATHER DRY LAND AND SEA
BREEZE PATTERN BY WEDNESDAY.

DURING THE SECOND HALF OF THE WEEK...THE ZONAL JET STREAM IS
FORECAST TO SHIFT SOUTHWARD...BRINGING A BETTER CHANCE OF A FRONT
SENDING CLOUDS AND SHOWERS DOWN THE ISLAND CHAIN AROUND CHRISTMAS.
HOWEVER...ITS TOO EARLY TO PIN DOWN ANY DETAILS.

&&

.MARINE...
A FAST-MOVING LOW NORTH OF THE STATE PRODUCED A NORTH SWELL THAT
WILL AFFECT THE STATE THROUGH THE WEEKEND. THE LOW WAS RELATIVELY
CLOSE TO THE ISLANDS...SO THE SWELL WILL BE SHORT PERIOD...AROUND 11
TO 12 SECONDS. SURF WILL REMAIN BELOW THE ADVISORY LEVEL ACROSS
NORTH FACING SHORES...BUT SURF ABOVE THE 8 FOOT ADVISORY LEVEL IS
EXPECTED ALONG EAST FACING SHORES EXPOSED TO THE NORTH SWELL. THE
SWELL SHOULD ALSO PRODUCE MODERATE SURGES IN NORTH FACING
HARBORS...NAMELY KAHULUI AND HILO. DUE TO THE INCREASING NORTH SWELL
A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR HAZARDOUS SEAS.

NORTHEAST WINDS BEHIND THE FRONT HAVE PEAKED AT MODERATE TO LOCALLY
FRESH LEVELS BUT REMAIN BELOW THE SMALL CRAFT ADVISORY...SCA LEVEL.
WINDS WILL BE ON A SLOW DECLINE THROUGH THE WEEKEND AND INTO NEXT
WEEK.

A LARGER NORTHWEST SWELL MAY PRODUCE SURF AROUND WARNING LEVEL FOR
NORTH AND WEST FACING SHORES DURING THE MIDDLE OF NEXT WEEK. THE
SYSTEM THAT WILL SEND THE FRONT TOWARD THE ISLANDS DURING THE END
OF NEXT WEEK MAY ALSO DELIVER A SIGNIFICANTLY LARGER NORTHWEST SWELL
AROUND CHRISTMAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FOR EAST FACING SHORES OF ALL ISLANDS EXCEPT
LANAI AND KAHOOLAWE THROUGH SUNDAY AFTERNOON.

SMALL CRAFT ADVISORY THROUGH TONIGHT FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-
ALENUIHAHA CHANNEL-BIG ISLAND WINDWARD WATERS.

&&

WROE