Flash Flood Warning 15, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST WED FEB 6 2008

.SYNOPSIS...AN UPPER LEVEL TROUGH OVER THE STATE WILL MOVE EAST OF
THE BIG ISLAND BY THURSDAY...BRINGING DRIER AND MORE STABLE
CONDITIONS FOR THE END OF THE WEEK AND THE WEEKEND. LIGHT TO
MODERATE TRADE WINDS WILL CONTINUE THROUGH THE WEEKEND.

&&

.DISCUSSION...WATER VAPOR IMAGERY SHOWS AN UPPER TROUGH NEAR
OAHU...WHICH IS MOVING TO THE EAST AT ABOUT 15 MPH. A WEAK LOW IS
ACTUALLY TAKING SHAPE IN THE TROUGH JUST NORTH OF THE STATE. THIS
EMBEDDED FEATURE IS HELPING TO ENHANCE SHOWERS AND A FEW
THUNDERSTORMS ACROSS THE NORTHERN OFFSHORE WATERS. THANKFULLY IT DID
NOT DEVELOP 150 MILES FARTHER SOUTH...OR THE STRONG CONVECTION WOULD
BE AFFECTING THE BIG ISLAND AGAIN. AS IT STANDS...THE BIG ISLAND HAS
BEEN FREE OF MOST RAINFALL THROUGH THE DAY. WITH THE FOCUS SHIFTING
NORTH OF THE STATE...WE WILL DROP THE FLASH FLOOD WATCH FOR THE BIG
ISLAND. ROAD CREWS ON THE MAUNA KEA SUMMIT REPORT DEEP SNOW DRIFTS
AND STRONG WINDS/BLOWING SNOW THAT IS CONTINUING TO MAKE TRAVEL
HAZARDOUS. THEREFORE WE WILL EXTEND THE WINTER WEATHER ADVISORY
THROUGH TONIGHT FOR THE MAIN HAZARD OF BLOWING SNOW. WINDS DIMINISH
TOMORROW AS THE TROUGH PASSES EAST OF THE STATE...AND CONDITIONS
WILL SLOWLY IMPROVE.

THE 00Z SOUNDINGS FROM PHLI/PHTO CONTINUE TO SHOW A MODERATELY
UNSTABLE AIRMASS. HOWEVER...MID LEVELS HAVE CONTINUED TO DRY OUT AT
PHLI. THE POES AMSU TOTAL PRECIPITABLE WATER SENSOR SHOWS VALUES
SIMILAR TO THE SOUNDINGS...WITH TPW VALUES DROPPING OFF FROM THE
NORTHWEST BEHIND THE UPPER TROUGH. THE CURRENT SPEED OF THE TROUGH
SUPPORTS THE 18Z GFS SOLUTION TAKING THE TROUGH EAST OF THE BIG
ISLAND EARLY THURSDAY. FORECAST SOUNDINGS SHOW THE AIRMASS REMAINING
MODERATE UNSTABLE...WITH NO REAL INVERSION DEVELOPING UNTIL THIS
WEEKEND.

HIGH PRESSURE FAR NORTHEAST OF THE STATE WILL SLOWLY WEAKEN THROUGH
THE REST OF THE WEEK AS A STRONG LOW PRESSURE SYSTEM PASSES ACROSS
THE NORTH PACIFIC. THE PRESSURE GRADIENT OVER THE STATE WILL FURTHER
WEAKEN TONIGHT AS A COLD FRONT ASSOCIATED WITH THE LOW IMPINGES UPON
THE SUBTROPICAL RIDGE. WIND SPEEDS WILL CONTINUE TO DIMINISH...BUT
TRADES WILL REMAIN AT LIGHT TO MODERATE LEVELS. WINDS MAY DIMINISH
ENOUGH FOR LOCALIZED SEA BREEZES ACROSS SHELTERED LEEWARD AREAS
EARLY NEXT WEEK. HOWEVER...BY THIS POINT FORECAST SOUNDINGS SHOW A
STRENGTHENING SUBSIDENCE INVERSION THAT SHOULD LIMIT CONVECTIVE
DEVELOPMENT. SHOWERS MAY STILL BECOME MORE FREQUENT ACROSS THE
INTERIOR OF THE ISLANDS EARLY NEXT WEEK...BUT HEAVY SHOWERS FROM
DEEP CONVECTION ARE NOT AS LIKELY.

OVER THE WEEKEND AND INTO NEXT WEEK...THE GFS TRIES TO SPIN UP A
SURFACE LOW OUT OF THE TROPICS. THE LAST COUPLE RUNS HAVE HAD A
SIMILAR FEATURE...BUT HAVE BEEN AT DIFFERENT LOCATIONS AND AT
DIFFERENT TIMES. THE 12Z AND 00Z ECMWF RUNS ALSO DEVELOP SIMILAR
FEATURES OVER THE WEEKEND. ONLY ONE 12Z GFS ENSEMBLE MEMBER IS
FARTHER NORTH WITH THE SURFACE WAVE THAN THE OPERATIONAL GFS. THE
CURRENT FORECAST KEEPS LIGHT TO MODERATE TRADES THROUGH THE
PERIOD...WHICH STILL LOOKS ON TRACK.

&&

.MARINE...THE MORNING QUIKSCAT PASS SHOWED THAT WINDS HAD WEAKENED
ACROSS THE CHANNELS...BUT THERE WAS STILL A LARGE AREA OF 25 KNOTS
IN THE ALENUIHAHA CHANNEL AND SOUTH OF THE BIG ISLAND. CLOUD MOTIONS
ARE ALSO NEAR 25 KNOTS ACROSS THE COASTAL WATERS THIS AFTERNOON. THE
18Z GFS SHOWS WINDS CONTINUING TO SUBSIDE THROUGH THE NIGHT. THE
SOUTH POINT ARC ALSO REPORTED WINDS OF 22G32KT THIS AFTERNOON.
THEREFORE WE WILL EXTEND THE EXISTING SMALL CRAFT ADVISORY THROUGH
THE NIGHT.

THE 18Z WAVEWATCH III GUIDANCE HAS TRENDED ABOUT A FOOT HIGHER WITH
THE NEXT NORTHWEST SWELL...WHICH PEAKS THURSDAY NIGHT/FRIDAY. THIS
SWELL IS EXPECTED TO PRODUCE ADVISORY LEVEL SURF ALONG NORTH AND
WEST FACING SHORES LATE THURSDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 6 AM HST THURSDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR KAIWI CHANNEL-
MAALAEA BAY-PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND
LEEWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

&&

BRAVENDER