Heavy Rain in Hawaii, February 29, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1006 AM HST WED FEB 29 2012

.SYNOPSIS...
HIGH PRESSURE FAR NORTHEAST OF THE HAWAIIAN ISLANDS WILL MAINTAIN A
TRADE WIND PATTERN INTO THE UPCOMING WEEKEND. CURRENT FRESH
TRADES WILL EASE OFF TO MODERATE SPEEDS BY THURSDAY NIGHT. SHOWERS
COMING IN WITH THE TRADES WILL BE MOST ACTIVE DURING THE NIGHT AND
MORNINGS...FAVORING THE WINDWARD AND MOUNTAIN AREAS. SHOWERS WILL
BE A LITTLE BIT MORE ENHANCED OVER THE NEXT WEEK DUE TO A COOLER
AIR MASS ALOFT.

&&

.DISCUSSION...
HIGH PRESSURE NORTHEAST OF THE ISLANDS ARE DRIVING FRESH TRADE
WINDS...WHICH ARE EXPECTED TO DROP TO MODERATE THURSDAY NIGHT AS
THE HIGH MOVES AWAY TO THE EAST. A SLIGHT SOUTH OF EAST COMPONENT
IN THE WIND FLOW WILL PERSIST INTO FRIDAY...BEFORE RETURNING TO A
MORE EASTERLY THIS WEEKEND.

EARLY MORNING SOUNDING SHOW THE AIR MASS OVER THE ISLANDS HAS
STARTED TO COOL ALOFT...WHICH IS WEAKENING THE TRADE WIND
INVERSION AND MAKING THINGS A LITTLE MORE ON THE UNSTABLE ACROSS
THE ISLANDS. RADAR IS ALSO SHOWING HIGHER VERTICAL TOPS...10 TO 15
THOUSAND FEET...AND A FEW MODERATE SHOWERS AROUND THE ISLANDS.
SLIGHTLY COLDER TEMPERATURES ALOFT WILL RESULT IN ENHANCEMENT OF
THE TRADE SHOWERS THROUGHOUT THE NEXT WEEK. THIS ENHANCEMENT IS
ONLY MARGINAL THOUGH AND WILL REALLY BE DEPENDENT ON AVAILABLE
MOISTURE. IMMEDIATELY UPSTREAM THERE DOESN/T APPEAR TO BE A WHOLE
LOT OF MOISTURE...SO WINDWARD AREAS WILL BE ISOLATED TO SCATTERED
COVERAGE FROM BIG ISLAND TO OAHU. MORE MOISTURE IS EVIDENT IN THE
LEEWARD WATERS WITHIN THE BIG ISLAND PLUME...WHICH IS PUSHING MORE
SHOWERS OVER KAUAI AND NIIHAU TODAY.

A PERSISTENT SURFACE TROUGH WEST OF THE STATE CONTINUES TO GENERATE
THUNDERSTORMS ABOUT 60 TO 100 MILES WEST OF KAUAI. UPPER LEVEL
WINDS ARE CARRYING THE DENSE CIRRUS FROM THE STORMS ACROSS THE
ISLAND SKIES...EFFECTIVELY CREATING OVERCAST SKIES ON KAUAI AND
OAHU...AND BROKEN OVER THE OTHER ISLANDS. THIS MID TO UPPER LEVEL
CLOUD COVER COULD LINGER INTO THURSDAY.

SO FROM NOW THROUGH SATURDAY...CAN EXPECT A SLOW AND GRADUAL BREAK
UP OF THE HIGH CLOUDS. FRESH TRADES AND COOLING ALOFT WILL BRING WINDWARD
AND MOUNTAIN SHOWERS...ISOLATED PASSING LEEWARD SHOWERS ON THE
SMALLER ISLANDS...WITH MORE OVERALL SHOWERS ON KAUAI. AS WITH A
TRADE WIND REGIME...SHOWERS WILL BE MORE ACTIVE DURING THE NIGHT
AND MORNINGS.

LONG TERM MODEL DISCREPANCY CONTINUES FROM SUNDAY ONWARD WITH THE
TIMING AND IMPACT OF A SHEAR LINE/FRONTAL BOUNDARY. THE ECMWF HAS
THIS FEATURE REACHING THE ISLANDS SUNDAY MORNING...WHILE THE GFS IS
LOOKING AT SUNDAY NIGHT...THROUGH IT IS ABOUT 6 TO 12 HOURS FASTER
THAN IN PREVIOUS MODEL RUNS.

ANOTHER DIFFERENCE IS THE LOCATION OF THE SURFACE LOW WHICH BOTH
MODELS DEPICT FORMING NORTHEAST OF THE STATE...BUT AT DIFFERENT
LOCATIONS...WHICH WILL IMPACT HOW MUCH EXPOSURE TO THE WINDS FROM
A STRONG SUBTROPICAL HIGH NORTH OF THE LOW. BOTH MODELS NOW BRING
THE FRONTAL MOISTURE OVER ALL THE ISLANDS AND DISSIPATE IT OVER
THE BIG ISLAND MONDAY NIGHT...WHICH IS MORE CONSISTENT THAN THEY
HAVE BEEN.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL AT MOST TERMINALS. EXPECT BRIEF MVFR
CEILINGS AND VISIBILITIES ASSOCIATED WITH PASSING TRADE WIND
SHOWERS. THUNDERSTORMS WEST OF KAUAI SENDING LAYERED MID AND HIGH
CLOUDS EASTWARD OVER THE ISLANDS.

&&

.MARINE...
BREEZY WINDS IN EXPOSED WATERS WILL RESULT IN ROUGH OCEAN CONDITIONS.
A SMALL CRAFT ADVISORY /SCA/ IS IN EFFECT UNTIL 6 AM HST THURSDAY.
THE SCA MAY EXTEND THROUGH THE DAY THURSDAY FOR SOME WINDY ZONES...BUT
SHOULD BE GONE BY THURSDAY NIGHT. THE SCA COVERS MOST MARINE ZONES
EITHER FOR WINDS OF AROUND 25 KT OR COMBINED SEAS OF 10 FEET.

ALSO...A HIGH SURF ADVISORY FOR THE EAST FACING SHORES OF KAUAI
OAHU MOLOKAI MAUI AND THE BIG ISLAND IS GOOD TO 6 AM HST THURSDAY.

WAVE WATCH IS INDICATING A VERY LARGE NORTHEAST SWELL SUNDAY NIGHT
AND EARLY NEXT WEEK...THE BY PRODUCT OF THE DEEP SURFACE LOW
NORTHEAST OF THE ISLANDS AS NOTED BY THE GFS. WITH THE CURRENT
MODEL DISCREPANCY BETWEEN THE GFS AND ECMWF IN THE LONG TERM...WILL
CONTINUE TO MONITOR THIS FOR CHANGES IN SIZE AND TIMING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST THURSDAY FOR EAST FACING SHORES
OF KAUAI OAHU MOLOKAI MAUI AND THE BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR ALL HAWAIIAN WATERS
EXCEPT THE LEEWARD WATERS OF KAUAI OAHU AND MAUI COUNTY.

&&

FOSTER