Heavy Rain in Hawaii, March 7 to 10, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST FRI MAR 7 2014

.SYNOPSIS...
SOUTHWEST WINDS WILL STRENGTHEN AS SHOWERS INCREASE AHEAD OF A FRONT
APPROACHING FROM THE NORTHWEST. THE FRONT ITSELF WILL BRING HEAVIER
RAIN AND NORTHERLY WINDS FROM THIS AFTERNOON INTO SATURDAY. TRADE
WINDS WILL RETURN AFTER THE FRONT PASSES...FOCUSING SHOWERS ONTO
WINDWARD SLOPES INTO TUESDAY. MORE TYPICAL TRADE-WIND WEATHER WILL
PREVAIL AROUND THE MIDDLE OF THE WEEK...BUT ANOTHER FRONT MAY
APPROACH KAUAI BY NEXT FRIDAY.

&&

.DISCUSSION...
PREVAILING WINDS AROUND THE MAIN HAWAIIAN ISLANDS CONTINUED THEIR
TRANSITION TO SOUTHWEST FLOW...BUT SPEEDS SHOWED LITTLE INCREASE
YET. LAND BREEZES ALSO WERE PRESENT...IF UNEXCEPTIONAL. THE AIR MASS
CONTINUED TO MOISTEN FROM THE SOUTHEAST AT THE LOWEST LEVELS...WHILE
CLOUD LAYERS AT SEVERAL HIGHER LEVELS APPROACHED FROM THE WEST AND
SOUTHWEST. THE INVERSION BECAME MORE COMPLEX.

SHOWERS REMAINED MORE ACTIVE THAN THEY WERE 24 HOURS AGO...SOME
ORGANIZING INTO BANDS OR STREETS ROUGHLY PARALLEL TO THE PREVAILING
SOUTHWEST FLOW. HOWEVER...OVERALL COVERAGE WAS QUITE UNEVEN. OAHU
RECEIVED SIX-HOUR ACCUMULATION APPROACHING AN INCH IN PLACES...WHILE
KAUAI REMAINED VERY NEARLY DRY...AND MOST OF THE OTHER ISLANDS
VARIED BETWEEN THESE EXTREMES. NONETHELESS...THE FRONTAL CLOUD BAND
APPROACHING FROM THE NORTHWEST STILL PROMISED SIGNIFICANT...AND
PERHAPS LOCALLY HEAVY...RAIN FOR MOST OF THE STATE LATER TODAY.

ALOFT...EXTREMELY BROAD TROUGHING PREVAILED OVER MOST OF THE CENTRAL
NORTH PACIFIC OCEAN JUST NORTH OF THE ISLANDS...THOUGH THE DEEPER
LOW REMAINED MUCH FURTHER TO THE NORTH. GUIDANCE AGREED REASONABLY
WELL THAT THE TROUGH WILL SHARPEN MODESTLY THROUGH THE DAY AND INTO
TONIGHT AS A MASSIVE RIDGE BUILDS OVER THE WEST COAST OF NORTH
AMERICA. THE SHARPER TROUGH WILL PROVIDE INCREASED SUPPORT FOR THE
FRONT APPROACHING THE ISLANDS. THIS SUPPORT WILL FOCUS A BIT TOO FAR
TO THE NORTHEAST TO BE OPTIMAL FOR THE STATE...BUT STILL SHOULD BE
MORE THAN ENOUGH TO ENHANCE RAINFALL LOCALLY. SIMILARLY...STABILITY
PROBABLY WILL NOT FALL LOW ENOUGH TO SUPPORT THUNDERSTORM
DEVELOPMENT HERE.

HEAVIER RAIN WILL ACCOMPANY THE FRONT ITSELF AS IT MOVES ACROSS THE
STATE FROM THIS AFTERNOON INTO SATURDAY...BUT THE RAGGED LEADING
EDGE OF THIS FEATURE PROBABLY WILL MAKE THE TRANSITION RATHER
GRADUAL. FROM AN EULERIAN PERSPECTIVE...IT MAY JUST SEEM THAT
SHOWERS BECOME MORE FREQUENT AND HEAVIER WITH TIME. ON THE OTHER
HAND...THE BACK EDGE OF THE BAND WAS BETTER DEFINED...LIKELY
REPRESENTING A MORE NOTICEABLE CHANGE TO A COOLER AND DRIER AIR MASS
WHEN IT ARRIVES. NORTHERLY WINDS FOLLOWING THE FRONT WILL BECOME
NORTHEASTERLY FAIRLY SOON...CARRYING ANY LINGERING MOISTURE ONTO
WINDWARD SLOPES THROUGH THE REST OF THE WEEKEND...ESPECIALLY OVER
THE BIG ISLAND...CLOSE TO THE DISSIPATING BOUNDARY.

BRIEF MID-LEVEL RIDGING ALOFT WILL PASS OVERHEAD FROM SUNDAY INTO
MONDAY...AND THEN A TROUGH ALOFT WILL DIG DOWN EAST OF THE STATE
FROM MONDAY INTO TUESDAY...PROVIDING SUPPORT FOR ANOTHER APPROACHING
FRONT. GUIDANCE HAD BEEN PORTRAYING THIS FEATURE AS WEAK FOR SOME
TIME NOW...BUT RECENTLY IT SUGGESTED THAT THE FRONT MAY NOT EVEN
REACH THE ISLANDS...ONLY GATHERING MOISTURE AND LIMITED SUPPORT FOR
ENHANCED PRE-FRONTAL SHOWERS. CERTAINLY THIS SCENARIO WAS MORE
COMPATIBLE WITH TRADE WINDS CONTINUING THROUGH THURSDAY...AS THE
MODELS HAD BEEN SHOWING. IN ANY CASE...SHOWER ACTIVITY WILL TREND
TOWARD CLIMATOLOGY THROUGH THE MIDDLE OF THE WEEK AS THE LINGERING
MOISTURE IS RAINED OUT OR BLOWN AWAY TO THE SOUTHWEST. RECENT RUNS
HAD STARTED AGREEING THAT ANOTHER FRONT MAY APPROACH KAUAI FROM THE
NORTHWEST BY NEXT FRIDAY.

&&

.MARINE...
A LARGE NORTHWEST SWELL WILL BUILD ACROSS THE ISLANDS THROUGH THE
DAY...AND THEN PEAK ABOVE WARNING LEVELS TONIGHT OR SATURDAY.
ADVISORY-LEVEL CONDITIONS MAY ARRIVE EARLIER IN THE DAY TODAY. BASED
ON THE DIFFERENT TIMING ACROSS THE STATE...THE HIGH SURF WARNING IS
SET TO BEGIN LATE THIS MORNING. ADVISORY-LEVEL SURF MAY REACH
WEST-FACING SHORES OF THE BIG ISLAND BY SATURDAY.

A SMALL CRAFT ADVISORY HAS BEEN ISSUED FOR LARGE COMBINED SEAS OVER
EXPOSED COASTAL WATERS. THESE CONDITIONS LIKELY WILL CONTINUE INTO
SUNDAY...BEFORE SUBSIDING BELOW 10 FEET.

&&

.AVIATION...
SHOWERS AHEAD OF A FRONT...MOVING UP FROM THE SW...CONTINUE TO
BRING PERIODS OF MVFR CONDITIONS...MAINLY TO MAUI COUNTY TERMINALS.
THE APPROACHING FRONT IS EXPECTED TO CARRY HEAVY SHOWERS WITH
EMBEDDED MVFR CONDITIONS TO THE SMALLER ISLANDS THROUGHOUT THE
DAY...BEGINNING ON KAUAI BY 2100Z.

AIRMET SIERRA IS UP FOR MTN OBSC OVER MAUI COUNTY DUE PASSING
SHOWERS AHEAD OF THE FRONT. AS THE FRONT APPROACHES...THIS MAY NEED
TO BE EXTENDED TO ACCOUNT FOR LOWER VIS AND CEILINGS FROM THE
EMBEDDED HEAVY SHOWERS.

AIRMET TANGO IS IN PLACE FOR TEMPO MOD TURB N AND NE OF KAUAI
THROUGH MAUI COUNTY. AS THE FRONT APPROACHES THE ISLANDS AND THE
INVERSION ERODES...CONDITIONS ARE EXPECTED TO END BY 2200Z.

AIRMET ZULU FOR MOD RIME ICING WILL GO INTO EFFECT AT 1600Z OVER
KAUAI...FOR LAYERS 140-FL200. THIS AIRMET IS EXPECTED TO EXTEND
EASTWARD AS THE FRONT MOVES OVER OAHU...WITH ICING CONDITIONS
LIKELY TO BEGIN BY 2200Z FOR LAYERS 140-FL200.

FOR BIG ISLAND...VFR CONDITIONS WILL CONTINUE THROUGH THE PERIOD.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING FROM 10 AM THIS MORNING TO 6 AM HST SUNDAY FOR
NIIHAU-KAUAI WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH
SHORE-OAHU KOOLAU-OLOMANA-MOLOKAI-MAUI WINDWARD WEST-WINDWARD
HALEAKALA.

SMALL CRAFT ADVISORY FROM 10 AM THIS MORNING TO 6 AM HST SUNDAY
FOR KAUAI NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD
WATERS.

&&

AVIATION...EATON
REST...RYSHKO