Heavy Rain in Hawaii, December 12, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
339 PM HST SAT DEC 12 2015

.SYNOPSIS...THANKS TO A FRONT APPROACHING FROM THE NORTHWEST...A
MIXED WEATHER PATTERN WILL AFFECT THE ISLANDS INTO EARLY NEXT
WEEK. LIGHT TO MODERATE SOUTHEASTERLIES ARE EXPECTED ACROSS
WINDWARD AREAS WHILE A MILD CONVECTIVE REGIME IS EXPECTED ACROSS
LEEWARD AREAS. THE FRONT WILL STALL BEFORE REACHING THE ISLANDS
AND THEN LIFT OFF TO THE NORTH EARLY NEXT WEEK. A NEW HIGH WILL
BUILD NORTHEAST OF THE ISLANDS AND BRING BACK LOCALLY BREEZY TRADE
WINDS BY THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...
THERE IS LITTLE OR NO CHANGE IN FORECAST PHILOSOPHY SINCE THIS
MORNING. A LOW 500 TO 600 MILES NORTHWEST OF KAUAI HAS AN
ASSOCIATED FRONT WHICH EXTENDS SOUTHWEST THROUGH 23N162W AND
20N170W...OR ABOUT 200 MILES NORTHWEST OF KAUAI. THE PROXIMITY OF
THIS FRONT HAS CAUSED WINDS TO VEER TO SOUTHEASTERLIES ACROSS
LOCAL WATERS WHILE ALSO KNOCKING DOWN WIND SPEEDS. THIS HAS SET UP
A MIXED WEATHER PATTERN ACROSS THE ISLANDS...WHERE LIGHT TO
MODERATE SOUTHEAST WINDS CONTINUE TO BRING LIMITED MOISTURE TO
MOST WINDWARD AREAS...BUT WHERE BIG ISLAND BLOCKING ALLOWS LOCAL
SEA AND LAND BREEZES TO FORM ACROSS LEEWARD AREAS. THE 1028 MB
HIGH FAR NORTHEAST OF THE ISLANDS IS BLOCKING ANY SIGNIFICANT
EASTWARD MOVEMENT OF THE FRONT.

MODELS SHOW THE LOW NORTHWEST OF KAUAI WILL LIFT NORTHWARD OVER
THE NEXT FEW DAYS...CAUSING THE FRONT TO COMPLETELY STALL AND
WEAKEN BEFORE REACHING THE MAIN ISLAND CHAIN. THE SUPPORTING UPPER
TROUGH IS ALSO EXPECTED TO FILL IN AS IT PASSES ACROSS THE ISLANDS
SUNDAY AND MONDAY...MINIMIZING ITS DESTABILIZING IMPACT. BEGINNING
MONDAY...HIGH PRESSURE WILL BEGIN TO BUILD BACK WESTWARD ACROSS
THE ISLANDS AS THE LOW LIFTS NORTHWARD...ALLOWING A GRADUAL
TRANSITION BACK TO EAST NORTHEAST TRADE FLOW WHICH WILL BE
COMPLETED BY WEDNESDAY. WINDS MAY BE RATHER STRONG AND GUSTY
WITHIN THE TRADITIONALLY WINDIER WATERS WEDNESDAY. TRADE FLOW
WILL THEN REMAIN IN PLACE THROUGH THE REST OF THE FORECAST PERIOD.

EXPECT LOCAL LAND AND SEA BREEZES ACROSS LEEWARD AREAS THROUGH THE
WEEKEND...WITH CLOUDIER CONDITIONS IN THE AFTERNOONS AND EVENINGS.
CLOUDS AND SPOTTY SHOWERS WILL CONTINUE ACROSS WINDWARD AREAS
WHERE LIGHT TO MODERATE SOUTHEAST WIND FLOW CAN REACH. SOME SHOWER
ENHANCEMENT MAY OCCUR THIS WEEKEND AS THE WEAKENING UPPER TROUGH
PASSES BY. THE LEEWARD CONVECTIVE PATTERN WILL WEAKEN AND CEASE AS
TRADE FLOW RETURNS AND STRENGTHENS NEXT WEEK.

&&

.AVIATION...
LIGHT WINDS ACROSS THE ISLANDS WITH A SURFACE TROUGH TO THE
NORTHWEST AND RIDGE NEARBY WILL LEAD TO A LOCALLY-DRIVEN CONVECTIVE
PATTERN...WITH ONSHORE WINDS DURING THE DAY GIVING WAY TO OFFSHORE
WINDS AT NIGHT. CLOUDS AND SHOWERS WILL BE FOCUSED MAINLY ACROSS
INTERIOR SECTIONS OF THE ISLANDS THROUGH THE EARLY EVENING...WITH
OCCASIONAL MVFR CEILINGS AND VISIBILITIES IN SHOWERS. BRIEF IFR
VISIBILITIES ARE ALSO POSSIBLE IN THE HEAVIEST SHOWERS...MAINLY OVER
MAUI COUNTY.

THIS WIND PATTERN WITH THE TROUGH TO THE NORTHWEST WILL ALSO LEAD TO
ENHANCED CONVERGENCE OVER THE EASTERN HALF OF THE STATE...GENERALLY
FROM MOLOKAI TO THE BIG ISLAND. THIS MAY LIMIT HOW MUCH THESE
ISLANDS CLEAR OUT TONIGHT...LEADING TO MVFR CONDITIONS AND MOUNTAIN
OBSCURATIONS THAT MAY PERSIST LONGER THAN WOULD OTHERWISE BE THE
CASE. KAUAI IS OUTSIDE OF THIS AREA AND IS MOST LIKELY TO CLEAR OUT
AS WOULD NORMALLY BE EXPECTED IN THIS TYPE OF LIGHT WIND PATTERN.
SURFACE OBSERVATIONS ACROSS OAHU ALSO SHOW AN INCREASING NUMBER OF
NORTHWESTERLY WIND REPORTS...WHICH INDICATE THAT THE MAIN
CONVERGENCE BAND IS EAST OF THE ISLAND AS WELL. WITH THIS IN
MIND...HAVE UPDATED THE AREA FORECAST TO GROUP OAHU WITH KAUAI
RATHER THAN MAUI COUNTY.

&&

.MARINE...
SLOPPY SHORT PERIOD SURF CONTINUES NEAR ADVISORY LEVEL ALONG EAST
FACING SHORES OF THE BIG ISLAND. WHILE DROPPING THE HIGH SURF
ADVISORY ELSEWHERE...WE WILL KEEP THE ADVISORY UP THROUGH THE
NIGHT FOR EAST FACING SHORES OF THE BIG ISLAND.

A NEW NORTHWEST SWELL IS EXPECTED TO ARRIVE LATE MONDAY AND CONTINUE
INTO WEDNESDAY. WAVE WATCH GUIDANCE SHOWS THIS SWELL WILL BRING SURF
NEAR ADVISORY LEVELS ALONG NORTH AND WEST FACING SHORES AT ITS PEAK.
THE LOW GENERATING THIS SWELL WILL BE QUITE DEEP...BUT THE LARGEST
SWELLS WILL PASS NORTH OF HAWAII.

A COMBINATION OF WINDS AND SEAS ABOVE SMALL CRAFT ADVISORY
THRESHOLD PERSISTS ACROSS WATERS EAST AND SOUTHEAST OF THE BIG
ISLAND THIS AFTERNOON. THEREFORE WE WILL KEEP THE SMALL CRAFT
ADVISORY UP THROUGH TONIGHT FOR THESE AREAS...BUT DROP IT
ELSEWHERE.

&&
.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST SUNDAY FOR SOUTH BIG ISLAND-
BIG ISLAND NORTH AND EAST.

SMALL CRAFT ADVISORY UNTIL 6 AM HST SUNDAY FOR BIG ISLAND
WINDWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

&&

DISCUSSION/MARINE...POWELL
AVIATION...BRAVENDER