High Surf Advisory 47, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
949 AM HST MON DEC 19 2011

.SYNOPSIS...
A LOW ALOFT SOUTH OF THE BIG ISLAND WILL BRING ENHANCED TRADE WIND
SHOWERS TO ALL ISLANDS THROUGH TODAY...AND THE THREAT OF HEAVY
SHOWERS AND THUNDERSTORMS TO THE BIG ISLAND OVER THE NEXT COUPLE OF
DAYS. TRADE WINDS WILL BE RATHER BREEZY THIS WEEK...DUE TO HIGH
PRESSURE FAR NORTHEAST OF THE ISLANDS...AND A WEAK AREA OF LOW
PRESSURE PASSING SOUTH OF THE BIG ISLAND.

&&

.DISCUSSION...
THE MAIN HAWAIIAN ISLANDS LIE WITHIN A RELATIVELY TIGHT PRESSURE
GRADIENT BETWEEN A SUBTROPICAL RIDGE ABOUT 600 MILES TO THE NORTH
OF HONOLULU...AND A TROUGH OF LOW PRESSURE ABOUT 200 MI SE OF THE
BIG ISLAND. THIS IS KEEPING TRADES BLOWING AT FRESH TO LOCALLY
STRONG LEVELS OVER AND AROUND THE STATE...AND COMBINED SEAS AROUND
THE 10 FOOT SMALL CRAFT ADVISORY THRESHOLD. UNSTABLE AIR AROUND AN
UPPER LEVEL LOW ABOUT 200 MILES SOUTH-SOUTHWEST OF THE BIG ISLAND
IS KEEPING THE ATMOSPHERE RELATIVELY MOIST AND UNSTABLE...
ESPECIALLY NEAR THE BIG ISLAND.

SATELLITE IMAGERY CONTINUES TO SHOW QUITE A BIT OF SHOWERY LOW
CLOUDS RIDING IN ON THE TRADES HEADED FOR THE BIG ISLAND...WITH
FEWER EMBEDDED SHOWER BANDS HEADING FOR THE SMALLER ISLANDS. THERE
IS STILL A RISK OF HEAVY DOWNPOURS MOVING ASHORE OVER THE HILO...
PUNA...AND POSSIBLY KAU DISTRICTS TODAY AND TONIGHT...AND A
LIKLIHOOD OF LOCALLY HEAVY AFTERNOON CONVECTION OVER THE BIG
ISLAND SLOPES AND SUMMITS THIS AFTERNOON. THE CURRENT FORECAST
LOOKS GOOD AND NO UPDATES APPEAR NECESSARY FOR NOW.

THE FORECAST CALLS FOR SURFACE HIGH PRESSURE TO PERSIST FAR NE OF
THE ISLANDS THROUGH THE WEEK...DRIVING A TRADE WIND FLOW OVER THE
ISLANDS. HOWEVER...THE PERSISTENT LOW ALOFT S OF THE BIG ISLAND WILL
CHANGE AND MOVE LITTLE OVER THE NEXT COUPLE OF DAYS...WITH GLOBAL
MODELS DROPPING SURFACE PRESSURES NEAR AND SE OF THE ISLANDS IN
RESPONSE. WHILE GFS RUNS HAVE BEEN FLIP-FLOPPING WITH RESPECT TO THE
DEVELOPMENT OF AN ACTUAL SURFACE LOW...THERE IS MODEL CONSENSUS THAT
LOWERING PRESSURES TO THE S WILL RESULT IN A TIGHTENED LOW LEVEL
PRESSURE GRADIENT NEAR THE ISLANDS...WITH LOW LEVEL TRADE WINDS
BECOMING RATHER STRONG...PEAKING AROUND MIDWEEK. WINDS MAY GET
STRONG ENOUGH TO WARRANT A WIND ADVISORY...AND GALE WARNINGS FOR
THE WINDIER CHANNELS...ESPECIALLY OVER THE S END OF THE ISLAND
CHAIN...BUT CONFIDENCE AND TIMING PRECLUDES THE ISSUANCE OF A HIGH
WIND/GALE WATCH AT THIS TIME.

REGARDLESS...INCREASED TRADE WIND FLOW IS THE EXPECTATION FOR THE
FIRST HALF OF THE WEEK. AS THE LOW ALOFT WEAKENS DURING THE SECOND
HALF OF THE WEEK...AND THE LOW LEVEL TROUGH MOVES W AWAY FROM THE
ISLANDS...TRADE WIND SPEEDS WILL EASE. EARLY INDICATIONS FOR THE
CHRISTMAS WEEKEND ARE THAT A NEW LOW ALOFT DEVELOPING SW OF THE
ISLANDS WILL LOWER SURFACE PRESSURES ONCE AGAIN WHILE A NEW HIGH
PASSES N OF THE ISLANDS...LEADING TO AN INCREASED TRADE WIND
FLOW...AND THE POTENTIAL FOR WETTER WEATHER OVER THE SE PART OF
THE ISLAND CHAIN ONCE AGAIN.

&&

.MARINE...
A MODERATE NW SWELL BUILDING LATER TODAY AND TUESDAY WILL PERSIST
FOR MOST OF THE WEEK...WITH RESULTANT SURF HEIGHTS BELOW ADVISORY
LEVELS ALONG N AND W FACING SHORES. A LARGER NW SWELL NEXT WEEKEND
MAY PUSH SURF TO NEAR ADVISORY LEVELS ALONG SELECT N AND W FACING
SHORES ON SUNDAY. IN THE MEANTIME...SURF ALONG E FACING SHORES WILL
BE TRENDING UPWARDS TODAY AND TUESDAY...PEAKING AROUND THE MIDDLE OF
THE WEEK WITH HEIGHTS WELL ABOVE ADVISORY LEVELS.

WAVE WATCH GUIDANCE INDICATES COMBINED SEAS WILL BE AT OR ABOVE THE
SMALL CRAFT ADVISORY /SCA/ CRITERION OF 10 FEET THROUGH THE WEEK...
WITH SHORT PERIOD TRADE WIND SWELLS BEING THE PRIMARY SOURCE
CONTRIBUTING TO THE TOTAL WAVE HEIGHT. THE SCA HAS THEREFORE BEEN
EXPANDED TO INCLUDE NEARLY ALL MARINE ZONES THROUGH THURSDAY.

&&

.AVIATION...
RADAR VAD WIND PROFILES AND ACARS SOUNDINGS OUT OF HONOLULU AND
KAHULUI SHOW 25-30 KNOTS OF WIND WITHIN THE MIXED LAYER. THE TRADE
WIND INVERSION IS VERY WEAK AND ELEVATED...WHICH IS NOT TYPICALLY
ASSOCIATED WITH WIDESPREAD LEEWARD TURBULENCE. HOWEVER...WE HAVE
RECEIVED TWO PILOT REPORTS OF MODERATE LOW-LEVEL TURBULENCE THIS
MORNING FROM NEAR HNL AND LNY...WHICH SUPPORTS THE CURRENT AIRMET
FOR LOW-LEVEL TURBULENCE. THE LOW-LEVEL GRADIENT IS EXPECTED TO
STRENGTHEN FURTHER ON TUESDAY...AND THE STRONGER WINDS /AND
RESULTING POTENTIAL FOR LOW-LEVEL TURBULENCE/ SHOULD CONTINUE
THROUGH THURSDAY BEFORE WEAKENING.

SHOWERY CONDITIONS WILL BE MOST PREVALENT ACROSS WINDWARD AND
SOUTHEAST SECTIONS OF THE BIG ISLAND. OCCASIONAL MVFR CONDITIONS ARE
EXPECTED WITH THE HEAVIER SHOWERS DUE TO CEILINGS AND VISIBILITIES.
AS THE MORE WIDESPREAD ACTIVITY MAY REMAIN BANKED ALONG THE
TERRAIN...AN AIRMET IS IN EFFECT FOR MOUNTAIN OBSCURATION FOR
WINDWARD AND SOUTHEAST BIG ISLAND. SHOWERS ACROSS THE SMALLER
ISLANDS WILL ALSO PRODUCE BRIEF MVFR CONDITIONS...BUT REMAIN MORE
TRANSIENT/PROGRESSIVE. THESE SHOWERS WILL BE FOCUSED MAINLY ACROSS
WINDWARD SECTIONS...BUT THE STRONG WINDS WILL ALSO SPREAD THEM
ACROSS LEEWARD AREAS AS WELL.

&&

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

FLASH FLOOD WATCH UNTIL 6 AM HST TUESDAY FOR BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR ALL CHANNELS AND
COASTAL WATERS EXCEPT FOR OAHU AND MAUI LEEWARD WATERS.

&&

R BALLARD/BRAVENDER