Heavy Rain and Winter Weather in Hawaii, December 19 to 20, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1055 PM HST SUN DEC 18 2011

.UPDATE...
MARINE FORECASTS HAVE BEEN UPDATED TO INDICATE AN EXPANSION OF THE
SMALL CRAFT ADVISORY...IN BOTH TIME AND AREA...TO INCLUDE NEARLY
ALL MARINE ZONES...AS ROUGH AND CHOPPY SEAS WILL BE IN EXCESS OF THE
10 FOOT CRITERION FOR A GOOD PORTION OF THE UPCOMING WEEK.

&&

.SYNOPSIS...
A LOW ALOFT SOUTH OF THE BIG ISLAND WILL BRING ENHANCED TRADE WIND
SHOWERS TO ALL ISLANDS THROUGH MONDAY...AND THE POSSIBILITY OF
THUNDERSTORMS FOR MAUI COUNTY AND 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 AN AREA OF LOW
PRESSURE PASSING SOUTH OF THE ISLANDS.

&&

.DISCUSSION...
HAVE MADE SOME TWEAKS TO THE OVERNIGHT AND EARLY MORNING MONDAY
FORECAST...AS AN AREA OF ENHANCED MOISTURE IS TRAVELLING BRISKLY
WESTWARD OVER WINDWARD WATERS OF ALL ISLANDS. CLOUD TOPS ARE
CURRENTLY AS HIGH AS 25 KFT IN THIS AREA...AND WITH STRONG TRADE
WINDS EXTENDING AS LEAST AS HIGH AS 10 KFT...THESE SHOWERS WILL HAVE
NO PROBLEM SPREADING LEEWARD ON THE SMALLER ISLANDS. THEREFORE...
ALTHOUGH POPS WERE ALREADY HIGH OVER THE ISLANDS...HAVE PUSHED POPS
TO THE CATEGORICAL LEVEL FOR A LARGE PORTION OF THE ISLANDS FROM
KAUAI TO MAUI.

OVERALL FORECAST PHILOSOPHY REMAINS UNCHANGED FROM PREVIOUS
DISCUSSION...WITH SOME MINOR DIFFERENCES INDICATED IN THE LATEST GFS
RUN WITH RESPECT TO SURFACE LOW DEVELOPMENT S OF THE ISLANDS LATER
IN THE WEEK. ALTHOUGH THE 00Z RUN NO LONGER DEPICTS THE DEVELOPMENT
OF A LOW PER SE...PRESSURES ARE STILL EXPECTED TO DROP IN
ASSOCIATION WITH A TROUGH PASSING S OF THE ISLANDS...AND THE
FORECAST CALLING FOR STRONG TO NEAR GALE FORCE WINDS REMAINS VALID.

&&

.PREV DISCUSSION ISSUED 4 PM HST...
A MODERATELY STRONG SURFACE HIGH PRESSURE SYSTEM IS POSITIONED FAR
NORTHEAST OF THE HAWAIIAN ISLANDS. IT HAS AN ASSOCIATED RIDGE
EXTENDING SOUTHWEST WHICH COMES WITHIN 550 MILES NORTH OF KAUAI AT
THE CLOSEST POINT. THE PRESSURE GRADIENT BETWEEN THIS RIDGE AND
EQUATORIAL LOW PRESSURE IS SUFFICIENTLY STEEP TO SUPPORT 20 TO 25
MPH TRADE WINDS ACROSS LOCAL WATERS. VISIBLE SATELLITE LOOP AND
RADAR SHOWS PASSING CUMULUS CLOUDS AND SHOWERS WITH SOME HEAVIER
SHOWERS UPSTREAM MOVING TOWARDS KAUAI AND OAHU THIS AFTERNOON.

THE WATER VAPOR LOOP SHOWS A CLOSED UPPER LOW AROUND 300 MILES SOUTH
OF THE BIG ISLAND. THIS LOW IS EMBEDDED WITHIN A LARGER UPPER TROUGH
POSITIONED ACROSS THE BIG ISLAND. AFTERNOON SOUNDINGS SHOW A BIT
MORE STABLE CONDITIONS PERSISTING OVER THE ISLANDS WHICH HAS ALLOWED
PASSING TRADE SHOWERS TO SOMEWHAT WEAKEN THROUGH THE DAY.

CLOUDS AND SHOWERS BROUGHT IN BY THE TRADES WILL REMAIN ENHANCED BY
THE UPPER LOW MAINLY OVER MAUI AND HAWAII COUNTY THROUGH TOMORROW
EVENING AND POSSIBLY INTO TUESDAY. SOME OF THESE SHOWERS COULD BE
LOCALLY HEAVY AT TIMES FOR MAUI AND THE BIG ISLAND AND THERE STILL
WILL BE A SLIGHT CHANCE FOR THUNDERSTORMS ACROSS EASTERN WATERS.

TRADE WINDS WILL BE FRESH TO STRONG THROUGH MONDAY NIGHT...
SUPPORTED BY HIGH PRESSURE WELL NORTH OF THE ISLANDS. MODELS
CONTINUE TO SHOW CLOSED SURFACE LOW DEVELOPING SOUTHEAST OF THE
ISLANDS MONDAY NIGHT. THE LOW IS EXPECTED TO TRACK WESTWARD AND CLIP
THE SOUTHEASTERN END OF THE STATE ON WEDNESDAY. THIS WILL NOT ONLY
BRING CONTINUED WET WEATHER TO THE BIG ISLAND AND MAUI...BUT
INCREASE THE LOW LEVEL PRESSURE GRADIENT OVER THE ISLANDS FURTHER
STRENGTHENING OF THE WINDS. NEAR GALE FORCE WINDS COULD BE FELT
ACROSS THE CHANNELS AND EXPOSED AREAS WEDNESDAY...BEFORE BACKING
THINGS DOWN THURSDAY NIGHTS AS THE LOW WEAKENS TO A TROUGH AND MOVES
WEST OF THE AREA. MODELS HINT AT ANOTHER FRONT IN A WEAKENED STATE
MOVING TOWARDS THE STATE FROM THE NORTHWEST NEXT WEEKEND.

&&

.AVIATION...
WE HAVE RECEIVED A FEW PILOT REPORTS OF LIGHT LOW-LEVEL TURBULENCE
WITH ONE MODERATE REPORT DOWNSTREAM OF THE KOHALA RANGE ON THE BIG
ISLAND. RADAR VAD WIND PROFILES AND ACARS SOUNDINGS OUT OF HONOLULU
AND KAHULUI SHOW A SOLID AREA OF 25 KNOT WINDS WITHIN THE MIXED
LAYER. THE TRADE WIND INVERSION IS VERY WEAK AND ELEVATED...WHICH
IS NOT TYPICALLY ASSOCIATED WITH WIDESPREAD LEEWARD TURBULENCE.
HOWEVER...WAVE CLOUDS ARE APPARENT EXTENDING DOWNSTREAM FROM THE
KOOLAU RANGE ON OAHU...WHICH DOES LEND SUPPORT TO THE CURRENT AIRMET
FOR LOW-LEVEL TURBULENCE. THE LOW-LEVEL GRADIENT IS EXPECTED TO
STRENGTHEN FURTHER MONDAY AND TUESDAY...AND THE STRONGER WINDS /AND
RESULTING POTENTIAL FOR LOW-LEVEL TURBULENCE/ SHOULD CONTINUE
THROUGH THURSDAY BEFORE WEAKENING.

SHOWERY CONDITIONS WILL CONTINUE TO AFFECT THE SMALLER ISLANDS.
BRIEF PERIODS OF MVFR VISIBILITIES AND CEILINGS ARE EXPECTED TO
CONTINUE...MAINLY ACROSS WINDWARD SECTIONS. WITH THE STRONG WINDS
THESE SHOWERS WILL ALSO SPREAD ACROSS LEEWARD AREAS AS WELL. AS IT
STANDS...THE SHOWERS HAVE BEEN PROGRESSIVE...ONLY PERSISTING ACROSS
ONE AREA FOR A BRIEF PERIOD. THIS BEARS WATCHING AS IF THEY BEGIN TO
ANCHOR OVER THE TERRAIN...WE WILL NEED AN AIRMET FOR MOUNTAIN
OBSCURATIONS.

&&

.MARINE...
HIGH SURF ADVISORY IS CANCELLED FOR NORTH AND WEST SHORES AS SURF
HEIGHTS HAVE FALLEN BELOW CRITERIA AND ARE EXPECTED TO CONTINUE TO
GRADUALLY DROP TONIGHT. ANOTHER MODERATE NORTHWEST SWELL WILL
REINFORCE THE DROPPING ONE ON MONDAY AND REMAIN INTO THE LATER PART
OF THIS WEEK...BUT WILL STAY BELOW HIGH SURF ADVISORY LEVELS ALONG
NORTH AND WEST SHORES. SURF WILL EVENTUALLY EXCEED THE THRESHOLD
AGAIN ON EAST FACING SHORES TUESDAY OR WEDNESDAY DUE TO THE STRONGER
WINDS TRADE WINDS EXPECTED IN.

WAVE WATCH GUIDANCE INDICATES COMBINED SEAS WILL BE AT OR ABOVE THE
SMALL CRAFT ADVISORY /SCA/ CRITERION OF 10 FEET THROUGH THE
WEEK...WITH TRADE WIND SWELLS BEING THE PRIMARY SOURCE CONTRIBUTING
TO THE TOTAL WAVE HEIGHT. CURRENTLY THE SCA COVERS TYPICAL WINDY
ZONES AROUND MAUI AND HAWAII COUNTY THROUGH MONDAY...BUT THIS WILL
LIKELY NEED TO BE EXTENDED AND EXPANDED BY MONDAY NIGHT OR TUESDAY
DUE TO THE STRENGTHENING TRADE WINDS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

SMALL CRAFT ADVISORY FROM 4 AM MONDAY TO 6 AM HST THURSDAY FOR
KAUAI NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD
WATERS-KAUAI CHANNEL-OAHU WINDWARD WATERS-KAIWI CHANNEL-MAUI
COUNTY WINDWARD WATERS-BIG ISLAND WINDWARD WATERS.

&&

UPDATE/DISCUSSION...BIRCHARD
PREV DISCUSSION/MARINE...FOSTER
AVIATION...BRAVENDER