Heavy Rain in Hawaii, March 24, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SAT MAR 24 2012

.SYNOPSIS...
STRONG HIGH PRESSURE FAR NORTH OF THE STATE WILL WEAKEN SLIGHTLY
TONIGHT AND SUNDAY...ALLOWING TRADE WINDS TO DROP OFF TO MORE
MODERATE LEVELS. AN UPPER LEVEL TROUGH INTERACTING WITH MOISTURE
FROM AN OLD FRONT WILL BRING UNSETTLED WEATHER TO THE ISLANDS...WITH
LOCALLY HEAVY SHOWERS POSSIBLE AND A SLIGHT CHANCE OF THUNDERSTORMS.
THE ATMOSPHERE WILL BEGIN TO STABILIZE ON MONDAY...AND DRIER TRADE
WIND WEATHER IS EXPECTED TO RETURN TUESDAY AND WEDNESDAY.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL TROUGH OVER THE STATE...
WITH ANOTHER SHORTWAVE NORTH OF THE ISLANDS DIGGING SOUTHWARD. AT
THE SURFACE...HIGH PRESSURE IS LOCATED FAR NORTH NORTHEAST OF THE
STATE. 00Z SOUNDINGS FROM HILO AND LIHUE SHOW AN UNSTABLE AIRMASS
ACROSS THE STATE...WITH 500MB TEMPERATURES -11C/-13C RESPECTIVELY. A
VERY WEAK SUBSIDENCE INVERSION IS NOTED ON THE LIHUE SOUNDING NEAR
8000 FT...BUT THERE IS NONE IN THE HILO SOUNDING. PRECIPITABLE
WATERS RANGE FROM 1 INCH AT LIHUE TO OVER 1.4 INCHES AT HILO.

IN THE NEAR TERM...A NUMBER OF CLOUD BANDS ARE APPARENT NEAR THE
ISLANDS. KAUAI IS LOCATED BETWEEN SHOWER BANDS TO THE NORTH AND
SOUTH. A LARGER BAND OF CLOUDS IS LOCATED NEAR MAUI COUNTY AND THE
BIG ISLAND...AND EXTENDS WELL UPSTREAM TO THE EAST SOUTHEAST FROM
THE BIG ISLAND. THIS BAND IS ALREADY SPREADING TO OAHU...AND
ACCOUNTS FOR THE INCREASE IN SHOWERS WE HAVE SEEN THIS AFTERNOON.
WHILE THE 00Z KAUAI SOUNDING IS INITIALLY DRY...MOISTURE WILL
INCREASE TONIGHT AS THE CLOUD BANDS FILL IN AND SPREAD WEST.
POPS WILL BE CATEGORICAL ACROSS WINDWARD/MAUKA SECTIONS TONIGHT.
WITH THE DEEP MOISTURE AND UNSTABLE AIRMASS...HEAVY RAIN IS
POSSIBLE. WHILE THE INSTABILITY IS NARROW AND SPREAD THROUGH THE
COLUMN...THERE IS STILL A CONCERN FOR THUNDER AND WILL KEEP A
MENTION OF SLIGHT CHANCE FOR TONIGHT.

ON THE BIG ISLAND...RANGERS ON MAUNA KEA REPORT SNOW BEGINNING TO
ACCUMULATE ON SUMMIT ROADS. WEB CAMS SHOW OCCASIONAL BURSTS OF
SNOW...BUT ALSO PERIODS OF LIQUID PRECIPITATION. TEMPERATURES AT ALL
OBSERVATION SITES ON MAUNA KEA ARE BELOW FREEZING...GENERALLY
29-31F. DEEPER SHOWERS WILL TAP INTO COLDER AIR AND ALLOW FOR
SUFFICIENT ICE NUCLEATION FOR SNOW...WHILE SHALLOWER SHOWERS WILL
REMAIN LIQUID...LEADING TO FREEZING RAIN/FREEZING DRIZZLE. THE
WINTER WEATHER ADVISORY ISSUED EARLIER THIS AFTERNOON STILL LOOKS
GOOD...AND COVERS BOTH SNOW AND FREEZING PRECIPITATION. THE ADVISORY
RUNS THROUGH SUNDAY MORNING.

THE DEEPER MOISTURE ASSOCIATED WITH THIS FRONTAL REMNANT WILL
DIMINISH ON SUNDAY. HOWEVER...AFTER THE BRIEF RESPITE ANOTHER
REMNANT BAND WILL MOVE ACROSS THE ISLANDS SUNDAY NIGHT/MONDAY.
MOISTURE ASSOCIATED WITH THIS NEXT BAND IS SHALLOWER. POES AMSU/SSMI
TOTAL PRECIPITABLE WATER IMAGERY SHOWS PW VALUES ARE MUCH LOWER THAN
THE CURRENT FRONTAL BAND. WHILE MOISTURE WILL NOT BE AS DEEP...THE
AIRMASS WILL REMAIN UNSTABLE. THE SECOND SHORTWAVE TROUGH NOTED
ABOVE WILL DIVE SOUTHWARD...REINFORCING THE MID-LEVEL COLD POOL OVER
THE STATE. WHEREAS 500MB TEMPERATURES ARE -11C TO -13C NOW...THE 18Z
GFS SHOWS THEM DROPPING TO NEAR -15C SUNDAY NIGHT. THE CURRENT
FORECAST DOES NOT HAVE THUNDER MENTIONED BEYOND TONIGHT...BUT THIS
MAY NEED TO BE REEVALUATED DEPENDING ON HOW THE SITUATION EVOLVES
TONIGHT.

THE UPPER TROUGH BEGINS TO MOVE EAST ON MONDAY...WITH MID-LEVEL
HEIGHTS REBUILDING AND TEMPERATURES WARMING. FORECAST SOUNDINGS SHOW
THE AIRMASS STABILIZING...WITH A TYPICAL SUBSIDENCE INVERSION
REDEVELOPING. MODERATE TRADE WINDS WILL CONTINUE THROUGH THE WEEK...
KEEPING SHOWERS FOCUSED ACROSS WINDWARD AND MAUKA AREAS. WITH DRIER
LOW-LEVEL AIR...RAINFALL CHANCES WILL RETURN TO NEAR NORMAL
CLIMATOLOGICAL VALUES.

&&

.AVIATION...
VFR CONDITIONS PREVAIL AT ALL TAF SITES WITH THE EXCEPTION OF PHTO
WHERE OCCASIONAL MVFR CONDITIONS ARE OCCURING. MVFR CONDITIONS ARE
LIKELY OCCURING ALONG WINDWARD SLOPES AND OVER MOUNTAINOUS TERRAIN
WITH SOME LOCAL IFR CONDITIONS POSSIBLE. THESE CONDITIONS WILL
CONTINUE THROUGH TONIGHT.

AIRMET TANGO REMAINS UP FOR LOW LEVEL TURBULENCE S AND W OF
MOUNTAINS OF ALL ISLANDS. THESE CONDITIONS WILL CONTINUE THROUGH
TONIGHT AS WELL.

&&

.MARINE...
A SMALL CRAFT ADVISORY IS IN EFFECT THROUGH TONIGHT FOR ALL HAWAIIAN
COASTAL WATERS FOR A COMBINATION OF THE BREEZY TRADE WINDS AND
COMBINED SEAS FROM THE WIND WAVES...NORTHERLY AND SOUTHERLY SWELLS.
BOTH THE TRADE WINDS AND SWELLS ARE EXPECTED TO SUBSIDE BY SUNDAY
TO DROP MOST OF THE ZONES OUT OF THE ADVISORY...LIKELY LEAVING ONLY
THE TYPICAL WINDY ZONES AROUND THE BIG ISLAND AND MAUI COUNTY
THROUGH THE DAY SUNDAY.

TRADE SWELL WILL BACK DOWN OVER THE NEXT FEW DAYS. WAVEWATCH III
GUIDANCE SHOWS THE SWELL REMAINING ABOVE 10 FEET INTO SUNDAY
AFTERNOON...AND THIS LOOKS REASONABLE WITH CURRENT COMBINED SEAS AT
MOKAPU AROUND 12 FEET. THEREFORE THE HIGH SURF ADVISORY HAS BEEN
EXTENDED THROUGH 4PM SUNDAY. ELSEWHERE MAINLY SMALL SURF IS
EXPECTED. NORTH SHORE WILL PULL IN SOME HEAD HIGH PLUS NORTHWEST
SWELL DURING THE SECOND HALF OF THE WEEK. THE CHEST-SHOULDER HIGH
BUMP ALONG THE SOUTH FACERS WILL DROP OVER THE NEXT FEW DAYS. A
STRONGER SHOT OF SOUTHERN HEMI ENERGY WILL FILL IN DURING THE SECOND
HALF OF THE WEEK WITH HEAD HIGH AND OCCASIONAL PLUS SURF EXPECTED.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 4 PM HST SUNDAY FOR THE EAST FACING SHORES
OF ALL ISLANDS EXCEPT LANAI AND NIIHAU.

WINTER WEATHER ADVISORY UNTIL NOON HST SUNDAY FOR BIG ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST SUNDAY FOR ALL HAWAIIAN WATERS.

&&

PUBLIC...BRAVENDER
AVIATION...BURKE
MARINE...FOSTER/DEJESUS