High Surf Advisory 23, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST WED APR 6 2011

.SYNOPSIS...
AN UPPER LOW WILL MOVE JUST SOUTH OF THE MAIN HAWAIIAN ISLANDS BY
EARLY THURSDAY. THE ATMOSPHERE IS ALREADY UNSTABLE AND INCREASING
MOISTURE WILL CIRCULATE UP FROM THE SOUTH. SHOWERS AND THUNDERSTORMS
ARE EXPECTED THROUGH THURSDAY. SOME AREAS MAY EXPERIENCE HEAVY
PRECIPITATION. MOISTURE WILL LINGER OVER THE ISLANDS INTO THE
WEEKEND BUT SHOWERS WILL NOT BE AS HEAVY BY THEN. WINDS WILL BECOME
SOUTHEASTERLY THURSDAY. TRADES WILL RETURN BRIEFLY SUNDAY WHEN THE
UPPER LOW SHIFTS WESTWARD BUT WINDS WILL AGAIN BE SOUTHEASTERLY FOR
THE FIRST PART OF NEXT WEEK WHEN ANOTHER LOW DEVELOPS TO THE
NORTHWEST.

&&

.DISCUSSION...
AN UPPER LOW PRESENTLY 400 MILES NORTHWEST OF KAUAI HAS AN
ASSOCIATED TROUGH EXTENDING TO ABOUT 250 MILES SOUTH OF THE ISLANDS.
BY EARLY THURSDAY THE LOW WILL HAVE CONSOLIDATED SOUTHWEST OF KAUAI.
SATELLITE IMAGERY SHOWS ACTIVE CONVECTION EMBEDDED IN A BROAD AREA
OF CLOUDS SOUTH AND SOUTHEAST OF THE ISLANDS. SATELLITE ESTIMATES
INDICATE  DEEP MOISTURE OVER THIS AREA. COMPUTER MODELS SEEM IN GOOD
AGREEMENT ON HANDLING THIS FEATURE OVER THE NEXT FEW DAYS.

HIGHER LEVEL CLOUDS AREA MOVING OVER THE ISLANDS AND WILL PERSIST
FOR SEVERAL MORE DAYS. THIS MAY ACT TO INHIBIT SURFACE HEATING OVER
THE ISLANDS THAT WOULD OTHERWISE PRODUCE STRONGER CONVECTIVE SHOWERS
OR THUNDERSTORMS.

AREAS OF HEAVY PRECIPITATION ARE OF MOST CONCERN OVER THE NEXT FEW
DAYS. INCREASING MOISTURE IS MODELED TO CIRCULATE AROUND THE LOW
ONTO THE MAIN HAWAIIAN ISLANDS THURSDAY AND CONTINUE INTO FRIDAY.
THE ATMOSPHERE IS QUITE UNSTABLE...TEMPERATURES NEAR 20 THOUSAND
FEET ARE NEAR MINUS 18 DEGREES CENTIGRADE. LATE THIS MORNING THE
WEATHER RADAR SHOWED ONLY ISOLATED MODERATE TO HEAVY SHOWERS OVER
THE ISLANDS BUT A FEW THUNDERSHOWERS WERE REPORTED EARLIER AND HEAVY
SHOWERS OCCURRED OVER THE KOOLAU MOUNTAINS OF OAHU. BEFORE DAWN THIS
MORNING THERE WERE ALSO VERY STRONG CONVECTIVE SHOWERS OVER OCEAN
AREAS WITHIN RANGE OF THE RADAR.

SCATTERED HEAVY SHOWERS AND THUNDERSTORMS COULD FORM MOST ANYWHERE
OVER THE ISLANDS TODAY. DUE TO THE INCREASE IN DEEPER MOISTURE
TONIGHT AND THURSDAY PRECIPITATION MAY BY EARLY THURSDAY ASSUME THE
CHARACTER OF CLOUDY RAINY WEATHER ALTHOUGH WITH A CONTINUING
POTENTIAL FOR HEAVY SHOWERS OR THUNDERSTORMS.

A WINTER WEATHER ADVISORY THE SUMMITS OF THE BIG ISLAND IS IN EFFECT
AND THE POSSIBILITY EXISTS OF UPGRADING TO A WINTER WEATHER WARNING
IF IT APPEARS THERE IS A GREATER CHANCE OF SIGNIFICANT SNOWFALL.
SOUTHEAST WINDS OVER THE SUMMITS OF THE BIG ISLAND WILL INCREASE
THURSDAY...PERHAPS TO THE 30 MPH THRESHOLD THAT WOULD REQUIRE A WIND
ADVISORY.

A BROAD SURFACE TROUGH WILL FORM JUST WEST OF KAUAI TONIGHT AND
SHIFT TOWARD OAHU THURSDAY. WINDS WILL BECOME MODERATE SOUTHEASTERLY
THURSDAY. TRADES APPEAR BRIEFLY FOR SUNDAY AFTER THE UPPER LOW MOVES
WEST AND WEAKENS BUT ANOTHER TROUGH WILL APPROACH FROM THE NORTHWEST
DURING THE FIRST PART OF THE WEEK CAUSING SURFACE WINDS TO AGAIN
BLOW FROM THE SOUTHEAST.

&&

.PREV FORECAST 4 AM...
WITH WINDS PRETTY LIGHT THROUGH MOST OF THE ATMOSPHERE DURING THIS
TIME...IT IS UNLIKELY THAT THE RESULTING THUNDERSTORMS WILL PRODUCE
DAMAGING WINDS AT THE SURFACE. HOWEVER...RAINFALL HEAVY ENOUGH TO
CAUSE LOCALIZED FLOODING CERTAINLY WILL BE POSSIBLE. AT HIGHER
ELEVATIONS...PRECIPITATION LIKELY WILL FALL AS SNOW OR ICE
PELLETS...AND HAIL COULD EVEN REACH SEA LEVEL IN STRONGER STORMS.

THIS CONVECTIVE PRECIPITATION ALSO WILL BE QUITE SPOTTY...WITH GREAT
DIFFERENCES IN ACCUMULATION ACROSS SMALL DISTANCES. HOWEVER...GIVEN
THE COMBINATION OF MOISTURE FROM THE SURFACE TROUGH AND ADVECTED UP
FROM THE DEEP TROPICS BY SOUTHEASTERLY FLOW...IT IS FAIR TO SAY THAT
MID-WEEK LOOKS PRETTY WET FOR MOST AREAS.

WHAT COURSE THE WEATHER TAKES FROM FRIDAY INTO THE WEEKEND IS LESS
CERTAIN...MAINLY BECAUSE THE MOTION OF CUT-OFF LOWS CAN BE
NOTORIOUSLY DIFFICULT TO PREDICT...BUT IF THIS FEATURE DOES LINGER
SOUTHWEST OF THE STATE AS MOST SOLUTIONS EXPECT...THEN STRATUS RAIN
PROBABLY WILL REPLACE THE CONVECTION GRADUALLY ACROSS THE
ARCHIPELAGO.

THE SURFACE TROUGH TO THE WEST MAY TEMPORARILY TIGHTEN THE PRESSURE
GRADIENT ENOUGH LOCALLY TO BOOST WIND SPEEDS OVER THE NORTHEAST
COASTAL WATERS FROM FRIDAY INTO SATURDAY...AND THE CURRENT FORECAST
REFLECTS THIS POSSIBILITY. HOWEVER...A SLIGHT DIFFERENCE IN THE
TRACK OF THE LOW COULD EASILY THROW THIS PART OF THE FORECAST OFF.

EVENTUALLY...THE WINDS WILL ASSUME MORE TYPICAL SPEEDS AND AN
EASTERLY TO NORTHEASTERLY ORIENTATION...THE BETTER TO CARRY AWAY TO
THE WEST MOST OF THE CONVERGED MOISTURE. RECENT RUNS PREDICT THIS
CHANGE OCCURRING NO EARLIER THAN MONDAY OR TUESDAY...OR PERHAPS
LATER IF AN APPROACHING FRONT TURNS THE FLOW SOUTHEASTERLY AGAIN
AROUND NEXT TUESDAY.

&&

.MARINE...
THE LINGERING NORTH SWELL HAS NECESSITATED THE EXTENSION OF THE HIGH
SURF FOR NORTH AND EAST SHORES OF THE WESTERN ISLANDS THROUGH THIS
AFTERNOON. SMALL CRAFT ADVISORIES FOR SEAS AT OR ABOVE 10 FEET FOR
THIS WESTERN AREA HAS ALSO BEEN EXTENDED THROUGH THIS AFTERNOON.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH LATE THURSDAY NIGHT FOR ALL HAWAIIAN
ISLANDS.

HIGH SURF ADVISORY UNTIL 6 PM HST TODAY FOR NORTH-FACING AND
EAST-FACING SHORES OF KAUAI AND OAHU...AND FOR NORTH-FACING SHORES
OF NIIHAU.

WINTER WEATHER ADVISORY UNTIL 6 AM HST FRIDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST TODAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-KAIWI CHANNEL.

&&

HOAG/RYSHKO