High Surf in Hawaii, September 26 to 30, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST SAT SEP 26 2015

.SYNOPSIS...
MODERATE TO LOCALLY BREEZY TRADE WINDS WILL KEEP MOST SHOWERS FOCUSED
OVER WINDWARD SLOPES INTO NEXT WEEK...THOUGH AFTERNOON SHOWERS ALSO
WILL OCCUR OVER SOME LEE SLOPES. A MOIST AND UNSTABLE AIR MASS FROM
THE DEEP TROPICS LIKELY WILL BRING HEAVY SHOWERS AND THUNDERSTORMS TO
THE BIG ISLAND AT LEAST THROUGH SUNDAY NIGHT.

&&

.DISCUSSION...
SURFACE OBSERVATIONS FROM AROUND THE MAIN HAWAIIAN ISLANDS CONTINUED TO
REPORT MODERATE TO LOCALLY STRONG TRADE WINDS. A SCATTEROMETER PASS
REPORTED THE HIGHEST SPEEDS IN THE USUAL WINDY PLACES AROUND MAUI AND
HAWAII COUNTIES...BUT ALSO NORTH OF KAUAI. THE MOIST...AND FURTHER
MOISTENING...AIR MASS OVER THE STATE SUPPORTED WELL-DEVELOPED MARINE
SHOWERS MOVING ASHORE ALONG WINDWARD SLOPES. LEE CONVECTION REMAINED
RATHER SUBDUED CONSIDERING THE AVAILABLE FUEL...AS AGREED WELL WITH
CLIMATOLOGY...BUT SHOWERS PASSING COMPLETELY OVER THE ISLANDS FROM
NORTHEAST TO SOUTHWEST STILL MANAGED TO REACH LEE AREAS. ALOFT...
CONDITIONS WERE NEARLY FLAT...WITH ONLY A HINT OF TROUGHING EXTENDING
TOWARD THE STATE FROM THE NORTHEAST.

GUIDANCE AGREED FAIRLY WELL IN PREDICTING ONLY GRADUAL CHANGE TO THE
OVERALL SYNOPTIC PATTERN THROUGH THE FORECAST PERIOD. WITH THE TRADE WINDS
FIRMLY IN PLACE...AND CHANGING ONLY SLIGHTLY IN INTENSITY...MOST
PRECIPITATION WILL OCCUR OVER WINDWARD SLOPES...THOUGH AFTERNOON CUMULUS
BUILDUPS WILL REMAIN POSSIBLE AS WELL. MODEL CONSENSUS ALSO FAVORED A
DRIER AIR MASS GRADUALLY WORKING ITS WAY SOUTH OVER THE STATE...MAINLY
FROM MONDAY NIGHT. A DRIER AIR MASS EVENTUALLY WILL LIMIT PRECIPITATION OVER
THE STATE AS A WHOLE...WITH THE GREATEST DECREASE OVER LEE SLOPES.

HOWEVER...THE TRACK AND INTENSITY OF TROPICAL CYCLONE NIALA WILL HAVE THE
POTENTIAL TO INFLUENCE THE FORECAST DECISIVELY. THE LATEST FORECAST FROM
CPHC PREDICTED NIALA TO CURVE WEST RAPIDLY AND PASS SOUTH OF THE BIG ISLAND
OVER THE WEEKEND. IN THIS SCENARIO...THE MAIN CHANGE TO THE WIND FIELD FROM
BASIC TRADE FLOW WILL CONSIST OF A MODEST INCREASE IN SPEED OVER MUCH OF
THE STATE AND COASTAL WATERS. THAT SAID...THERE WILL BE SOME RISK THAT WINDS
OVER AND SOUTH OF THE BIG ISLAND MAY REACH TROPICAL STORM FORCE IN PLACES...
ESPECIALLY IF NIALA DOES NOT END UP FOLLOWING THE FORECAST TRACK.

STILL...THE LARGER INFLUENCE ON ISLAND WEATHER WILL COME FROM THE SURGE OF
MOISTURE ACCOMPANYING NIALA FROM THE DEEP TROPICS. THIS SURGE ALREADY
REACHED THE BIG ISLAND...AND WILL ONLY INCREASE INTO TONIGHT. SOUTHEAST
SLOPES LIKELY WILL BEAR THE BRUNT OF THE RESULTING HEAVY RAIN AND POSSIBLE
THUNDERSTORMS...BUT THE WHOLE ISLAND WILL BE AT RISK INTO NEXT WEEK.
GUIDANCE DISAGREED ON HOW FAR NORTH THE DEEP MOISTURE WILL REACH...BUT
TRADE WIND FLOW WILL HELP TO SHIELD THE SMALLER ISLANDS. A FLASH FLOOD
WATCH REMAINED IN EFFECT ACCORDINGLY.

&&

.MARINE...
ALTHOUGH TROPICAL STORM NIALA IS EXPECTED TO PASS SOUTH OF THE BIG
ISLAND AND WEAKEN OVER THE WEEKEND AS IT CONTINUES ON A WESTWARD
TRACK...COASTAL IMPACTS ARE ANTICIPATED...ESPECIALLY ALONG THE
SOUTHEAST COAST OF THE BIG ISLAND AND THE ADJACENT WATERS. THE
LOCALLY RUN HIRES WAVE MODEL GUIDANCE DRIVEN BY THE OFFICIAL CENTRAL
PACIFIC HURRICANE CENTER WIND FORECAST ASSOCIATED WITH NIALA
SUPPORTS THIS AND INDICATES SEAS RAPIDLY RISING THROUGH THE DAY
SATURDAY...PEAKING BETWEEN TONIGHT AND SUNDAY NIGHT IN THE 10 TO
15 FT RANGE OVER THE SOUTHEAST BIG ISLAND WATERS...THEN SLOWLY
SUBSIDING THROUGH THE FIRST HALF OF THE UPCOMING WEEK AS NIALA
WEAKENS AND PASSES SOUTH OF THE ISLANDS.

ADVISORY LEVEL SURF...THAT COULD NEAR WARNING LEVELS ALONG THE
SOUTHEAST FACING BEACHES OF THE BIG ISLAND SATURDAY NIGHT THROUGH
SUNDAY NIGHT...IS EXPECTED THROUGH THIS TIME ALONG THE EAST FACING
SHORES OF EACH ISLAND. WAVE RUNUP COULD BECOME A CONCERN AND WILL
NEED TO BE MONITORED THROUGH THE WEEKEND...MAINLY IN THE TYPICALLY
VULNERABLE LOCATIONS ALONG THE SOUTHEAST COAST OF THE BIG ISLAND.

MODERATE TO LOCALLY STRONG TRADE WINDS ASSOCIATED WITH HIGH PRESSURE
NORTH OF THE MARINE AREA ARE ANTICIPATED BY MID WEEK. SEAS WILL
REFLECT THIS AND GRADUALLY SUBSIDE.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM 6 AM THIS MORNING TO 6 AM HST MONDAY FOR
KAUAI WINDWARD-OAHU KOOLAU-OLOMANA-MOLOKAI WINDWARD-MAUI
WINDWARD WEST-WINDWARD HALEAKALA.

FLASH FLOOD WATCH THROUGH MONDAY AFTERNOON FOR BIG ISLAND.

TROPICAL STORM WATCH FOR BIG ISLAND.

TROPICAL STORM WATCH FOR BIG ISLAND WINDWARD WATERS-BIG ISLAND
LEEWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST MONDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI CHANNEL-MAUI
COUNTY WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS-MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL.

&&

RYSHKO