Heavy Rain in Hawaii, September 25 to 27, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
800 PM HST FRI SEP 25 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 FROM TONIGHT INTO NEXT WEEK.

&&

.DISCUSSION...
SURFACE OBSERVATIONS FROM AROUND THE MAIN HAWAIIAN ISLANDS CONTINUED TO
REPORT MODERATE TO LOCALLY STRONG TRADE WINDS. SPEEDS WERE HIGHEST IN THE
USUAL WINDY PLACES AROUND MAUI AND HAWAII COUNTIES. THE RELATIVELY
MOIST AIR MASS OVER THE STATE SUPPORTED WELL-DEVELOPED MARINE SHOWERS
MOVING ASHORE ALONG WINDWARD SLOPES. AFTERNOON LEE CONVECTION WAS RATHER
SUBDUED CONSIDERING THE AVAILABLE FUEL...HOWEVER. 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 WOULD 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.

THE LARGER INFLUENCE ON ISLAND WEATHER WILL COME FROM THE SURGE OF MOISTURE
EXPECTED TO ACCOMPANY NIALA FROM THE DEEP TROPICS. THIS SURGE ALREADY
APPROACHED THE BIG ISLAND TONIGHT...AND WILL ONLY INCREASE INTO SATURDAY
NIGHT. 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...
TRADE WINDS WILL BECOME FRESH TO STRONG OVER THE NEXT SEVERAL DAYS
AS PASSING TROPICAL CYCLONE NIALA TIGHTENS THE PRESSURE GRADIENT. A
TROPICAL STORM WATCH WAS IN EFFECT FOR THE FOLLOWING COASTAL WATERS...
BIG ISLAND WINDWARD...BIG ISLAND LEEWARD...AND BIG ISLAND SOUTHEAST.
A SMALL CRAFT ADVISORY REMAINED IN EFFECT THROUGH SUNDAY NIGHT FOR THE
REST OF THE HAWAIIAN COASTAL WATERS NOT IN THE TROPICAL STORM WATCH.

NIALA WILL DIRECT SHORT-PERIOD SWELL PRIMARILY TOWARD EAST-FACING
SHORES OF THE BIG ISLAND...THOUGH SOME ALSO WILL REACH EAST SHORES OF
KAUAI...OAHU...MOLOKAI...AND MAUI. SURF AND RUN UP INFORMATION IS
CONTAINED IN THE TROPICAL STORM LOCAL STATEMENT PRODUCT...HLSHFO. A
HIGH SURF ADVISORY WILL GO INTO EFFECT FOR EAST-FACING SHORES OF
KAUAI...OAHU...MOLOKAI...AND MAUI AT 6 AM HST SATURDAY. WIND...SURF...
AND SEAS ARE EXPECTED TO DECLINE GRADUALLY EARLY NEXT WEEK AS NIALA
WEAKENS AND MOVES AWAY FROM THE STATE.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM 6 AM SATURDAY 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