Flash Flood and Heavy Rain in Hawaii, November 18 to 21, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
841 PM HST WED NOV 18 2015

.SYNOPSIS...
ENHANCED MOISTURE WILL MOVE OVER THE ISLANDS FROM THE SOUTHEAST
DURING THE NEXT COUPLE OF DAYS...WHICH WILL BRING AN INCREASING
THREAT OF LOCALLY HEAVY RAINFALL. INITIALLY...THE HEAVY SHOWERS
WILL FAVOR WINDWARD SECTIONS OF THE BIG ISLAND AND PARTS OF MAUI
COUNTY FROM TONIGHT INTO THURSDAY...THEN KAUAI AND OAHU BY
THURSDAY NIGHT OR FRIDAY. AS A WEAK LOW DEVELOPS IN THE VICINITY
OF THE ISLANDS LATER THIS WEEK...WINDS WILL SHIFT TO THE
SOUTH...AND SHOWERS... SOME LOCALLY HEAVY...WILL ALSO AFFECT
LEEWARD AREAS. THE UNSETTLED WET WEATHER PATTERN MAY CONTINUE OVER
PORTIONS OF THE STATE INTO EARLY NEXT WEEK.

&&

.DISCUSSION...
THE ROUNDED WESTERN END OF A SURFACE RIDGE IS LOCATED ABOUT 490
MILES NORTH OF HONOLULU EARLY THIS EVENING. ALSO...A SURFACE
FRONT REMAINS STALLED ABOUT 340 MILES NORTHWEST OF LIHUE...WHILE A
WEAK SURFACE TROUGH IS LOCATED JUST SOUTH OF SOUTH POINT ON THE
BIG ISLAND. THESE WEATHER FEATURES CONTINUE TO DISRUPT OUR TYPICAL
TRADE WIND WEATHER PATTERN. ALOFT...A MIDDLE TO UPPER LONG-WAVE
TROUGH AXIS EXTENDS FROM NEAR 40N 166W TO 23N 177E...OR MORE THAN
1400 MILES WEST OF LIHUE. THIS FEATURE...WHICH APPEARS TO BE
PROPAGATING SLOWLY EASTWARD...HAS LIKELY BEEN SUPPORTING THE
STATIONARY FRONT LOCATED WEST OF THE ISLANDS. ALSO...ABUNDANT
MOISTURE RIDING UP IN THE SOUTHEASTERLY FLOW EAST OF THE SURFACE
TROUGH IS CURRENTLY MOVING  OVER THE WINDWARD BIG ISLAND THIS
EVENING...BRINGING LOTS OF LOW CLOUDS AND SHOWERS TO THE LOWER
ELEVATIONS OF THE BIG ISLAND. THE LATEST FORECAST SHOWS THE
MOISTURE WILL BE SUFFICIENTLY DEEP TO REQUIRE A FLASH FLOOD WATCH
AND WINTER WEATHER ADVISORY FOR THE BIG ISLAND. THE PRIMARY WINTER
WEATHER THREAT FOR THE SUMMITS IS EXPECTED TO BE FREEZING RAIN.

IN REGARDS TO THE FORECAST...THE LATEST MODEL GUIDANCE CONTINUES
TO SUGGEST THAT WE WILL BE IN A WET WEATHER PATTERN AT LEAST
THROUGH THE END OF THIS WEEK. THE MIDDLE AND UPPER LEVEL TROUGH
ALOFT IS EXPECTED TO REMAIN NORTHWEST OF THE ISLANDS INTO NEXT
WEEK. THE PROXIMITY OF THIS FEATURE TO THE ISLANDS WILL LIKELY
BRING UNSETTLED WEATHER TO PARTS OF THE STATE. IN ADDITION...THE
AREA OF ENHANCED MOISTURE CURRENTLY IN THE VICINITY OF THE BIG
ISLAND WILL SPREAD SLOWLY WEST-NORTHWESTWARD OVER THE STATE DURING
THE NEXT FEW DAYS. THE SURFACE TROUGH IS ALSO EXPECTED TO BECOME
BETTER DEFINED AS IT MOVES WEST-NORTHWEST AND BEGINS TO BE
INFLUENCED BY THE MID TO UPPER LEVEL LONG-WAVE TROUGH. THEREFORE...
THE FORECAST MODELS INDICATE THAT THERE IS AN INCREASING THREAT OF
LOCALLY HEAVY RAINFALL. NOTE THAT MODEL SOLUTIONS CONTINUE TO LACK
CONSISTENCY IN WHERE AND WHEN HEAVY RAIN MAY OCCUR. THE GENERAL
TREND HAS BEEN FOR THE MOISTURE TO AFFECT THE EASTERN ISLANDS
INITIALLY...AND THEN SPREAD UP TOWARD OAHU AND KAUAI FROM THURSDAY
NIGHT INTO THE WEEKEND.

AS MENTIONED ABOVE...THE MODELS SHOW THE SURFACE TROUGH WILL
LIKELY BECOME BETTER DEFINED. EVENTUALLY THIS SURFACE TROUGH IS
EXPECTED TO DEVELOP INTO A SURFACE LOW...POSSIBLY NORTHWEST OF
KAUAI. SINCE THERE IS QUITE A BIT OF UNCERTAINTY IN EXACTLY WHERE
THIS LOW MIGHT FORM...THE WIND FORECAST REMAINS PROBLEMATIC. THE
CURRENT FORECAST STILL ANTICIPATES CYCLOGENESIS NORTHWEST OF
KAUAI...WITH WINDS OVER THE WESTERN END OF THE ISLAND CHAIN
SHIFTING TO THE SOUTHEAST AND THEN SOUTH. THIS DEVELOPING LOW IS
FORECAST TO MOVE NORTH OF THE ISLANDS INTO EARLY NEXT WEEK. AN
ASSOCIATED CONVERGENCE BAND WILL LIKELY DEVELOP OVER PORTIONS OF
THE ISLAND CHAIN BY THIS WEEKEND...AND THEN REMAIN IN PLACE INTO
EARLY NEXT WEEK. THIS CONTINUES TO LEND CONFIDENCE IN THE EVENTUAL
DEVELOPMENT OF A SIGNIFICANTLY WET WEATHER REGIME THAT WOULD
AFFECT BOTH WINDWARD AND LEEWARD AREAS OF THE ISLANDS FROM LATER
THIS WEEK INTO NEXT WEEK

&&

.AVIATION... ABUNDANT MOISTURE SPREADING OVER THE WINDWARD AND SE
PORTIONS OF THE BIG ISLAND WILL PRODUCE AREAS OF MVFR CONDITIONS AND
MOUNTAIN OBSCURATION WITH OCCASIONAL IFR THERE OVERNIGHT INTO
THURSDAY... AND AIRMET SIERRA IS IN EFFECT FOR THAT AREA. CLOUDS AND
SOME POSSIBLE FROZEN PRECIP WILL AFFECT THE BIG ISLAND SUMMITS
OVERNIGHT AS WELL. THIS MOISTURE WILL SPREAD ACROSS MAUI COUNTY ON
THURSDAY...AND AREAS OF MOUNTAIN OBSCURATION MAY DEVELOP PRIMARILY
ACROSS WINDWARD AND SE FACING SLOPES.

OTHERWISE...VFR CONDITIONS WILL PREVAIL OVERNIGHT AND THURSDAY...
WITH ISOLATED MVFR CONDITIONS POSSIBLE ACROSS WINDWARD SECTIONS FROM
MOLOKAI THROUGH KAUAI.

&&

.MARINE...
THE COMBINATION OF MODERATE TO LOCALLY STRONG EASTERLY TRADE
WINDS...A BUILDING NORTHWEST SWELL...AND EASTERLY SWELLS ARRIVING
IN THE AREA FROM AN EXTENSIVE BELT OF TRADE WINDS EAST OF THE
AREA...IS LEADING TO COMBINED SEAS NEAR THE SMALL CRAFT ADVISORY
/SCA/ CRITERIA OF 10 FEET. THEREFORE...MOST HAWAIIAN WATERS ARE
INCLUDED IN THE SCA THROUGH THURSDAY. EAST SWELLS WILL CONTINUE TO
ADVECT INTO ISLAND WATERS THROUGH THE REMAINDER OF THE WEEK. BASED
ON THE LATEST FORECAST...WINDS AND SEAS ARE EXPECTED TO DROP BELOW
THE SCA THRESHOLD IN MOST MARINE ZONES BY THURSDAY NIGHT. HOWEVER...
EAST TO SOUTHEAST WINDS INTERACTING WITH...AND ACCELERATING
AROUND...THE TERRAIN OF THE BIG ISLAND MAY REMAIN STRONG ENOUGH TO
REQUIRE AN EXTENSION OF THE SCA IN MARINE ZONES SURROUNDING THE
BIG ISLAND.

AS MENTIONED ABOVE...ELEVATED EAST SWELL WILL CONTINUE ALONG THE
WINDWARD BIG ISLAND. OBSERVATIONS AND GUIDANCE INDICATE HIGH SURF
WILL PERSIST HERE DURING THE NEXT COUPLE OF DAYS. ALSO...A
BUILDING NORTHWEST SWELL WILL LIKELY PEAK LATER TONIGHT OR
THURSDAY MORNING BEFORE GRADUALLY DIMINISHING. OBSERVED SWELL
HEIGHTS AT BUOYS 51001/51101 NORTHWEST OF KAUAI HAVE BEEN LARGER
THAN ANTICIPATED...WHICH PROMPTED A HIGH SURF ADVISORY FOR NORTH
AND WEST FACING SHORES OF KAUAI AND NIIHAU. THIS SWELL IS
EXPECTED TO PEAK OVERNIGHT ON OAHU AND MAUI WITH SURF HEIGHTS
REMAINING JUST BELOW THE HIGH SURF ADVISORY CRITERIA. THE PEAK OF
THE SWELL IS EXPECTED ON THE KONA SHORES ON THURSDAY...AGAIN WITH
SURF HEIGHTS NEAR...BUT BELOW THE HIGH SURF ADVISORY THRESHOLD.

A SMALLER PULSE OF NORTH-NORTHWEST SWELL IS EXPECTED THURSDAY AND
FRIDAY. ALSO...A SMALL NORTHWEST SWELL IS EXPECTED THIS WEEKEND.
NO MAJOR NORTHWEST SWELLS ARE ANTICIPATED THROUGH EARLY NEXT WEEK
BASED ON THE LATEST GUIDANCE. ELSEWHERE...A SLIGHT BUMP IN THE
SOUTH SWELL IS EXPECTED ON FRIDAY AND SATURDAY. SEE THE LATEST
OAHU SURF DISCUSSION /SRDHFO/ FOR MORE DETAILS ON THE SOURCES OF
THE SWELLS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL NOON HST THURSDAY FOR NIIHAU-KAUAI
WINDWARD-KAUAI LEEWARD.

FLASH FLOOD WATCH THROUGH LATE THURSDAY NIGHT FOR BIG ISLAND.

HIGH SURF ADVISORY UNTIL 6 AM HST FRIDAY FOR SOUTH BIG ISLAND-
BIG ISLAND NORTH AND EAST.

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

SMALL CRAFT ADVISORY UNTIL 6 PM HST THURSDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-
ALENUIHAHA CHANNEL-BIG ISLAND WINDWARD WATERS-BIG ISLAND LEEWARD
WATERS-BIG ISLAND SOUTHEAST WATERS.

&&

DISCUSSION/MARINE...HOUSTON
AVIATION...JACOBSON