Heavy Rain in Hawaii, April 22, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1005 AM HST WED APR 22 2015

.SYNOPSIS...
MODERATE TO LOCALLY BREEZY TRADE WINDS WILL PREVAIL NEAR THE BIG
ISLAND INTO THE WEEKEND...DELIVERING A FEW CLOUDS AND SHOWERS TO
WINDWARD AND SOUTHEAST SLOPES. LIGHT TO LOCALLY MODERATE SOUTHEAST
WINDS WILL PREVAIL OVER THE REMAINDER OF THE ISLAND CHAIN THROUGH
FRIDAY...WITH WINDS WEAKENING STATEWIDE AND SHIFTING TO THE SOUTH
AHEAD OF A FRONT THIS WEEKEND. THE SOUTHEAST WINDS WILL SPREAD
VOLCANIC HAZE TO THE OTHER SMALLER ISLANDS THE NEXT COUPLE OF DAYS.
THE SOUTHEAST WINDS WILL BRING PERIODS OF CLOUDS AND SHOWERS...
MAINLY TO WINDWARD AND SOUTHEAST SLOPES...BUT WILL ALSO ALLOW A FEW
SHOWERS TO AFFECT LEEWARD AREAS. THE WEAKENING FRONT IS EXPECTED TO
MOVE OVER THE ISLANDS THIS WEEKEND AND EARLY NEXT WEEK...ACCOMPANIED
BY MORE WIDESPREAD CLOUDS AND SHOWERS.

&&

.DISCUSSION...
LATEST SURFACE ANALYSIS DEPICTS A RATHER STRONG 1035 MB HIGH
CENTERED ABOUT 1500 MILES NORTHEAST OF THE ISLANDS...AND A
DEVELOPING DEEP-LAYER LOW ABOUT 1000 MILES TO THE NW. AN ASSOCIATED
FRONTAL BOUNDARY IS ABOUT 600 MILES NORTHWEST OF KAUAI...WITH
LAYERED CLOUDS AND SHOWERS AND EMBEDDED THUNDERSTORMS...EXTENDING
SEVERAL HUNDRED MILES EAST OF THE FRONT. ALOFT...A HIGHLY AMPLIFIED
PATTERN PREVAILS OVER THE CENTRAL PACIFIC...WITH A CLOSED LOW
SUPPORTING THE SURFACE LOW TO THE NORTHWEST...AND A STRONG
NORTH-SOUTH ORIENTED RIDGE ALONG ABOUT 152W. VISIBLE IMAGERY SHOWS
CLOUDS AND SHOWERS HEADED TOWARD KAUAI FROM THE SOUTHEAST...WITH LOW
CLOUDS FAVORING WINDWARD AND SOUTHEAST SLOPES OF THE OTHER ISLANDS.
FIRST PERIOD FORECASTS WERE UPDATED TO ACCOUNT FOR THE MOST RECENT
OBSERVATIONS.

LATEST FORECAST MODELS CONTINUE THE TREND PRESENTED BY EARLIER
RUNS...AND THUS THE FORECAST PHILOSOPHY REMAINS MUCH THE SAME.
FOR TODAY THROUGH FRIDAY...THE UNUSUALLY DEEP TROUGH OVER THE
CENTRAL PACIFIC WILL MOVE VERY SLOWLY EASTWARD...DRAGGING THE
ASSOCIATED SURFACE COLD FRONT SLOWLY EASTWARD AS WELL ACROSS THE
NORTHWESTERN HAWAIIAN ISLANDS. STRONG HIGH PRESSURE WILL REMAIN
ANCHORED FAR TO THE NORTHEAST. WHILE LOW LEVEL FLOW OVER THE WESTERN
PORTION OF THE STATE WILL BECOME INCREASINGLY SOUTHEASTERLY BETWEEN
THESE FEATURES...MAINLY E WINDS WILL PERSIST NEAR THE BIG ISLAND.

THIS WIND REGIME IS NOTORIOUSLY DIFFICULT FOR ANTICIPATING CLOUD AND
SHOWER COVERAGE ON THE SMALLER ISLANDS...AND ALSO MAKES IT DIFFICULT
TO ANTICIPATE WINDS ON THE SMALLER ISLANDS. THIS IS DUE TO THE FACT
THAT THE SMALLER ISLANDS ARE IN THE SHADOW OF THE BIG ISLAND...AND
EDDIES DEVELOPING DOWNSTREAM OF THE BIG ISLAND CAN HAVE SIGNIFICANT
IMPACTS ON SENSIBLE WEATHER. OVERNIGHT AND EARLY THIS MORNING WAS A
PERFECT EXAMPLE...AS LOCALLY HEAVY SHOWERS BRIEFLY IMPACTED WINDWARD
OAHU FROM NEAR TURTLE BAY TO HAUULA...WITH SKIES THEN NEARLY CLEAR
AT SUNRISE. ALSO...LOCALLY GUSTY OVERNIGHT WINDS AT LIHUE WENT
COMPLETELY CALM...AND ALLOWED EARLY MORNING LAND BREEZES TO
DEVELOP. SO WHILE THE ONGOING FORECAST ATTEMPTS TO DESCRIBE THE
LOCAL EFFECTS KNOWN TO TYPICALLY OCCUR IN THIS FLOW REGIME...
FORECAST CONFIDENCE IS LOWER THAN NORMAL...ESPECIALLY FOR THE FINE
SCALE DETAILS. HAVING SAID THAT...SHOWERS AND CLOUDS SHOULD CONTINUE
TO FAVOR WINDWARD AND MAUKA AREAS...BUT THIS PATTERN TENDS TO ALSO
FAVOR ISLAND PLUMES WHICH CAN BRING SHOWERS TO SOME TYPICALLY DRIER
LEEWARD SECTIONS AT TIMES...ESPECIALLY ON OAHU AND KAUAI. PERSISTENT
SHOWERS IN ISLAND PLUMES MAY PRODUCE SOME ISOLATED HEAVY AMOUNTS AT
TIMES.

FOR THE WEEKEND...MODELS CONTINUE TO KEEP THE COLD AIR POOL AND
STRONGEST MOISTURE/FORCING ASSOCIATED WITH THE EASTWARD MOVING
MID/UPPER TROUGH TO THE NORTH OF THE STATE...WHILE DRAGGING A
WEAKENING SURFACE FRONT OR TROUGH FROM JUST WEST OF KAUAI ON
SATURDAY ACROSS AT LEAST KAUAI AND OAHU SATURDAY NIGHT AND SUNDAY.
THIS WOULD CAUSE WINDS TO BECOME QUITE LIGHT FOR A TIME...THEN
INCREASE OUT OF THE NORTH/NORTHEAST BEHIND THE FRONT. A BAND OF
ENHANCED SHOWER ACTIVITY SEEMS LIKELY TO BE PRESENT ALONG AND
AHEAD OF THE FRONT DURING THIS TIME. THUNDERSTORMS APPEAR UNLIKELY
AT THIS POINT DUE TO EXPECTED WARM/STABLE MID LEVELS...BUT THERE
WOULD BE THE POTENTIAL FOR SOME LOCALLY HEAVIER DOWNPOURS. WILL
CONTINUE TO MONITOR LATER MODEL RUNS DURING THE NEXT COUPLE OF
DAYS.

TRADES SHOULD RESUME EARLY NEXT WEEK AS A NEW SURFACE HIGH BUILDS
TO OUR NORTHWEST AND NORTH. SOME LINGERING MOISTURE ALONG THE
DISSIPATING TROUGH NEARBY MAY CONTINUE TO PRODUCE ENHANCED SHOWERS
OVER WINDWARD AND MAUKA AREAS.

&&

.AVIATION...
VFR CONDITIONS PREVAIL AT MOST TAF SITES WITH OCCASIONAL MVFR
CONDITIONS OCCURING AT PHLI AND PHTO DUE TO LOW CLOUDS AND SHRA.
EXPECT THESE CONDITIONS TO CONTINUE OFF AND ON THROUGHOUT THE DAY.

AIRMET SIERRA REMAINS POSTED ALONG THE WINDWARD SLOPES ON THE BIG
ISLAND DUE TO LOW CLOUDS AND SHRA. AIRMET TANGO REMAINS POSTED FOR
LOW LEVEL TURB W AND NW OF MOUNTAIN RANGES AS LOW LEVEL WINDS BLOW
FROM THE E AND SE.

&&

.MARINE...
A HIGH SURF ADVISORY REMAINS IN EFFECT THROUGH TONIGHT FOR EAST
FACING SHORES OF MOST ISLANDS. ALTHOUGH LOCAL WINDS WILL BE OUT OF
THE SOUTHEAST OVER THE NEXT COUPLE OF DAYS...ESPECIALLY OVER THE
WESTERN END OF THE ISLAND CHAIN...A LONG FETCH OF FRESH TO STRONG
EAST WINDS WILL REMAIN UPSTREAM. WAVEWATCH GUIDANCE RESPONDS BY
INDICATING THAT THE POTENTIAL FOR HIGH SURF WILL PERSIST INTO
FRIDAY...AND THE ADVISORY WILL LIKELY NEED TO BE EXTENDED IN TIME.
OBSERVATIONS ALSO INDICATE AN ONGOING SOUTH SWELL...WITH RESULTING
SURF JUST BELOW ADVISORY LEVELS ON SOUTH FACING SHORES. THIS SWELL
WILL LINGER FOR A COUPLE OF DAYS AS IT GRADUALLY SUBSIDES. SMALL
SWELLS FROM THE NORTHWEST SWELLS WILL PREVAIL INTO THE WEEKEND...
WITH A LARGER NORTH-NORTHWEST SWELL POSSIBLE FROM SUNDAY INTO
MONDAY...WITH PEAK SURF HEIGHTS POTENTIALLY REACHING ADVISORY
LEVELS...ESPECIALLY ON N FACING SHORES.

OVERNIGHT ASCAT PASS FOUND LARGE AREAS OF 25 KT WINDS IN THE
CHANNELS...SOUTH OF THE BIG ISLAND...AND ACROSS THE WINDWARD
WATERS...IN GOOD AGREEMENT WITH THE EXISTING SMALL CRAFT ADVISORY
/SCA/ CONFIGURATION AND LATEST HI-RES WIND GUIDANCE. WITH ONLY
SLIGHT CHANGES IN THE OVERALL SYNOPTIC PATTERN THE NEXT COUPLE OF
DAYS...THE SCA WILL LIKELY BE EXTENDED IN TIME. HOWEVER...MAALAEA
BAY HAS BEEN REMOVED FROM THE SCA AS THIS WIND DIRECTION SHOULD
PRECLUDE WINDS FROM GETTING VERY STRONG THERE.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST THURSDAY FOR EAST FACING SHORES
OF KAUAI...OAHU...MOLOKAI...MAUI AND BIG ISLAND.

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

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...BURKE