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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST MON APR 20 2015

.SYNOPSIS...
MODERATE TO STRONG TRADE WINDS WILL WEAKEN AND BECOME MORE 
SOUTHEASTERLY THROUGH MID-WEEK...AND THEN CHANGE LITTLE INTO THE 
WEEKEND. MOST CLOUDS AND RAIN WILL OCCUR OVER SLOPES FACING INTO 
THE WIND...BUT SHOWERS WILL BE MORE LIKELY TO DRIFT LEEWARD LATER IN 
THE WEEK...ESPECIALLY AT NIGHT. SOUTHEAST WINDS ALSO MAY BRING 
VOLCANIC HAZE FROM KILAUEA TO THE SMALLER ISLANDS BY THE WEEKEND.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED 
MODERATE TO STRONG TRADE WINDS. SOUNDING DATA REVEALED A FAIRLY 
TYPICAL VERTICAL PROFILE...WITH A MODESTLY STRONG INVERSION AT NEAR 
THE USUAL HEIGHT ABOVE A SURFACE AIR MASS HOLDING WATER VAPOR JUST 
SLIGHTLY LESS THAN SUGGESTED BY CLIMATOLOGY. ALOFT...A PROMINENT 
DOME OF HIGH HEIGHTS SUPPORTED THE SURFACE RIDGE TO THE NORTH...AND 
ALSO RESISTED ANY RISE OF THE INVERSION LOCALLY.

UNSURPRISINGLY...CLOUDS ACROSS THE STATE TENDED TO ENHANCE MAINLY 
OVER WINDWARD SLOPES...WHERE OROGRAPHIC FORCING WAS MOST 
SIGNIFICANT. ALTHOUGH SATELLITE DATA ESTIMATED A SOMEWHAT DRIER AIR 
MASS FURTHER UPWIND...THE REST OF THE SCENARIO SUPPORTED THE 
CONTINUATION OF PRESENT CONDITIONS THROUGH MID-WEEK...AND PERHAPS 
EVEN A SLIGHT INCREASE IN WINDWARD PRECIPITATION OVERNIGHT BASED ON 
CLIMATICALLY FAVORED DIURNAL CYCLES. ACCUMULATION SHOULD REMAIN ON 
THE LOW SIDE NONETHELESS.

BY WAY OF CONTRAST WITH THIS STEREOTYPICAL STATUS QUO...MOST 
SOLUTIONS AGREED IN PREDICTING A RATHER ODD DEVELOPMENT FOR THE 
SECOND HALF OF THE WEEK. A MID-LEVEL TROUGH APPROACHING RAPIDLY FROM 
THE NORTHWEST WILL SLOW DOWN AND DEEPEN AS IT CUTS OFF NEAR 170W 
AROUND WEDNESDAY. BY THURSDAY...THIS FEATURE WILL SUPPORT THE 
DEVELOPMENT OF A SIGNIFICANT SURFACE REFLECTION...INCLUDING AN 
ELONGATED TROUGH OR FRONT HOLDING TOGETHER SOMETHING LIKE A 
BAROCLINIC LEAF CLOUD.

SURFACE FLOW OVER THE STATE WILL WEAKEN AND TURN MORE SOUTHEASTERLY 
AS THE SURFACE RIDGE TO THE NORTH RECEDES EAST AHEAD OF THE NEW 
SURFACE FEATURES. LEE PLUME CLOUDS FROM ONE ISLAND MAY MOVE ASHORE 
OVER OTHER ISLANDS DOWNWIND...BUT THE LARGE AREA OF HEAVY RAIN 
ASSOCIATED WITH THE DEEPLY STACKED LOW SHOULD REMAIN OFFSHORE TO THE 
WEST UNTIL THE FEATURE WEAKENS AND RECEDES TOWARD THE NORTHWEST OVER 
THE WEEKEND. WHILE THIS PROGNOSIS BODES WELL FOR THE STATE...IT IS 
WORTH MENTIONING THAT THE HIGHLY STRUCTURED FEATURE PRODUCING 
INTENSE RAIN WILL MISS KAUAI BY 100 MILES OR PERHAPS LESS. IN THE 
VASTNESS OF THE PACIFIC OCEAN...THIS DISTANCE IS NOT SO GREAT. IF 
THE LOW ALOFT PARKS A BIT FURTHER EAST THAN THE MODELS PREDICT... 
THEN PARTS OF THE STATE COULD BE AT RISK FOR EXCESSIVE RAINFALL.

&&

.AVIATION...
HIGH PRESSURE NORTHEAST OF THE ISLANDS WILL KEEP A BREEZY EAST TO 
NORTHEAST TRADE WIND FLOW IN PLACE THROUGH TUESDAY...WITH CLOUDS AND 
SHOWERS PRIMARILY FOCUSED OVER MOUNTAIN AND NORTH THROUGH EAST 
SECTIONS OF THE ISLANDS. BRIEF MVFR CIGS/VSBYS WILL BE POSSIBLE AT 
PHTO...PHOG...PHMK...AND HLI THROUGH MID MORNING TUESDAY. 
ELSEWHERE...VFR CONDITIONS ARE EXPECTED TO PREVAIL.

AIRMET TANGO REMAINS IN EFFECT FOR LOW LEVEL TURBULENCE OVER AND TO 
THE SOUTH THRU WEST OF THE MOUNTAINS...AND THESE CONDITIONS ARE 
EXPECTED TO CONTINUE THROUGH TUESDAY.

AIRMET SIERRA MAY EVENTUALLY BE NEEDED FOR PORTIONS OF THE ISLANDS 
LATER TONIGHT DUE TO MOUNTAIN OBSCURATION. 

&&

.MARINE...
STRONG WINDS WILL PERSIST OVER THE COASTAL WATERS THROUGH TUESDAY. A 
GALE WARNING REMAINED IN EFFECT THROUGH 6 AM TUESDAY FOR THE 
ALENUIHAHA CHANNEL. A SMALL CRAFT ADVISORY /SCA/ CURRENTLY IN EFFECT 
FOR THE REST OF THE COASTAL WATERS LIKELY WILL BE NEEDED FOR ALL 
WATERS ON TUESDAY...AND LATER INTO THE WEEK FOR SELECT ZONES.

WINDS SHOULD WEAKEN SLIGHTLY AND VEER SOUTHEASTERLY OVER THE AREA 
FROM TUESDAY NIGHT. THE SOUTHEAST FLOW THEN WILL CONTINUE THROUGH 
THE REST OF THE WEEK INTO THE WEEKEND.

THE STRONG TRADE WINDS WILL CONTINUE TO BRING ADVISORY-LEVEL ROUGH 
AND CHOPPY SURF TO EAST-FACING SHORES FOR THE NEXT COUPLE OF DAYS. 
THEREFORE...A HIGH SURF ADVISORY WILL REMAIN IN EFFECT AT LEAST 
THROUGH TUESDAY NIGHT...AND MAY BE NEEDED LATER INTO THE WEEK.

A LONG-PERIOD SOUTH SWELL BUILDING SLOWLY TONIGHT WILL PEAK AROUND 
TUESDAY...BRINGING SURF NEAR THE THRESHOLD FOR AN ADVISORY TO 
SOUTH-FACING SHORES...AND THEN DIMINISHING SLOWLY THROUGH THE REST 
OF THE WEEK. ANOTHER SMALL NORTHWEST SWELL EXPECTED ON TUESDAY 
SHOULD PRODUCE PEAK SURF HEIGHTS WELL BELOW THE ADVISORY LEVELS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST WEDNESDAY FOR KAUAI WINDWARD-
OAHU KOOLAU-OLOMANA-MOLOKAI WINDWARD-MAUI WINDWARD WEST-WINDWARD 
HALEAKALA-SOUTH BIG ISLAND-BIG ISLAND NORTH AND EAST.

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY 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-BIG ISLAND WINDWARD WATERS-BIG ISLAND LEEWARD 
WATERS-BIG ISLAND SOUTHEAST WATERS.

GALE WARNING UNTIL 6 AM HST TUESDAY FOR ALENUIHAHA CHANNEL.

&&

$$

DISCUSSION/MARINE...RYSHKO
AVIATION...JELSEMA



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