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Hawaiian Islands Synoptic Discussion and Guidance

824
FXHW60 PHFO 161913
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST WED APR 16 2014

.SYNOPSIS...
TRADE WIND WEATHER WILL PERSIST THROUGH THE FORECAST PERIOD AS HIGH
PRESSURE DOMINATES TO OUR NORTH. WINDS WILL INCREASE LATE SUNDAY
INTO EARLY NEXT WEEK AS THIS HIGH PRESSURE STRENGTHENS. CLOUDS AND
SHOWERS WILL CONTINUE TO FAVOR WINDWARD AND MAUKA AREAS. LEEWARD
AREAS OF THE BIG ISLAND WILL SEE INCREASED CLOUDS AND SHOWERS EACH
AFTERNOON AND EARLY EVENING.

&&

.DISCUSSION...
THE AXIS OF A SURFACE RIDGE NORTH OF THE MAIN HAWAIIAN ISLANDS
EXTENDS THROUGH 35N140W AND 27N180W...SPANNING THE CENTRAL NORTH
PACIFIC. ANCHORED BY A 1027 MB HIGH NEAR 35N140W...THIS RIDGE WILL
PLAY A KEY ROLE IN OUR WEATHER THROUGH THE NEXT WEEK BY KEEPING
TRADE WINDS BLOWING ACROSS LOCAL WATERS. WATER VAPOR LOOP SHOWS A
NORTHEAST TO SOUTHWEST ORIENTED UPPER TROUGH ACROSS OUR AREA...WITH
AN AXIS EXTENDING THROUGH 31N140W AND 15N164W. OVERNIGHT SOUNDINGS
SHOW EVIDENCE OF BOTH WEATHER FEATURES...WITH STRONG TRADE
INVERSIONS MARKING SUBSIDENCE ASSOCIATED WITH A LOW-LEVEL RIDGE AND
NEGATIVE LIFTED INDEX VALUES MARKING THE MILD INSTABILITY ASSOCIATED
WITH THE UPPER TROUGH.

THE PRESSURE GRADIENT BETWEEN THE RIDGE TO OUR NORTH AND EQUATORIAL
LOW PRESSURE IS SUFFICIENTLY STEEP TO SUPPORT 20 TO 25 MPH NORTHEAST
TRADE WINDS ACROSS LOCAL WATERS THIS MORNING. WINDS ARE SLIGHTLY
STRONGER IN THE CHANNELS AND SMALL CRAFT ADVISORIES ARE IN EFFECT
FOR MOST HAWAIIAN WATERS. WIND WAVES ARE HIGH ENOUGH TO KEEP A HIGH
SURF ADVISORY UP THROUGH THIS AFTERNOON ALONG MOST EAST-FACING
SHORES. MODELS SHOW THE RIDGE IS EXPECTED TO WEAKEN SLIGHTLY OVER
THE NEXT FEW DAYS AS A DEVELOPING LOW TRACKS NORTHEASTWARD WITHIN
THE STORM TRACK TO ITS NORTH. AS A RESULT...WE EXPECT LOCAL TRADE
WINDS WILL DROP OFF A BIT AFTER TODAY. A STRONGER HIGH IS EXPECTED
TO DEVELOP ALONG THE RIDGE WEST OF 160W BY SUNDAY...THEN CONTINUE TO
STRENGTHEN AS IT TRACKS EASTWARD EARLY NEXT WEEK. THIS WILL CAUSE
TRADE WINDS TO STRENGTHEN AS WELL.

CLOUDS AND SHOWERS EMBEDDED WITHIN THE TRADES WILL FAVOR WINDWARD
AND MAUKA AREAS THROUGH THE FORECAST PERIOD. HOWEVER...RETURN FLOW
AND DAYTIME HEATING WILL ALSO CAUSE INCREASED CLOUDS AND SHOWERS
ACROSS LEEWARD BIG ISLAND AREAS EACH AFTERNOON AND EARLY EVENING.
CONTINUED LOW LEVEL RIDGING WILL TEND TO LOWER THE INVERSION HEIGHT
OVER TIME...SO WE EXPECT OVERALL SHOWER ACTIVITY WILL DIMINISH.
HOWEVER...MODELS SHOW A MODEST PLUME OF MORE HUMID AIR WILL MOVE
OVER THE EASTERN ISLANDS FROM THURSDAY THROUGH FRIDAY...BRIEFLY
INCREASING TRADE SHOWER COVERAGE ACROSS THOSE AREAS.

IN SHORT...WE EXPECT TRADE WIND WEATHER THROUGH THE FORECAST
PERIOD...WITH MINOR VARIATIONS IN TRADE WIND SPEEDS AND TRADE SHOWER
COVERAGE.

&&

.AVIATION...
MODERATE TO LOCALLY STRONG TRADE WINDS FROM SFC TO ABOUT 15K WITH
SUBSIDENCE INVERSIONS BETWEEN 8K AND 10K ALOFT EXPECTED TO PERSIST
THROUGH 24 HOURS AND BEYOND. WILL BE KEEPING AIRMET TANGO FOR
MECHANICAL TURB IN EFFECT INDEFINITELY.

CLOUDS AND SHOWERS WILL FAVOR THE TYPICAL WINDWARD AND MOUNTAIN
AREAS WITH A FEW OF THESE SHOWERS DRIFTING ACROSS LEEWARD PORTIONS
OF THE SMALLER ISLANDS. THE KONA COAST WILL SEE A FEW SHOWERS
DEVELOP THIS AFTERNOON THEN DISSIPATE AFTER SUNSET. VFR FLYING
CONDITIONS WILL NEVERTHELESS BE PREDOMINANT ACROSS ALL AREAS. MVFR
CIGS IN PASSING SHOWERS SHOULD BE ISOLATED AND LIMITED TO THE NE
COAST AND SLOPES OF THE BIG ISLAND.

&&

.MARINE...
A SMALL CRAFT ADVISORY IS IN EFFECT THROUGH THIS AFTERNOON
FOR THE CHANNELS AND WATERS AROUND KAUAI AND OAHU...AND CONTINUE
THROUGH FRIDAY FOR TYPICALLY WINDIER AREAS AROUND MAUI AND THE BIG
ISLAND. TRADE WINDS WILL DIMINISH SOMEWHAT TODAY...BUT THEN REMAIN
MODERATE TO FRESH THROUGH FRIDAY. WIND WAVES PRODUCED BY THE
PERSISTENT TRADE WINDS ALSO WILL CONTINUE TO REQUIRE A HIGH SURF
ADVISORY FOR EAST-FACING SHORES THROUGH THIS AFTERNOON.

A SMALL SWELL FROM THE WEST NORTHWEST WILL ARRIVE LATE TODAY...AND
THEN PEAK THURSDAY BELOW THE ADVISORY LEVELS. NO OTHER SIGNIFICANT
SWELLS ARE EXPECTED.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST THIS EVENING 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 PM HST FRIDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THIS EVENING FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI CHANNEL-OAHU
WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-BIG
ISLAND WINDWARD WATERS.

&&

$$

MARINE/PUBLIC...POWELL
AVIATION...BEDAL





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