429
FXHW60 PHFO 201322
AFDHFO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
322 AM HST SAT APR 20 2019
SYNOPSIS
SEASONAL TRADE WIND CONDITIONS WILL PREVAIL THROUGH THE WEEKEND AND
MUCH OF NEXT WEEK AS HIGH PRESSURE REMAINS IN PLACE TO THE NORTH.
CLOUDS AND SHOWERS WILL FAVOR THE TYPICAL WINDWARD LOCATIONS EACH
DAY.
DISCUSSION
THE LATEST SURFACE ANALYSIS SHOWED A 1029 MB AREA OF HIGH PRESSURE
CENTERED OVER THE NORTHEAST PACIFIC AND A RIDGE EXTENDING WEST-
SOUTHWEST FROM IT TO AN AREA NORTH OF THE ISLANDS. WATER VAPOR
IMAGERY SHOWED AN UPPER RIDGE POSITIONED OVER THE REGION AND
MOSTLY DRY AIR ALOFT. SATELLITE DATA AND UPPER AIR SOUNDINGS
DEPICTED MOISTURE LEVELS NEAR TO SLIGHTLY BELOW NORMAL FOR THIS
TIME OF YEAR AND A STRONG SUBSIDENCE INVERSION BETWEEN SIX AND
EIGHT THOUSAND FEET. RAIN GAGES SUPPORT THE DRY CONDITIONS WITH
MINIMAL ACCUMULATIONS MEASURED OVER THE PAST 12-HRS (THROUGH 2 AM
HST). PEAK TOTALS FOR THIS PERIOD HAVE BEEN ON THE BIG ISLAND
ALONG THE WINDWARD COAST FROM HAMAKUA TO NORTH KOHALA (.22 TO .7
INCHES).
GUIDANCE SUPPORTS THE MOSTLY DRY AND STABLE TRADE WIND PATTERN
CONTINUING THROUGH MUCH OF NEXT WEEK. TRADES ARE FORECAST TO
TREND UP TODAY THROUGH SUNDAY DUE TO THE RIDGE STRENGTHENING NORTH
OF THE ISLANDS. BREEZY CONDITIONS WILL RESULT, ESPECIALLY THROUGH
THE LATE MORNING AND AFTERNOON HOURS. THE UPPER RIDGE IS FORECAST
TO REMAIN OVER THE REGION INTO NEXT WEEK WITH MOSTLY DRY MID- TO
UPPER-LEVEL AIR HOLDING IN PLACE. THESE UPPER CONDITIONS COMBINED
WITH A STRONG SUBSIDENCE INVERSION WILL LIMIT THE RAINFALL
POTENTIAL/COVERAGE, EVEN FOR WINDWARD LOCATIONS. THE EXCEPTION
WILL BE DUE TO POCKETS OF HIGHER MOISTURE MOVING THROUGH
PERIODICALLY WITHIN THE TRADES, WHICH WILL TRANSLATE TO BETTER
WINDWARD RAINFALL COVERAGE.
FOR THE EXTENDED PERIOD, GUIDANCE DEPICTS LOWERING UPPER HEIGHTS AND
THE SURFACE RIDGE TO THE NORTH WEAKENING DUE TO A WEAK SHORTWAVE
TROUGH AND SURFACE FRONT MOVING INTO THE REGION LATE NEXT WEEK.
FORECAST CONFIDENCE THIS FAR OUT REMAINS LOW, BUT IF THIS SCENARIO
DOES EVOLVE, TRADES WOULD TREND DOWN, POTENTIALLY BECOMING LIGHT
AND VARIABLE FOR SOME LOCATIONS.
AVIATION
A HIGH PRESSURE RIDGE NORTH OF THE STATE WILL KEEP THE ISLANDS IN
A MODERATE TO BREEZY TRADE WIND WEATHER PATTERN THROUGH MONDAY.
NEAR SURFACE WIND SPEEDS ARE STRONG ENOUGH TO SUPPORT MODERATE LOW
LEVEL TURBULENCE OVER AND SOUTH THROUGH WEST OF MOUNTAIN RANGES
OF ALL ISLANDS. SCATTERED SHOWER ACTIVITY WILL FAVOR WINDWARD AND
MOUNTAIN SECTIONS WITH BRIEF PERIODS OF MVFR CONDITIONS MAINLY
DURING THE OVERNIGHT HOURS.
AIRMET TANGO REMAINS IN EFFECT FOR TEMPO MODERATE TURBULENCE BELOW
080. THIS AIRMET WILL LIKELY REMAIN IN EFFECT THROUGH THE
WEEKEND.
MARINE
NO SIGNIFICANT CHANGES TO THE FORECAST WITH THE MORNING PACKAGE.
THE RIDGE TO THE NORTH OF THE ISLANDS WILL MAINTAIN FRESH TO
STRONG TRADE WINDS INTO EARLY NEXT WEEK. THE SMALL CRAFT ADVISORY
(SCA) IS IN EFFECT FOR MOST OF THE WATERS NEAR MAUI COUNTY AND THE
BIG ISLAND THROUGH TONIGHT, AND THROUGH TOMORROW FOR THE MORE
TYPICAL WINDIER AREAS. WILL WAIT TO SEE HOW THINGS PAN OUT TODAY
BEFORE MAKING ANY CHANGE IN THE TIMING OF THE SCA.
SURF ALONG ALL SHORES IS EXPECTED TO REMAIN BELOW ADVISORY LEVELS
FOR THE NEXT WEEK. THE CURRENT NORTH-NORTHWEST SWELL WILL
CONTINUE TO DECLINE THROUGH THE WEEKEND. A SMALL, WEST-NORTHWEST
SWELL IS POSSIBLE EARLY NEXT WEEK. BACKGROUND SOUTH-SOUTHWEST
SWELLS WILL KEEP SURF SMALL ALONG SOUTH FACING SHORES THROUGH THE
WEEKEND. AN UPWARD TREND ALONG SOUTH FACING SHORES IS POSSIBLE
EARLY NEXT WEEK, DUE TO RECENT ACTIVITY SOUTHEAST OF NEW ZEALAND.
SURF ALONG EAST FACING SHORES WILL SEE A BUMP OVER THE WEEKEND
IN RESPONSE TO THE TRADE WINDS. SEE THE LATEST OAHU SURF
DISCUSSION (SRDHFO) FOR ADDITIONAL DETAILS ON SURF AND SWELL.
HFO WATCHES/WARNINGS/ADVISORIES
SMALL CRAFT ADVISORY UNTIL 6 PM HST SUNDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.
SMALL CRAFT ADVISORY UNTIL 6 AM HST SUNDAY FOR KAIWI CHANNEL-
MAUI COUNTY WINDWARD WATERS-BIG ISLAND WINDWARD WATERS.
DISCUSSION...GIBBS
AVIATION...BOHLIN
MARINE...M BALLARD
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