646
FXHW60 PHFO 201329
AFDHFO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
329 AM HST MON APR 20 2026
SYNOPSIS
AN UPPER LEVEL TROUGH PASSING THROUGH THE HAWAII REGION TODAY
WILL KEEP HIGH AND MIDDLE LEVEL CLOUDS OVER THE ISLANDS WITH
SCATTERED SHOWERS AND CHANCES FOR A FEW THUNDERSTORMS MAINLY OVER
THE WESTERN ISLANDS. A SERIES OF LOW PRESSURE SYSTEMS DEVELOPING
NORTHWEST TO NORTH OF THE ISLANDS FOR THE FIRST HALF OF THIS WEEK
WILL WEAKEN THE RIDGE NORTH OF THE STATE, PRODUCING WEAK
SOUTHEASTERLY WIND FLOW LASTING THROUGH WEDNESDAY. THE RIDGE
BUILDS BACK IN NORTH OF THE STATE BY THURSDAY, ALLOWING MODERATE
EASTERLY TRADE WINDS WITH PASSING WINDWARD SHOWERS TO RETURN
LASTING THROUGH THE UPCOMING WEEKEND.
DISCUSSION
LOOKING INTO THE INFRARED SATELLITE IMAGERY THIS MORNING WE SEE A
MESOSCALE CONVECTIVE SYSTEM WITH MODERATE TO HEAVY SHOWERS AND
THUNDERSTORMS COLLAPSING AS SCATTERED THUNDERSTORMS RIDING IN ON AN
OUTFLOW BOUNDARY FADES AS IT APPROACHES THE WESTERN COASTAL
WATERS OF KAUAI. A FEW OF THESE THUNDERSTORMS WILL LIKELY REACH
THESE WESTERN COASTAL WATERS EARLY THIS MORNING, HOWEVER THE
ENTIRE SYSTEM WILL LIKELY COLLAPSE INTO A MORE MODERATE RAIN BAND
BY THE TIME IT REACHES THE ISLANDS OF KAUAI AND NIIHAU AROUND
SUNRISE. ELSEWHERE THE PASSING UPPER LEVEL TROUGH SHOWN ON
SATELLITE WATER VAPOR IMAGERY WILL INCREASE SHOWER TRENDS TODAY
ACROSS THE STATE, ESPECIALLY ALONG SOUTHEASTERN SLOPES OF
HALEAKALA ON MAUI AND THE SOUTHEAST SLOPES OF MAUNA LOA ON THE BIG
ISLAND THIS AFTERNOON. RAINFALL AND THUNDERSTORM COVERAGE IN THE
NEXT 24 HOURS OF THE STATEWIDE FORECAST WAS ADJUSTED UPWARD TO
ACCOUNT FOR THESE SMALLER SCALE WEATHER IMPACTS.
AN UPPER LEVEL TROUGH WILL SWIFTLY PASS FROM WEST TO EAST ACROSS
THE STATE TODAY IN SOUTHEASTERLY WIND FLOW BRIEFLY ENHANCING
SHOWER ACTIVITY AS IT PASSES OVER THE ISLAND CHAIN. AN UPPER
LEVEL RIDGE WILL BUILD IN BEHIND THE TROUGH AS THE TROUGH MOVES
AWAY FROM THE ISLANDS FROM TONIGHT INTO TUESDAY. EXPECT DRYING
TRENDS AS THE UPPER LEVEL RIDGE BUILDS STABILIZING SUBSIDENCE,
DOWNWARD MOVING WARMING AND DRYING AIR, STRENGTHENING AND LOWERING
THE SUBSIDENCE INVERSION HEIGHTS ACROSS THE HAWAII REGION.
STRONGER SUBSIDENCE WILL BUILD THE SURFACE RIDGE NORTH OF THE
ISLANDS, ALLOWING TRADE WINDS TO RETURN TO THE REGION FROM
WEDNESDAY NIGHT ON INTO THE UPCOMING WEEKEND. SUBSIDENCE INVERSION
HEIGHTS WILL RANGE FROM 5,000 TO 7,000 FEET DURING THIS TIME
PERIOD, HIGH ENOUGH FOR BRIEF PASSING SHOWERS FAVORING THE TYPICAL
WINDWARD AND MOUNTAIN AREAS IN THE OVERNIGHT TO EARLY MORNING
HOURS INTO SUNDAY.
AVIATION
LIGHT EASTERLY WINDS PERSIST OVER WINDWARD ASPECTS OF THE STATE
TODAY AMONG SOME LIGHT AND VARIABLE WINDS ASSOCIATED WITH LAND-SEA
BREEZES OVER LEEWARD AND INTERIOR REGIONS. WINDS WILL BEGIN TO
VEER MORE ESE LATE THIS AFTERNOON INTO THE EVENING HOURS. LIGHT
SHOWERS REMAIN POSSIBLE ACROSS WINDWARD AND MAUKA AREAS, AS WELL
AS OVER ISLAND INTERIORS DURING AFTERNOON HOURS. AS THE UPPER-
LEVEL DISTURBANCE AND ASSOCIATED SURFACE TROUGH APPROACH THE
HAWAIIAN ISLANDS FROM THE WEST, EXPECT CLOUD AND SHOWER COVERAGE
TO INCREASE STATEWIDE, WITH LOW-MVFR/IFR CONDITIONS POSSIBLE
WITHIN HEAVY, ISOLATED SHOWERS. OTHERWISE, VFR CONDITIONS PREVAIL.
NO AIRMETS ARE CURRENTLY IN EFFECT.
MARINE
A TROUGH OF LOW PRESSURE PASSING NORTHWEST OF THE STATE WILL
ACT TO WEAKEN AND VEER WINDS EAST-SOUTHEASTERLY THROUGH MID-WEEK
ACROSS THE LOCAL WATERS. LIGHT WIND FLOW SHOULD ALLOW FOR
AFTERNOON SEA BREEZE DEVELOPMENT NEAR THE SHORES. ISOLATED
THUNDERSTORMS ARE EXPECTED TO CONTINUE OVER THE NORTHWESTERN
WATERS IN THE VICINITY OF THE TROUGH THROUGH TONIGHT. BY THURSDAY,
HIGH PRESSURE RIDGING BUILDS BACK IN NORTH OF THE STATE WITH
FRESH NORTHEASTERLY TRADE WINDS RETURNING TO THE REGION INTO NEXT
WEEKEND.
SWELL AND SURF WILL REMAIN SMALL ALONG ALL SHORES THROUGH LATE
TODAY.
A SMALL, MEDIUM-PERIOD NORTHWEST SWELL ARRIVES TONIGHT, BRINGING
AN UPTICK IN SURF ALONG NORTH AND WEST FACING SHORES FOR TUESDAY
BEFORE PEAKING ON WEDNESDAY. A MODERATE, MEDIUM- PERIOD NORTH-
NORTHEAST SWELL ARRIVES ON TUESDAY AND PEAKS ALSO ON WEDNESDAY,
BRINGING MODERATE SURF TO NORTH AND NORTHEAST FACING SHORES,
BEFORE SLOWLY DECLINING THROUGH THE END OF THE WEEK.
ANOTHER SMALL, LONG-PERIOD WEST-NORTHWEST SWELL WILL ENTER THE WATERS
WEDNESDAY NIGHT AND PROVIDE A SLIGHT BUMP UP FOR SURF ALONG WEST AND
NORTHWEST FACING SHORES. SURF WILL CONTINUE TO BE SMALL ALONG
SOUTH FACING SHORES THROUGH THE PERIOD. EAST FACING SHORES WILL
SEE AN INCREASE IN SURF WITH THE ARRIVAL OF THE AFOREMENTIONED
NORTH-NORTHEASTERLY SWELL DESPITE BELOW-SEASONAL AVERAGE TRADES.
HFO WATCHES/WARNINGS/ADVISORIES
NONE.
DISCUSSION...BOHLIN
AVIATION...PIERCE
MARINE...QUESADA
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