125
FXHW60 PHFO 171358
AFDHFO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
358 AM HST THU JUL 17 2025
SYNOPSIS
A NEAR STATIC AND QUIET WEATHER PATTERN WILL UNDERGO CHANGE THE
NEXT FEW DAYS AS RAIN CHANCES INCREASE GOING INTO AND THROUGH THE
WEEKEND. THE WESTERN HALF OF THE STATE WILL LIKELY EXPERIENCE A
MORE WET WEEKEND WHILE LIGHT TO LOCALLY BREEZY REGIONAL TRADE
WINDS REMAIN IN PLACE THROUGH THE END OF THE WEEK. TODAY'S
SHOWERS WILL FOCUS ALONG WINDWARD EXPOSURES, UPSLOPE MAUKA AND
WITHIN HIGHER ELEVATIONS. STABLE AND DRIER WEATHER IS EXPECTED TO
RETURN EARLY NEXT WEEK.
DISCUSSION
THE CURRENT SYNOPTIC SUMMERTIME PATTERN REMAINS ENTRENCHED OVER
THE HAWAIIAN ISLANDS. THIS BEING OF HIGH PRESSURE RIDGING AT ALL
LEVELS POSITIONED NORTH NORTHEAST OF THE STATE. LIGHT, RANDOM
SHOWERS PASSING WITHIN THE TRADE BELT HAS WETTENED MANY WINDWARD
LOCALES WITH BRIEF LIGHT RAIN EPISODES. THE WINDWARD SLOPES OF
HAMAKUA, HILO AND PUNA REGIONS OF BIG ISLAND HAVE RECEIVED THE
MOST 24 HOUR RAINFALL OF TYPICALLY A QUARTER OF AN INCH OR LESS.
THE REMAINDER OF THE STATE HAS PICKED UP NO MORE THAN A FEW
HUNDREDTHS TO A TENTH OF AN INCH OVER EAST-FACING REGIONS WHILE
MANY LEEWARD AREAS HAVE REMAINED BONE DRY. A SURGE OF HIGHER MID
TO LOW LEVEL MOISTURE PASSING THROUGH TODAY WILL THICKEN UP CLOUDS
AND INCREASE PRIMARILY WINDWARD AND UPSLOPE MAUKA/HIGH ELEVATION
SHOWER ACTIVITY. THE CAVEAT WILL BE ALONG MORE WIND- PROTECTED BIG
ISLAND KONA SLOPES WHEREAS THE ENHANCED AFTERNOON SEA BREEZE WILL
INCREASE UPSLOPE CLOUDS AND AREAL RAIN CHANCES. THE LARGE SCALE
DOWNSTREAM PRESSURE GRADIENT FROM A 1035 MB HIGH CENTERED NEAR
45N145W HAS BEEN TIGHT ENOUGH TO PRODUCE STATEWIDE INTERIOR LIGHT
TO BREEZY TRADES. DOWNSLOPE OR CHANNELED WINDS HAVE BEEN LOCALLY
STRONG OVER SUCH AREAS AS THE CENTRAL MAUI VALLEY (KEALALOLOA
RIDGE) AND NORTH KOHALA'S NORTHERNMOST TIP (UPOLU POINT) WHERE
WINDS OF 25 TO 30 MPH SUSTAINED WITH GUSTS TO OVER 40 MPH HAVE
BEEN COMMONPLACE THE PAST SEVERAL DAYS. LITTLE CHANGE IN THE
STRENGTH AND POSITION OF THE SURFACE HIGH WILL GUARANTEE ANOTHER
FEW DAYS OF THIS WIND BEHAVIOR.
GEOPOTENTIAL HEIGHTS WILL BEGIN TO GRADUALLY FALL FROM THE
NORTHWEST AS A WEAK MID TO UPPER LOW DEVELOPS WEST OF THE ISLANDS
THE NEXT SEVERAL DAYS. HEIGHT FALLS OF AROUND 400 METERS AND
INCREASED 500 MB VORTICITY, ALONG WITH A SURGE OF UNSEASONABLY
HIGHER MOISTURE (1.6 INCH PWATS) ADVECTING IN FROM THE EAST, WILL
DECREASE STABILITY AND PICK UP RAIN CHANCES MAINLY NORTH OF
21N OF BETTER FOCUSING ON OAHU AND KAUAI FROM LATE FRIDAY THROUGH
EARLY SUNDAY. DETERMINISTIC GUIDANCE IS FORECASTING DEEPENING
MOISTURE WHERE YESTERDAY'S 4K FT RESIDENT BOUNDARY LAYER DEPTH
(INVERSION HEIGHT) SHOULD DOUBLE TO AROUND 8K FT BY SATURDAY. THE
MAJORITY OF THE GFS AND EC ENSEMBLE GUIDANCE QPF MEMBERS DEPICT A
DISTINCT SWATH OF HIGHER RAIN PARALLELING NEAR 20 N LATITUDE.
WHILE CONFIDENCE IS MODERATELY HIGH THAT MORE WESTERN ISLANDS WILL
EXPERIENCE A MORE WET WEEKEND INTO EARLY NEXT WEEK, THE CHANCE OF
THUNDERSTORMS AND CONFIDENCE OF OCCURRENCE REMAINS LOW. NWP
GUIDANCE DOS NOT COOL UPPER LEVEL TEMPERATURES ENOUGH (AROUND
MINUS 7 C AT 500 MB) TO PROVIDE A STRONG ENOUGH INSTABILITY SIGNAL
TO PLACE THUNDER IN KAUAI OR OAHU'S WEEKEND FORECAST. THE
PROXIMITY OF THE LOW ACROSS THE FAR NORTHERN OFFSHORE WATERS WILL
LIKELY BE THE IMPETUS TO ISOLATED STORMS THIS WEEKEND. AS OF NOW,
THE HIGHEST CONFIDENCE LAYS IN HIGHER RAIN PROBABILITIES N OF 20N
WITH PERIODS OF HEAVY RAIN FROM AS EARLY AS TOMORROW (FRIDAY)
NIGHT THROUGH AS LATE AS EARLY MONDAY. TIMING WILL DEPEND UPON THE
EVOLUTION OF THE UPPER LOW IN ACCORDANCE WITH THE HIGHEST
MOISTURE ADVECTION.
A MORE STABLE SUMMER-LIKE TRADE WIND PATTERN IS EXPECTED TO
RETURN BY LATE MONDAY. AS GREATER MOISTURE EXITS WEST ON MONDAY A
DRIER AIR MASS WILL REACH THE EASTERN HALF OF THE STATE. THE
UPPER LEVEL LOW WILL MOVES FURTHER AWAY NORTHEAST OF THE REGION...
WEAKENING INTO A TROUGH AS RIDGING EXPANDS IN FROM THE SOUTH.
AVIATION
MODERATE TO LOCALLY BREEZY TRADES THROUGH TOMORROW. LOW CIGS AND
SHRA SHOULD MAINLY IMPACT WINDWARD AND MAUKA LOCATIONS, AS WELL AS
THE SOUTH KONA SLOPES OF THE BIG ISLAND. TRADES WILL EASE
SLIGHTLY FRIDAY AND SATURDAY, AND SHRA ACTIVITY MAY INCREASE AS
THE ATMOSPHERE BECOMES LESS STABLE.
AIRMET SIERRA IS IN EFFECT FOR MTN OBSC FOR WINDWARD SECTIONS OF
ALL ISLANDS. CONDS SHOULD IMPROVE IN THE MORNING HOURS.
AIRMET TANGO IS IN EFFECT FOR LOW LEVEL TURB BLW 070 DOWNWIND OF
ISLAND TERRAIN.
MARINE
HIGH PRESSURE WILL REMAIN FAR NORTHEAST OF THE STATE THROUGH THE
REST OF THE WEEK. FRESH TO LOCALLY STRONG TRADES WILL PERSIST
THROUGH THE REST OF TODAY, THEN DROP SLIGHTLY AS A WEAK LOW LEVEL
TROUGH DEVELOPS IN THE EASTERLY FLOW AND MOVES ACROSS THE STATE
TONIGHT INTO THE WEEKEND. A SMALL CRAFT ADVISORY (SCA) REMAINS IN
EFFECT FOR THE TYPICAL WINDY AREAS AROUND MAUI COUNTY AND THE BIG
ISLAND THROUGH 6AM FRIDAY.
A FADING MEDIUM PERIOD SOUTH SWELL COMBINED WITH A FADING SOUTH-
SOUTHEAST SWELL WILL MAINTAIN BELOW SUMMER AVERAGE SURF ON THE
SOUTH FACING SHORES THROUGH TODAY. A SLIGHT BUMP IN SURF IS
EXPECTED THIS WEEKEND AS A NEW LONG-PERIOD SOUTHWEST SWELL ARRIVES
FROM THE TASMAN SEA, ALONG WITH A NEW SMALL SHORT-PERIOD SOUTH-
SOUTHEAST SWELL. THE TASMAN SWELL IS EXPECTED TO PEAK ON SATURDAY,
WITH SURF HEIGHTS TOPPING NEAR THE SUMMER AVERAGES BEFORE
GRADUALLY DECLINING INTO EARLY NEXT WEEK.
SURF ALONG EAST FACING SHORES WILL CONTINUE TO REMAIN ROUGH AND
CHOPPY DUE TO THE FRESH TO LOCALLY STRONG TRADE WINDS AND A SMALL
SHORT-PERIOD NORTHEAST SWELL THAT IS EXPECTED TO FILL IN THIS
WEEKEND. SURF ALONG NORTH FACING SHORES WILL REMAIN NEARLY FLAT
THROUGH THE WEEKEND.
MINOR COASTAL FLOODING ALONG THE SHORELINE AND IN LOW-LYING
COASTAL AREAS WILL BE POSSIBLE AROUND THE DAILY HIGH TIDE
STARTING MONDAY OF NEXT WEEK DUE TO PEAK MONTHLY TIDES AND HIGHER
THAN PREDICTED WATER LEVELS AT SELECT LOCATIONS.
FIRE WEATHER
TRADE WINDS WILL REMAIN IN THE MODERATE TO LOCALLY BREEZY RANGE
THROUGH THURSDAY BEFORE WEAKENING SLIGHTLY INTO THE WEEKEND.
SHOWER ACTIVITY IS EXPECTED TO INCREASE FROM FRIDAY AND HOLD
THROUGH THE WEEKEND. A WEAK UPPER LEVEL DISTURBANCE WILL NEAR
NORTHWEST KAUAI LATE THURSDAY AND FRIDAY, INCREASING INSTABILITY
SLIGHTLY. AT THE SAME TIME A AREA OF LOW LEVEL MOISTURE WILL MOVE
FROM EAST TO WEST ACROSS THE STATE, INCREASING CHANCES OF SHOWERS
MAINLY FOR WINDWARD A MAUKA AREAS BUT COULD ALSO SPILL OVER TO
LEEWARD AREAS OF THE SMALLER ISLANDS AT TIMES AS THE TEMPERATURE
INVERSION HEIGHTS INCREASE. DRIER AND MORE STABLE WEATHER IS
EXPECTED TO GRADUALLY RETURN FROM EAST TO WEST SUNDAY INTO EARLY
NEXT WEEK. BORDERLINE FIRE WEATHER CONCERNS OVER LEEWARD AREAS MAY
BE OF CONCERN MIDDLE OF NEXT WEEK AS BREEZY, DRY AND STABLE TRADE
WIND PATTERN DEVELOPS.
HFO WATCHES/WARNINGS/ADVISORIES
SMALL CRAFT ADVISORY UNTIL 6 AM HST FRIDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.
DISCUSSION...BLOOD
AVIATION...WALSH
MARINE...WALSH
FIRE WEATHER...BLOOD
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