199
FXHW60 PHFO 190207
AFDHFO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
407 PM HST FRI APR 18 2025
SYNOPSIS
AN ACTIVE CONVECTIVE PATTERN WITH MOSTLY LIGHT WINDS WILL CONTINUE
THROUGH THIS EVENING, WITH SHOWERS AND THUNDERSTORMS FAVORING
ISLAND INTERIORS, THEN TRANSITIONING TO MOSTLY WINDWARD AREAS
OVERNIGHT. MODERATE TRADES ARE EXPECTED TO RETURN TOMORROW AND
PERSIST THROUGH THE WEEKEND, WITH LINGERING MOISTURE AND
INSTABILITY MAINTAINING A RATHER SHOWERY PATTERN ACROSS THE ISLAND
CHAIN. A NEW STORM SYSTEM WILL DISRUPT THE TRADES ONCE AGAIN AND
BRING ANOTHER CHANCE FOR HEAVY RAINFALL AND THUNDERSTORMS TO THE
ISLANDS TUESDAY THROUGH THURSDAY.
DISCUSSION
A COLD CORE UPPER LEVEL LOW REMAINS CENTERED NEAR MAUI COUNTY
THIS AFTERNOON AND WILL GRADUALLY BEGIN TO PUSH EASTWARD TONIGHT.
THE INSTABILITY THIS UPPER LEVEL FEATURE IS PROVIDING TO THE
STATE COMBINED WITH AMPLE LINGERING MOISTURE (PWATS OVER 1.5
INCHES AS MEASURED ON THIS AFTERNOON'S UPPER AIR SOUNDINGS) HAS
LEAD TO HEAVY SHOWERS AND THUNDERSTORMS IMPACTING THE MAJORITY OF
THE STATE THROUGHOUT THE DAY TODAY, WHICH CAUSED FLASH FLOODING
ACROSS PORTIONS OF KAUAI, OAHU AND MAUI COUNTY. HIGH RESOLUTION
MODELS INDICATE THAT HEAVY RAIN AND THUNDERSTORMS WILL PERSIST
THROUGH THIS EVENING. IN ADDITION, SNOW IS EXPECTED ACROSS THE
HIGH ELEVATION SUMMITS OF THE BIG ISLAND AND THE WINTER WEATHER
ADVISORY HAS BEEN EXTENDED THROUGH 6 AM SATURDAY FOR ELEVATIONS
ABOVE 11,000 FEET.
AT THE SURFACE, A STRONG HIGH PRESSURE SYSTEM TO FAR NORTH AND
WEAKENING SURFACE TROUGH RETREATING TO WEST WILL BRING LIGHT TO
MODERATE SOUTHEAST WINDS THROUGH THIS EVENING. THIS LIGHTER FLOW
WILL ALLOW LAND BREEZES TO HELP TO PUSH SOME OF THE SHOWER AND
THUNDERSTORM ACTIVITY OFFSHORE OVERNIGHT. SHOWERS CAUGHT UP IN
THE SOUTHEAST FLOW WILL ALSO PERIODICALLY MOVE INTO WINDWARD
AREAS, PARTICULARLY DURING THE OVERNIGHT HOURS, AND OCCASIONALLY
PASS OVER INTO LEEWARD AREAS.
OVER THE WEEKEND, MODERATE EASTERLY TRADES WILL RETURN AS THE
SURFACE TROUGH PUSHES FURTHER AWAY AND DISSIPATES, ALLOWING THE
HIGH PRESSURE SYSTEM TO THE NORTH TO TAKE MORE CONTROL OF THE
LOCAL WINDS. ALOFT, WE'LL LOSE SOME OF THE INSTABILITY AS THE
UPPER LOW OPENS INTO A TROUGH, WEAKENS AND PUSHES FURTHER EASTWARD,
BUT SHOWERS ARE EXPECTED TO STICK AROUND AS MOISTURE LINGERS. AS
A RESULT, EXPECT A SHOWERY TRADEWIND PATTERN THROUGH THE WEEKEND,
WITH SHOWERS OCCASIONALLY SPILLING OVER TO LEEWARD AREAS.
A NEW, POTENT STORM SYSTEM WILL DISRUPT THE TRADEWIND FLOW
MUCH OF NEXT WEEK BRINGING LIGHT TO MODERATE SOUTHEASTERLY WINDS
BACK TO THE AREA LATE MONDAY INTO TUESDAY AND ANOTHER ROUND OF
POTENTIALLY SIGNIFICANT RAINFALL TUESDAY THROUGH THURSDAY.
AVIATION
INTERIOR SHOWERS AND THUNDERSTORMS ARE BRINGING WIDESPREAD MVFR
TO OAHU THROUGH MAUI AND PORTIONS OF THE BIG ISLAND WITH POCKETS
OF IFR WITHIN MORE INTENSE ACTIVITY. SHRA/TSRA WILL GRADUALLY
DISSIPATE THIS EVENING WITH THE LOSS OF DIURNAL HEATING. THE
DEVELOPMENT OF LAND BREEZES TONIGHT WILL ACCELERATE CLEARING,
THOUGH CONVECTIVE DEBRIS WILL LIKELY CONTRIBUTE TO A MID-LEVEL/VFR
CLOUD DECK THAT WILL PERSIST WELL INTO TONIGHT. MODERATE TRADES
RETURN EARLY SATURDAY, BUT WILL NOT BE STRONG ENOUGH TO ELIMINATE
THE INLAND SHOWER THREAT. INSTEAD, THE RESULTING HYBRID SEA BREEZE
PATTERN WILL FOCUS MODERATE TO LOCALLY HEAVY SHOWERS OVER WESTERN
PORTIONS OF THE SMALLER ISLANDS, PARTICULARLY OAHU AND MAUI.
AIRMET SIERRA REMAINS IN EFFECT FOR THE ISLANDS OF OAHU, MOLOKAI,
AND MAUI.
MARINE
HIGH PRESSURE FAR TO THE NORTHEAST WILL GRADUALLY STRENGTHEN
THROUGH THE WEEKEND. LIGHT EAST TO SOUTHEAST WINDS ACROSS HAWAII WILL
BECOME LIGHT TO MODERATE EAST TO NORTHEAST WINDS BY TOMORROW
NIGHT. TRADE WIND FLOW WILL PREDOMINATE SUNDAY AND MONDAY, BUT A
DEEP LOW DEVELOPING TO THE NORTHWEST OF THE STATE WILL DISRUPT
THAT FLOW ON TUESDAY. WINDS WILL YET AGAIN WEAKEN AND SHIFT TO BE
OUT OF THE SOUTH OR SOUTHEAST AND REMAIN SO THROUGH MOST OF THE
WEEK.
MEANWHILE, A PASSING UPPER LEVEL DISTURBANCE HAS TRIGGERED
ISOLATED TO SCATTERED THUNDERSTORMS ACROSS MUCH OF THE STATE.
THESE STORMS ARE CAPABLE OF PRODUCING STRONG GUSTY WINDS, LOCALLY
HEAVY DOWNPOURS, AND SMALL HAIL. THE POTENTIAL FOR STRONG
THUNDERSTORMS WILL PERSIST THROUGH TONIGHT AND POSSIBLY BEYOND.
A LINGERING NORTH-NORTHWEST SWELL THAT PEAKED LAST NIGHT WILL
CONTINUE ITS GRADUAL DECLINE THROUGH SATURDAY. ON SUNDAY,
SURF ALONG NORTH AND WEST FACING SHORES WILL DIP BELOW AVERAGE,
THEN REMAIN SO WELL INTO NEXT WEEK. GUIDANCE DOES DEPICT A GALE
FORMING ON MONDAY NEAR THE KURILS, THEN TRACKING NORTHEAST TO THE
WESTERN ALEUTIANS BY WEDNESDAY. IF THIS MATERIALIZES, HAWAII COULD
EXPERIENCE AN UPTICK IN THE NORTHWEST SWELL NEXT WEEKEND.
SURF ALONG EAST FACING SHORES WILL REMAIN BELOW THE APRIL AVERAGE
THROUGH MOST OF NEXT WEEK DUE TO THE LACK OF CONSISTENT TRADE
WIND FLOW.
A SMALL LONG PERIOD BACKGROUND SOUTH SWELL WILL MOVE THROUGH THIS
WEEKEND, BUT WILL HAVE LITTLE IMPACT ON SURF HEIGHTS. A LARGER
PULSE OF LONG-PERIOD SOUTH- SOUTHWEST SWELL IS EXPECTED FOR THE
COMING WEEK.
HFO WATCHES/WARNINGS/ADVISORIES
WINTER WEATHER ADVISORY UNTIL 6 AM HST SATURDAY FOR BIG ISLAND
SUMMITS.
DISCUSSION...FARRIS
AVIATION...JVC
MARINE...BEDAL
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