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Hawaiian Islands Synoptic Discussion and Guidance

327
FXHW60 PHFO 281336
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
336 AM HST SAT FEB 28 2015

.SYNOPSIS...
A WEAKENING FRONT CURRENTLY NEAR MAUI WILL DRIFT SLOWLY EASTWARD
TODAY. MOISTURE ALONG THIS FRONT COMBINED WITH INSTABILITY ALOFT
DUE TO AN UPPER TROUGH WILL PRODUCE LOCALLY HEAVY DOWNPOURS ALONG
WITH A SLIGHT CHANCE FOR THUNDERSTORMS ACROSS THE EASTERN HALF OF
THE STATE. DRIER WEATHER WILL PREVAIL ACROSS THE WESTERN SECTIONS.
TRADE WINDS WILL SPREAD ACROSS THE STATE ON SUNDAY AS THE FRONT
DISSIPATES...BUT LINGERING ENHANCED MOISTURE AND INSTABILITY WILL
REMAIN NEAR THE BIG ISLAND WITH LOCALLY HEAVY RAINS ONCE AGAIN
POSSIBLE. A WET AND BREEZY TRADE WIND PATTERN WILL PREVAIL ACROSS
THE STATE FROM MONDAY THROUGH AT LEAST THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...
WIDELY SCATTERED HEAVY SHOWERS AND ISOLATED THUNDERSTORMS HAVE
PERSISTED AROUND THE BIG ISLAND DURING THE NIGHT...ALTHOUGH MOST
OF THE ACTIVITY HAS REMAINED OFFSHORE. RADAR RETURNS STREAMING
OVER MAUI COUNTY APPEAR TO BE CAUSED BY THICK MID LEVEL CLOUDS...
WITH RAIN GAUGES SHOWING VERY LITTLE RECENT PRECIP THERE. MOSTLY
DRY CONDITIONS PREVAIL FURTHER WEST OVER OAHU AND KAUAI...WITH
JUST SOME ISOLATED LIGHT RAINFALL AMOUNTS MEASURED DURING THE
NIGHT. SURFACE ANALYSIS DEPICTS A WEAK FRONT THROUGH THE CENTER
OF THE STATE...LIKELY LOCATED NEAR MAUI AT THIS TIME. NORTHERLY
LOW LEVEL WINDS HAVE SPREAD OVER KAUAI AND OAHU TO THE WEST OF
THIS BOUNDARY...WHILE LIGHT NE TO E WINDS PREVAIL TO THE EAST.
ALOFT...THE STATE LIES UNDER THE EXIT REGION OF A 100+ KT JET
STREAK OVER THE CENTRAL PACIFIC...AND IN THE SOUTHEAST QUADRANT OF
A MID LEVEL TROUGH. THE 12Z LIHUE SOUNDING MEASURED COLD 500 MB
TEMPS OF -14C...BUT A RATHER MEAGER PWAT OF ONLY 1.06 INCHES...AND
AN INVERSION BASED AROUND 8000 FEET WITH A VERY DRY PROFILE ABOVE.
IN CONTRAST...THE 12Z HILO SOUNDING WAS NOTABLY MOIST AND
UNSTABLE...WITH PWAT OF 1.61 INCHES...SBCAPE OVER 1800 J/KG...
NO CONVECTIVE INHIBITION...500 MB TEMP OF -11C AND A LIFTED INDEX
OF -6.

MIMIC-TPW SATELLITE IMAGERY SHOWS A BAND OF ENHANCED MOISTURE
OVER THE CENTRAL AND EASTERN PARTS OF THE STATE...WITH ESTIMATED
PWATS UP TO 1.7 INCHES. A CONSIDERABLY DRIER AIRMASS HAS PUSHED
INTO KAUAI AND IS SPREADING INTO OAHU.

FOR TODAY...THE WEAK FRONT/TROUGH WILL CONTINUE DRIFTING EAST TO
NEAR THE BIG ISLAND...WITH N/NE LOW LEVEL FLOW SPREADING ACROSS
MAUI COUNTY IN ITS WAKE. ENHANCED LOW/MID LEVEL MOISTURE WILL REMAIN
POOLED ALONG AND TO THE EAST OF THE TROUGH...AND THE ATMOSPHERE
WILL REMAIN UNSTABLE ALOFT AS THE MID LEVEL TROUGH AND THE UPPER
LEVEL JET STREAM EXIT REGION MOVES ONLY SLIGHTLY EASTWARD. AS A
RESULT...A CHANCE FOR SCATTERED HEAVY SHOWERS AND ISOLATED
THUNDERSTORMS WILL PERSIST OVER THE BIG ISLAND AND MAUI COUNTY.
MODELS HAVE TRENDED TO A DRIER SOLUTION FOR OAHU...AND HAVE
REMOVED MENTION OF HEAVY RAINS THERE. MAINLY DRY WEATHER WILL
CONTINUE ACROSS KAUAI TODAY...WITH SCATTERED SHOWERS CONFINED TO
THE NORTHERN SLOPES AND MOUNTAINS.

TONIGHT AND SUNDAY...THE DISSIPATING FRONT/TROUGH WILL CONTINUE
DRIFTING SLOWLY EASTWARD. ENHANCED MOISTURE ALONG THE FRONT
COMBINED WITH CONTINUED INSTABILITY ALOFT AND THE NEARBY JET
STREAM WILL MAINTAIN THE CHANCE FOR LOCALLY HEAVY RAINS AND
ISOLATED THUNDERSTORMS OVER EASTERN SECTIONS OF THE STATE. FURTHER
TO THE WEST...SCATTERED SHOWERS WILL BECOME FOCUSED OVER WINDWARD
AND MAUKA SECTIONS OF OAHU AS TRADE WINDS INCREASE. DRIER
CONDITIONS WILL CONTINUE ON KAUAI WITH JUST SOME LIGHT SHOWERS
POSSIBLE IN THE MOUNTAINS AND ON THE WINDWARD SLOPES.

A WET AND BREEZY TRADE WIND PATTERN WILL LIKELY DEVELOP STATEWIDE
SUNDAY NIGHT AND MONDAY...THEN PERSIST THROUGH AT LEAST THE
MIDDLE OF NEXT WEEK. A LINGERING MID/UPPER LEVEL TROUGH OVERHEAD
WILL MAINTAIN SOMEWHAT UNSTABLE CONDITIONS ALOFT WITH THE USUAL
MID LEVEL INVERSION LIKELY REMAINING WEAK OR ABSENT...AND MODEL
CONSENSUS SHOWS PLENTY OF AVAILABLE LOW LEVEL MOISTURE BEING
PUSHED AGAINST THE TERRAIN BY BRISK TRADE WIND FLOW.

THE LATEST ECMWF AND GFS DIVERGE SOMEWHAT TOWARD THE END OF NEXT
WEEK. BOTH MODELS KEEP STRONG TRADES IN PLACE...BUT THE EC PUSHES
A DRIER AIRMASS SOUTH ACROSS THE STATE WHILE THE GFS KEEPS
MOISTURE FURTHER NORTH IN RESPONSE TO A DEVELOPING LOW PRESSURE
AREA NEAR THE DATELINE.

&&

.AVIATION...
A WEAK FRONT/SURFACE TROUGH CONTINUES TO BE THE FOCUS FOR ENHANCED
SHOWER ACTIVITY. THE FRONT IS CURRENTLY POSITIONED OVER MAUI COUNTY
AND IS FORECAST TO MOVE SLOWLY EASTWARD...REACHING THE BIG ISLAND BY
EVENING. LOCALIZED HEAVY SHOWERS AND POSSIBLE LIGHTING ARE
ASSOCIATED WITH THIS FEATURE.

TEMPO MVFR CONDS AND SCATTERED SHOWERS ARE EXPECTED TO PERSIST
ACROSS MAUI COUNTY THROUGH SATURDAY NIGHT. GRADUAL IMPROVEMENT IS
EXPECTED ON OAHU WHERE SHOWERS WILL BECOME ISOLATED BY NOON AND VFR
CONDS WILL PREDOMINATE. KAUAI...WELL WITHIN THE DRIER NORTHERLY
FLOW...WILL SEE ONLY PATCHY VFR CLOUDS AND ISOLATED SHOWERS.

ON THE BIG ISLAND...AFTERNOON BREEZES WILL ENCOURAGE CUMULUS BUILDUPS
AND ISOLATED SHOWERS OVER THE ISLAND/S INTERIOR. THEN TOWARDS
EVENING TEMPO MVFR CONDS ARE EXPECTED TO DEVELOP ACROSS WINDWARD
BIG ISLAND IN RESPONSE TO THE APPROACHING FRONT/TROUGH AND THE
REASSERTION OF EASTERLY FLOW.

THE HIGH CLOUDS BETWEEN 13000 AND 30000 FT OAHU SOUTHEASTWARD ARE
ASSOCIATED WITH A SLOW MOVING UPPER LEVEL TROUGH. THIS HIGH LEVEL
OVERCAST IS EXPECTED TO PERSIST ACROSS THE EASTERN HALF OF THE STATE
THROUGH SATURDAY NIGHT AND BEYOND. TEMPO LIGHT ICE WITHIN THIS LAYER
UP TO FL250.

&&

.MARINE...
THE FRIDAY EVENING ASCAT PASS FOUND N/NE WINDS FROM OAHU WESTWARD
BEHIND THE FRONT...WITH SOME 20 KT WINDS AROUND KAUAI. LIGHT NE/E
WINDS PREVAILED OVER THE EASTERN AND CENTRAL PARTS OF THE STATE.
NE WINDS WILL SPREAD SLOWLY EASTWARD AND STRENGTHEN TODAY AND
TONIGHT...THEN INCREASE FURTHER ON SUNDAY WITH A GOOD CHANCE FOR
SMALL CRAFT ADVISORY CONDITIONS IN THE CHANNELS. BREEZY TRADES
APPEAR LIKELY TO CONTINUE THROUGH NEXT WEEK...WITH SCA CONDITIONS
IN THE NORMALLY WINDIER SPOTS POSSIBLY BECOMING MORE WIDESPREAD
DURING THE SECOND HALF OF THE WEEK...WHEN THE TRADES MAY INCREASE
ANOTHER NOTCH.

EASTERLY SWELL AROUND 5 FT AND 10 SECONDS WILL REMAIN THE
DOMINANT SOURCE OF SEAS ACROSS MOST AREAS TODAY...THOUGH FRESH
NORTHERLY WINDS DEVELOPING AROUND KAUAI AND OAHU WILL PRODUCE
SOMEWHAT LARGER SEAS IN THOSE AREAS. SURF ALONG EAST FACING SHORES
WILL REMAIN BELOW HIGH SURF ADVISORY LEVELS AS THE EAST SWELL
GRADUALLY DECLINES DURING THE WEEKEND. SURF ALONG NORTH FACING
SHORES WILL REMAIN LOW THROUGH SUNDAY. A NORTH-NORTHWEST SWELL
WILL PUSH NORTH SHORE SURF CLOSE TO MINIMAL ADVISORY LEVELS LATE
SUNDAY NIGHT AND MONDAY. THIS SWELL WILL PEAK MONDAY THEN FADE
AWAY THROUGH MIDWEEK. SEE THE COLLABORATIVE NEARSHORE SWELL AND
WIND FORECAST /SRDHFO/ FOR MORE DETAILS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

DISCUSSION/MARINE...JACOBSON
AVIATION...BEDAL

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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