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Hawaiian Islands Synoptic Discussion and Guidance

517
FXHW60 PHFO 040211
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
411 PM HST TUE MAR 3 2015

.SYNOPSIS...
HIGH PRESSURE TO THE NORTHWEST WILL KEEP MODERATE TO FRESH TRADE
WINDS BLOWING OVER THE ISLANDS TONIGHT...BECOMING STRONGER THROUGH
FRIDAY. A TROUGH ALOFT NEAR THE STATE WILL RESULT IN HEAVY RAIN
AND THUNDERSTORMS ON THE BIG ISLAND AND MAUI THROUGH THURSDAY.
TRADE WINDS WILL FOCUS RAINFALL OVER THE WINDWARD SLOPES...WITH
SOME SHOWERS CARRYING OVER TO LEEWARD AREAS.

&&

.DISCUSSION...
A 1038 MB SURFACE HIGH IS CENTERED 1500 MILES NORTHWEST OF KAUAI
AND IS PRODUCING MODERATE TO FRESH TRADE WINDS ACROSS THE ISLANDS.
A BROAD TROUGH ALOFT IS LOCATED NORTH OF THE STATE WHILE A WEST
TO EAST ORIENTED JET STREAM RUNS ALONG THE BASE OF THIS TROUGH.
INFRARED SATELLITE SHOWS THE JET STREAM CARRYING A THICK BAND OF
LAYERED AND HIGH CLOUDS OVER HAWAII FROM THE WEST LATE THIS
AFTERNOON.

CURRENT RADAR SHOWS THUNDERSTORMS OVER THE LEEWARD AND INTERIOR
SECTION OF THE BIG ISLAND. THESE THUNDERSTORMS WERE TRIGGERED BY A
COMBINATION OF DAYTIME HEATING...INSTABILITY CAUSED BY THE
INCOMING TROUGH ALOFT AND ABUNDANT MOISTURE.

THE BROAD AND DEEP TROUGH ALOFT WILL PERSIST NORTH OF THE AREA
FOR AT LEAST THE NEXT WEEK. A SHORT WAVE TROUGH WITHIN THE LARGER
TROUGH WILL APPROACH THE ISLANDS FROM THE NORTHWEST TONIGHT. AS THE
TROUGH APPROACHES...THE ATMOSPHERE WILL BECOME MORE UNSTABLE AND
THE DEEP LAYERED MOISTURE WILL SPREAD OVER THE ISLANDS. THE FOCUS
OF THIS MOISTURE AND INSTABILITY WILL BE ACROSS MAUI AND THE BIG
ISLAND THROUGH WEDNESDAY NIGHT. THUS...WE EXPECT HEAVY RAIN WITH
EMBEDDED THUNDERSTORMS TONIGHT THROUGH WEDNESDAY NIGHT. A FLASH
FLOOD WATCH HAS BEEN ISSUED FOR THE BIG ISLAND AND MAUI AND A
WINTER STORM WARNING HAS BEEN ISSUED FOR THE BIG ISLAND SUMMITS...BOTH
PRODUCTS GO THROUGH WEDNESDAY NIGHT. CONCERNING THE FLASH FLOOD
WATCH...WHILE BOTH ISLANDS ARE ENTIRELY INCLUDED WITHIN THE
WATCH...TRADE WINDS WILL FOCUS THE MORE PERSISTENT RAIN ACROSS THE
WINDWARD SLOPES. STRONG WESTERLY WINDS ALOFT WILL STRENGTHEN OVER
THE BIG ISLAND AS THE SHORTWAVE DEPARTS ON THURSDAY...AND MAY
RESULT IN ADVISORY OR WARNING LEVEL WINDS FOR THE BIG ISLAND
SUMMITS FROM THURSDAY INTO NEXT WEEK.

FOR KAUAI...OAHU AND THE REMAINDER OF MAUI COUNTY...TRADE WINDS
WILL STRENGTHEN OVER THE NEXT COUPLE OF DAYS AS THE SURFACE HIGH
FAR NORTHWEST OF THE STATE STRENGTHENS AND MOVES CLOSER. THE
TRADES WILL USHER IN SCATTERED SHOWERS THROUGH THURSDAY WITH THE
FOCUS OVER THE WINDWARD SLOPES. WE ANTICIPATE OVERALL LESS
MOISTURE ACROSS THE WESTERN HALF OF THE STATE...THUS LESS SHOWER
COVERAGE AND LIGHTER PRECIPITATION CAN BE EXPECTED. TRADE WINDS
WILL REMAIN BREEZY THROUGH FRIDAY NIGHT WHEN THE HIGH BEGINS TO
TRACK BACK TO THE NORTH.

THE WEEKEND COULD BRING ANOTHER ROUND OF HEAVY RAIN AND
THUNDERSTORMS OVER THE EASTERN HALF OF THE STATE. WE WILL CONTINUE
TO MONITOR FOR MORE DETAILS IN THE COMING DAYS.


&&

.AVIATION...
THE AFTERNOON CONVECTION ON THE BIG ISLAND SLOPES...LEADING TO CB
TOPS TO FL300 AND LIGHTNING...WILL BEGIN TO DIMINISH WITH SUNSET.
ELSEWHERE...SHOWERS EMBEDDED WITHIN THE TRADE WIND FLOW CONTINUES TO
FOCUS OVER WINDWARD AND MAUKA AREAS. AIRMET SIERRA IS POSTED FOR
MOUNTAIN OBSCURATION OVER N THRU E SECTIONS OF OAHU AND MAUI. THESE
MORE ACTIVE SHOWERS ARE EXPECTED TO CONTINUE OVER THE NEXT FEW
PERIODS. THE EASTERN ISLANDS...ESPECIALLY WINDWARD BIG ISLAND AND
WINDWARD MAUI...WILL LIKELY EXPERIENCE HEAVY SHOWERS...POSSIBLY
SEEING PREVAILING MVFR WITH ISOL TO TEMPO IFR CONDITIONS STARTING
LATE TONIGHT. AIRMET SIERRA WILL LIKELY BE EXPANDED LATER THIS
EVENING. THE POTENTIAL FOR THUNDERSTORMS REMAINS OVER THE BIG ISLAND
THIS EVENING AND THROUGH WEDNESDAY NIGHT AND OVER MAUI COUNTY LATE
TONIGHT.

THE UPPER LEVEL JET OVER THE ISLANDS IS CARRYING ENOUGH SPEED SHEAR
TO KEEP AIRMET TANGO POSTED FOR TEMPO MOD TURB BETWEEN FL250 AND
FL400. ADDITIONALLY...A THICK MID TO UPPER LEVEL CLOUD DECK PASSING
OVER THE ISLANDS ALONG THE JET. THEY MAY BE THICK ENOUGH TONIGHT TO
PROMPT THE ISSUANCE OF AIRMET ZULU FOR LIGHT TO MOD RIME ICING ABOVE
130.


&&

.MARINE...
WINDS WILL REMAIN BELOW SMALL CRAFT ADVISORY SPEEDS TONIGHT BUT
THE WINDS WILL STRENGTHEN TOMORROW AS A STRONG HIGH BUILDS
NORTHWEST OF THE AREA. THIS WILL LIKELY REQUIRE A SMALL CRAFT
ADVISORY FOR STRONG TRADE WINDS AND ROUGH SEAS BY WEDNESDAY. THE
BREEZY TRADE WINDS WILL PRODUCE ROUGH SURF ALONG EAST FACING
SHORES AS WELL OVER THE NEXT SEVERAL DAYS...POSSIBLY RESULTING IN
A HIGH SURF ADVISORY LATER IN THE WEEK. OTHERWISE SURF IS
EXPECTED TO REMAIN BELOW THE ADVISORY THRESHOLDS.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH LATE WEDNESDAY NIGHT FOR MAUI AND THE
BIG ISLAND.

WINTER STORM WARNING UNTIL 6 AM HST THURSDAY FOR BIG ISLAND
SUMMITS.


&&

$$

DISCUSSION/MARINE...FOSTER
AVIATION...EATON



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