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

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Hawaiian Islands Synoptic Discussion and Guidance

598
FXHW60 PHFO 042008
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST FRI SEP 4 2015

.SYNOPSIS...
A HUMID AND UNSTABLE AIR MASS WILL SUPPORT SPOTTY...BUT LOCALLY
VERY HEAVY...SHOWERS AND THUNDERSTORMS THROUGH THIS AFTERNOON. WEAK
TRADE WINDS AND SOMEWHAT MORE STABLE CONDITIONS WILL SPREAD FROM THE
EAST TONIGHT AND SATURDAY...THOUGH THE WEATHER WILL REMAIN HOTTER
AND MORE HUMID THAN USUAL. PREVAILING WINDS WILL WEAKEN EARLY NEXT
WEEK...LEADING TO STAGNANT CONDITIONS AND INCREASED AFTERNOON
SHOWERS AGAIN.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL TROUGH NEAR KAUAI/OAHU...
WITH A RIDGE JUST EAST OF THE BIG ISLAND. HURRICANE IGNACIO IS FAR
TO THE NORTH-NORTHWEST OF THE STATE...AND JIMENA IS FAR TO THE EAST.
ELSEWHERE AT THE SURFACE...A TROUGH IS LOCATED JUST WEST OF KAUAI.
HIGH PRESSURE IS LOCATED FAR NORTH NORTHEAST OF THE STATE...WITH A
RIDGE EXTENDING BACK ACROSS THE ISLANDS. THE 12Z LIHUE SOUNDING
SHOWS AN UNSTABLE PROFILE WITH AN UNMODIFIED MIXED-LAYER CAPE OVER
1600 J/KG. THE 12Z HILO SOUNDING IS SOMEWHAT DRIER AND MORE
STABLE...WITH A PRECIPITABLE WATER OF 1.6 INCHES VS 2.0 INCHES AT
LIHUE...AND A WEAK INVERSION AROUND 5KFT. EARLY MORNING MIMIC TOTAL
PRECIPITABLE WATER IMAGERY IS STILL ON THE HIGH SIDE COMPARED TO THE
RAOBS...BUT SHOWS QUALITATIVELY DRIER AIR BETWEEN THE BIG ISLAND AND
ABOUT 151W.

MANY AREAS STARTED THE MORNING FREE OF LOW CLOUDS...AIDED BY
OFFSHORE LAND BREEZES OVERNIGHT...THOUGH CONVECTION WAS ACTIVE
OFFSHORE. CONDITIONS ARE ALREADY STARTING TO CLOUD UP THIS
MORNING...WITH ACTIVITY FILLING IN BY AFTERNOON. INSTABILITY IS
GREATEST ACROSS THE WESTERN HALF OF THE STATE...CLOSER TO THE UPPER
LEVEL SUPPORT AND GREATER MOISTURE POOLED ALONG THE SURFACE TROUGH.
BOTH THE GLOBAL AND HIGH-RESOLUTION MESOSCALE MODELS SHOW THIS BEING
THE MAIN FOCUS AREA FOR ACTIVITY TODAY...AS LARGE SCALE CONVERGENCE
WORKS WITH THE AVAILABLE INSTABILITY TO SPARK ADDITIONAL CONVECTION.
THE HIGH-RESOLUTION WRF-ARW AND WRF-NMM RUNS FROM NCEP ALSO SHOW
ADDITIONAL AFTERNOON CONVECTION ACROSS THE INTERIOR OF THE BIG
ISLAND...AND HAVE UPDATED TO ADD A SLIGHT CHANCE OF THUNDERSTORMS
THERE AS WELL.

THE GFS AND ECMWF SHOW THE THE UPPER LEVEL TROUGH NORTHWEST OF THE
STATE EVOLVING INTO A CUTOFF LOW OVER THE WEEKEND. THIS LOW WILL
EVENTUALLY SHIFT WESTWARD EARLY NEXT WEEK...BUT THE LOWER HEIGHTS
AND INVERTED SURFACE TROUGH JUST WEST OF THE STATE WILL REMAIN
NEARBY FOR MUCH OF THE WEEKEND. FORECAST SOUNDINGS AND CROSS
SECTIONS SHOW LITTLE CHANGE IN THE ATMOSPHERE OVER THE ISLANDS OVER
THE NEXT COUPLE DAYS...WITH DEEP SATURATION OVER THE WESTERN HALF OF
THE STATE AND DRIER MID-LEVELS AND WEAK INVERSIONS OVER THE EAST. IT
IS NOT UNTIL EARLY NEXT WEEK THAT THE LOW SHIFTS WEST...ALLOWING
HEIGHTS TO REBUILD OVER THE STATE AND BRINGING IN A SURGE OF DRIER
AIR. THE FLASH FLOOD WATCH MAY NEED TO BE EXTENDED FOR KAUAI/OAHU...
BUT WILL LIKELY BE ABLE TO BE DROPPED AFTER TODAY FOR MAUI COUNTY
AND THE BIG ISLAND. WILL SEE HOW CONVECTIVE ACTIVITY PROGRESSES
THROUGH THE DAY AND MAKE THE DETERMINATION ABOUT THE WESTERN ISLANDS
WITH THE AFTERNOON PACKAGE.

WINDS WILL STILL BE RELATIVELY LIGHT OVER THE ISLANDS...WITH A
SURFACE RIDGE NEARLY STATIONARY OVER US. LOW-LEVEL FLOW WILL BE
GENERALLY EASTERLY OR NORTHEASTERLY NEAR THE BIG ISLAND AND
SOUTHEASTERLY NEAR KAUAI. THE WEAK BACKGROUND FLOW WILL STILL ALLOW
FOR LOCALIZED ONSHORE SEA BREEZES...WITH CONVERGENCE AND AFTERNOON
CLOUDS/SHOWERS FOCUSED ACROSS INTERIOR AND LEEWARD SECTIONS. PARTIAL
CLEARING IS EXPECTED AT NIGHT...BUT THE WEAK BACKGROUND FLOW SHOULD
BE STRONG ENOUGH TO KEEP SOME CLOUDS IN PLACE ACROSS WINDWARD
SECTIONS.

HURRICANE JIMENA IS EXPECTED TO TRACK NORTH-NORTHWEST FOR THE NEXT
COUPLE DAYS...THEN TAKE A TURN WESTWARD BY THE END OF THE FORECAST
PERIOD. SEE THE BULLETINS FROM THE CENTRAL PACIFIC HURRICANE CENTER
FOR THE LATEST DETAILS. INTO OUR EXTENDED PERIOD...JIMENA /OR THE
REMNANTS OF JIMENA/ WILL LIKELY CONTINUE WESTWARD AS IT WEAKENS.
GFS ENSEMBLE MEMBERS SHOW A WIDE SPREAD IN TRACK...WHETHER IT HEADS
NORTHWESTERLY OR SOUTHWESTERLY. A TRACK OF THE SYSTEM NORTH OF THE
ISLANDS WOULD CUT OF THE WINDS AGAIN...AND BRING ANOTHER SURGE OF
MOISTURE ACROSS US. IF IT TRACKS CLOSER TO THE ISLANDS WE COULD SEE
MORE SIGNIFICANT IMPACTS...SO RESIDENTS SHOULD STAY UP TO DATE WITH
THE FORECAST FOR NEXT WEEK.

&&

.AVIATION...
AN AREA OF LOW LEVEL CONVERGENCE OVER THE WESTERN PART OF THE
STATE....COMBINED WITH AN UPPER LEVEL TROUGH AND COPIOUS TROPICAL
MOISTURE...MAY GENERATE SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS
THIS AFTERNOON. AREAS OF MVFR AND LOCALIZED IFR WILL BE POSSIBLE
WITHIN THE HEAVIER SHOWERS AND THUNDERSTORMS.

NO AIRMETS ARE CURRENTLY IN EFFECT...BUT AIRMET SIERRA FOR MOUNTAIN
OBSCURATION WILL LIKELY BE NEEDED FOR SOME OF THE ISLANDS LATER
TODAY.

&&

.MARINE...
INCOMING EAST SWELL FROM HURRICANE JIMENA CONTINUED TO REQUIRE A
HIGH SURF WARNING FOR EAST-FACING SHORES OF MOST ISLANDS. EAST-SHORE
SURF WILL REMAIN NEAR WARNING LEVELS THROUGH THE WEEKEND...AND
LIKELY INTO EARLY NEXT WEEK.

A SMALL CRAFT ADVISORY ALSO REMAINED IN EFFECT FOR HAZARDOUS SEAS
PRODUCED BY THE SWELLS FROM IGNACIO AND JIMENA. WINDS ARE EXPECTED
TO REMAIN BELOW THE ADVISORY THRESHOLD THIS WEEK.

A LONG-PERIOD SWELL FROM THE SOUTH-SOUTHWEST WILL PRODUCE
ADVISORY-LEVEL SURF ALONG SOUTH-FACING SHORES THIS WEEKEND. PLEASE
REFER TO THE LATEST COLLABORATIVE NEARSHORE SWELL AND WIND FORECAST
/SRDHFO/ FOR MORE DETAILS ON THE SWELLS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH UNTIL 6 PM HST THIS EVENING FOR ALL ISLANDS.

HIGH SURF WARNING UNTIL 6 PM HST SATURDAY FOR EAST FACING SHORES OF
KAUAI...OAHU...MOLOKAI...MAUI...AND BIG ISLAND.

HIGH SURF ADVISORY UNTIL 6 PM HST SATURDAY FOR SOUTH FACING SHORES
OF NIIHAU...KAUAI...OAHU...MOLOKAI...LANAI...KAHOOLAWE...MAUI...AND
BIG ISLAND.

SMALL CRAFT ADVISORY UNTIL 6 PM HST THIS EVENING FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI CHANNEL-OAHU
WINDWARD WATERS-KAIWI CHANNEL-MAUI COUNTY WINDWARD WATERS-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND WINDWARD WATERS-
BIG ISLAND SOUTHEAST WATERS.

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...FOSTER




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