Flash Flood Warning 36, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST TUE MAR 26 2013

.SYNOPSIS...
A PASSING UPPER LEVEL TROUGH IS BRINGING POTENTIAL FOR HEAVY
RAIN...THUNDERSTORMS AND FLASH FLOODING...MAINLY OVER KAUAI AND
OAHU THROUGH WEDNESDAY. A BRIEF PERIOD OF DRIER WEATHER WILL RETURN
ON THURSDAY...THEN AN APPROACHING TROUGH AND FRONTAL BOUNDARY SYSTEM
WILL BRING ANOTHER ROUND OF UNSETTLED WEATHER DURING THE WEEKEND.

&&

.DISCUSSION...
A BROAD UPPER LVL TROUGH IS BEING OBSERVED ON WV SATELLITE IMAGERY
APPROACHING THE ISLAND CHAIN FROM THE WEST. A SHORTWAVE FEATURE
MOVING ALONG THE BASE OF THE TROUGH IS LOCATED AROUND 300 MILES WNW
OF KAUAI AND PUSHING E AT 20 MPH. THIS IS BRINGING INCREASING
INSTABILITY ACROSS THE WESTERN EDGE OF THE ISLAND CHAIN...WITH KAUAI
EXPERIENCING WIDESPREAD RAIN COVERAGE DURING THE LAST COUPLE OF
HOURS. A FLASH FLOOD WARNING HAS BEEN ISSUED FOR KAUAI THROUGH 915
PM HST TONIGHT...ESPECIALLY FOR THE NORTHERN AND EASTERN HALF OF THE
ISLAND. NATIONAL WEATHER SERVICE DOPPLER RADAR DATA WAS SHOWING
RAINFALL RATE ESTIMATES OF 2 INCHES AN HOUR ACROSS MUCH OF KAUAI.
MEANWHILE...A WEAKENING FRONTAL BOUNDARY REMAINS NEARLY STATIONARY
JUST NORTH OF THE ISLANDS...WITH A WEAK SFC TROUGH NEAR OAHU.
THE COMBINATION OF THESE FEATURES WILL KEEP VERY FAVORABLE
CONDITIONS FOR RAIN...FLOODING POTENTIAL AND SOME POSSIBLE
THUNDERSTORM ACTIVITY LATER TONIGHT...MAINLY OVER KAUAI COUNTY AND
OAHU. A FLASH FLOOD WATCH ALSO REMAINS IN EFFECT FOR KAUAI COUNTY
AND OAHU THROUGH WEDNESDAY EVENING.

THE SHORTWAVE WNW OF THE ISLANDS SHOULD REACH KAUAI TONIGHT AFTER
MIDNIGHT...AND MOVING INTO THE CENTRAL PORTIONS OF THE ISLAND CHAIN
BY WEDNESDAY. ALTHOUGH THE CORE OF THE SHORTWAVE SHOULD REMAIN TO
THE N OF THE ISLANDS...LATEST SOUNDING DATA...MIMIC MOISTURE
ANALYSES AND MODEL CAPE DEPICT AN AIRMASS THAT WILL STILL PROVIDE
ENOUGH INSTABILITY TO JUSTIFY CARRYING CHANCES OF THUNDERSTORMS IN
TONIGHT/S FORECAST FOR KAUAI AND OAHU. A DRIER AIRMASS WILL KEEP
LOWER CHANCES OF RAIN OVER THE BIG ISLAND...BUT MAUI COUNTY COULD
SEE SOME ENHANCED SHOWER ACTIVITY ON WEDNESDAY.

A BRIEF DRIER PERIOD WILL ESTABLISH FROM THURSDAY THROUGH EARLY
FRIDAY...THEN MODELS SHOW ANOTHER SHORTWAVE TROUGH APPROACHING THE
ALOHA STATE DURING THE WEEKEND WITH A COLD FRONT PASSAGE POSSIBLY
BRINGING ANOTHER ROUND OF UNSETTLED WEATHER.

&&

.AVIATION...
AIRMET SIERRA FOR MTN OBSCURATION FOR KAUAI AND THE BIG ISLAND WILL
REMAIN POSTED THROUGH THE FORECAST PERIOD WITH THE POSSIBILITY OF
EXTENDING IT TO OTHER ISLANDS.

FREQ MVFR FOR PHLI WILL CONTINUE OVERNIGHT WITH THESE CONDITIONS
SPREADING E TO OAHU THEREAFTER. OTHERWISE...VFR WILL PREVAIL AT ALL
OTHER TERMINALS FROM MOLOKAI TO THE BIG ISLAND WITH BRIEF MVFR
CONDITIONS AT PHTO.

SIGMET QUEBEC FOR HIGH LVL MOD-SEV CAT IS IN EFFECT IMT NW OF KAUAI
MOVING EWD. THE STRONGEST TURB LIKELY WILL NOT AFFECT THE LOCAL FA
HOWEVER AIRMET TANGO FOR AT LEAST MOD CHOP ALF IS LIKELY AFT 27/1000
UTC AS THE OFFENDING TROUGH NEARS HAWAII.

&&

.MARINE...
A NORTHWEST SWELL WILL CONTINUE TO SLOWLY DIMINISH THROUGH
WEDNESDAY. A NEW LONG PERIOD NORTHWEST SWELL ARRIVING LATE FRIDAY
WILL POTENTIALLY BRING SURF NEAR ADVISORY LEVELS FOR NORTH AND WEST
FACING SHORES OVER THE WEEKEND.

ALTHOUGH WINDS AND SEAS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT
ADVISORY LEVELS UNTIL THE WEEKEND...MARINERS SHOULD REMAIN ALERT FOR
THE POTENTIAL FOR HAZARDOUS MARINE WEATHER...PRIMARILY TONIGHT AND
WEDNESDAY AROUND KAUAI AND OAHU...WITH THE GREATEST THREATS BEING
THUNDERSTORMS AND GUSTY ERRATIC WINDS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH WEDNESDAY AFTERNOON FOR NIIHAU KAUAI AND
OAHU.

&&

DISCUSSION/MARINE...REYNES
AVIATION...LUTU-MCMOORE