Flash Flood Warning 28, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST MON MAY 9 2011

.SYNOPSIS...
UNSETTLED WEATHER WILL CONTINUE THROUGH TUESDAY...AS AN UPPER LEVEL
TROUGH KEEPS THE ATMOSPHERE UNSTABLE. LOCALLY HEAVY SHOWERS AND
ISOLATED THUNDERSTORMS ARE POSSIBLE. BY THURSDAY...TRADE WINDS AND
MORE PLEASANT WEATHER SHOULD RETURN.

&&

.DISCUSSION...
EXCEPT FOR A LOT OF THUNDERSTORMS AROUND NIIHAU AND KAUAI...ISLAND
WEATHER IS DECEPTIVELY QUIET. THIS DOES NOT MEAN THAT THE FLOOD
THREAT IS OVER HOWEVER...BECAUSE THE ATMOSPHERE IS STILL WET AND
UNSTABLE...AND WILL REMAIN SO AT LEAST THROUGH TUESDAY AFTERNOON.

NOTHING HAS CHANGED IN THE OVERALL GAME PLAN. AN UPPER LEVEL LOW
NORTHWEST OF THE STATE WILL MOVE TO THE NORTH...AND BY WEDNESDAY
WILL BE INCORPORATED INTO A MID-LATITUDE LOW ABOUT 1000 MILES
NORTHWEST OF KAUAI. BECAUSE OF THE LOW/S SLOW MOVEMENT...IT WILL
TAKE AT LEAST ANOTHER DAY FOR THE AIRMASS OVER THE ISLANDS TO BECOME
SLIGHTLY MORE STABLE.

MEANWHILE...A SURFACE LOW IS FORMING NORTHWEST OF KAUAI IN A NORTH
TO SOUTH ORIENTED TROUGH. THE LOW WILL FOLLOW THE UPPER LEVEL LOW
AND MOVE TO THE NORTH...BUT THE TROUGH WILL REMAIN WEST OF THE
STATE. THE MOVEMENT OF THE LOWS AND THE LACK OF MOVEMENT OF THE
SURFACE TROUGH...THOUGH SOMEWHAT COMPLEX...CAN BE SIMPLIFIED TO THE
FOLLOWING. SOUTH AND SOUTHEAST LOW LEVEL FLOW SOUTHEAST OF THE LOWS
AND THE TROUGH WILL CONTINUE TO DRAW ABUNDANT MOISTURE UP OVER THE
STATE. COMBINED WITH COLD TEMPERATURES ALOFT AND THE RESULTANT
DESTABILIZED ATMOSPHERE...THE CHANCE OF HEAVY SHOWERS AND
THUNDERSTORMS IS STILL PRESENT. THE FLASH FLOOD WATCH THEREFORE
REMAINS IN EFFECT THROUGH TUESDAY AFTERNOON.

THE 6 PM HST TUESDAY END OF THE FLASH FLOOD WATCH IS NOT ABSOLUTE.
THE WATCH MAY IN FACT NEED TO BE EXTENDED INTO WEDNESDAY IF THE
ATMOSPHERE REMAINS MORE UNSTABLE THAN EXPECTED. IF THE WATCH IS
EXTENDED...IT IS LIKELY THAT IT WILL BE LIMITED TO KAUAI SINCE IT IS
CLOSEST TO THE LOWS AND TROUGH. WILL BE LOOKING AT THE POSSIBLE
EXTENSION OVERNIGHT.

MODELS AGREE THAT THE SURFACE TROUGH WILL START TO MOVE WEST AWAY
FROM THE STATE ON WEDNESDAY...ALLOWING A RIDGE TO BUILD SLOWLY TO
OUR NORTH. AS THIS HAPPENS...THE TRADE WINDS WILL RETURN...AND THE
MOISTURE WILL DECREASE. BY THE END OF THE WORKWEEK...IT IS LIKELY
THAT PLEASANT WEATHER WILL FINALLY RETURN TO THE ISLANDS.

&&

.MARINE...
A MARINE WEATHER STATEMENT REMAINS POSTED FOR MOST HAWAIIAN COASTAL
WATERS FOR POSSIBLE THUNDERSTORMS AND HEAVY SHOWERS...AND THE
ACCOMPANYING LIGHTNING...GUSTY WINDS AND LOCALLY HAZARDOUS SEAS.
ALSO...A WATERSPOUT OR TWO WILL BE POSSIBLE.

THE SOUTH SWELL THAT ARRIVED SUNDAY WILL PERSIST THROUGH TUESDAY.

&&

.AVIATION...
AIRMET ZULU REMAINS UP FOR MODERATE RIME ICING IN CLOUDS IN A LAYER
BETWEEN 12000 FEET AND 22000 FEET OVER KAUAI AND THE ADJACENT
WATERS. HEAVY SHOWERS AND THUNDERSTORMS REMAIN OVER AND AROUND THE
GARDEN ISLE WHICH MAY RESULT IN SOME IFR AND MVFR CONDITIONS.
OTHERWISE VFR CONDITIONS PREVAIL AROUND THE STATE WITH BRIEF
LOCALIZED IFR AND MVFR CONDITIONS IN ISOLATED MODERATE TO HEAVY
SHOWERS.

&&

.HYDROLOGY...
THE THREAT OF HEAVY RAIN WILL CONTINUE THROUGH TUESDAY. WITH MOIST
AND UNSTABLE AIR OVERHEAD...THE THREAT OF FLOODING WILL CONTINUE NOT
ONLY IN AREAS WHERE IT IS RAINING...BUT ALSO DOWNSTREAM. FOR THOSE
AREAS THAT HAVE ALREADY RECEIVED RAIN...THE GROUND IS SATURATED AND
ANY FUTURE RAIN COULD EASILY LEAD TO FLASH FLOODING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH UNTIL 6 PM HST TUESDAY FOR NIIHAU...KAUAI...
OAHU...MOLOKAI...LANAI...KAHOOLAWE AND MAUI.

WINTER WEATHER ADVISORY UNTIL 6 AM HST TUESDAY FOR THE SUMMITS OF
MAUNA KEA AND MAUNA LOA.

&&

DISCUSSION/MARINE/HYDROLOGY...CRAIG
AVIATION...BALLARD