Flash Flood Warning 29, NWS Honolulu, HI

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
540 AM HST MON DEC 31 2007

.SYNOPSIS...INCREASED SHOWER ACTIVITY CAN BE EXPECTED FROM OAHU
TO THE BIG ISLAND THIS MORNING AS A WEAK TROUGH LINGERS OVER THE
CENTRAL ISLANDS. A WEAK SHEAR LINE NEAR KAUAI TODAY WILL GRADUALLY
WORK ITS WAY DOWN THE ISLAND CHAIN AS IT DISSIPATES OVER THE CENTRAL
ISLANDS DURING THE MIDDLE OF THE WEEK. DRIER AIR WILL FILTER
THROUGH THE ISLANDS BEHIND THE SHEAR LINE WITH NORTHEAST WINDS
PREVAILING THROUGH THE WEEK. A TYPICAL TRADE WIND PATTERN OF
WINDWARD AND MAUKA SHOWERS IS EXPECTED FOR THE START OF 2008.

&&

.UPDATE...UPDATED FORECASTS TO INCLUDE FLASH FLOOD WATCH HEADLINE
FOR MAUI COUNTY AND THE BIG ISLAND. AT 530 THIS MORNING...SHOWER
ACTIVITY LOOKS TO BE TAPERING OFF ON OAHU. LOW LEVEL CIRCULATION
REMAINS IN THE VICINITY OF MOLOKAI. LOCALLY HEAVY SHOWERS CONTINUE
TO LINGER AROUND MAUI COUNTY AND WILL CONTINUE TO AFFECT
LANAI...MOLOKAI AND MAUI UNDER VERY MOIST CONDITIONS TODAY. AN
UPPER SHORTWAVE SWINGING OVERHEAD IS A POSSIBLE MECHANISM THAT CAN
ENHANCE SHOWER ACTIVITY TODAY. LOCAL TERRAIN EFFECTS AND DAYTIME
HEATING ARE OTHER FACTORS THAT WILL COME INTO PLAY. BIG ISLAND WAS
POSTED MAINLY FOR THE KONA SIDE AS MOIST SOUTHWEST FLOW UPSLOPE
COULD TRIGGER HEAVY SHOWERS AND THE POTENTIAL FOR FLOODING.

&&

.DISCUSSION...THE WEATHER MADE A TURN FOR THE WORSE OVERNIGHT WITH
HEAVY RAINS FALLING ACROSS THE CENTRAL ISLANDS. A VARIETY OF
FACTORS CAME INTO PLAY WHEN THIS ACTIVE WEATHER FLARED UP. AN
INTERESTING PATTERN WAS NOTICED AT THE LOWER LEVELS WHICH WAS SEEN
ON RADAR AND SATELLITE IMAGERY. A LOW LEVEL CIRCULATION WITH
ITS CENTER BETWEEN OAHU AND MOLOKAI WAS EVIDENT. WEATHER ON KAUAI
WAS QUIET WITH A STABLE NORTHERLY FLOW IN PLACE. MOST OF THE ACTION
WAS FROM OAHU TO MAUI COUNTY AND TO SOME EXTENT ON THE BIG ISLAND.
SHOWERS WERE MOVING IN A CYCLONIC FASHION AROUND THE APPARENT
CENTER OF CIRCULATION. CLOUD MOTION WAS TOWARD THE EAST...NORTH OF
MOLOKAI AND OAHU...WHILE CLOUD MOTION WAS WESTWARD SOUTH OF THESE
ISLANDS. LINGERING INSTABILITY AND AN APPROACHING SHORTWAVE TROUGH
ALOFT HELPED TO ENHANCE SHOWER ACTIVITY OVER THE ISLANDS.

SEVERAL RAIN GAGES ON THE WINDWARD SIDE OF OAHU REPORTED RAINFALL
RATES OF 1 TO 3 INCHES PER HOUR. RADAR DERIVED RAIN RATES WERE NEAR
5 INCHES PER HOUR IN SOME LOCATIONS. FLOOD ADVISORIES WERE INITIALLY
ISSUED FOR OAHU AND PORTIONS OF MOLOKAI...LANAI AND MAUI. FURTHER
HEAVY RAINFALL PROMPTED THE ISSUANCE OF FLASH FLOOD WARNINGS FOR
WINDWARD OAHU AND THE EAST SIDE OF MAUI. THE STREAM GAGE AT OHEO
GULCH QUICKLY REACHED A STAGE OF 7 FEET DURING THE TIME OF HEAVIEST
ACTIVITY. MOST RAIN GAGE SITES BETWEEN PUNALUU AND OLOMANA ON THE
WINDWARD SIDE OF OAHU CAME IN WITH 2 TO 3 INCHES OF RAINFALL DURING
A 3 HOUR PERIOD ENDING AT 2 AM THIS MORNING.

FORECAST MODELS PICKED UP ON THIS SURFACE TROUGH/CIRCULATION AND
SHOWS IT WEAKENING AS IT GETS SHIFTED SOUTH. SHOWERS ASSOCIATED
WITH THIS FEATURE WILL CONTINUE TODAY...AT LEAST FOR THE MORNING
HOURS. AT 330 AM...MOST OF THE SERIOUS ACTIVITY HAS QUIETED DOWN.
ALTHOUGH THERE ARE STILL SHOWERS OUT THERE AND ANY OF THEM CAN BE
ENHANCED AND PRODUCE LOCALLY HEAVY RAINFALL IN SOME AREAS. OAHU AND
MAUI COUNTY ALONG WITH THE BIG ISLAND WILL CONTINUE TO RECEIVE
SHOWERS THIS MORNING. LOCALLY HEAVY RAINFALL POSSIBLE THIS
MORNING...BUT AN IMPROVING TREND IS EXPECTED THIS AFTERNOON.

THE TAIL END OF A FRONTAL BOUNDARY ASSOCIATED WITH A DEEPENING LOW
IN THE GULF OF ALASKA WAS JUST NORTHWEST OF KAUAI THIS MORNING.
FORECAST MODELS CONTINUE TO SHOW THIS FEATURE DISSIPATING AS IT
SLOWLY MAKES ITS WAY TOWARD THE CENTRAL ISLANDS TUESDAY AND
WEDNESDAY. MODERATE NORTHEAST WINDS ARE EXPECTED TO SLOWLY FILTER
ACROSS THE ISLAND CHAIN AS THE SHEAR LINE PASSES. A HIGH WILL TAKE
UP A POSITION NORTH NORTHWEST OF THE STATE...NORTHEAST WINDS WILL
FILTER ACROSS THE ISLANDS AND TYPICAL TRADE WIND WEATHER IS EXPECTED
FOR FIRST WEEK OF 2008.

&&

.MARINE...THE KAILUA BAY BUOY WAS REPORTING SEAS OF 7-8 FEET THIS
MORNING. SEAS CONTINUE TO SUBSIDE ON THE EAST SIDE OF THE ISLAND
CHAIN. THE HIGH SURF ADVISORY HAS BEEN CANCELLED AND THE SMALL
CRAFT ADVISORY FOR HAZARDOUS SEAS WAS DROPPED AS WELL. MODERATE
NORTH TO NORTHEAST WINDS WILL REQUIRE A SMALL CRAFT ADVISORY TO
REMAIN POSTED FOR KAUAI NORTHWEST AND WINDWARD WATERS. A COUPLE OF
MODERATE SIZE NORTH NORTHWEST SWELLS ARE EXPECTED THIS WEEK WITH THE
FIRST ARRIVING TUESDAY AND THE SECOND ON THURSDAY. THIS MAY CAUSE
SEAS TO REMAIN AROUND THE 10 FOOT SMALL CRAFT ADVISORY THRESHOLD
ESPECIALLY OVER WATERS NORTH OF THE ISLANDS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

FLASH FLOOD WATCH FOR MAUI COUNTY AND BIG ISLAND.

SMALL CRAFT ADVISORY FOR KAUAI NORTHWEST AND WINDWARD WATERS.

&&

ELAU