Heavy Rain in Hawaii, November 22 to 23, 2004

HAWAIIAN ISLANDS FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST MON NOV 22 2004

.SYNOPSIS...
SURFACE TROUGHING WILL KEEP WINDS LIGHT AS AN UPPER LEVEL LOW
CONTINUES TO SLIDE SOUTHWARD ACROSS THE ISLANDS. COLD AIR ALOFT
ACCOMPANYING THIS UPPER LOW WILL DESTABILIZE THE ATMOSPHERE...
SETTING OFF HEAVY SHOWERS AND THUNDERSTORMS FROM THE BIG ISLAND
THROUGH OAHU...AND POSSIBLY KAUAI. THE HEAVIEST RAINFALL WILL
OCCUR ON THE BIG ISLAND OF HAWAII THROUGH TONIGHT...AS TROPICAL
MOISTURE IS ADDED INTO THE MIX THERE.

&&
.DISCUSSION...
HAVE JUST ISSUED A WINTER STROM WATCH FOR THE SUMMITS OF MAUNA KEA
AND MAUNA LOA ON THE BIG ISLAND DUE TO INCREASING DEEP CONVECTION IN
THE AREA. GRIDS AND ZONES WERE UPDATED TO INCLUDE THIS HEADLINE...
BUT NO OTHER CHANGES TO THE FORECAST WERE INCORPORATED AT THIS TIME.
HEAVY SHOWERS ARE AFFECTING WINDWARD BIG ISLAND...WITH THUNDERSTORMS
DEVELOPING NORTHEAST CLOCKWISE THROUGH SOUTHEAST OF THE ISLAND. THE
UPPER LOW IS BEHAVING AS EXPECTED...PROGRESSIVELY MOVING TOWARDS THE
SOUTH AT 10 TO 15 MPH...CURRENTLY CENTERED JUST NORTH OF MAUI. WHILE
MOST OF THE HEAVY RAIN IS CURRENTLY NEAR THE BIG ISLAND...SHOWERS
ARE INCREASING ALONG A WEAK BOUNDARY THAT EXTENDS NORTHWARD FROM
OAHU. DEVELOPING CONVECTION ALONG WINDWARD BIG ISLAND WILL RESULT IN
LOCALLY HEAVY DOWNPOURS. CELLS WILL BE MONITORED FOR POSSIBLE
TERRAIN ANCHORING...WHICH MAY RESULT IN LOCALIZED FLOODING. THE
RADAR AT SOUTH KOHALA/UPOLU POINT IS INCONVENIENTLY OUT OF
SERVICE...SO SATELLITE AND RAINGAGE DATA...AS WELL AS RADAR DATA
FROM SOUTH POINT...WILL BE UTILIZED TO DETERMINE THE EXTENT OF THE
HEAVY RAINFALL.

.PREV DISCUSSION...
AN UPPER LOW NEAR 24N 156W...ABOUT 215 MILES NORTHEAST OF
HONOLULU...AT 200 PM HAS BEEN MOVING SOUTH SOUTHWEST AROUND 15 MPH.
A WEAK SURFACE LOW HAS BEEN NOTED IN ASSOCIATION WITH THIS UPPER LOW
NEAR 25N 156W. THUNDERSTORMS ASSOCIATED WITH THIS SYSTEM HAVE BEEN
DRAWN NORTHWARD MAINLY TO THE EAST AND SOUTH OF THE BIG ISLAND OF
HAWAII. THIS UPPER LOW IF FORECAST TO CONTINUE MOVING SOUTHWARD OVER
THE NEXT 30 HOURS...TO BE NEAR 23N 157W OR NEAR OAHU AND MOLOKAI...
BY TUESDAY EVENING. AS THE UPPER LOW SINKS SOUTHWARD IT WILL LIKELY
CONTINUE PULLING WARM MOIST AIR UP FROM THE TROPICS AND WRAPPING
THIS MOISTURE AROUND ITS NORTHEAST QUADRANT. THIS PUTS THE BIG
ISLAND OF HAWAII IN THE MOST LIKELY POSITION FOR HEAVY RAIN AND
THUNDERSTORMS THROUGH TUESDAY AND INTO TUESDAY NIGHT. THUS A FLASH
FLOOD WATCH HAS BEEN ISSUED FOR THE BIG ISLAND OF HAWAII FOR TONIGHT
AND TUESDAY. THIS WATCH MAY BE EXTENDED TO SOME OR ALL OF THE OTHER
ISLANDS TUESDAY IF NECESSARY.

ISOLATED CONVECTION HAS DEVELOPED FROM MOISTURE IN A CONVERGENCE
BAND EXTENDING SOUTHWEST FROM THE UPPER LOW AND OVER KAUAI AND OAHU.
THESE SHOWERS ARE ON THE LIGHT TO MODERATE SIDE THIS AFTERNOON AND
SHOULD END AFTER DARK. HOWEVER...AS THE UPPER LOW MOVES DOWN OVER
THE CENTRAL HAWAIIAN ISLANDS LATE IN THE DAY TUESDAY INTO TUESDAY
NIGHT...SCATTERED SHOWERS...SOME HEAVY...WITH ISOLATED THUNDERSTORMS
WILL ALSO AFFECT THE OTHER ISLANDS OF MAUI COUNTY AND OAHU...AND
POSSIBLY KAUAI...AS WELL. THESE SHOWERS WILL ALSO BE BRIEFLY
HEAVY...BUT ARE NOT EXPECTED TO CAUSE FLASH FLOODING AT THE PRESENT
TIME. HOWEVER...THIS SITUATION WILL BE CLOSELY WATCHED FOR FURTHER
THREATS OF FLASH FLOODING.

THE DEEP UNSTABLE MOISTURE WRAPPING AROUND THIS UPPER LOW WILL
EVENTUALLY BE PULLED WESTWARD OVER ALL THE HAWAIIAN ISLANDS...
LIKELY LATE TUESDAY NIGHT INTO WEDNESDAY. FORECAST CHARTS ARE
TENDING TO WEAKEN THE UPPER LOW AND KICK IT OFF TO THE EAST OF THE
STATE ON THANKSGIVING DAY...WITH SOME CLEARING LIKELY...AT LEAST BY
AFTERNOON ACROSS THE STATE.

.LONG TERM...AS THE UPPER LOW WEAKENS AND MOVES OFF TO THE EAST...
TRADE WINDS ARE EXPECTED TO BUILD BACK OVER THE STATE THROUGH THE
COMING WEEKEND AND INTO NEXT WEEK. WITH PLENTY OF MOISTURE OFF TO
OUR EAST WITH THE DISTURBANCE IN THE UPPER ATMOSPHERE...EXPECT
TRADES TO BE ON THE WET SIDE INTO EARLY NEXT WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FOR BIG ISLAND OF HAWAII TONIGHT AND TUESDAY.
WINTER STORM WATCH FOR BIG ISLAND SUMMITS TUESDAY THROUGH
THANKSGIVING DAY.

BIRCHARD