Heavy Rain in Hawaii, December 18 to 19, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST THU DEC 18 2008

.SYNOPSIS...
A STORM SYSTEM WEST OF THE STATE WILL PRODUCE A THREAT OF HEAVY
SHOWERS MAINLY OVER KAUAI AND NIIHAU OVERNIGHT...WITH OAHU SEEING
INCREASING CLOUDS AND SHOWERS. THE REMAINING ISLANDS WILL EXPERIENCE
INTERMITTENT SHOWERS MOVING IN FROM THE SOUTHEAST. THE STORM SYSTEM
WILL MOVE WESTWARD TOMORROW LEAVING THE ISLANDS UNDER SOUTHEAST
WINDS THROUGH THE WEEKEND. TRADE WINDS ARE EXPECTED TO RETURN BY THE
MIDDLE OF NEXT WEEK.
&&

.DISCUSSION...
THE PRIMARY CONCERN THROUGH TONIGHT WILL BE THE IMPRESSIVE
CONVECTION WEST OF THE STATE ASSOCIATED WITH THE KONA LOW ANCHORED
WELL TO THE NW...AND HOW FAR EASTWARD IT REACHES. THE CONVECTION IS
TAKING PLACE ALONG A CONVERGENT BAND ORIENTED SOUTH TO NORTH ABOUT
60 NM WEST OF KAUAI. THERE ARE ISOLATED SHOWERS AND THUNDERSTORMS
MOVING NORTHWARD AHEAD OF THE LINE RECEIVING SOME EXTRA ISENTROPIC
FORCING WHERE THE LOW LEVEL FLOW REMAINS NORMAL TO THE THETA-E
GRADIENT. BOTH AREAS APPEAR TO BE IN AN AREA OF FAIR UPPER LEVEL
DIVERGENCE IN THE PROXIMITY OF THE RIGHT REAR ENTRANCE REGION OF A
90 KT JET MAX. GUIDANCE CONTINUES TO KEEP A SERIES OF LOW TO MID
LEVEL VORT MAXES WEST OF THE AREA AS THEY MOVE NWD...WHICH SUGGEST
THE HEAVIEST RAIN SHOULD REMAIN WEST OF FORECAST AREA.
HOWEVER...CONVECTION HAS ALREADY PROGRESSED FURTHER EAST THAN
MODELS SUGGEST.  DUE TO THE CONTINUED SLOW EWD MOVEMENT OF THE
CONVECTIVE LINE WILL KEEP THE POSSIBILITY OF HEAVY SHOWERS AND
ISOLATED TSTORMS IN THE FORECAST FOR NIIHAU AND KAUAI FOR NOW. WILL
ADJUST THE FORECAST THROUGH THE SHORT TERM AS THE CONVECTION EVOLVES
FURTHER. THE REMAINDER OF THE STATE WILL REMAIN IN ESE FLOW WITH
ISOLATED SHOWERS OVERNIGHT WITH THE SMALL CRAFT ADVISORY CONTINUING
THROUGH 6AM TOMORROW MORNING FOR ZONES MENTIONED IN W/W/A BELOW.

THERE HAS BEEN NO CHANGE TO THE REMAINDER OF THE FORECAST IN THE
PAST 4 HOURS.

&&

.PREV DISCUSSION 400 PM HST...
THE MAIN FORECAST ISSUE CONTINUES TO BE THE KONA STORM CURRENTLY
CENTERED 740 MILES WEST OF KAUAI. THE HEAVIEST SHOWERS AND
THUNDERSTORMS CONTINUE TO FORM WITHIN THE SOUTHEAST THROUGH EAST
QUADRANTS OF THE STORM...INCLUDING TWO IMPRESSIVE THUNDERSTORM
AREAS 240 AND 500 MILES SOUTHWEST OF KAUAI. CLOSER IN...A BAND OF
HEAVY SHOWERS LIES BETWEEN 80 AND 140 MILES WEST OF KAUAI. THIS BAND
HAS NOT MOVED MUCH CLOSER TO NIIHAU AND KAUAI OVER THE PAST 6 TO 8
HOURS. ON THE OTHER SIDE OF THE STATE...BUILDING HIGH PRESSURE IS
KEEPING THE KONA LOW AT BAY...PREVENTING OR AT THE VERY LEAST
SLOWING ITS EASTWARD MOTION.

AT UPPER LEVELS OF THE ATMOSPHERE...A LOW ALOFT IS PROVIDING AMPLE
INSTABILITY TO SUSTAIN THE KONA LOW. SATELLITE DERIVED WINDS SHOW A
VERY HEALTHY DIVERGENCE AREA ALMOST DIRECTLY OVER WHERE THE HEAVIEST
SHOWERS AND THUNDERSTORMS ARE OCCURRING. ALOFT AND EAST OF THE
ISLANDS...A SHARP UPPER ATMOSPHERIC RIDGE HAS PREVENTED THE UPPER
LOW FROM MOVING ANY CLOSER TO THE ISLANDS. FORECAST MODELS ARE IN
GOOD AGREEMENT THE STORM WILL WOBBLE AROUND IN ITS CURRENT LOCATION
OVER THE NEXT 24 HOURS BEFORE STARTING TO MOVE WESTWARD AND AWAY
FROM THE ISLANDS. SATELLITE IMAGES AND OTHER DATA CURRENTLY SUPPORT
THIS SCENARIO.

THE FORECAST CHALLENGE OVERNIGHT IS WHETHER AN INCOMING
SHORTWAVE...CURRENTLY WRAPPING UP ON THE SOUTHWEST SIDE OF THE LOW
ALOFT...WILL HAVE ENOUGH PUNCH TO NUDGE THE KONA LOW FURTHER
EASTWARD...THEREBY MOVING HEAVY SHOWERS AND THUNDERSTORMS OVER
NIIHAU AND KAUAI. FORECAST MODELS SUGGEST OTHERWISE...INDICATING THE
SHORTWAVE WILL MERGE WITH THE MAIN LOW ALOFT AND ACTUALLY NUDGE THE
SYSTEM TOWARD THE WEST. NONETHELESS...BELIEVE THE THREAT STILL
EXISTS AND WILL MAINTAIN THE POSSIBILITY OF LOCALLY HEAVY SHOWERS
AND A SLIGHT CHANCE OF THUNDERSTORMS AROUND NIIHAU AND KAUAI
OVERNIGHT. STILL DO NOT BELIEVE THE CHANCE IS HIGH ENOUGH OVER KAUAI
AND NIIHAU TO WARRANT A FLASH FLOOD WATCH. RAINFALL ESTIMATES OVER
KAUAI DO NOT TAKE INTO ACCOUNT THE HEAVY SHOWERS AND THUNDERSTORMS
DUE TO THE UNCERTAINTY OF THESE ACTUALLY MOVING OVER THE ISLAND.
LIGHT TO MODERATE SHOWERS CONTINUE TO FORM AROUND NIIHAU AND EXPECT
THESE SHOWERS WILL SPREAD TO KAUAI THIS EVENING. OAHU WILL SEE
INCREASING CLOUDS AND SCATTERED SHOWERS MOVING IN WITH THE SOUTHEAST
FLOW AND DO NOT ANTICIPATE ANYTHING HEAVY. THE REMAINING ISLANDS
WILL BE PARTLY TO MOSTLY CLOUDY WITH MAINLY ISOLATED SHOWERS.

ONCE THE KONA STORM STARTS TO MIGRATE WESTWARD TOMORROW... THE
ISLANDS WILL REMAIN SANDWICHED BETWEEN LOW PRESSURE TO THE WEST AND
HIGH PRESSURE TO THE EAST...TRANSLATING INTO SOUTHEAST FLOW THROUGH
THE WEEKEND AND INTO NEXT WEEK. THE SOUTHEAST FLOW WILL MAINTAIN
MUGGY CONDITIONS ACROSS THE STATE. OUR FRIENDS THE TRADE WINDS WILL
RETURN FROM VACATION BY THE MIDDLE OF NEXT WEEK.

&&

.MARINE...
A SMALL CRAFT ADVISORY FOR STRONG AND GUSTY SOUTHEAST WINDS AROUND
THE BIG ISLAND AND THE LEEWARD MAUI COUNTY WATERS WILL REMAIN
OVERNIGHT. SATELLITE DERIVED WINDS SHOW SPEEDS TO 25 KTS IN THESE
AREAS. AS THE LOW PRESSURE SYSTEM STARTS TO MOVE WEST AND AWAY FROM
THE ISLANDS TOMORROW...EXPECT THE WINDS TO DECREASE BELOW ADVISORY
LEVELS AND REMAIN SOUTHEAST THROUGH THE WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST FRIDAY FOR MAUI COUNTY
LEEWARD WATERS-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

&&

DISCUSSION/MARINE...DEJESUS/BIRCHARD
PREVIOUS...TANABE