Special Marine Warning 21, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST SAT MAR 18 2006

.SYNOPSIS...
AN UPPER TROUGH TO THE WEST OF THE ISLANDS WILL CONTINUE THROUGH THE
FORECAST PERIOD. THIS WILL KEEP WINDS GENERALLY LIGHT WITH A FEW
LOCALLY HEAVY SHOWERS FORMING MAINLY DURING THE AFTERNOON AND
EVENING HOURS FAVORING MAUKA AND CENTRAL AREAS OF THE ISLANDS.

&&

.DISCUSSION...
AN UPPER TROUGH REMAINS OFF TO THE WEST OF THE MAIN ISLAND CHAIN.
THIS TROUGH REMAINS OFF TO THE NORTHWEST...AND THE GFS STARTS TO
DEVELOP A CUTOFF IN THE MID AND UPPER LEVELS LATE ON
SUNDAY...HOWEVER SOUTHWEST FLOW CONTINUES ACROSS THE ISLAND CHAIN.
VERY LITTLE CHANGE IS FORECASTED IN THE MID TO UPPER LEVELS. THIS
SUPPORTS A CONTINUATION OF THE CONVECTIVE PATTERN OVER THE STATE OF
HAWAII.

A BOUNDARY REMAINS OVER THE WESTERN ISLANDS AND IS WELL NOTED THIS
MORNING AS CONVECTION IS DEVELOPING ALONG IT. THE GFS DEVELOPS A LOW
SLOWLY...ORGANIZES AND SLIDES NORTHEAST TOWARDS THE ISLAND CHAIN.
THE OTHER MODELS SOLUTIONS ARE...THE NAM AND ECMWF JUST LEAVES A
WEAK TROUGH OVER THE WESTERN ISLANDS...THE NOGAPS DEVELOPS THE
SURFACE LOW WELL NORTH OF A KAUAI. ALL THE MODELS AGREE THAT A
BOUNDARY REMAINS IN PLACE OVER THE WESTERN ISLANDS. AGREE WITH HPC
THAT THE GFS IS TRYING TO OVER-DO THE SOLUTION FOR THE SURFACE
LOW...SPINNING IT INTO A CLOSED CIRCULATION. FEEL THAT GFS DOESN/T
HAVE A GOOD HANDLE ON IT. GOING TO LEAN TOWARDS ECMWF FOR THE UPPER
PATTERNS THROUGH THE FORECAST PERIOD...AND IN THE SHORT TERM GOING
TO HAVE TO RELY ON THE SATELLITE TO PICK UP ON THE MINOR FEATURES
DRIVING THE CONVECTION AND EXTRAPOLATE THEM FOR THE FORECAST.

FOR THIS MORNING WILL HAVE TO UPDATE THE FORECAST FOR CURRENT TRENDS
ACROSS THE STATE. CONVECTION IS FIRING ALONG THE BOUNDARY WHICH IS
ALIGNED FROM SOUTHWEST OF KAUAI...OVER THE ISLAND EXTENDING
NORTHEASTWARD. THE CONVECTION SHOULD BE ENHANCED BY THE SHORTWAVE
WHICH IS DIGGING INTO THE UPPER TROUGH. AS THIS WAVE PASSES THROUGH
THE AREA IT STRENGTHENS. THE LOW LEVEL CONVERGENCE ALONG THE SURFACE
BOUNDARY WILL BE THE FOCUS WHERE THE CONVECTION WILL FIRE. THE MAIN
PROBLEM WILL BE THE MOVEMENT OF THE BOUNDARY IF ANY EAST OR WEST
FROM IT/S CURRENT LOCATION. IF THE BOUNDARY REMAINS IN PLACE KAUAI
COULD BE IN FOR MORE CONVECTION AND POSSIBLE HEAVY RAINFALL OVER THE
REGION. THIS MORNING/S SOUNDING OUT OF LIHUE IS A LITTLE MORE STABLE
THAN IN THE RECENT HISTORY...HOWEVER WITH THAT SAID STILL IS
SUPPORTING AMPLE CONVECTIVE AVAILABLE POTENTIAL ENERGY...AND FAIR
CONVECTIVE PARAMETERS. EVEN THE HILO SOUNDING IS UNSTABLE THIS
MORNING. A RECENT ACARS SOUNDING ON AN OUTBOUND FLIGHT FROM HONOLULU
INTERNATIONAL SHOWS AN UNSTABLE ATMOSPHERE FOR THE CENTRAL ISLANDS.
GOING TO KEEP THE CURRENT SCATTERED WORDING IN THE FORECAST FOR
SHOWERS AND UPDATE TO ADD A CHANCE OF THUNDER THIS AFTERNOON WITH
LOCALLY HEAVY RAINFALL POSSIBLE.

FLASH FLOOD WATCH IS SET TO EXPIRE THIS AFTERNOON. THE HIGHEST
PROBABILITY OF FLOODING IS OVER THE WESTERN ISLANDS...BUT THE
REMAINING ISLANDS ARE NOT OUT OF THE WOODS WITH ALL THE INSTABILITY
AROUND. THE QUESTION REMAINING WITH THE FORECAST TEAM IS WHETHER TO
EXTEND THE FLOOD WATCH...AND FOR WHICH REGIONS. MAUI COUNTY AND THE
BIG ISLAND HAVE NOT SEEN ANY FLOODING IN THE PAST FEW DAYS. THE
CONVECTIVE PATTERN FAVORS MAUKA AND CENTRAL REGIONS OF THE ISLANDS
AND LESS ON THE WINDWARD. THEREFORE EVEN OAHU IS QUESTIONABLE SINCE
THE WINDWARD AREA ISN/T AS LIKELY TO RECEIVE THE HEAVY RAIN AS THE
CENTRAL AND LEEWARD OAHU. WILL HOLD OFF UNTIL THIS AFTERNOON FOR ANY
CHANGES TO THE WATCH. DO EXPECT THAT THE WESTERN ISLANDS WILL REMAIN
IN THE WATCH...HOWEVER COULD ALLOW THE WATCH TO EXPIRE FOR THE
EASTERN AND CENTRAL ISLANDS.

TONIGHT THROUGH MONDAY...THE UPPER TROUGH CONTINUES TO SPIN IMPULSES
THROUGH THE FLOW TOWARDS THE ISLANDS AND THE SURFACE BOUNDARY WILL
MOVE VERY LITTLE THEREFORE EXPECT MORE OF THE SAME.

TUESDAY THROUGH THE REMAINDER OF THE WEEK...THE UPPER TROUGH REMAINS
TO THE WEST AND THE SURFACE BOUNDARY WILL CONTINUE OVER THE ISLAND
CHAIN. THIS WILL LEAD TO MORE SHOWERS AND LOCALLY HEAVY RAINFALL AT
TIMES...THEREFORE LITTLE CHANGE IS EXPECTED.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...

FLASH FLOOD WATCH...ALL ISLANDS THROUGH THIS AFTERNOON.

&&

PROTON