AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
944 AM HST WED MAY 11 2011

.SYNOPSIS...
THE UNSETTLED WEATHER WILL CONTINUE TO EASE OUT THROUGH THURSDAY.
EAST-SOUTHEASTERLY WINDS WITH PERIODS OF SEA AND LAND BREEZES WILL
PREVAIL THROUGH THE WEEKEND.

&&

.DISCUSSION...
STATIONARY MID/UPR LEVEL LOW PRESSURE RESIDES NW OF THE ISLANDS. A
WEAK ATTENDANT FRONTAL BOUNDARY/CONVERGENCE ZONE ALONG WITH
SUFFICIENT BUOYANCY IS MAINTAINING SCATTERED MODERATE SHOWERS OVER
THE WESTERN PORTION OF THE STATE. THE POSSIBILITY OF LOCALLY HEAVY
RAINFALL AND ISOLATED TSTORMS WILL CONTINUE FOR KAUAI/OAHU THROUGH
THIS AFTERNOON. THE BIG ISLAND/MAUI COUNTY ARE WEAKLY CAPPED BY A
MODEST ISOTHERMAL MIXED LAYER FROM 6-9 KFT. ISOLATED TSTORMS ALONG
THE BIG ISLAND SLOPES AND SNOW SHOWERS FOR THE SUMMITS ARE STILL
POSSIBLE THIS AFTERNOON.

THE UPPER LOW WILL LINGER NW OF THE STATE HAVING VARIABLE EFFECTS ON
RAINFALL COVERAGE/INTENSITY. THE ASSOCIATED SURFACE
REFLECTION/TROUGH W OF THE STATE AND HIGH PRESSURE TO THE NE WILL
MAINTAIN E-ESE FLOW WITH PERIODS OF LIGHT WINDS THROUGH THE WEEKEND.
SHOWERS SHOULD REMAIN PRIMARILY FOCUSED ON WINDWARD TERRAIN WITH
ISOLATED AFTERNOON LEEWARD/INTERIOR CONVECTION. ENHANCED
CONVERGENCE AND LINGERING INSTABILITY OVER THE FAR WESTERN PORTION
OF THE STATE/COASTAL WATERS COULD STILL PRODUCE PERIODS OF
WIDESPREAD MODERATE RAINFALL THROUGH SUN.

UNCERTAINTIES IN THE GFS/ECMWF SOLUTIONS APPEAR LATE IN THE WEEKEND
REGARDING THE PERSISTENCE/LOCATION OF THE SURFACE TROUGH W OF
HAWAII. THIS HAS SIGNIFICANT EFFECTS ON THE WESTWARD
EXTENT/STRENGTH OF THE SURFACE RIDGE AND RESULTANT LOCAL TRADE
FLOW. STILL EXPECTING LIGHT ESE FLOW TO PREVAIL INTO NEXT WEEK WITH
A HYBRID DIURNAL/TRADE POP REGIME. WILL BE MAKING SOME SUBTLE
ADJUSTMENTS TO THE GRIDS TO ACCOUNT FOR MODEL TRENDS.

&&

.PREVIOUS DISCUSSION.../ISSUED 330 AM HST WED MORNING MAY 11/
WE HAVE GONE AHEAD TO CANCEL THE FLASH FLOOD WATCH /FFW/ FOR OAHU
AND KAUAI COUNTIES...DUE TO THE LACK OF WIDESPREAD HEAVY SHOWERS AND
THUNDERSTORMS DURING THE PAST 36 HOURS. WE HAVE HAD SCATTERED
SHOWERS ON PARTICULARLY OAHU DURING THIS PERIOD BUT WE HAVE NOT HAD
WIDESPREAD HEAVY ACTIVITY. THIS CANCELLATION OF THE FFW DOES NOT
KEEP US FROM ISSUING A FLASH FLOOD WARNING IF ONE DOES ARISE.

ANOTHER REASON TO DROP THE FFW IS BECAUSE THE MORNING SOUNDINGS FROM
HILO INCLUDING SEVERAL AIRCRAFT SOUNDINGS...INDICATED A MORE STABLE
AIR MASS WITH AN INVERSION BETWEEN 5 AND 10 THOUSAND FEET EAST OF
KAUAI. THE PRESENCE OF THIS INVERSION WILL MINIMIZE IF NOT ELIMINATE
THE THREAT OF WIDESPREAD HEAVY SHOWERS AND THUNDERSTORMS. ON THE
OTHER HAND...KAUAI IS STILL UNSTABLE. WITH THIS IN MIND...WE WILL
GRADUALLY PHASE OUT THE TSTORMS. WE WILL REDRAW THE TSTORM LINE
FROM W OF OAHU TODAY TO W OF KAUAI AND NIIHAU TONIGHT.

WE HAVE ALSO DROPPED THE WINTER ADVISORY FOR THE BIG ISLAND SUMMITS.
THERE SHOULD BE ICE UP THERE NOW WITH AIR TEMPS BETWEEN 25 AND 30 F
WITH AN MODERATELY HIGH RH. BUT THERE SHOULD NOT BE ANY SIGNIFICANT
ACCUMULATION OF ICE. ALSO...DESPITE THE FORECAST FOR AN AFTERNOON
SNOW SHOWER...NO SIGNIFICANT ACCUMULATION OF SNOW IS EXPECTED.

THE FORECAST FOR THE WINDS ARE AS FOLLOWS...LIGHT VARIABLE TO LIGHT
SLY W OF MOLOKAI FOR TODAY AND TONIGHT...THEN LIGHT SE WINDS TO THE
WRN HALF OF THE STATE THURSDAY. ELSEWHERE...WILL HAVE LIGHT TO
MODERATE TRADES. BY SATURDAY...WE HAVE LIGHT TRADES FOR THE AREA W
OF MOLOKAI AND LIGHT TO MODERATE TRADES ELSEWHERE. ON SUNDAY...LIGHT
SE WINDS RETURNS TO KAUAI COUNTY. THEREAFTER...WE WILL SIDE WITH THE
ECMWF AND NOGAPS WHERE IT SUGGESTS LIGHT TO MODERATE TRADES.

BELIEVE THE TRADES WILL BE LIGHT ENOUGH TO PROMOTE LOCAL SEA BREEZES
DURING THE DAYTIME. BELIEVE THERE IS ENOUGH INSTABILITY TO SQUEEZE
OUT AN AFTERNOON SHOWER OR TWO ACROSS INTERIOR AND LEEWARD AREAS.

THE CURRENT SYNOPTIC PICTURE HAS A SURFACE LOW NEAR 31N164W OR 650
MILES NNW OF KAUAI. THE LOW CONTINUES TO MOVE NNW. THERE IS A TROUGH
EXTENDING S FROM THE LOW THROUGH 25N163W OR 270 MILES WNW OF KAUAI.
ALOFT...THE MAIN UPPER LOW IS NEAR 32N171W OR 1000 MILES NW OF
KAUAI. THERE IS A SECONDARY LOW IN THE SE QUADRANT MOVING NWD.

&&

.MARINE...
WEATHER OVER THE WESTERN COASTAL ZONES HAS SETTLED DOWN
SOME...HOWEVER THE CHANCES FOR ISOLATED TSTORMS WILL PERSIST THROUGH
TODAY. ANY DEVELOPING TSTORMS MAY SILL PRODUCE DANGEROUS LIGHTNING
AT TIMES. ELSEWHERE NO MARINE HAZARDS ARE EXPECTED IN THE SHORT
TERM. SURF WILL STAY ON THE MODEST SIDE FOR ALL SHORES OVER THE NEXT
SEVERAL DAYS.

&&

.AVIATION...
IN THE NEAR TERM...MAIN HAWAIIAN ISLAND TAFS LOOK FINE WITH VFR
CONDITIONS CONTINUING. CONDITIONS WILL INCLUDE MVFR CEILINGS AT
HIGHER ELEVATION INTERIOR TERMINALS...SUCH AS LANAI...BY THIS
AFTERNOON AS CLOUDS BUILDUP WITH SEA BREEZE CONVERGENCE. OTHER
TERMINALS WILL REMAIN VFR WITH LOCALIZED MVFR VISIBILITY POSSIBLE
WITH ANY AFTERNOON SHOWERS...AND THESE SHOWERS ARE MOST LIKELY OVER
KAUAI AND OAHU. CONDITIONS ARE EXPECTED TO IMPROVE AFTER SUNSET AS
LAND BREEZES DEVELOP.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$