Flash Flooding and Heavy Rain in Hawaii, May 6 to 10, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
827 PM HST THU MAY 5 2011

.SYNOPSIS...
THERE IS A SLIGHT CHANCE OF THUNDERSTORMS TONIGHT AS ENHANCED
SHOWERS MOVE THROUGH THE STATE. BREEZY...WET TRADE WINDS WILL
CONTINUE THROUGH SATURDAY WITH HIGH PRESSURE PROVIDING THE
TRADES...AND INSTABILITY ALOFT ALLOWING FOR WET WINDWARD SHOWERS.
SOME SHOWERS WILL BLOW OVER TO LEEWARD LOCATIONS OF THE SMALLER
ISLANDS. THE LEEWARD BIG ISLAND CAN EXPECT AFTERNOON CONVECTIVE
SHOWERS. WINDS WILL DECREASE SUNDAY AS A TROUGH APPROACHES FROM
THE NORTHWEST...BRINGING UNSETTLED WEATHER TO THE STATE SUNDAY
AND INTO NEXT WEEK.

&&

.DISCUSSION...
AN AREA OF CLOUDS...SHOWERS AND ISOLATED THUNDERSTORMS IS APPROACHING
FROM THE EAST AND WILL BRING WET WEATHER TO THE MAJORITY OF THE
STATE TONIGHT...WITH A FOCUS OVER WINDWARD LOCATIONS. THE ISOLATED
THUNDERSTORMS CAN BE SEEN UPSTREAM OF THE ISLANDS WITH INFRARED
SATELLITE ESTIMATING CLOUD TOPS REACHING 30 THOUSAND FEET. AN
UPPER LEVEL TROUGH THAT HAS MAINTAINED AN UNSTABLE ATMOSPHERE FOR
THE PAST FEW DAYS...REMAINS IN PLACE OVER THE ISLANDS. THE
AFTERNOON SOUNDINGS SHOWED 500 MB TEMPERATURES OF -12 TO -14
DEGREES CELSIUS...AND THIS COLD AIR ALOFT IS PROVIDING THE
THERMODYNAMIC LIFT TO DRIVE THE WEAK THUNDERSTORMS AND ENHANCED
SHOWERS.

A 1030 MB HIGH PRESSURE LOCATED FAR NORTH OF THE STATE IS
PROVIDING THE BREEZY TRADE WINDS WILL REMAIN IN PLACE THROUGH
SATURDAY. THE UPPER TROUGH ALSO REMAINS...AND IT WILL CONTINUE
THE PRESENCE OF AN UNSTABLE ATMOSPHERE THROUGH THE WEEKEND AND
LINGER INTO NEXT WEEK. SO...BOTH LOW LEVEL MOISTURE AND
INSTABILITY ALOFT WILL CONTINUE THE WET TRADE WINDS INTO THE
WEEKEND. AFTERNOON CONVECTION WILL CONTINUE TO FIRE ALONG THE
LEEWARD SLOPES OF THE BIG ISLAND AS DAYTIME LAND HEATING AT HIGHER
ELEVATIONS IS ABLE TO TAP INTO THE INSTABILITY IN THE UPPER
ATMOSPHERE. A SLIGHT CHANCE OF AFTERNOON THUNDERSTORMS IS
FORECASTED FOR THE LEEWARD SLOPES THROUGH THE WEEKEND.

THE LINGERING UPPER TROUGH WILL CONTINUE TO ALLOW PRECIP TO REACH
THE BIG ISLAND SUMMITS...AND ANY PRECIPITATION TONIGHT WILL FALL
IN THE FORM OF FREEZING RAIN OR SNOW...CREATING HAZARDOUS DRIVING
CONDITIONS. THE WINTER WEATHER ADVISORY WILL BE IN EFFECT THROUGH
THE NIGHT AND WILL LIKELY BE EXTENDED INTO FRIDAY NIGHT AT THE 4AM
FORECAST ISSUANCE...AS SIMILAR CONDITIONS ARE EXPECTED.

TRADE WINDS ARE FORECASTED TO WEAKEN ON SUNDAY AND TAKE AN EASTERLY
DIRECTION WITH A SLIGHT SOUTHERLY COMPONENT. THE WEAKENING AND
TURNING OF THE TRADES IS DUE TO A SURFACE TROUGH APPROACHING FROM
THE NW. BOTH THE GFS AND ECMWF CONTINUE TO HAVE SOME DIFFICULTY
WITH THE STRENGTH AND PLACEMENT OF THIS TROUGH WITH RESPECT TO THE
ISLANDS ON SUNDAY AND INTO NEXT WEEK. HOWEVER...THEY HAVE BOTH
REMAINED CONSISTENT WITH THE IDEA OF THE TROUGH TURNING ISLAND
WINDS AND BRINGING INCREASED MOISTURE UP FROM THE TROPICS. THE
INSTABILITY FROM THE UPPER LOW AND INCREASED MOISTURE WOULD BRING
THE POSSIBILITY OF THUNDERSTORMS STATE-WIDE SUNDAY AND INTO THE
MIDDLE OF NEXT WEEK.

&&

.AVIATION...
UPPER LOW CONTINUES TO PRODUCE SOME ATMOSPHERIC INSTABILITY.
SCATTERED MODERATE TO LOCALLY HEAVY SHOWERS ARE CROSSING ISLANDS IN
THE FRESH TRADE WIND FLOW. TCU TOPS RANGE FROM 20 TO 25 THOUSAND
FEET. NIGHTTIME COOLING SEEMS TO HAVE PROVIDED NECESSARY CONVECTIVE
BOOST TO CREATE A FEW THUNDERSTORMS THAT MAY AFFECT THE SMALLER
ISLANDS TONIGHT. DAYTIME CONVECTIVE ACTIVITY EXPECTED TO BE WEAKER
UNDER THE TRADE WIND FLOW ACROSS THE SMALLER ISLANDS BUT AFTERNOON
HEATING OVER THE BIG ISLAND SLOPES GIVES THE CHANCE FOR
THUNDERSTORMS.

AIRMET FOR WINDWARD MOUNTAINS OBSCURED BY CLOUDS AND SHOWERS WILL
LIKELY BE ISSUED FOR THE SMALLER ISLANDS AND COULD CONTINUE INTO
MID MORNING BEFORE IMPROVING.

STRONG HIGH PRESSURE FAR TO THE NORTHEAST IS CAUSING 20 TO 25 KNOT
WINDS IN LOWER ATMOSPHERE MAINLY ACROSS SMALLER ISLANDS. HENCE
AIRMET FOR TURBULENCE TO THE LEE OF THE ISLAND MOUNTAINS WILL
CONTINUE.

&&

.MARINE...
SMALL CRAFT ADVISORY REMAINS FOR ALL WATERS DUE TO WINDS AND SEAS.

HIGH SURF ADVISORY FOR EAST SHORES OF MOST ISLANDS REMAINS AS THE
STRONG TRADE WINDS SEND ROUGH AND CHOPPY SURF INTO THE COASTS. THE
TRADE SWELL IS EXPECTED TO DIMINISH OVER THE WEEKEND.

A SMALL SOUTH SWELL WILL PERSIST THROUGH THE WEEKEND...FOLLOWED
BY A SLIGHTLY LARGER SOUTH SWELL EARLY NEXT WEEK. A SMALL NORTHWEST
SWELL WILL DIMINISH FRIDAY...AND FADE ON SATURDAY. ANOTHER SMALL
NORTHWEST SWELL MAY BOOST SURF INTO THE HEAD HIGH RANGE SUNDAY...THEN
DECLINE MONDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST FRIDAY FOR KAUAI WINDWARD-OAHU
KOOLAU-OLOMANA-MOLOKAI WINDWARD-MAUI WINDWARD WEST-WINDWARD
HALEAKALA-SOUTH BIG ISLAND-BIG ISLAND NORTH AND EAST.

WINTER WEATHER ADVISORY UNTIL 9 AM HST FRIDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR ALL HAWAIIAN
WATERS-

&&

PUBLIC...MORRISON
AVIATION...HOAG