Dust Devil in Hawaii, April 21, 2010

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE HONOLULU HI
930 AM HST WED APR 21 2010

.SYNOPSIS...
HIGH PRESSURE TO THE NORTH OF THE STATE WILL CONTINUE TO GENERATE
TRADE WINDS THROUGH THE REST OF THIS WEEK AND INTO THE NEXT. THE
STRENGTH OF THE TRADE WINDS WILL BE TURNING LIGHTER FRIDAY AND
REMAIN IN THE LIGHT TO MODERATE RANGE INTO NEXT WEEK. AN UPPER
LEVEL TROUGH MOVING THROUGH THE ISLANDS DURING THE NEXT 12 TO 36
HOURS WILL ENHANCE THE TRADE WIND SHOWERS BENEATH IT. THE SMALLER
ISLANDS WILL EXPERIENCE SOME FREQUENT PASSING SHOWERS THANKS TO THE
BREEZY TRADES. SHOWER ACTIVITY WILL TREND BACK TO NORMAL FREQUENCY
BY FRIDAY NIGHT AS A MORE STABLE AIR MASS SETTLES OVER THE AREA.

&&

.DISCUSSION...
A SURFACE HIGH FAR NORTHEAST OF THE STATE WILL CONTINUE ON ITS
EASTWARD MOVEMENT. THIS ACTION WILL RELAX THE PRESSURE GRADIENT
ACROSS THE HAWAIIAN ISLANDS STARTING THURSDAY NIGHT. THE HIGH HAS
A RIDGE EXTENDING WESTWARD...AND THIS RIDGE WILL BE SUPPRESSED
SOUTHWARD TO JUST A COUPLE OF HUNDRED MILES NORTH OF KAUAI BY
SATURDAY NIGHT. THE TRADES WILL STRENGTHEN JUST A TAD SUNDAY NIGHT
AS A NEW SURFACE HIGH APPROACHES THE AREA NORTH OF KAUAI FROM THE
WEST.

IN THE MEAN TIME...THE TRADES WILL BE MODERATE TO STRONG...AND
SHOWERS STAYING PRETTY MUCH OVER THE WINDWARD AND MOUNTAIN AREAS.
SATELLITE IMAGERY REVEAL A BAND OF SHOWERY LOOKING CLOUDS STRETCHED
ACROSS THE ISLAND/S WINDWARD WATERS...LIKELY THE REMNANTS OF A OLD
FRONT OR SHEAR LINE. SO...DURING THE NEXT 6 TO 12 HOURS...THESE
SHOWERY CLOUDS WILL GLANCE THE WINDWARD AREAS OF THE SMALLER ISLANDS
EXCEPT KAUAI WHERE IT IS SITTING IN THE DIRECT PATH OF THESE
SHOWERS. THE LEEWARD DISTRICTS OF ALL THE SMALLER ISLANDS WILL
EXPERIENCE A FEW PASSING SHOWERS AIDED BY THE BREEZY TRADES.

SHOWERS WILL BECOME MORE FREQUENT STARTING AROUND SUNSET AND LASTING
THROUGH MOST OF THURSDAY. THE LEEWARD SECTIONS WILL NOT BE IMMUNE.
THE SHOWERS WILL BE QUITE COMMON OVER MOST LEEWARD COMMUNITIES. THIS
IS DUE TO LOW LEVEL MOISTURE UPWIND OF THE OLD SHEAR LINE...COMING
IN WITH THE TRADE WIND FLOW AND BEING ENHANCED BY AN UPPER LEVEL
TROUGH MOVING FROM THE NORTH AND NORTHWEST.

THE UPPER LEVEL TROUGH WILL ALSO AFFECT THE BIG ISLAND...NOT
TODAY...BUT THURSDAY. THE UPPER LEVEL TROUGH WILL LIFT THE INVERSION
OVER THE BIG ISLAND TO 18000 FEET BY THURSDAY AFTERNOON...JUST IN
TIME FOR SOME AFTERNOON CONVECTION AND SCATTERED SHOWERS ACROSS THE
ISLAND/S LEEWARD DISTRICTS. THE SUMMITS MAY ALSO RECEIVE A COUPLE OF
SHOWERS. WE WILL BE KEEPING AN EYE ON THE TEMPERATURES THERE AS THE
GFS SOLUTION IS SUGGESTING TEMPS NEAR FREEZING.

THE UPPER LEVEL TROUGH IS SLATED TO EASE SOUTH AND EAST OF THE BIG
ISLAND FRIDAY NIGHT. AT THAT POINT...WE WILL SEE THE TRADE SHOWERS
RETURNING TO THEIR NORMAL FREQUENCY. ADDITIONALLY...WITH THE CALMER
TRADES...THE SHOWERS WILL BE RESTRICTED TO THE WINDWARD AND MOUNTAIN
AREAS.

&&

.AVIATION...
SATELLITE IMAGERY SHOWS MOUNTAIN WAVES DOWN WIND OF THE KOOLAU MTNS
OF OAHU. THIS IS AN INDICATION OF LOW LEVEL TURBULENCE. SO THE
AIRMET FOR TURBULENCE WILL REMAIN IN EFFECT THROUGH TODAY AND LIKELY
BE EXTENDED THROUGH TONIGHT.

THE SHOWERS AFFECTING THE WINDWARD AND MOUNTAIN AREAS CONTAINS SOME
BRIEF MVFR CONDITIONS.

&&

.MARINE...
THE SMALL CRAFT ADVISORY /SCA/ REMAINS IN EFFECT UNTIL 6 AM THURSDAY
FOR THE TYPICALLY WINDIER COASTAL ZONES...NEAR THE BIG ISLAND AND
MAUI COUNTY. WE WILL LIKELY START SCALING BACK THE AREAS UNDER THE
SCA THEREAFTER TO WHERE ALL HAWAIIAN WATERS BE FREE OF THE SCA BY
FRIDAY.

THE CURRENT NORTHWEST SWELL WILL BE REINFORCED BY A NEW NORTHWEST
SWELL STARTING TONIGHT. THIS NEW SWELL MAY BE BIG ENOUGH WHERE SURF
MAY REACH THE ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES
THURSDAY. A SERIES OF SOUTH SWELLS IS EXPECTED TO REACH THE ISLANDS
FROM THURSDAY TO EARLY NEXT WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST THURSDAY FOR MAALAEA
BAY-PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-
BIG ISLAND SOUTHEAST WATERS.

&&

LAU