Heavy Rain in Hawaii, May 22, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 AM HST THU MAY 22 2014

.SYNOPSIS...
LIGHT SOUTHEAST TO SOUTH WINDS WILL PREVAIL INTO THE WEEKEND...
WITH LAND AND SEA BREEZES OVER THE ISLANDS. THIS WIND PATTERN WILL
MAINTAIN WARM AND MUGGY CONDITIONS...AND SUPPORT CLOUDS AND SHOWERS
OVER ISLAND INTERIORS DURING AFTERNOONS AND EVENINGS...WITH SOME
CLEARING AT NIGHT. HOWEVER...AN AREA OF MOISTURE NEAR THE BIG ISLAND
THIS MORNING IS EXPECTED TO MOVE OVER KAUAI AND OAHU FROM THE SOUTH
TONIGHT THROUGH FRIDAY NIGHT...INTERRUPTING THE LAND AND SEA BREEZE
WEATHER PATTERN. ADDITIONALLY...HEAVY RAIN AND THUNDERSTORMS ARE
POSSIBLE ON KAUAI AND OAHU THIS WEEKEND AS THE MOIST ATMOSPHERE
BECOMES INCREASINGLY UNSTABLE. TRADE WINDS ARE EXPECTED TO RETURN BY
MONDAY...AND BECOME BREEZY ON TUESDAY AND WEDNESDAY...WITH A WET
TRADE WIND PATTERN FOR MOST OF NEXT WEEK...WITH SHOWERS FAVORING
WINDWARD AREAS.

&&

.DISCUSSION...
CURRENTLY...A SURFACE RIDGE OF HIGH PRESSURE EXTENDS SW FROM A HIGH
CENTERED FAR NE OF THE ISLANDS...WITH THE RIDGE ESTIMATED TO BE
WITHIN ABOUT 200 MILES N OF KAUAI. OVERNIGHT ASCAT PASS MISSED MOST
ISLAND WATERS...BUT SHOWED A LIGHT TO MODERATE ESE FLOW OVER WATERS
E OF MAUI AND THE BIG ISLAND...WITH ACCELERATION TO NEAR 20 KT ALONG
THE HAMAKUA COAST. MOST ISLAND LOCATIONS ARE REPORTING LIGHT WINDS...
WITH OFFSHORE FLOWING LAND BREEZES IN MANY AREAS. AN EXTENSIVE AREA
OF BROKEN TO OVERCAST LOW CLOUDS...WITH EMBEDDED SHOWERS...EXTENDS
FROM WINDWARD WATERS OF MAUI COUNTY SOUTHWARD TO OVER 400 MILES S
OF THE BIG ISLAND...WITH THIS MOISTURE TRACKING SLOWLY TOWARD THE NW
AND N. SHOWERS HAVE BEEN ESPECIALLY ACTIVE OVER THE KAU COAST AND
SLOPES AND ALONG THE WINDWARD HALEAKALA SLOPES OVERNIGHT...WITH A
FEW SHOWERS ALSO AFFECTING WINDWARD OAHU AND KAUAI.

FOR THE SHORT TERM...SHOWERS ARE EXPECTED TO PERSIST THROUGH MUCH OF
THE MORNING ALONG THE WINDWARD AND SE SLOPES OF THE ISLANDS...WITH
THE FOCUS FOR SHOWERS MOVING OVER ISLAND INTERIORS THIS AFTERNOON AS
ISLAND-SCALE SEA BREEZES DEVELOP UNDER THE LIGHT SYNOPTIC SCALE WIND
REGIME.

THE SURFACE RIDGE IS FORECAST TO SLIDE S TODAY AS LOW PRESSURE TO
THE DISTANT NW BEGINS TO INCH CLOSER TO THE ISLANDS...WITH THE LOW
LEVEL FLOW VEERING TO THE SE AND S THROUGH FRIDAY. THIS WILL KEEP
LIGHT WINDS IN PLACE...WITH A LAND AND SEA BREEZE WEATHER PATTERN
PREVAILING OVER THE ISLANDS...WITH SOME CAVEATS. ONE POTENTIAL
INTERRUPTION TO THE GENERAL CLEARING EXPECTED OVER LAND AT
NIGHT IS RELATED TO THE MOISTURE CLUMP NOW S AND SW OF THE BIG
ISLAND. GFS HAS INITIALIZED WELL WITH THIS MOISTURE...AND DEPICTS IT
ROUNDING THE RIDGE AND FUELING SHOWERS THAT MOVE OVER KAUAI AND OAHU
FROM THE S FROM TONIGHT THROUGH FRIDAY NIGHT. OTHERWISE...THE FOCUS
FOR CLOUDS AND SHOWERS SHOULD BE OVER THE INTERIOR AND NW PORTIONS
OF THE ISLANDS DURING THE AFTERNOONS. ALTHOUGH SKIES OVER THE
SMALLER ISLANDS HAVE BEEN RELATIVELY FREE OF VOG RECENTLY...THE
EXPECTATION IS THAT THE DEVELOPING SE TO S FLOW WILL BEGIN TO ADVECT
GREATER CONCENTRATIONS OF VOG OVER THE SMALLER ISLANDS BY THIS
AFTERNOON.

AS WE BEGIN THE WEEKEND...INCREASED MOISTURE IS FORECAST TO MOVE
OVER THE WESTERN PART OF THE ISLAND CHAIN...AS AN APPROACHING FRONT
STALLS NEAR KAUAI AND TRANSITIONS TO A N-S ORIENTED TROUGH. THIS
WILL KEEP A MOIST S FLOW OVER KAUAI AND OAHU...WITH A DRIER AIR MASS
IN PLACE OVER MAUI COUNTY AND THE BIG ISLAND. A CLOSED LOW ALOFT
DEVELOPING W AND NW OF THE ISLANDS WILL ACCOMPANY THIS INCREASED
MOISTURE...BRINGING THE THREAT OF HEAVY SHOWERS AND THUNDERSTORMS.
FORECAST PWAT VALUES...500 MB TEMPS AND LOW-MID LEVEL LAPSE RATES
INDICATE THE GREATEST POTENTIAL FOR UNSETTLED WEATHER OVER KAUAI AND
OAHU AND ADJACENT WATERS. UNCERTAINTY CONTINUES REGARDING TIMING AND
PLACEMENT OF THE POTENTIAL HEAVY RAIN/THUNDERSTORM THREAT...BUT
LATEST GUIDANCE SHOWS THE PEAK POTENTIAL CENTERED ON SATURDAY...
MAINLY FOR KAUAI...AND TO A LESSER EXTENT...OAHU. IF FORECAST MODELS
CONTINUE TO INDICATE THIS THREAT...THEN A FLASH FLOOD WATCH MAY BE
ISSUED LATER TODAY OR TONIGHT.

THE LOW-LEVEL TROUGH BEGINS TO WEAKEN AND SLIDE W OF THE ISLANDS
LATE THIS WEEKEND...AS THE CLOSED LOW ALOFT OPENS INTO AN E-W
ORIENTED TROUGH NEAR/OVER THE ISLANDS. A NEW AND RELATIVELY STRONG
HIGH IS FORECAST TO BUILD FAR N OF THE ISLANDS AT THE SAME TIME...
AND TRADE WINDS GRADUALLY RETURNING LATE SUNDAY WILL BECOME
ESTABLISHED MONDAY...AND BOLSTERED EVEN FURTHER ON TUESDAY AND
WEDNESDAY. THE PRESENCE OF THE LINGERING TROUGH ALOFT LIKELY SPELLS
A WET AND BREEZY TRADE WIND PATTERN FOR MOST OF NEXT WEEK.

&&

.AVIATION...
THE AREA OF LOW CLOUDS HAVE CLEARED WINDWARD MAUI COUNTY AND ITS
ADJACENT WATERS SOMEWHAT...THOUGH MORE CLOUDS HAVE REACHED THE
SOUTHERN PORTION OF MAUI COUNTY COASTAL WATERS OVERNIGHT. IN
FACT...SOME OF THESE CLOUDS MAY SPREAD TO SOUTHERN OAHU COASTAL
WATERS LATER THIS MORNING. SOME CLOUDS HAVE ALSO MOVED TO KAUAI
VICINITY EARLY THIS MORNING...WHILE LINGERING LOW CLOUDS CONTINUE TO
AFFECT PORTIONS OF WINDWARD AND SOUTH BIG ISLAND AND ADJACENT
COASTAL WATERS. REGARDLESS...MAINLY VFR CONDITIONS WILL PREVAIL OVER
THE SITES...THOUGH ISOL MVFR CEILING AND VIS CAN BE EXPECTED DUE TO
CLOUDS AND SHRA. MORNING HILO SOUNDING INDICATED LOW LEVEL MOISTURE
EXTENDING TO OVER 9 THOUSAND FEET.

THE LIGHT SOUTHEASTERLY BACKGROUND FLOW WILL CONTINUE ACROSS THE
STATE TODAY. NIGHT TIME LAND BREEZES WILL PEAK IN THE NEXT FEW
HOURS THEN SLOWLY SWITCH TO SEA BREEZES LATER UNDER INCREASING DAY
TIME HEATING.

&&

.MARINE...
WINDS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY /SCA/ LEVELS
THROUGH THE WEEKEND. SCA WINDS ARE EXPECTED OVER THE WINDIER MARINE
ZONES OF THE BIG ISLAND AND MAUI BY MONDAY...LIKELY LASTING FOR
MOST OF NEXT WEEK...AND POTENTIALLY EXPANDING TO OTHER ZONES.

SMALL TO MODERATE SWELLS FROM THE SOUTH WILL PERSIST UNTIL THE
WEEKEND...WHEN A SLIGHTLY LARGER AND LONGER PERIOD SOUTH SWELL WILL
BRING SURF HEIGHTS CLOSE TO ADVISORY LEVELS ALONG SOUTH FACING
SHORES. NO OTHER SIGNIFICANT SWELLS ARE EXPECTED THROUGH THE
FORECAST PERIOD.

&&

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

&&

DISCUSSION/MARINE...BIRCHARD
AVIATION...HUI