Flash Flood and Heavy Rain in Hawaii, September 10 to 15, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
410 PM HST THU SEP 10 2015

.SYNOPSIS...
THE PREVAILING WARM AND HUMID CONDITIONS WILL CONTINUE AS LIGHT
AND VARIABLE WINDS PREVAIL THROUGH EARLY NEXT WEEK. LOCALLY HEAVY
SHOWERS AND A SLIGHT CHANCE FOR THUNDERSTORMS ARE EXPECTED THROUGH
SATURDAY AS MOISTURE ASSOCIATED WITH POST TROPICAL CYCLONE JIMENA
AND AN UPPER LEVEL TROUGH APPROACHING FROM THE NORTHWEST WILL
COMBINE TO BRING UNSETTLED WEATHER. THE HEAVIEST RAIN WILL FOCUS
ACROSS THE WESTERN HALF OF THE ISLAND CHAIN.

&&

.DISCUSSION...
POST-TROPICAL CYCLONE JIMENA IS CENTERED ABOUT 150 MILES NORTH OF
KAUAI AND MOVING WEST-SOUTHWEST AT ABOUT 15 MPH. JIMENA/S TRACK
WILL GRADUALLY SHIFT THE GENTLE FLOW AROUND THE ISLANDS DURING THE
NEXT COUPLE OF DAYS. THE GRADIENT WILL LIKELY BE LIGHT ENOUGH FOR
LOCALIZED NIGHTTIME LAND BREEZES AND DAYTIME SEA BREEZES FOR
SOME PORTIONS OF THE STATE. WATER VAPOR DEPICTS A MID TO UPPER
LEVEL RIDGE OVER THE AREA. 00Z SOUNDINGS SHOW MIDLEVEL INVERSIONS
AT AROUND 9 KFT WITH ABOVE NORMAL PRECIPITABLE WATER /PW/ VALUES
AROUND 1.7 TO 1.8 INCHES. RADAR SHOWS SCATTERED MODERATE TO HEAVY
SHOWERS ACROSS THE STATE THIS AFTERNOON.

FOR TONIGHT...SOUTHERLY FLOW WILL KEEP WARM AND HUMID CONDITIONS
AND CARRY THE VOG PLUME ONTO MAUI COUNTY AND OAHU. THE ABUNDANT
MOISTURE LINGERING ACROSS THE AREA WILL BRING UNSETTLED WEATHER
THROUGH SATURDAY. MODELS SHOW PW VALUES INCREASING TO WELL ABOVE
NORMALS WITH 2 INCHES OR MORE ACROSS THE ISLAND CHAIN.
MEANWHILE...AN UPPER LEVEL TROUGH / LOW COMPLEX WILL APPROACH THE
STATE FROM THE NORTHWEST ON FRIDAY WHICH WILL ERODE THE MIDLEVEL
INVERSION AND DESTABILIZE THE MOIST AIRMASS.

WITH THE MOISTURE AND INSTABILITY IN PLACE...LOCALLY HEAVY RAIN
AND A SLIGHT CHANCE FOR THUNDERSTORMS WILL BE POSSIBLE THROUGH
SATURDAY. KAUAI...OAHU AND ISLANDS OF MAUI COUNTY ARE LIKELY HAVE
THE GREATEST THREAT FOR FLASH FLOODING RAINS...PARTICULARLY FRIDAY
AND SATURDAY AS MODELS DEPICT CONVERGENCE ZONES OVER THE CENTRAL
AND WESTERN PART OF THE STATE. A FLASH FLOOD WATCH IS IN EFFECT
FOR THESE AREAS...AND FLOODING WILL BE MORE OF A THREAT DUE TO THE
ALREADY SATURATED GROUNDS. THE BIG ISLAND WILL ALSO SEE AN UPTICK
IN SHOWERS THROUGH TONIGHT...WITH THE FOCUS SHIFTING WESTWARD BY
FRIDAY.

WINDS WILL REMAIN LIGHT...BUT SHIFT SOUTHEAST ON FRIDAY THROUGH
SUNDAY...THEN GRADUALLY SHIFT BACK TO EASTERLY MONDAY AND
TUESDAY. WARM AND HUMID CONDITIONS WILL ALSO CONTINUE THROUGH THE
WEEKEND AS WINDS KEEP BACKING TO A MORE SOUTHEAST FLOW. AFTERNOON
HEAT INDEX VALUES WILL APPROACH 100 DEGREES AT THE LOWER
ELEVATIONS OVER THE NEXT SEVERAL DAYS. TRADES WILL BUILD BY THE
MIDDLE OF NEXT WEEK WITH THE POSSIBILITY OF AN UPPER LEVEL LOW
MOVING TOWARDS THE STATE FROM THE NORTHEAST LATE NEXT WEEK.

&&

.AVIATION...
SOUTHEAST FLOW CONTINUES TO BRING MOIST UNSTABLE CONDITIONS TO THE ISLANDS.
AIRMET SIERRA REMAINS POSTED FOR MTN OBSCURATION ALONG THE SE COAST OF
THE BIG ISLAND INCLUDING THE KAU DISTRICT. VFR WILL DOMINATE THE TERMINALS
THROUGH THE REST OF TODAY...BUT OCCASIONAL MFR CIGS AND VIS ARE POSSIBLE
LATER WITHIN SHOWERS.

&&

.MARINE...
THE NORTH SWELL ASSOCIATED WITH POST-TROPICAL CYCLONE JIMENA WILL
BEGIN DECLINING TONIGHT. A SMALL LONG-PERIOD SOUTH SWELL WILL CONTINUE
THROUGH SATURDAY FOLLOWED BY A SMALL...BUT LONGER-PERIOD
SOUTH-SOUTHWEST SWELL ON SUNDAY.

WINDS WILL REMAIN GENTLE AND VARIABLE THROUGH THE FORECAST PERIOD.
WINDS WILL BE OUT OF THE SOUTH THROUGH FRIDAY AND SHIFTING TO A
MORE SOUTHEASTERLY FLOW OVER THE WEEKEND. WINDS AND SEAS WILL
REMAIN BELOW SMALL CRAFT ADVISORY LEVELS THROUGH AT LEAST THE
WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH SATURDAY AFTERNOON FOR NIIHAU-KAUAI-
OAHU-MOLOKAI-LANAI-KAHOOLAWE-MAUI.

&&

DISCUSSION/MARINE...FOSTER
AVIATION...POWELL