Flash Flood Warning 27, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST MON AUG 24 2015

.SYNOPSIS...
AN EXTREMELY MOIST AIR MASS OVER THE STATE...TOGETHER WITH LOW
PRESSURE IN THE MIDDLE ATMOSPHERE TO THE NORTH...RAISED THE THREAT
OF HEAVY RAIN AND THUNDERSTORMS ACROSS THE MAIN HAWAIIAN ISLANDS
THROUGH THE EARLY PART OF THE WEEK. LIGHT SOUTHEAST WINDS WILL
STRENGTHEN AND BECOME MORE EASTERLY SLOWLY THROUGH THE WEEK. ANOTHER
PERIOD OF UNSETTLED WEATHER MAY OCCUR NEXT WEEKEND.

&&

.DISCUSSION...
AN EXTREMELY AND UNSEASONABLY MOIST AIR MASS CONTINUED TO STREAM
OVER THE MAIN HAWAIIAN ISLANDS FROM THE SOUTHEAST. AT THE SAME
TIME...BROAD BUT SHALLOW TROUGHING ALOFT TO THE NORTH OF THE
STATE HELPED TO DESTABILIZE THE LOCAL ATMOSPHERE. SOUNDING DATA
RECORDED TEMPERATURES AT 500 MB NOT MUCH COLDER THAN USUAL
OVERHEAD...AND THE CORE OF THE COLD POOL REMAINED TO THE
NORTHEAST...BUT THE AIR MASS AT LOWER LEVELS WAS SO MOIST THAT ANY
FURTHER STEEPENING OF THE ENVIRONMENTAL LAPSE RATE SUFFICED.

THE TROUGHING ALOFT ALSO LIMITED SUPPORT FOR THE SUBTROPICAL RIDGE
AT THE SURFACE. THUS...THE WEST END OF THE RIDGE RETREATED GRADUALLY
TOWARD THE EAST...KEEPING WINDS SOUTHEASTERLY TO SOUTHERLY OVER THE
ISLANDS. GUIDANCE AGREED WELL IN MAINTAINING THIS SITUATION THROUGH
AT LEAST THE MIDDLE OF THE WEEK. INTERESTINGLY...SATELLITE ESTIMATES
DETECTED A SIGNIFICANTLY DRIER AIR MASS EAST OF 150W AT THE LATITUDE
OF THE STATE. HOWEVER...THE ORIENTATION OF THE PREVAILING FLOW WILL
KEEP PRECIPITABLE WATER AMOUNTS APPROACHING TWO AND A HALF INCHES IN
PLACE OVER THE ISLANDS AT LEAST THROUGH WEDNESDAY.

BY THE END OF THE WEEK...GUIDANCE GENERALLY AGREED THAT THE MOSTLY
ZONAL MID-LEVEL TROUGH TO THE NORTH WILL SHIFT SOUTH IN SOME WAY...
ALTHOUGH SPECIFICS DIFFERED. SOME SOLUTIONS...SUCH AS THE ECMWF...
DEPICTED THE AXIS OF THE MERIDIONALLY DIGGING TROUGH TO REMAIN
MOSTLY EAST OF THE STATE. ON THE OTHER HAND...OTHER SOLUTIONS...LIKE
THE GFS...DEVELOPED THE TROUGH OVER OR SLIGHTLY WEST OF THE ISLANDS.
DESPITE SOME LIMITED DRYING NEAR THE SURFACE...THE COLDER AIR AND
DYNAMIC SUPPORT ALOFT STILL IMPLIED ANOTHER PERIOD OF POSSIBLE DEEP
CONVECTION FOR NEXT WEEKEND. FOR NOW...A SLIGHT CHANCE OF
THUNDERSTORMS HAS BEEN INTRODUCED FOR THE MIDDLE OF THE STATE AROUND
NEXT SATURDAY.

FOR THE TIME BEING HOWEVER...THE CONVECTION CURRENTLY POPPING OVER
THE STATE REPRESENTED THE BIGGEST CONCERN IN THE FORECAST.

&&

.AVIATION...
SOUTHEASTERLY FLOW WILL CONTINUE TO STEER BANDS OF LOW CLOUDS AND
SHOWERS TOWARDS THE ISLANDS. PRECIPITATION WILL FAVOR EAST AND
SOUTHEAST FACING SLOPES AND COASTS BUT GIVEN THE HIGHLY MOIST
UNSTABLE AIRMASS OVER THE STATE...THE POTENTIAL FOR LOCALIZED HEAVY
DOWNPOURS EXISTS EVERYWHERE. THE AIRMET FOR MTN OBSCURATION FOR
EASTERN BIG ISLAND WILL LIKELY REMAIN NECESSARY THROUGH AT LEAST
MONDAY MORNING.

THUNDERSTORMS HAVE ALREADY BEEN REPORTED THIS EVENING FROM ONE END
OF THE STATE TO THE OTHER WITH THE GREATEST ACTIVITY THUS FAR
OCCURING OVER LEEWARD AREAS. NOW HOWEVER...A DENSE BAND OF
THUNDERSTORMS...TRIGGERED BY A SOUTHEASTWARD MOVING UPPER LEVEL
DISTURBANCE...IS VISIBLE ON SATELLITE JUST TO THE NORTH OF THE
ISLANDS. SIGMET NOVEMBER FOR EMBEDDED THUNDERSTORMS HAS BEEN ISSUED
AND COULD NEED TO EXPANDED TO INCLUDE MUCH OF THE STATE LATER
THIS EVENING. WHILE VFR CONDITIONS ARE FORECAST TO PREVAIL AT ALL
TAF SITES THE POTENTIAL FOR LIGHTNING AND BRIEF PERIODS OF IFR CIGS
AND/OR VIS SHOULD BE CONSIDERED TO EXIST ALL AREAS.

&&

.MARINE...
A SMALL TO MODERATE W SWELL WILL CONTINUE OVER THE ISLANDS THROUGH
THE MIDDLE OF THE WEEK. A SLIGHTLY LARGER VERY LONG-PERIOD WNW
SWELL FROM DISTANT TYPHOON ATSANI IS EXPECTED TO ARRIVE AROUND
WEDNESDAY...AND THEN TO PEAK THURSDAY INTO FRIDAY. THIS SWELL MAY BE
LARGE ENOUGH TO PRODUCE ADVISORY-LEVEL SURF ALONG WEST-FACING SHORES
OF THE BIG ISLAND.

MARINERS MAY ENCOUNTER HEAVY SHOWERS OR THUNDERSTORMS AS A VERY
MOIST AND UNSTABLE AIR MASS LINGERS ACROSS THE AREA THROUGH THE NEXT
COUPLE OF DAYS. A MARINE WEATHER STATEMENT IS IN EFFECT TO HIGHLIGHT
THE THREAT TO MARINERS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH MONDAY AFTERNOON FOR NIIHAU-KAUAI-OAHU-
MOLOKAI-LANAI-MAUI-BIG ISLAND.

SEVERE THUNDERSTORM WATCH 1 IN EFFECT UNTIL 4 AM HST MONDAY FOR
OAHU...MAUI COUNTY...AND ADJACENT WINDWARD WATERS.

&&

AVIATION...BEDAL
DISCUSSION/MARINE...RYSHKO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
424 AM HST MON AUG 24 2015

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0353 AM     FLASH FLOOD      MAKIKI                  21.31N 157.84W
08/24/2015                   HONOLULU           HI   LAW ENFORCEMENT

            H-1 WEST AT WILDER OFF RAMP CLOSED DUE TO FLOODING.

0406 AM     FLASH FLOOD      WAIKIKI                 21.28N 157.83W
08/24/2015                   HONOLULU           HI   NWS EMPLOYEE

            HOBRON LANE FROM ALA MOANA BLVD IS FLOODED OVER 1.5 FEET
            OF WATER AND IMPASSABLE.

&&

HFO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
546 AM HST MON AUG 24 2015

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0412 AM     FLOOD            HONOLULU                21.31N 157.86W
08/24/2015                   HONOLULU           HI   BROADCAST MEDIA

            STREETS FLOODED AT KEEAUMOKU AND MAKALOA.

0434 AM     FLOOD            WAIMALU                 21.40N 157.94W
08/24/2015                   HONOLULU           HI   LAW ENFORCEMENT

            H-1 LANE CLOSURE...TWO LANES CLOSEST TO THE CENTER MEDIAN
            ALONG H-1 WEST BOUND NEAR THE WAIMALU OFF-RAMP ARE CLOSED
            DUE TO FLOODING.

0534 AM     FLOOD            HONOLULU                21.31N 157.86W
08/24/2015                   HONOLULU           HI   BROADCAST MEDIA

            H1 WEST BOUND VINEYARD OFF-RAMP CLOSED DUE TO FLOODING.

&&

HFO

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
556 AM HST MON AUG 24 2015

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0550 AM     FLOOD            WAIKANE                 21.50N 157.88W
08/24/2015                   HONOLULU           HI   EMERGENCY MNGR

            KAMEHAMEHA HIGHWAY CLOSED AT WAIKANE STREAM.

&&

HFO