Flash Flood Warning 16, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT JUL 19 2014

.SYNOPSIS...
CLOUDS...LOCALLY HEAVY SHOWERS...AND POSSIBLY THUNDERSTORMS WILL
SPREAD OVER THE STATE TONIGHT...AND THEN LINGER THROUGH MONDAY.
CONDITIONS WILL CLEAR FROM MONDAY INTO WEDNESDAY...AND THEN REMAIN
MORE STABLE INTO LATE NEXT WEEK. TRADE WINDS ARE EXPECTED TO
CONTINUE THROUGHOUT.

&&

.DISCUSSION...
WHAT ONCE WAS WALI NOW CONSTITUTED A SHAPELESS MASS OF SHEARED CLOUD
LAYERS AND COMPLEX PRESSURE TROUGHS SPREADING OVER 1500 MILES ACROSS
THE CENTRAL PACIFIC OCEAN. THE LEADING EDGE OF THE CLOUD LAYERS HAD
PASSED OVER THE STATE AND REACHED KAUAI...BUT THE BULK OF THE SOUPY
MASS REMAINED FURTHER TO THE EAST...AT LEAST THROUGH THE SHORT RUN.
POWERFUL THUNDERSTORMS RAGED OFFSHORE TO THE NORTHEAST OF MAUI AND
HAWAII COUNTIES...UNDERLINING THE VERY REAL RISKS POSED BY THIS
SCENARIO. THUS...THE RELATIVELY CALM CONDITIONS OVER OAHU THIS
AFTERNOON COULD BE DESCRIBED BEST AS MISLEADING.

TO PUT MATTERS INTO PERSPECTIVE...MICROWAVE ESTIMATES FROM SATELLITE
AND THE HILO SOUNDING BOTH MEASURED PRECIPITABLE WATER VALUES AROUND
TWO AND ONE QUARTER INCHES OVER AND NORTHEAST OF THE BIG ISLAND...
APPROACHING TWO TIMES THE SEASONABLE VALUE...FOR A RELATIVELY HUMID
SEASON. STILL...BY ITSELF THE THERMODYNAMIC ENERGY PACKED INTO THIS
AIR MASS REPRESENTED ONLY A POTENTIAL. THE MID-LEVEL TROUGH
EXTENDING FROM SOUTHWEST TO NORTHEAST JUST TO THE NORTH OF THE STATE
PROVIDED A MECHANISM FOR UNLEASHING THIS ENERGY. SATELLITE IMAGES
SHOWED LITTLE PUFFS OF TOWERING CUMULI POPPING UNDER THE TROUGH FROM
THE DRIER AIR MASS LOCATED TO THE NORTHWEST OF KAUAI...AND
PRESUMABLY THE CONVECTION WILL BE THAT MUCH MORE IMPRESSIVE ONCE THE
MORE HUMID AIR MASS STARTS TO INTERACT WITH THE TROUGH.

THIS ATMOSPHERIC COLLISION WILL CONTINUE THROUGH THE REST OF THE
WEEKEND AND INTO EARLY NEXT WEEK AS MORE AND MORE OF WALI ARRIVES
FROM THE EAST SOUTHEAST. WAVES OF MOISTURE WILL CONTINUE TO FUEL
CONVECTION...WHILE MINOR TROUGHS EMBEDDED IN THE CLOUD MASS WILL
PROVIDE LOCALIZED CONVERGENCE. THUS...SHOWER ACTIVITY WILL INCREASE
AND DECREASE WITH TIME...BUT LOCALLY HEAVY RAIN WILL REMAIN POSSIBLE
IN MOST AREAS THROUGH ABOUT MONDAY.

THE SUBTROPICAL RIDGE SHOULD KEEP MODEST TRADE WINDS BLOWING THROUGH
THE FORECAST PERIOD. HOWEVER...PASSING LOW-LEVEL TROUGHS AND LOCAL
THUNDERSTORM OUTFLOWS MAY PRODUCE PERIODS OF VARIABLE WIND. ALSO...
SOME INCREASE IN OVERALL WIND SPEED WILL BE POSSIBLE AS THE
SUBTROPICAL RIDGE PEAKS FROM TUESDAY INTO WEDNESDAY.

FROM MONDAY INTO MID-WEEK...GUIDANCE PREDICTED A MID-LEVEL RIDGE TO
BUILD OVER THE STATE FROM THE EAST. ALSO...THE LAST OF WALI SHOULD
PASS THE ISLANDS DURING THIS TIME. THE RESULTING SUBSIDENCE FROM
ABOVE AND A DRIER AIR MASS UPWIND WILL PUSH CONDITIONS STRONGLY
TOWARD CLIMATOLOGY. THUS...MORE TYPICAL TRADE-WIND WEATHER WILL
RETURN...WITH MOST PRECIPITATION SHIFTING TO WINDWARD SLOPES ALL
DAY...AND SELECT LEE SLOPES EACH AFTERNOON.

&&

.MARINE...
WIND SPEEDS REMAINED JUST BELOW THE THRESHOLD FOR A SMALL CRAFT
ADVISORY /SCA/ IN TYPICALLY WINDY AREAS AROUND MAUI AND THE BIG
ISLAND. SPEEDS THERE SHOULD REMAIN BELOW THIS THRESHOLD THROUGH
MONDAY NIGHT BEFORE TRENDING UPWARD...POSSIBLY INCREASING ENOUGH TO
REQUIRE AN ADVISORY BY TUESDAY NIGHT OR WEDNESDAY.

SEVERAL SMALL SOUTH AND SOUTHEAST SWELLS WILL ARRIVE IN THE COMING
WEEK. HIGH PRESSURE FAR NORTH OF THE ISLANDS WILL SUPPORT TRADE
WINDS...WHICH IN TURN WILL PRODUCE SHORT-PERIOD CHOPPY SURF ALONG
EAST-FACING SHORES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH MONDAY AFTERNOON FOR ALL THE MAIN
HAWAIIAN ISLANDS.

&&

RYSHKO

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE HONOLULU HI
1023 PM HST SAT JUL 19 2014

HIZ005>011-201115-
CENTRAL OAHU-OAHU KOOLAU-OAHU NORTH SHORE-OAHU SOUTH SHORE-OLOMANA-
WAIANAE COAST-WAIANAE MOUNTAINS-
1023 PM HST SAT JUL 19 2014

AT 1015 PM HST...A STRONG THUNDERSTORM JUST NORTH OF PEARL HARBOR
MOVED NORTHEAST AT 15 MPH. EXPECT INTENSE LIGHTNING...WIND GUSTS TO
30 MPH...AND NICKEL-SIZE HAIL FROM THIS STORM. DO NOT STAY IN THE
OPEN NOR SEEK SHELTER UNDER TREES WHEN LIGHTNING THREATENS...MOVE
INDOORS WHEN A THUNDERSTORM APPROACHES. VERY HEAVY RAIN WILL FALL
FROM THIS STORM...CAUSING WATER TO POND ON ROADS...FLOOD AREAS WITH
POOR DRAINAGE...AND CAUSE RAPID RISES ON SMALL STREAMS AND CREEKS.

RYSHKO/POWELL

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE HONOLULU HI
1200 AM HST SUN JUL 20 2014

HIZ008-201300-
OAHU KOOLAU-
1200 AM HST SUN JUL 20 2014

AT 1148 PM HST...A STRONG THUNDERSTORM WAS LOCATED 17 MILES NORTH OF
HONOLULU. THIS STORM PRODUCED LIGHTNING AND VERY HEAVY RAIN...AND
MAY PRODUCE PENNY-SIZED HAIL.