Flash Flooding and Heavy Rain in Hawaii, January 16 to 17, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1210 AM HST MON JAN 16 2012

.UPDATE...FOR HAZE...PRECIPITATION...AND THUNDERSTORM COVERAGE.

&&

.SYNOPSIS...
HAZY CONDITIONS WITH LOCALLY HEAVY SHOWERS ALONG SOUTHERLY SLOPES
WILL PREVAIL THROUGH WEDNESDAY AS A STALLED FRONT LINGERS NORTHWEST
OF THE STATE. TRADE WINDS AND MORE USUAL WEATHER WILL RETURN
GRADUALLY FROM WEDNESDAY INTO FRIDAY.

&&

.DISCUSSION...
THE LARGEST CHANGE IN THE UPDATE IS WIDE EXPANSION OF HAZE COVERAGE.
THUNDERSTORM COVERAGE...WHILE IMPORTANT...IS FAIRLY SMALL IN AREA...
AND LIMITED MAINLY TO WATERS AT THE EXTREME NORTHWEST AND SOUTHEAST
EDGES OF THE STATE. THERE IS NO CHANGE IN OVERALL FORECAST THINKING.

A COMPLEX SERIES OF FRONTS WAS LOCATED NORTHWEST OF THE MAIN
HAWAIIAN ISLANDS. ALTHOUGH THUNDERSTORMS ALONG THE COLD FRONT
NEAREST THE ISLANDS WERE CLOSE ENOUGH TO SHOW UP AT THE EDGE OF
RADAR COVERAGE...THEY ARE NOT EXPECTED TO GET MUCH CLOSER. ON THE
OTHER HAND...THUNDERSTORMS LIKELY WILL CONTINUE TO FORM OFF OF CAPE
KUMUKAHI THROUGH THE EARLY MORNING.

DEEP AND BROAD TROUGHING ALOFT SUPPORTED THE SERIES OF FRONTS...BUT
A RATHER STEEP HEIGHT GRADIENT OVER THE STATE LED TO A SUBSTANTIALLY
UNYIELDING RIDGE TO THE EAST CLOCKWISE THROUGH SOUTHWEST. AS THE
MID-LEVEL TROUGH ROTATES NORTHEAST FROM THIS EVENING INTO
WEDNESDAY...ITS SUPPORT FOR THE SURFACE FRONTS WILL WEAKEN...AND
THEN GIVE OUT COMPLETELY. THUS...LARGE-SCALE CONDITIONS AT THE
SURFACE WILL BE RATHER STATIC IN THE SHORT RUN...BUT THEN THE FRONTS
WILL BREAK UP AND GET SWEPT AWAY TO THE NORTHWEST FROM AROUND
MID-WEEK.

THUS...NO FRONTAL PASSAGE IS EXPECTED FOR THE ISLANDS THIS WEEK...
BUT MODERATE TO LOCALLY STRONG...CONVERGENT...SOUTHERLY WINDS MAY
CAUSE SOME DIFFICULTIES. ALTHOUGH WIND SPEEDS LIKELY WILL REMAIN
BELOW THE THRESHOLD FOR A WIND ADVISORY...INFRASTRUCTURE AND
VEGETATION OPTIMIZED TO RESIST NORTHEASTERLY WINDS MAY NOT FAIR SO
WELL AGAINST SOUTHERLIES...ESPECIALLY WHERE THE WINDS ACCELERATE
DOWNSLOPE. CONVERGENCE IN THE WIND FIELD AHEAD OF THE FRONTS HAS
PRODUCED BATCHES OF SHOWERS ALREADY THIS EVENING...AND WILL CONTINUE
TO DO SO AT LEAST THROUGH MONDAY. AS THEY APPROACH FROM THE
SOUTHEAST TO SOUTHWEST...THESE SHOWERS WILL MOST AFFECT AREAS THAT
OFTEN REMAIN RELATIVELY DRY DURING TRADE-WIND WEATHER.

THE CONVERGENCE SHOWERS HAVE THE POTENTIAL TO PRODUCE HEAVY RAINFALL
FOR THREE MAIN REASONS. FIRST...THOUGH MID-LEVEL RIDGING DID PREVAIL
OVER THE STATE...TEMPERATURES ALOFT WERE QUITE LOW...REDUCING
STABILITY...AND PARTLY EXPLAINING THE HIGH INVERSION AND COMPLEX
STRUCTURE OF THE SURFACE MIXED LAYER IN THE SOUNDINGS. SECOND...
SOUTHERLY FLOW WILL BRING UP WATER VAPOR FROM THE DEEP TROPICS.
PRECIPITABLE WATER MAY NOT HAVE BEEN MUCH ABOVE TYPICAL VALUES IN
THE 00Z SOUNDINGS...BUT SATELLITE ESTIMATES OF THIS VALUE TO THE
SOUTH OF THE ISLANDS SUGGESTED THAT IT CAN ONLY RISE THROUGH MONDAY
AT LEAST. FINALLY...VERY SLOW CHANGES IN THE PREVAILING FLOW MAY
KEEP ZONES OF CONVERGENCE OVER THE SAME LOCATIONS FOR EXTENDED
PERIODS OF TIME. TRAINING HEAVY SHOWERS CAN DROP LARGE AMOUNTS OF
RAIN...BUT THOUGH THE CURRENT SITUATION CALLS FOR VIGILANCE...IT
DOES NOT WARRANT A FLASH FLOOD WATCH.

SOUTHERLY WINDS ALSO WILL BRING UP VOLCANIC HAZE PREVIOUSLY CARRIED
SOUTHWEST FROM THE BIG ISLAND. THE FORECAST MAY NEED AN EXPANSION OF
THE AREAS AFFECTED BY HAZE...BUT RAIN SHOULD START TO CLEAR THE AIR
SOON ENOUGH.

AFTER WEDNESDAY...WINDS WILL START TO TURN MORE EASTERLY AGAIN AS
THE FRONTAL REMNANTS RETREAT TO THE NORTHWEST AND THE SUBTROPICAL
RIDGE SHIFTS NORTH. BY FRIDAY...MORE TYPICAL TRADE-WIND WEATHER WILL
RETURN.

&&

.MARINE...
NO MARINE WARNINGS OR ADVISORIES ARE IN EFFECT. SOUTHERLY WINDS ARE
EXPECTED TO REMAIN BELOW THE ADVISORY CRITERION OF 25 KT THROUGH THE
FORECAST PERIOD...BUT THEY MAY APPROACH THIS LIMIT NEAR KAUAI AND
OAHU THROUGH MONDAY NIGHT. HOWEVER...COMBINED SEAS IN THAT AREA ARE
EXPECTED TO EXCEED THE THRESHOLD OF 10 FEET BY EARLY MONDAY
MORNING...SO A SMALL CRAFT ADVISORY WILL BE IN EFFECT FROM 6 AM
MONDAY THROUGH TUESDAY AFTERNOON.

A MODERATE NW SWELL WILL ARRIVE ON MONDAY...AND THEN PEAK FROM
MONDAY NIGHT INTO TUESDAY. THIS SWELL WILL BUILD SURF HEIGHTS TO
NEAR ADVISORY LEVELS ALONG EXPOSED N AND W SHORES...BUT IT DOES NOT
APPEAR LIKELY THAT SURF WILL QUITE REACH THOSE THRESHOLDS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY FROM 6 AM MONDAY TO 6 PM HST TUESDAY FOR
KAUAI NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD
WATERS.

&&

RYSHKO