Flash Flood and Heavy Rain in Hawaii, July 10 to 12, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SUN JUL 10 2011

.SYNOPSIS...
A HIGH PRESSURE SYSTEM FAR NORTH OF THE ISLANDS WILL MAINTAIN
MODERATE TRADE WINDS THROUGH THIS WEEK. AN UPPER LEVEL LOW
NORTHEAST OF THE ISLANDS WILL BRING AN INCREASE IN TRADE SHOWERS
THROUGH WEDNESDAY MORNING. THESE SHOWERS WILL FAVOR THE WINDWARD AND
MOUNTAIN AREAS...BUT A FEW OF THEM WILL DRIFT OVER INTO
LEEWARD SECTIONS AT TIMES. THESE SHOWERS WILL BE MORE NUMEROUS
DURING NIGHTS AND MORNINGS. THE KONA SIDE OF THE BIG ISLAND WILL
HAVE AN UPTICK IN AFTERNOON AND EVENING SHOWERS MONDAY AND TUESDAY.
THE SECOND HALF OF THIS WEEK WILL INVOLVE FEWER SHOWERS DUE TO A
STABLE AIR MASS BUILDING OVER THE ISLANDS.

&&

.DISCUSSION...
A 1032 MB SUBTROPICAL HIGH IS CENTERED NEAR 40N 154W...OR ABOUT
1330 MILES NORTH OF HONOLULU. THE HIGH IS DRIVING MODERATE TRADES
ACROSS THE STATE THIS EVENING. THE HIGH IS EXPECTED TO MOVE AWAY
TO THE NORTH OVER THE NEXT COUPLE DAYS...BUT STRENGTHEN AT THE
SAME TIME. THE CURRENT WIND SPEEDS WILL MAINTAIN THROUGHOUT THIS
WEEK AS AN UPPER LEVEL LOW LOCATED EAST-NORTHEAST OF THE STATE
THIS EVENING WILL STAY BETWEEN US AND THE HIGH. THIS WILL RESULT
IN A SLIGHTLY WEAKER PRESSURE GRADIENT OVER THE STATE THAN WHAT IT
WOULD BE NORMALLY WITH A 1032 MB HIGH AT THAT DISTANCE.

THE UPPER LOW IS CURRENTLY CENTERED AT 21.5N 150W...OR ABOUT 360
MILES EAST NORTHEAST OF HILO THIS EVENING AND IS NEARLY STATIONARY.
THIS LOW IS CREATING SOME INSTABILITY ALOFT AND ENHANCING THE
TRADE WIND SHOWERS FOR WINDWARD SIDES OF OAHU THROUGH THE BIG
ISLAND. SOME HEAVY SHOWERS WERE OBSERVED OVER THE KONA SIDES OF
THE BIG ISLAND EARLIER THIS AFTERNOON. THE HILO SOUNDING FROM THIS
AFTERNOON SHOWS AN ELEVATED INVERSION HEIGHT AROUND 13000 FEET AND
NEAR NORMAL PRECIPITABLE WATER VALUES. SATELLITE AND RADAR SHOWS
NUMEROUS WINDWARD SHOWERS MOVING ONTO WINDWARD SIDES OF THE
ISLANDS FROM UPSTREAM. EXPECTING THESE SHOWERS TO WANE A BIT BY
LATE MONDAY MORNING DUE TO CLOUD TOP WARMING AND LOWER HUMIDITY.
THE UPPER LOW IS EXPECTED TO VERY SLOWLY BEGIN TRACKING NORTHWARD
AWAY FROM THE AREA BEGINNING MONDAY AND WILL NOT BE OUT OF OUR
AREA OF INFLUENCE UNTIL WEDNESDAY. SO THIS TREND OF ENHANCED
WINDWARD/MAUKA SHOWERS...MOSTLY DURING NIGHTS AND EARLY MORNING
WILL CONTINUE LIKELY THROUGH WEDNESDAY. LEEWARD SIDES WILL GET
SHADOWED FROM MUCH OF THIS RAIN...BUT OCCASIONAL PASSING SHOWERS
MAY BE POSSIBLE. THE SEA BREEZE DOMINATED KONA SLOPES ON THE BIG
ISLAND WILL ALSO EXPERIENCE A BIT MORE SHOWER ACTIVITY AS
WELL...BUT WILL MAXIMIZE DURING THE AFTERNOONS AND EVENINGS MONDAY
AND TUESDAY.

A MORE STABLE...WARMER AIR MASS SHOULD BE BUILDING ACROSS THE
ISLANDS BEGINNING WEDNESDAY AFTERNOON. THE MARINE LAYER IS SLATED TO
BECOME SHALLOWER IN TIME...FROM THE CURRENT 15000 TO 20000 FEET
TO 10000 TO 15000 FEET TUESDAY TO 8000 TO 12000 FEET WEDNESDAY
EVENING TO 7000 TO 10000 FEET FRIDAY. WE WILL STILL HAVE THE TRADE
WIND SHOWERS BUT THEY SHOULD BE FEWER AND LESS INTENSE.

&&

.AVIATION...
INCREASING SHOWERS TONIGHT WILL INVOLVE SOME MVFR CEILINGS AND VISIBILITY
ESPECIALLY FOR TERMINALS IN WINDWARD AREAS.

&&

.MARINE...
THE CURRENT SOUTH SWELL WILL CONTINUE TO LOWER TONIGHT. ANTICIPATING
A MODERATE SWELL FROM THE TASMAN SEA TO BUILD LATE FRIDAY AND
SATURDAY...BUT WILL LIKELY BE BELOW ADVISORY CRITERIA.

&&

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

&&

DISCUSSION/MARINE...FOSTER
AVIATION...FUJUII