AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
952 AM HST TUE AUG 9 2011

.SYNOPSIS...
TRADES WILL FOCUS RAINFALL ALONG WINDWARD TERRAIN AS THEY GRADUALLY
EASE THIS WEEK. HIGH CLOUDS WILL BEGIN TO INCREASE TONIGHT AND
PERSIST FOR A FEW DAYS. REMNANTS OF TROPICAL CYCLONE EUGENE WILL
BRING ENHANCED RAINFALL TO THE STATE THURSDAY NIGHT THROUGH
SATURDAY. TYPICAL AUGUST TRADE WEATHER WILL FOLLOW SUNDAY THROUGH
WEDNESDAY.

&&

.DISCUSSION...
THE NIGHT SHIFT HANDED OFF A GOOD FORECAST THIS MORNING WITH VERY
LITTLE CHANGE BEYOND THE NEAR TERM. DRIER AIR IMMEDIATELY UPSTREAM
COMBINED WITH DAYTIME MIXING/HEATING WILL GIVE WAY TO A MOSTLY SUNNY
DAY WITH MODERATE TRADES. EXPECT THE USUAL DIURNAL TRENDS ALONG THE
LEEWARD BIG ISLAND SLOPES/COAST.

GOES WV IMAGERY SHOWS DEEPER LAYERED HIGH PRESSURE OVER THE ENPAC
BECOMING PROGRESSIVELY MORE CYCLONIC. NRN/SRN STREAM JETS WILL
PHASE ALOFT DEVELOPING INTO A POSITIVELY TILTED TROUGH JUST N/NW OF
HAWAII. THIS WILL ADVECT HIGH LEVEL CONVECTIVE DEBRIS OVER THE STATE
FROM THE S LEADING TO MOSTLY CLOUDY SKIES MUCH OF THE WORK WEEK.
THIS WILL OCCUR COINCIDENT WITH THE ARRIVAL/PASSING OF THE REMNANTS
OF TC EUGENE THU-SAT ALONG WITH A TRADE WIND MINIMUM. THE THU-EARLY
SUN FORECAST DEPICTS HIGHER POPS/SKY ALTHOUGH SOME UNCERTAINTY
REMAINS THE RESULTANT BOUNDARY LAYER MOISTURE FIELDS AND AMOUNT OF
INTERACTION WITH ANY MID/UPR LEVEL DYNAMICS/INSTABILITY. ANY
ADJUSTMENTS TO THE FORECAST WILL BE SUBTLE TODAY AND LIKELY FOCUS ON
TIMING.

GRADUAL HEIGHT/PRESSURE RISES OVER THE CNTRL/ENPAC LATE IN THE
WEEKEND SUGGEST INCREASING TRADES LOCALLY TO HIGH END MODERATE
LEVELS SUN-WED. A MORE TYPICAL AUG AIRMASS WILL NOSE IN FROM THE E
WITH THE BUILDING TRADES.

&&

.PREVIOUS DISCUSSION.../ISSUED 330 AM HST TUE MORNING AUG 9/
AFTER DROPPING SLOWLY BUT STEADILY SINCE SATURDAY...ISLAND SURFACE
PRESSURE TRENDS ARE NOW FLAT OVER THE PAST 24 HOURS. THE LOWERING
PRESSURES HAD BEEN DUE TO THE WEAKENING OF THE TRADE WIND-SUPPLYING
HIGH...CURRENTLY CENTERED ABOUT 1200 MILES NNE OF THE ISLANDS...WITH
A CENTRAL PRESSURE NEAR 1030 MB. AS THE HIGH WEAKENED...TRADE WINDS
DIMINISHED TO THEIR CURRENT MODERATE SPEEDS...AND A GRADUAL
WEAKENING TREND IS STILL ANTICIPATED THROUGH THE WORK WEEK.

SATELLITE IMAGERY SHOWS A LOOSELY ORGANIZED...NW-SE ORIENTED...BAND
OF BROKEN LOW CLOUDS OVER THE ISLANDS...AND ADJACENT WINDWARD WATERS
OUT TO 75 MILES. CLOUDS WITHIN THIS BAND INCREASED DRAMATICALLY
MONDAY EVENING...BUT ARE ON A DECIDED DOWNWARD TREND EARLY THIS
MORNING. ALTHOUGH A GOOD PORTION OF THE BAND IS COMPRISED OF STABLE
STRATOCUMULUS...ISOLATED COOL CLOUD TOPS WITHIN THE BAND NEAR KAUAI
AND OAHU REPRESENT SMALL POCKETS OF HEAVY SHOWERS...AND WINDWARD
OAHU RECEIVED THE BULK OF THE RAINFALL OVERNIGHT. THE SHORT TERM
FORECAST CALLS FOR THIS CLOUD BAND TO CONTINUE TO DIMINISH THROUGH
MID-MORNING...GIVING WAY TO A MOSTLY DRY DAY STATEWIDE.

AT THE SURFACE...HIGH PRESSURE CENTERED FAR NNE OF THE ISLANDS WILL
GRADUALLY WEAKEN THROUGH THE WEEK...EVENTUALLY GETTING SPLIT INTO
TWO WEAKER CENTERS BY THURSDAY...AS A LOW DEVELOPS BETWEEN THE TWO.
THE LARGE SCALE PRESSURE GRADIENT WILL LOOSEN SOMEWHAT OVER THE
ISLANDS IN RESPONSE...BUT A GENERALLY MODERATE TRADE WIND FLOW IS
EXPECTED...WITH A MINIMA IN WIND SPEEDS FRI-SAT. THIS WILL KEEP
PASSING CLOUDS AND SHOWERS FOCUSED OVER WINDWARD SLOPES. WINDS ARE
EXPECTED TO TREND A LITTLE STRONGER TO START NEXT WEEK AS HIGH
PRESSURE BUILDS TO THE N...BUT NOT SIGNIFICANTLY.

THE OTHER SURFACE/LOW-LEVEL FEATURE OF INTEREST IS A WEAKENING LOW
PRESSURE SYSTEM...THE REMNANT CIRCULATION OF WHAT ONCE WAS HURRICANE
EUGENE...NOW CENTERED ABOUT 1075 MILES E OF THE BIG ISLAND...
TRAVELLING W NEAR 10 MPH. LATEST MODEL GUIDANCE WEAKENS THE SURFACE
LOW TO A TROUGH OVER THE NEXT TWO DAYS...BEFORE DISSIPATING THE
SURFACE TROUGH OVER WATERS E OF THE ISLANDS. DESPITE THIS WEAKENING
TREND...A SLUG OF LOW-LEVEL MOISTURE WILL CONTINUE TO TRACK W
WITHIN THE TRADE FLOW...AND IS EXPECTED TO TRAVERSE THE ISLAND CHAIN
FROM THURS NIGHT THROUGH SAT. TIMING AND IMPACTS OF THIS MOISTURE
WILL NEED TO BE FINE-TUNED...BUT WE CAN GENERALLY EXPECT INCREASED
WINDWARD SHOWERS AND HIGHER DEW POINTS TO COINCIDE WITH LIGHTER
TRADE WINDS...LIKELY MAKING IT FEEL MUGGY STATEWIDE. THIS MOISTURE
SHOULD BE W OF ALL ISLANDS BY SUN...WITH TYPICAL SUMMERTIME TRADE
WIND WEATHER RESUMING.

ALOFT...A BROAD AND WEAK E-W ORIENTED TROUGH AXIS IS CURRENTLY
LOCATED WELL N OF THE ISLANDS NEAR 29N. THE TROUGH WILL REMAIN IN
PLACE FOR THE NEXT COUPLE OF DAYS WITH LITTLE CHANGE...BUT A LOW
DEVELOPING WITHIN THE TROUGH IS FORECAST TO DIG S ON THU AND
FRI...MOVING INTO A POSITION NW OF KAUAI. WHILE THIS LOW MOVES NE ON
SAT AND SUN...A SHARP TROUGH AXIS ALOFT WILL PERSIST JUST NW OF
KAUAI INTO NEXT WEEK. INCREASED HIGH-LEVEL MOISTURE IS EXPECTED TO
BEGIN MOVING OVER THE ISLANDS AS EARLY AS TONIGHT...AND PERSISTING
INTO FRI...INCREASING THE POTENTIAL FOR CLOUDIER THAN NORMAL SKIES
DUE TO THICKENING HIGH CLOUDS. THE PRESENCE OF THE UPPER LOW TO THE
NW AND THE LOW-LEVEL MOISTURE INCREASE FROM THE E COULD LEAD TO
POCKETS OF HEAVY SHOWERS ON FRI/SAT...BUT AS NOTED WITHIN THE
HAWAII EXTENDED FORECAST DISCUSSION /PMDHI/ ISSUED BY HPC...NONE OF
THE CURRENT SUITE OF MODELS FORECASTS HEAVY RAIN.

&&

.AVIATION...
ISOL MVFR CEILINGS AND VIS FOR WINDWARD TERMINALS. OTHERWISE...VFR
CONDS WILL PREVAIL.

&&

.MARINE...
SURF WILL REMAIN ON THE SMALL SIDE UNTIL THE SECOND HALF OF THE
WEEK. SOUTHERN HEMI GROUNDSWELL WILL PUSH SURF BACK INTO THE CHEST
TO SHOULDER HIGH RANGE FRI-SAT. TOP AREAS MAY EVEN PULL IN SOME HEAD
HIGH-1 FT OVERHEAD SETS ON THE RIGHT TIDE. THE SURF SITUATION IS
LOOKING GOOD FOR MID MONTH. PLEASE CHECK OUT THE SURF FORECAST
PRODUCT FOR GREATER DETAIL...NEXT UPDATE WED 3PM HST.

MODERATE TRADES/SEAS WILL PREVAIL THIS WEEK WITH NO SCA CRITERIA
EXPECTED...NOT EVEN IN THE CHANNELS.

&&

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

&&

$$