Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 220605
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST THU AUG 21 2014

.SYNOPSIS...
TRADE WINDS WILL CONTINUE WITH MINOR CHANGES IN SPEED INTO NEXT
WEEK. SHOWERS GENERALLY WILL BE LIGHT...AND LIMITED TO SLOPES OF THE
BIG ISLAND EACH AFTERNOON...AND WINDWARD SLOPES ACROSS THE STATE ALL
DAY. HOWEVER...SHOWERS MAY BE SLIGHTLY MORE ACTIVE FROM SUNDAY NIGHT
INTO MONDAY.

&&

.DISCUSSION...
ALTHOUGH SATELLITE MEASUREMENTS AND SOME IN-SITU DATA STILL DETECTED
THE BUBBLE OF MOIST AIR AROUND THE ISLANDS...NOT ALL OF THIS
MOISTURE MADE IT ASHORE. MORE IMPORTANTLY...SUBSIDENCE FROM A
MID-LEVEL RIDGE EXPANDING OVER THE STATE FROM THE NORTHEAST HELPED
TO SUPPRESS SHOWER DEVELOPMENT. THUS...THE SHORT TERM MAY TURN OUT A
BIT LESS SHOWERY THAN GUIDANCE HAD SUGGESTED EARLIER...ESPECIALLY
OVER MAUI AND HAWAII COUNTIES.

HOWEVER...SOLUTIONS LARGELY AGREED THAT THE EXPANSION OF THE RIDGE
ALOFT WILL BE BRIEF...AND THAT BROAD...SHALLOW TROUGHING THEN WILL
EXPAND OVER THE STATE FROM THE SOUTHWEST INTO THE WEEKEND. THE
RESULTING REDUCTION IN SUBSIDENCE WILL TEND TO COUNTER A DECREASE IN
PRECIPITABLE WATER UPWIND...SO THE PRACTICAL CHANGE IN TRADE SHOWERS
LIKELY WILL BE...SUBTLE. THE SHALLOW TROUGH AND WEAK RIDGE THEN WILL
ALTERNATE EXPANDING OVER THE STATE AND RETREATING...RESULTING IN
LITTLE NET EFFECT LOCALLY.

FROM SUNDAY INTO MONDAY...GUIDANCE STILL AGREED THAT A MOISTER AIR
MASS MAY PASS OVER THE ISLANDS. HOWEVER...THE PHASE OF THE
ALTERNATING WEAK RIDGING AND WEAK TROUGHING ALOFT WILL DETERMINE
WHETHER SHOWERS CAN UTILIZE THE FULL POTENTIAL OF THIS PRECIPITABLE
WATER. AS IS TYPICAL DURING TRADE-WIND WEATHER...SOLUTIONS PROBABLY
EXAGGERATED THE AMPLITUDE OF THESE CHANGES.

OTHERWISE...GUIDANCE PREDICTED THE SUBTROPICAL RIDGE TO THE NORTH TO
REMAIN IN PLACE THROUGH THE FORECAST PERIOD...KEEPING THE TRADE
WINDS BLOWING OVER THE ISLANDS. SIMILARLY...PRECIPITATION WILL FOCUS
OVER SLOPES WHERE OROGRAPHIC FORCING IS STRONGEST.

&&

.AVIATION...
TRADE SHOWERS MAY BE A BIT MORE ACTIVE THAN NORMAL OVERNIGHT ACROSS
THE WINDWARD SECTIONS OF OAHU AND KAUAI...WHERE AN AREA OF DEEPER
MOISTURE IS MOVING THROUGH. AIRMET SIERRA FOR MOUNTAIN OBSCURATION
MAY BECOME NECESSARY IN THOSE AREAS LATER ON IF SHOWERS AND CLOUDS
INCREASE.

OTHERWISE...EXPECT VFR CONDITIONS TO PREVAIL ACROSS THE STATE
THROUGH FRIDAY.

&&

.MARINE...
WINDS LIKELY WILL PERSIST JUST BELOW THE THRESHOLD FOR A SMALL CRAFT
ADVISORY IN TYPICALLY WINDY AREAS AROUND MAUI COUNTY AND THE BIG
ISLAND THROUGH THE REST OF THE WEEK.

A HIGH SURF ADVISORY IS IN EFFECT FOR MOST SOUTH-FACING SHORES
THROUGH FRIDAY FOR SURF PRODUCED BY A SOUTH SWELL. A SERIES OF
SMALLER SOUTH SWELLS WILL CONTINUE TO ARRIVE THROUGH EARLY NEXT
WEEK...BUT THEY SHOULD PRODUCE SURF BELOW ADVISORY LEVELS.
SHORT-PERIOD TRADE-WIND SWELL WILL CONTINUE ALONG EAST-FACING
SHORES. NO SIGNIFICANT NORTHWEST SWELL IS EXPECTED THROUGH THE
WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST FRIDAY FOR NIIHAU-KAUAI
WINDWARD-KAUAI LEEWARD-OAHU SOUTH SHORE-WAIANAE COAST-MOLOKAI
LEEWARD-LANAI-KAHOOLAWE-MAUI LEEWARD WEST-MAUI CENTRAL VALLEY-
LEEWARD HALEAKALA-KONA-SOUTH BIG ISLAND-KOHALA.

&&

$$

DISCUSSION/MARINE...RYSHKO
AVIATION...JACOBSON




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