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Hawaiian Islands Synoptic Discussion and Guidance

684
FXHW60 PHFO 010637
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
837 PM HST THU JUL 31 2014

.SYNOPSIS...
HIGH PRESSURE FAR NORTH OF THE STATE WILL KEEP TRADE WINDS BLOWING
THROUGH THE WEEKEND AND INTO NEXT WEEK. ENHANCED MOISTURE MOVING
ACROSS THE ISLANDS FROM LATE TONIGHT THROUGH SATURDAY WILL RESULT
IN AN INCREASE IN SHOWERS...WITH SOME LOCALLY HEAVY RAINFALL
POSSIBLE. DRIER AND MORE STABLE WEATHER IS EXPECTED FROM SUNDAY
THROUGH THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...
RADAR LOOPS AND RAIN GAUGE DATA INDICATE A MOSTLY DRY EVENING THUS
FAR ACROSS THE HAWAIIAN ISLANDS...WITH LIGHT RAINFALL AMOUNTS OVER
SOME WINDWARD AREAS. SURFACE ANALYSIS DEPICTS AN ELONGATED HIGH
PRESSURE RIDGE IN ITS TYPICAL LOCATION OVER THE NORTHEAST PACIFIC
OCEAN...AND THE REMNANT LOW OF FORMER TROPICAL DEPRESSION
GENEVIEVE CENTERED ABOUT 520 MILES SOUTH SOUTHEAST OF HILO.
MODERATE TO LOCALLY BREEZY TRADE WINDS PREVAIL ACROSS THE STATE.
MIMIC-TPW SATELLITE IMAGERY INDICATES A SIGNIFICANT MOISTURE
GRADIENT OVER THE AREA...WITH ESTIMATED PWATS OF UP TO 2 INCHES
JUST SOUTHEAST OF THE BIG ISLAND...AND PWATS OF 1.3-1.5 INCHES
FROM MAUI THROUGH KAUAI. THE 00Z SOUNDINGS INDICATE THIS
MOISTURE GRADIENT AS WELL...ALONG WITH RELATIVELY WEAK INVERSIONS
BASED NEAR 7000 FEET. ALOFT...A WEAK MID/UPPER LEVEL TROUGH
EXTENDS FROM NEAR 30N160W SOUTHWARD TO THE WEST OF KAUAI...AND A
MID LEVEL RIDGE AXIS EXTENDS WESTWARD ALONG 20N TO NEAR THE BIG
ISLAND.

OVERNIGHT...THE LEADING AREA OF ENHANCED TROPICAL MOISTURE WILL
CONTINUE SPREADING ACROSS THE BIG ISLAND AND MAUI. WHILE PWATS
WILL BE WELL ABOVE NORMAL (1.5 TO 2 INCHES)...IT APPEARS THAT
THE DEEPEST TROPICAL MOISTURE (PWATS GREATER THAN 2 INCHES) WILL
REMAIN SOUTH OF THE BIG ISLAND DURING THE NIGHT. THERE SHOULD BE
SUFFICIENT MOISTURE TO PRODUCE MORE WIDESPREAD SHOWERS ACROSS THE
FAVORED WINDWARD AREAS LATE TONIGHT. IN THE EVENING FORECAST
UPDATE...HAVE DELAYED THE TIMING OF HIGH POPS AND LOCALLY HEAVY
RAINFALL FOR WINDWARD BIG ISLAND AND MAUI UNTIL LATE TONIGHT.
FURTHER WEST FROM MOLOKAI THROUGH KAUAI...WOULD EXPECT A TYPICAL
TRADE WIND PATTERN OVERNIGHT WITH SCATTERED MOSTLY LIGHT SHOWERS
FAVORING THE LATE NIGHT HOURS.

ON FRIDAY...ANOTHER BAND OF MORE SUBSTANTIAL TROPICAL MOISTURE
WHICH IS CURRENTLY LOCATED NEAR 20N 150W WILL MOVE WESTWARD
TOWARD THE BIG ISLAND. LOCALLY HEAVY RAINS WILL REMAIN POSSIBLE
ACROSS THE EASTERN HALF OF THE STATE...WITH WINDWARD CLOUDS
AND SHOWERS GENERALLY INCREASING STATEWIDE.

TROPICAL MOISTURE SHOULD PEAK FRIDAY EVENING ACROSS THE BIG
ISLAND...AND LATE FRIDAY NIGHT THROUGH SATURDAY MORNING ACROSS THE
SMALLER ISLANDS. MODERATE INSTABILITY WILL BE IN PLACE...BUT THE
RISK FOR THUNDERSTORMS WOULD STILL SEEM TO BE QUITE LOW DUE TO
RELATIVELY WARM MID LEVEL TEMPERATURES. WITH NO UPPER TROUGH IN
PLACE TO OUR WEST...IT APPEARS THAT THIS WILL PROBABLY BE MORE OF
AN OROGRAPHICALLY DRIVEN EVENT WITH LOCALLY HEAVY RAINS MOST
LIKELY ON THE WINDWARD AREAS...IN CONTRAST TO THE MORE WIDESPREAD
HEAVY RAINS AND THUNDERSTORMS THAT WERE OBSERVED A COUPLE OF WEEKS
AGO WHEN A PREVIOUS BOUT OF TROPICAL MOISTURE MOVED THROUGH THE
STATE. THE FLASH FLOODING THREAT APPEARS TO BE LOW...BUT WOULD NOT
BE SURPRISED TO SEE SOME FLOOD ADVISORIES ISSUED DURING THE NEXT
COUPLE OF DAYS.

THE AREA OF ENHANCED MOISTURE WILL SHIFT WEST OF THE STATE
SATURDAY NIGHT. A RATHER DRY MODERATE TRADE WIND PATTERN SHOULD
THEN PREVAIL FROM SUNDAY THROUGH AT LEAST THE MIDDLE OF NEXT
WEEK...WITH GENERALLY BELOW NORMAL PWATS AND RATHER STRONG RIDGING
ALOFT PREVAILING.

&&

.AVIATION...
INCREASING TROPICAL MOISTURE IS MOVING INTO THE BIG ISLAND THIS
EVENING FROM THE EAST...BRINGING INCREASED LOW CLOUDS AND SHOWERS
THAT WILL PRODUCE REDUCED VISIBILITIES AND LOWER CEILINGS. IN
ANTICIPATION OF THESE DETERIORATING CONDITIONS OVER THE WINDWARD BIG
ISLAND...AIRMET SIERRA HAS BEEN ISSUED FOR MOUNTAIN OBSCURATION FROM
UPOLU POINT TO CAPE KUMUKAHI TO SOUTH CAPE. ELSEWHERE...VFR
CONDITIONS ARE EXPECTED TO PREVAIL ACROSS MOST OF THE SMALLER
ISLANDS TONIGHT...WITH BRIEF MVFR CONDITIONS POSSIBLE ALONG WINDWARD
FACING SLOPES OF THESE ISLANDS...ESPECIALLY AFTER MIDNIGHT HST.

AS A RESULT OF THE MODERATE TO LOCALLY BREEZY LOW-LEVEL TRADE WIND
FLOW...AIRMET TANGO REMAINS IN EFFECT FOR LOW-LEVEL TURBULENCE BELOW
7 KFT IMMEDIATELY SOUTH THROUGH WEST OF THE HIGHER TERRAIN ON ALL
ISLANDS. THIS AIRMET WILL LIKELY REMAIN IN EFFECT AT LEAST THROUGH
FRIDAY AFTERNOON.

&&

.MARINE...
A 2027Z ASCAT PASS DID NOT FIND ANY 25 KNOT WINDS ACROSS THE
EASTERN HALF OF THE HAWAIIAN COASTAL WATERS. HOWEVER IT IS
POSSIBLE THAT THE PRESSURE GRADIENT OVER THE AREA HAS TIGHTENED
SOMEWHAT SINCE THEN...AS THE REMNANT LOW OF FORMER TROPICAL
DEPRESSION GENEVIEVE MOVES WESTWARD TOWARD ITS CLOSEST APPROACH
POINT WELL TO THE SOUTH OF THE BIG ISLAND. WILL MAINTAIN THE
EXISTING SMALL CRAFT ADVISORY FOR THE TYPICALLY WINDIER AREAS
AROUND MAUI COUNTY AND BIG ISLAND.

MODERATE SOUTH SWELL AND SURF WILL CONTINUE ON SOUTH FACING
SHORES THROUGH FRIDAY...THEN A REINFORCING SOUTH SWELL SHOULD
INCREASE SURF FURTHER ON SATURDAY...POSSIBLY APPROACHING ADVISORY
LEVELS. THIS SURF EVENT SHOULD PEAK LATER SATURDAY THEN SLOWLY
DECREASE SUNDAY THROUGH MONDAY. OTHERWISE...PREVAILING TRADE
WINDS WILL CONTINUE TO PRODUCE SHORT PERIOD SWELL AND CHOPPY
SURF ALONG EAST FACING SHORES THROUGH THE NEXT SEVERAL DAYS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST SUNDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

&&

$$

DISCUSSION/MARINE...JACOBSON
AVIATION...HOUSTON




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