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Hawaiian Islands Synoptic Discussion and Guidance

274
FXHW60 PHFO 260654
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SAT OCT 25 2014

.SYNOPSIS...
LOW CLOUDS AND SHOWERS ASSOCIATED WITH AN OLD FRONTAL BAND ARE
MOVING DOWN OVER THE ISLAND CHAIN THROUGH EARLY SUNDAY. A WEAKENING
SURFACE RIDGE NORTH OF THE ALOHA STATE WILL MAINTAIN LIGHT TO
MODERATE TRADE WINDS THROUGH THE MIDDLE OF NEXT WEEK. THE TRADES
WILL KEEP LOW CLOUDS AND SHOWERS FOCUSED ACROSS WINDWARD AND MAUKA
AREAS. A COLD FRONT IS EXPECTED TO PUSH THROUGH THE ISLANDS LATE
NEXT WEEK...BRINGING ANOTHER INCREASE IN SHOWERS FROM THURSDAY NIGHT
INTO FRIDAY. HEADING INTO NEXT WEEKEND...STRONGER TRADES...COOLER
AIR...AND LOWER HUMIDITY WILL FOLLOW BEHIND THE FRONT.

&&

.DISCUSSION...
A SURFACE RIDGE EXTENDS WEST SOUTHWEST FROM A 1026 MB SURFACE HIGH
NEAR 34N 141W THROUGH A POINT ABOUT 350 MILES NORTH OF HONOLULU. THE
PRESSURE GRADIENT SOUTH OF THIS SURFACE RIDGE WILL MAINTAIN LIGHT TO
MODERATE TRADE WINDS ACROSS THE ISLAND CHAIN. A BAND OF LOW CLOUDS
AND SHOWERS ASSOCIATED WITH AN OLD FRONT IS CURRENTLY LOCATED OVER
THE COASTLINES AND ADJACENT WINDWARD WATERS OF THE ISLANDS FROM THE
BIG ISLAND TO MOLOKAI. THIS BAND ALSO EXTENDS FAR EAST NORTHEAST
FROM THE BIG ISLAND. NOTE THAT ISOLATED SHOWERS AHEAD OF THIS BAND
ARE ALREADY MOVING INTO WINDWARD OAHU.

WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE OVER THE STATE...WITH
AN UPPER LEVEL TROUGH FAR TO THE EAST AND AN UPPER LEVEL LOW WEST
SOUTHWEST OF KAUAI. THE RIDGE ALOFT IS MAINTAINING RELATIVELY STABLE
ATMOSPHERIC CONDITIONS ACROSS THE REGION. EVEN WITH THESE STABLE
CONDITIONS...THE EARLY AFTERNOON SOUNDINGS FROM HILO AND LIHUE
INDICATED WEAK AND ELEVATED INVERSIONS NEAR 9 TO 10 THOUSAND FEET.
THE MIMIC PRECIPITABLE WATER VALUES SHOW HIGHER VALUES ASSOCIATED
WITH THE OLD FRONT ARE NOW LOCATED OVER THE WINDWARD BIG ISLAND...
EVEN THOUGH THE VALUES AT HILO FROM 200 PM HST WERE ONLY 1.09
INCHES.

THIS BAND OF LOW CLOUDS IS FORECAST TO PUSH THROUGH MOST OF THE AREA
BY SUNDAY AFTERNOON/EVENING. THE FORECAST MODEL GUIDANCE SHOWS
STABLE CONDITIONS AND DECREASING MOISTURE DEPTH DURING SUNDAY NIGHT
AND MONDAY. THE SURFACE RIDGE NORTH OF THE ISLANDS WILL MAINTAIN
LIGHT TO MODERATE TRADE WINDS. THE LOW-LEVEL TRADE WIND FLOW WILL
KEEP SHOWERS FOCUSED MAINLY ACROSS WINDWARD AND MAUKA SECTIONS OF
THE STATE. WITH THE UPPER LEVEL RIDGE REMAINING IN THE AREA...WE
EXPECT RAINFALL AMOUNTS WILL MEAGER DUE TO THE DRIER AND MORE STABLE
AIR MASS EARLY NEXT WEEK. CONDITIONS START TO CHANGE BY THE MIDDLE
OF NEXT WEEK AS THE UPPER LEVEL LOW TO THE WEST SOUTHWEST STARTS
MOVING TOWARD THE ISLANDS. THIS LOW ALOFT IS EXPECTED TO WEAKEN AS
IT MOVES INTO THE VICINITY OF THE STATE. HOWEVER...THE MAIN
INFLUENCE HAS THE MAIN EFFECT OF WEAKENING AND RAISING THE INVERSION
HEIGHT BY TUESDAY/WEDNESDAY AND ENHANCING TRADE WIND SHOWERS.

TOWARD THE END OF NEXT WEEK...A FRONT APPROACHING FROM THE NORTH MAY
BRING A NOTICEABLE CHANGE IN LOCAL WEATHER CONDITIONS. AS A NEW HIGH
PASSES FAR NORTH OF THE STATE...GUIDANCE REMAINS IN GOOD AGREEMENT
THAT A COLD FRONT WILL PUSH DOWN THROUGH THE ISLANDS. THIS FRONT
WILL BRING A BAND OF CLOUDS AND SHOWERS FROM THURSDAY NIGHT INTO
FRIDAY. BREEZY TRADES AND COOLER...DRIER AIR WILL FOLLOW BEHIND THIS
FRONT LATE NEXT WEEK. THE FORECAST REFLECTS THE CURRENT TIMING OF
THE FRONT...WITH AN INCREASE IN SHOWERS FROM LATE THURSDAY INTO
FRIDAY. THERE ARE STILL SOME UNCERTAINTIES IN THE TIMING OF THIS
FRONT AND ITS ASSOCIATED CHANGES TO OUR WEATHER...SO THERE MAY BE
ADJUSTMENTS TO THE FORECAST UNTIL WE GET CLOSER TO THE EVENT.

&&

.AVIATION...
AN AREA OF WIDESPREAD LOW CLOUDS AND SHOWERS ASSOCIATED WITH AN OLD
FRONT CONTINUES TO SAG DOWN OVER THE ISLANDS THIS EVENING. EXPECT
FREQUENT SHOWERS ALONG WINDWARD AND MAUKA AREAS OF THE BIG ISLAND
AND MAUI COUNTY EARLY THIS EVENING. THESE SHOWERS WILL EVENTUALLY
SPREAD OVER OAHU AND KAUAI LATER THIS EVENING. BRIEF PERIODS OF MVFR
DUE TO LOW CEILINGS AND REDUCED VISIBILITIES ARE LIKELY IN SOME OF
THE HEAVIER SHOWERS...ESPECIALLY ALONG WINDWARD FACING SLOPES AND
HIGHER TERRAIN. SOME OF THESE SHOWERS WILL BE CARRIED OVER TO
LEEWARD SECTIONS OF THE SMALLER ISLANDS.

AIRMET SIERRA HAS BEEN POSTED FOR MOUNTAIN OBSCURATION FOR THE
HIGHER TERRAIN ABOVE 2500 FEET ON THE ISLANDS FROM THE BIG ISLAND TO
OAHU DUE TO THE LOW CLOUDS AND SHOWERS ASSOCIATED WITH THE OLD
FRONTAL BAND. THIS AIRMET MAY NEED TO BE EXTENDED TO KAUAI LATER
TONIGHT.

&&

.MARINE...
THE GENERAL TRADE WIND SPEEDS ARE EXPECTED TO REMAIN IN THE LIGHT TO
MODERATE CATEGORY...WITH LOCALLY STRONGER WINDS IN THE TYPICALLY
WINDIER WATERS ADJACENT TO THE ISLANDS OF MAUI AND HAWAII
COUNTIES...THROUGH THE MIDDLE OF NEXT WEEK. NO SMALL CRAFT ADVISORY
/SCA/ CONDITIONS ARE IN THE FORECAST THROUGH WEDNESDAY. THE TRADE
WIND SPEEDS ARE FORECAST TO RAMP UP NEXT THURSDAY AND FRIDAY AS A
STRONG SURFACE HIGH BUILDS NORTH OF THE STATE. THIS WILL LIKELY
REQUIRE THE ISSUANCE OF A SCA FOR PORTIONS OF MOST OF THE COASTAL
WATERS LATE NEXT WEEK.

A SERIES OF SMALL NORTH AND NORTHWEST SWELLS WILL CONTINUE TO
PRODUCE MODEST SURF ALONG NORTH FACING SHORES OF THE ISLAND CHAIN
INTO WEDNESDAY. A SLIGHTLY LARGER NORTH SWELL IS EXPECTED DURING THE
SECOND HALF OF NEXT WEEK. ALSO...LARGER SHORT-PERIOD EASTERLY SWELLS
ARE EXPECTED BY FRIDAY AS THE TRADE WINDS STRENGTHEN. AS A
RESULT...SURF HEIGHTS MAY APPROACH THE HIGH SURF ADVISORY CRITERIA
ALONG EAST FACING SHORES LATE NEXT WEEK.

&&

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

&&

$$

HOUSTON



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