Weather Service Report


913 
FXHW60 PHFO 172052 AAA
AFDHFO

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
1052 AM HST THU APR 17 2014

UPDATED AVIATION SECTION

.SYNOPSIS...
TRADE WIND WEATHER WILL CONTINUE ACROSS THE MAIN HAWAIIAN ISLANDS 
THROUGH THE FORECAST PERIOD AS HIGH PRESSURE DOMINATES TO OUR NORTH. 
CLOUDS AND SHOWERS WILL FAVOR WINDWARD AND MAUKA AREAS. CLOUDS AND 
SHOWERS WILL INCREASE EACH AFTERNOON AND EVENING ACROSS LEEWARD 
PORTIONS OF THE BIG ISLAND. TOTAL RAINFALL WILL BE LIGHT.

&&

.DISCUSSION...
A 1025 MB HIGH NEAR 30N148W...ABOUT 870 MILES NORTHEAST OF THE MAIN 
HAWAIIAN ISLANDS...HAS AN ASSOCIATED RIDGE EXTENDING EAST THROUGH 
30N140W AND WEST THROUGH 25N180W. THE PRESSURE GRADIENT BETWEEN THIS 
RIDGE AND EQUATORIAL LOW PRESSURE IS SUFFICIENTLY STEEP TO SUPPORT 
MODERATE TO LOCALLY STRONG EASTERLY TRADE WINDS ACROSS LOCAL WATERS 
THIS MORNING. SMALL CRAFT ADVISORIES REMAIN IN EFFECT FOR THE 
TRADITIONALLY WINDIER CHANNELS AND WATERS AROUND MAUI AND THE BIG 
ISLAND THROUGH FRIDAY AFTERNOON. SHOWERS EMBEDDED WITHIN THE TRADES 
CONTINUE TO SEE ENHANCEMENT FROM A PERSISTENT UPPER TROUGH ACROSS 
THE ISLANDS. A RAGGED BAND OF HIGH CLOUDS EXTENDING SOUTHWEST 
THROUGH NORTHEAST ACROSS THE BIG ISLAND AND ADJACENT WATERS MARKS 
THE EASTERN FLANK OF THIS UPPER TROUGH. AIRMASS STABILITY VARIES 
FROM SLIGHTLY UNSTABLE NEAR KAUAI WITH CAPE OVER 600 J/KG AND A 
NEGATIVE LIFTED INDEX VALUE...TO STABLE OVER THE BIG ISLAND WITH 
CAPE LESS THAN 100 J/KG AND A POSITIVE LIFTED INDEX VALUE. THE TRADE 
INVERSION FROM YESTERDAY IS HIGHER AND WEAKER THIS 
MORNING...INDICATING THAT THE UPPER TROUGH DOES INDEED SEEM TO BE 
HAVING A DESTABILIZING EFFECT. HOWEVER...LIMITED MOISTURE WEST OF 
MAUI HAS KEPT SHOWERS ISOLATED AND LIGHT ACROSS THE WESTERN HALF OF 
THE STATE. A LOOSE BAND OF LOW CLOUDS AND SHOWERS ACROSS MAUI COUNTY 
AND THE BIG ISLAND IS STEADILY MOVING WEST AND MAY INCREASE RAINFALL 
SLIGHTLY ACROSS OAHU AND KAUAI LATER TODAY.

MODELS SHOW STRONGER HIGH PRESSURE WILL BUILD ALONG THE RIDGE 
NORTHWEST OF THE MAIN ISLANDS OVER THE WEEKEND...THEN PUSH EASTWARD 
EARLY NEXT WEEK. THIS WILL GIVE A SLIGHT BOOST TO TRADE WINDS ACROSS 
LOCAL WATERS FROM SUNDAY ONWARD AS THE GRADIENT STEEPENS. THE UPPER 
TROUGH WILL PERSIST INTO THE MIDDLE OF NEXT WEEK BEFORE 
FILLING...LIKELY KEEPING TRADE SHOWERS ACTIVE ESPECIALLY AT NIGHT. 

IN GENERAL...EXPECT TRADE WIND WEATHER TO CONTINUE THROUGH THE REST 
OF THE FORECAST PERIOD WITH RAINFALL FOCUSING OVER WINDWARD AND 
MOUNTAIN AREAS. THE MAIN EXCEPTION WILL BE THE LEEWARD SIDE OF THE 
BIG ISLAND...WHERE AFTERNOON AND EARLY EVENING CLOUDS AND SHOWERS 
ARE EXPECTED EACH DAY AS ONSHORE FLOW DEVELOPS. 

&&

.AVIATION...
UPDATE...STRONG WIND SHEAR AND DEFORMATION ACCOMPANY THE UPPER 
TROUGH THAT EXTENDS ACROSS THE STATE. GFS GUIDANCE HIGHLIGHTS THE 
GREATEST POTENTIAL FOR CLEAR-AIR TURBULENCE IN THE AREA TO THE 
NORTHEAST OF THE STATE. HOWEVER...RECENT WATER VAPOR IMAGERY SHOWS 
THIS AREA MOVING WESTWARD. HAVE UPDATED THE AIRMET TO INCLUDE A 
MENTION OF MODERATE HIGH-LEVEL TURBULENCE ACROSS THE STATE...AND 
ALSO BROUGHT SIGMET OSCAR FOR OCCASIONAL SEVERE TURBULENCE INTO THE 
WINDWARD COASTAL WATERS AS WELL. THIS POTENTIAL FOR HIGH-LEVEL 
TURBULENCE MAY CONTINUE THROUGH THE NIGHT. 

RADAR VAD WIND PROFILES AND 17Z-18Z ACARS SOUNDINGS FROM HONOLULU 
SHOW 20-25 KT OF WIND BELOW THE INVERSION. WE RECEIVED A COUPLE 
REPORTS OF MODERATE TURBULENCE YESTERDAY BUT NONE SO FAR TODAY. WILL 
KEEP THE AIRMET GOING FOR MODERATE LOW-LEVEL TURBULENCE. GFS MODEL 
CROSS SECTIONS SHOW LOW-LEVEL WINDS REACHING A MINIMUM THIS 
AFTERNOON INTO EARLY FRIDAY...THEN STRENGTHENING AGAIN AS WE HEAD 
INTO THE WEEKEND. IF WE DO END UP DROPPING THE AIRMET...IT WILL ONLY 
BE FOR A BRIEF PERIOD OF TIME AS CONDITIONS CONDUCIVE TO TURBULENCE 
QUICKLY REDEVELOP. 

CLOUDS AND SHOWERS REMAIN FOCUSED PRIMARILY ACROSS WINDWARD 
SECTIONS. CONDITIONS WILL BE PREDOMINATELY VFR...WITH OCCASIONAL 
MVFR CEILINGS AND VISIBILITIES IN SHOWERS. AN AREA OF MOISTURE EAST 
OF MAUI COUNTY WILL SPREAD WESTWARD...ENHANCING SHOWERS AND BRINGING 
MORE FREQUENT MVFR CONDITIONS TONIGHT INTO FRIDAY MORNING. WE MAY 
EVENTUALLY NEED AN AIRMET FOR MOUNTAIN OBSCURATION IF THE LOW 
CEILINGS BECOME MORE WIDESPREAD AND PERSISTENT. 

&&

.MARINE...
A SMALL CRAFT ADVISORY REMAINS IN EFFECT THROUGH FRIDAY AFTERNOON 
FOR TYPICALLY WINDIER AREAS AROUND MAUI AND THE BIG ISLAND. 

A SMALL SWELL FROM THE WEST NORTHWEST WILL PEAK BELOW ADVISORY 
LEVELS LATER TODAY. NO OTHER SIGNIFICANT SWELLS ARE EXPECTED.

&&

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

&&

$$

DISCUSSION/MARINE...POWELL
AVIATION...BRAVENDER



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