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Hawaiian Islands Synoptic Discussion and Guidance

916
FXHW60 PHFO 260145
AFDHFO

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

.SYNOPSIS...
REMNANT MOISTURE FROM AN OLD FRONT WILL INCREASE CLOUDS AND SHOWERS
OVER THE ISLANDS TONIGHT AND EARLY SUNDAY. A WEAKENING RIDGE NORTH
OF THE STATE WILL KEEP MODERATE TO LIGHT TRADE WINDS THROUGH MID
WEEK...WITH SHOWERS REMAINING FOCUSED ACROSS WINDWARD AND MAUKA
AREAS. A COLD FRONT IS EXPECTED TO PUSH THROUGH THE ISLANDS LATE IN
THE WEEK...BRINGING ANOTHER INCREASE IN SHOWERS THURSDAY NIGHT INTO
FRIDAY. HEADING INTO NEXT WEEKEND...STRONGER TRADES...COOLER
AIR...AND LOWER HUMIDITY WILL FOLLOWING BEHIND THE FRONT.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE OVER THE STATE...WITH
A TROUGH FAR TO THE EAST AND A LOW TO THE WEST-SOUTHWEST. AT THE
SURFACE...HIGH PRESSURE IS LOCATED FAR NORTHEAST OF THE STATE...WITH
A RIDGE PASSING ABOUT 300 MILES NORTH OF KAUAI. A REMNANT CLOUD BAND
FROM AN OLD FRONT IS LOCATED OVER THE WINDWARD COASTAL AND OFFSHORE
WATERS...AND ALSO EXTENDS FAR EAST-NORTHEAST FROM THE BIG ISLAND.
00Z SOUNDINGS FROM HILO AND LIHUE SHOW WEAK AND ELEVATED INVERSIONS
NEAR 9-10KFT...ALTHOUGH PRECIPITABLE WATERS RANGE FROM 1.47 AT LIHUE
TO 1.09 AT HILO. THIS UNUSUALLY DRY AIR OVER THE EASTERN HALF OF THE
STATE /AVERAGE PW AT HILO IS 1.45 INCHES IN OCTOBER/ WILL BE SHORT
LIVED...AS EARLY AFTERNOON MIMIC TOTAL PRECIPITABLE WATER IMAGERY
SHOWS HIGHER VALUES ASSOCIATED WITH THE OLD FRONT LOCATED JUST
NORTHEAST AND EAST OF THE STATE.

RAINFALL AMOUNTS HAVE BEEN ON THE LOW SIDE THIS AFTERNOON...AT LEAST
ACROSS THE EASTERN HALF OF THE STATE WHERE MOISTURE HAS BEEN MORE
LIMITED. EVEN ACROSS KAUAI AND OAHU...RAINFALL AMOUNTS HAVE BEEN
ABOUT A QUARTER INCH OR LESS OVER THE PAST 12 HOURS. WINDWARD CLOUDS
HAVE ALL BUT DISSIPATED ACROSS EAST MAUI AND THE BIG ISLAND.
HOWEVER...THE BAND OF UNSTABLE SHOWERY CLOUDS ASSOCIATED WITH THE
OLD FRONT IS LOCATED JUST OFFSHORE OF THE WINDWARD COASTS...AND A
LINEAR EXTRAPOLATION OF THE CLOUD MOTION BRINGS THEM INTO THE
WINDWARD SECTIONS WITHIN THE NEXT 1-2 HOURS. THEREFORE WILL START
THE FORECAST WITH HIGH POPS THIS EVENING AS CONDITIONS WILL
DETERIORATE LATE THIS AFTERNOON.

THIS MOISTURE AXIS WILL PUSH THROUGH MOST OF THE AREA BY SUNDAY
AFTERNOON/EVENING. FORECAST SOUNDINGS AND TIME SECTIONS SHOW
CONDITIONS STABILIZING/MOISTURE DEPTH DECREASING FOR SUNDAY
NIGHT/MONDAY. LIGHT TO MODERATE TRADE WINDS WILL KEEP SHOWERS
FOCUSED MAINLY ACROSS WINDWARD SECTIONS...BUT RAINFALL AMOUNTS WILL
BE ON THE LIGHT SIDE DUE TO THE DRIER AND MORE STABLE AIRMASS.
CONDITIONS START TO CHANGE BY THE MIDDLE OF THE WEEK AS THE LOW TO
OUR SOUTHWEST MOVES TOWARD THE ISLANDS. THIS LOW IS EXPECTED TO
WEAKEN AS IT MOVES INTO THE RIDGE OVER AND TO THE EAST OF THE STATE.
HOWEVER...IT HAS THE EFFECT OF WEAKENING AND RAISING THE INVERSION
HEIGHT BY TUESDAY/WEDNESDAY AND ENHANCING TRADE WIND SHOWERS.

BY THE END OF THE WEEK...A FRONT APPROACHING FROM THE NORTH MAY
BRING A NOTICEABLE CHANGE IN AIRMASS. AS A NEW HIGH PASSES FAR NORTH
OF THE STATE...GUIDANCE REMAINS IN GOOD AGREEMENT PUSHING A COLD
FRONT THROUGH THE ISLANDS WITH A BAND OF CLOUDS AND SHOWERS THURSDAY
NIGHT INTO FRIDAY. BREEZY TRADES AND COOLER DRIER AIR LOOKS TO
FOLLOW BEHIND IT FOR THE WEEKEND. THE FORECAST REFLECTS THE CURRENT
TIMING OF THE FRONT...WITH AN INCREASE IN SHOWERS THURSDAY
NIGHT/FRIDAY...BUT THIS MAY NEED TO BE ADJUSTED AS TIMING OR
POSITION OF THE EXPECTED FRONT CHANGES.

&&

.AVIATION...
ISOLATED SHOWERS CONTINUE TO FOCUS OVER WINDWARD AND MAUKA
AREAS...WITH SOME BLOW OVER TO LEEWARD AREAS. BRIEF PERIODS OF MVFR
ARE LIKELY IN SHOWERS. AN AREA OF ENHANCED MOISTURE ASSOCIATED WITH
AN OLD FRONT CONTINUES TO MOVE TOWARDS THE ISLANDS. EXPECT FREQUENT
SHOWERS ALONG WINDWARD AND MAUKA AREAS OF THE BIG ISLAND AND MAUI
THIS EVENING AND OVER OAHU AND KAUAI TONIGHT. AIRMET FOR MOUNTAIN
OBSCURATION WILL LIKELY BE POSTED THIS EVENING AND SPREAD DOWN THE
ISLAND CHAIN AS LOWER CLOUDS AND SHOWERS INCREASE.

&&

.MARINE...
LARGE SCALE TRADE WINDS ARE EXPECTED TO REMAIN IN THE LIGHT TO
MODERATE CATEGORY...WHICH MEANS LOCALLY FRESH TRADES IN THE WINDIER
CHANNELS THROUGH THE MIDDLE OF THE WEEK. TRADE WINDS ARE EXPECTED TO
INCREASE NOTICEABLE THURSDAY AND FRIDAY AS A STRONG HIGH BUILDS IN
TO THE NORTH OF THE STATE.

A SERIES OF SMALL NORTH SWELLS WILL CONTINUE THROUGH WEDNESDAY...
WITH A SLIGHTLY LARGER NORTH SWELL EXPECTED DURING THE SECOND HALF
OF THE WEEK. LARGER SHORT PERIOD EASTERLY SWELLS AREA EXPECTED LATE
IN THE WEEK AS TRADE WINDS STRENGTHEN.

&&

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

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...EATON



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office