Small Craft Advisory 42, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1013 AM HST TUE NOV 1 2011

.SYNOPSIS...
BOUTS OF UNSETTLED WEATHER ARE IN STORE THIS WEEK. AN UPPER LEVEL
DISTURBANCE OVER THE STATE WILL POSE A THREAT FOR HEAVY
SHOWERS...WITH THUNDERSTORMS POSSIBLE OVER THE BIG ISLAND AND MAUI
TODAY. WITH GENTLE TO MODERATE TRADE WINDS BLOWING...SHOWERS WILL
FAVOR WINDWARD AND MAUKA AREAS...THOUGH LEEWARD AREAS WILL SEE
THEIR FAIR SHARE OF SHOWERS AS WELL. STRENGTHENING TRADE WINDS TONIGHT
AND WEDNESDAY WILL CARRY A MOIST AIR MASS OVER THE ISLANDS...
BRINGING IN WET TRADE WIND WEATHER THROUGH THE WEEKEND...WHILE
UNSTABLE CONDITIONS MAY LEAD TO PERIODS OF LOCALLY HEAVY SHOWERS.

&&

.DISCUSSION...
LOCALLY HEAVY RAINFALL REMAINS A THREAT TODAY FOR THE STATE WITH
THUNDERSTORMS POSSIBLE OVER MAUI AND THE BIG ISLAND. A FLASH FLOOD
WATCH WILL CONTINUE THROUGH TODAY OVER THE BIG ISLAND WITH THE
MOST ATMOSPHERIC INSTABILITY OVER THE ORCHID ISLE. CURRENT
RADAR...AS OF 9AM...SHOWS THUNDERSTORMS OVER NE AND SE COASTAL
WATERS OF THE BIG ISLAND...AND PASSING HEAVY SHOWERS OVER MOLOKAI
OAHU AND KAUAI...AND THE ASSOCIATED WINDWARD WATERS.

THERE ARE TWO MAIN CULPRITS FOR THE INCREASED RAINFALL AND
THUNDERSTORMS. THE FIRST...IS AN UPPER LEVEL LOW THAT HAS
BROUGHT COLD TEMPERATURES ALOFT OVER THE STATE. THE COLD AIR HAS
DESTABILIZED THE ATMOSPHERE...REMOVING OUR NORMAL TRADE WIND
INVERSION THAT CAPS CLOUD GROWTH. THE SECOND IS ENHANCED MOISTURE
MOVING ALONG WITH THE TRADE WINDS INCREASING THE CHANCE OF LOCALLY
HEAVY RAINFALL. OVERNIGHT SOUNDINGS SHOWED AN AVERAGE PRECIPITABLE
WATER /PW/ OF 1.7 INCHES AND SATELLITE PW ESTIMATES AGREE SHOWING
ABOVE NORMAL MOISTURE UPSTREAM OF THE ISLANDS.

WET CONDITIONS ARE EXPECTED TODAY AND INTO THE WEEKEND AS THE
UPSTREAM MOISTURE CONTINUES TO RIDE IN ON STRENGTHENING TRADE
WINDS. TRADES WILL GRADUALLY STRENGTHEN AS A STRONG SURFACE HIGH
PRESSURE SETS UP FAR NORTH OF THE ISLANDS. THE STRONGER TRADES
WILL FOCUS CLOUDS AND SHOWERS OVER WINDWARD AND MAUKA
AREAS...WHILE SHOWERS WILL PERIODICALLY BE BLOWN OVER LEEWARD
SECTIONS OF THE SMALLER ISLANDS. THE ATMOSPHERE WILL BE SOMEWHAT
UNSTABLE AS THE UPPER LOW CONTINUES TO HOVER OVER THE STATE...WITH
LOCALLY HEAVY SHOWERS REMAINING A POSSIBILITY.

&&

.AVIATION...
A MOIST AND UNSTABLE AIRMASS ASSOCIATED WITH THE UPPER LOW TRACKING
SLOWLY WEST ACROSS THE STATE TODAY...WILL CONTINUE TO BRING SHOWERY
AND CLOUDY WEATHER TO ESPECIALLY WINDWARD LOCATIONS. MVFR CIGS WILL
BE RULE THERE WITH ISOLATED IFR VIS IN HEAVIER SHOWERS. TEMPO MT
OBSC IN THE LARGER ROVING BANDS OF SHOWERS WILL CONTINUE TO BE AN
ISSUE TODAY. IN ADDITION...THE THREAT FOR ISOLATED THUNDERSTORMS
REMAINS OVER THE AREA...WITH THE ASSOCIATED RIME ICING ABOVE IN THE
12000 TO 17000 FOOT LAYER. AIRMET ZULU IN EFFECT FOR WINDWARD WATERS
FROM MOLOKAI SOUTHEAST TO THE BIG ISLAND...MAY NEED TO BE EXTENDED
AND OR EXPANDED AFTER 2200Z.

LIGHT TRADES ACROSS THE ISLANDS WILL ALLOW LOCAL SEA BREEZES TO
DEVELOP OVER SHELTERED LEEWARD LOCATIONS THIS AFTERNOON. THIS WILL
INITIATE ADDITIONAL SHOWERS AND A CHANCE OF THUNDERSTORMS IN THESE
LOCATIONS. WINDS AT PHNL AND PHJR WILL BE A HYBRID OF LIGHT
EASTERLY TRADES AND SEA BREEZE...RESULTING IN SOUTHEASTERLY WINDS
OCCASIONALLY INTERRUPTED BY LIGHT AND VARIABLE WINDS THROUGH THE
AFTERNOON.

&&

.MARINE...
THE CURRENT MODERATE NORTHWEST SWELL IS PEAKING AND WILL DIMINISH
TODAY AND WEDNESDAY. FORERUNNERS FROM ANOTHER MODERATE NORTHWEST
SWELL WILL ARRIVE LATE FRIDAY AND FILL IN THROUGH THE DAY ON
SATURDAY. A ROUGH...SHORT-PERIOD EASTERLY TRADE SWELL WILL BUILD BY
MID-WEEK AS THE TRADE WINDS INCREASE.

WINDS SHOULD REMAIN BELOW THE 25 KT SMALL CRAFT ADVISORY THRESHOLD
TODAY. AS TRADES INCREASE LATER TONIGHT AND WEDNESDAY...SMALL CRAFT
WINDS ARE EXPECTED FOR THE TYPICALLY WINDY WATERS AROUND THE BIG
ISLAND AND MAUI. ALSO...AS THE TRADE SWELL BUILD BY MID-WEEK...SEAS
MAY ALSO APPROACH THE 10 FT THRESHOLD FOR SMALL CRAFT ADVISORY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH UNTIL 6 PM HST THIS EVENING FOR KONA-SOUTH BIG
ISLAND-BIG ISLAND NORTH AND EAST-KOHALA-BIG ISLAND INTERIOR.

&&

DISCUSSION...MORRISON
AVIATION...BRENCHLEY