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Hawaiian Islands Synoptic Discussion and Guidance

331
FXHW60 PHFO 190208
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
345 PM HST THU DEC 18 2014

.SYNOPSIS...
HIGH PRESSURE CENTERED NORTH OF THE ISLANDS WILL MAINTAIN LOCALLY
BREEZY TRADE WINDS THROUGH FRIDAY. SHOWERS WILL CONTINUE OVER MOST
WINDWARD AND MOUNTAIN AREAS TONIGHT...WITH ISOLATED SHOWERS POSSIBLE
OVER LEEWARD LOCATIONS INCLUDING THE HONOLULU METRO AREA. THE TRADES
ARE EXPECTED TO DECREASE DURING THE WEEKEND AS THE HIGH MOVES
FURTHER AWAY FROM THE ALOHA STATE AND A COLD FRONT APPROACHES THE
ISLANDS FROM THE NORTHWEST. THE FRONT WILL REACH THE ISLAND CHAIN ON
MONDAY AND LINGER THROUGH TUESDAY WITH INCREASING SHOWERS AND
POTENTIAL FOR LOCALIZED HEAVY RAIN. STRONG NORTHERLY WINDS ARE
EXPECTED BEHIND THE FRONT BY THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...
HIGH PRESSURE NE OF THE ISLANDS WILL CONTINUE TO MIGRATE E DURING
THE WEEKEND AS A SHORT WAVE TROUGH FAR NW OF THE AREA PUSHES A COLD
FRONT BOUNDARY TOWARDS THE ISLANDS. AS THE HIGH MOVES FURTHER
AWAY...EXPECT THE TRADES TO GRADUALLY DIMINISH AND ALLOWING FOR
PERIODS OF SEA/LAND BREEZES TO DEVELOP SAT...SUN AND POSSIBLY ON
MON. SHOWERS WILL CONTINUE TO AFFECT WINDWARD AREAS...WITH LEEWARD
AND INTERIOR AREAS EXPERIENCING PERIODS OF CLOUDY SKIES AND
SCATTERED SHOWERS AS SEA BREEZES PUSH INLAND EACH AFTERNOON.
HOWEVER...SOUNDING/MODEL SOLUTIONS SUGGEST A RELATIVELY DRY AIRMASS
PERSISTING ALOFT DURING THE NEXT COUPLE OF DAYS...KEEPING VERTICAL
DEVELOPMENT OF SHOWERS TO A MINIMUM.

AS THE TROUGH/FRONT COMPLEX APPROACHES THE ISLANDS...INTENSITY
DIFFERENCES BETWEEN THE GLOBAL MODELS...MAINLY GFS AND ECMWF...ARE
MAKING THE FORECAST FOR MON AND MON NIGHT RATHER DIFFICULT. TIMING
WISE...BOTH ARE GRADUALLY BECOMING IN BETTER AGREEMENT IN TERMS OF
BRINGING THE BASE OF THE TROUGH INTO THE W HALF OF THE ISLAND CHAIN
LATE MON INTO TUE. THE SFC FRONT...HOWEVER...IS LOOKING WEAKER ON
GFS. 500 MB TEMPS ON GFS FOR THAT TIME PERIOD ARE AROUND -13C...
WHILE ECMWF SHOWS A LITTLE BETTER UPPER LVL SUPPORT WITH TEMPS
AROUND -17C.

REGARDLESS OF THE FINAL OUTCOME...THERE WILL BE AN INCREASE IN
SHOWER ACTIVITY ACROSS THE ALOHA STATE WITH THE FROPA STARTING MON
EVENING...AND POTENTIAL FOR LOCALIZED HEAVY RAIN. WILL LEAVE MENTION
OF THUNDERSTORMS OUT OF THE FORECAST ATTM...BUT UPCOMING GUIDANCE
WILL LIKELY SUPPORT MODIFYING THE FORECAST TO INCLUDE MENTION OF
SLIGHT CHANCES OF THUNDERSTORMS FOR MON NIGHT.

FOR THE LONG TERM FORECAST...THE MODELS REMAIN IN DISAGREEMENT
REGARDING THE SOLUTION FOR THE UPPER TROUGH. ECMWF KEEPS THE TROUGH
AXIS CLOSER OR OVER THE ISLANDS THROUGH LATE TUE WITH CHANCES OF
ENHANCED SHOWERS AND MAYBE SLIGHT CHANCES OF THUNDERSTORMS. GFS HAS
A FASTER SOLUTION...PUSHING THE TROUGH FURTHER TO THE NE OF THE
ISLANDS BY LATE TUE...SUGGESTING A DECREASE IN SHOWERS AND LITTLE
OR NO SUPPORT FOR DEEP CONVECTION BY THAT TIME. HOWEVER...BOTH
MODELS SHOW THE SFC FRONTAL BOUNDARY LINGERING ACROSS THE CENTRAL
PORTIONS OF THE STATE ON TUE...MAYBE REACHING MAUI COUNTY IN THE
AFTERNOON HOURS. THEREFORE...EXPECT AT LEAST A WETTER PATTERN
STARTING MON AFTERNOON AND EVENING...CONTINUING THROUGH TUE.

ONCE THE FRONTAL BOUNDARY AND THE TROUGH CLEAR THE AREA...MUCH
STRONGER N WINDS ARE ANTICIPATED...BRINGING PERIODS OF BREEZY TO
POSSIBLY WINDY CONDITIONS ACROSS THE ISLANDS FOR THE MIDDLE OF NEXT
WEEK. THE SITUATION WILL CONTINUE TO BE CLOSELY MONITORED AND
CHANGES TO THE FORECAST WILL CERTAINLY BE INCORPORATED AS NEW MODEL
GUIDANCE BECOMES AVAILABLE.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL OVER LEEWARD AREAS. WINDWARD AND
MAUKA...CLOUDS AND SHOWERS WILL MOVE IN WITH THE TRADE WINDS ARE
CAUSING ISOL MVFR CIGS AND MTN OBSC. COVERAGE MAY INCREASE TONIGHT
AND MAY REQUIRE AN AIRMET FOR MTN OBSC.

BREEZY TRADE WINDS ARE ALSO RESULTING IN LEEWARD MECHANICAL
TURBULENCE OVER ALL ISLANDS. AIRMET TANGO IS IN EFFECT FOR THIS...
AND WILL LIKELY CARRY ON THROUGH THE DAY FRIDAY.

&&

.MARINE...
THE SMALL CRAFT ADVISORY /SCA/ HAS BEEN EXPANDED TO INCLUDE
BASICALLY ALL COASTAL WATERS THROUGH FRIDAY 6 PM HST FOR A
COMBINATION OF INCREASING NORTHWEST SWELL AND STRONG WINDS
PERSISTING OVER THE SOUTHERN WATERS. CONDITIONS SHOULD THEN SUBSIDE
FRIDAY EVENING...THEN ANOTHER LARGE NORTHWEST SHOULD BRING BACK SCA
CONDITIONS BY SATURDAY NIGHT.

AN EVEN LARGER NW SWELL IS EXPECTED TO ARRIVE ON SUNDAY...LIKELY
PUSHING SURF HEIGHTS INTO WARNING LEVELS OVER BOTH NORTH AND WEST
FACING SHORES OF MOST ISLANDS. THE SOURCE OF THIS SWELL IS FROM A
RATHER BROAD STORM LOW CURRENTLY LOCATED OFF THE EAST COAST OF
JAPAN. IN ADDITION...A LOW PRESSURE SYSTEM FAR NORTH OF THE ALOHA
STATE MAY KEEP SURF ELEVATED OVER THE NORTH FACING SHORES...PROBABLY
AT ADVISORY LEVELS...THROUGH THE SECOND HALF OF NEXT WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST FRIDAY FOR NIIHAU-KAUAI
WINDWARD-KAUAI LEEWARD-WAIANAE COAST-OAHU NORTH SHORE-OAHU
KOOLAU-MOLOKAI-MAUI WINDWARD WEST-WINDWARD HALEAKALA.

SMALL CRAFT ADVISORY UNTIL 6 PM HST FRIDAY FOR ALL HAWAIIAN
WATERS-

&&

$$

DISCUSSION/MARINE...REYNES
AVIATION...FOSTER





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