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Hawaiian Islands Synoptic Discussion and Guidance

169
FXHW60 PHFO 191340
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
340 AM HST SAT APR 19 2014

.SYNOPSIS...
A TRADE WIND WEATHER PATTERN WILL CONTINUE THROUGH THE NEXT
SEVERAL DAYS ACROSS HAWAII...THANKS TO HIGH PRESSURE NORTH OF THE
STATE. SHOWERS WILL FAVOR WINDWARD AND MAUKA AREAS WITH A FEW
SHOWERS POSSIBLE OVER SOME LEEWARD LOCATIONS.

&&

.DISCUSSION...
SHOWERS HAVE BECOME RATHER SPARSE OVER THE STATE DURING THE PAST
SEVERAL HOURS...WITH RADAR AND RAIN GAUGE DATA SHOWING ONLY VERY
LIGHT RAINFALL AMOUNTS ACROSS SOME WINDWARD AREAS. THE 12Z
SOUNDINGS SHOW A LOWERING OF THE INVERSION AS EXPECTED...WITH THE
BASE DOWN TO BETWEEN 5700 AND 6700 FEET...FROM ABOUT 9000 FEET 12
HOURS EARLIER. THIS IS CONSISTENT WITH MIMIC-TPW SATELLITE DATA
WHICH INDICATE A DRIER AIRMASS SPREADING INTO THE STATE FROM THE
EAST. EARLIER MODEL DATA SUGGESTED THAT THIS DRYING WOULD TAKE
PLACE PRIMARILY IN THE 850-700 MB LAYER...AGAIN CONSISTENT WITH
THE 12Z RAOB DATA. SATELLITE IMAGERY DEPICTS ABUNDANT HIGH
LEVEL CLOUDINESS STREAMING NORTHWARD ACROSS THE STATE...TO THE
EAST OF A MID/UPPER LEVEL TROUGH CENTERED SEVERAL HUNDRED MILES
TO THE SOUTHWEST. AT THE SURFACE...A 1023 MB HIGH IS CENTERED WELL
NORTHEAST OF THE STATE...WITH A STRONGER 1028 MB HIGH BUILDING
EASTWARD FROM THE DATELINE NORTH OF 30N.

FOR TODAY...EXPECT SHOWER COVERAGE TO REMAIN FAIRLY SPARSE WITH
LIGHT RAINFALL AMOUNTS EVEN ACROSS WINDWARD SECTIONS...AS STABLE
AND DRY CONDITIONS CONTINUE. TONIGHT...SATELLITE AND MODEL DATA
INDICATE THAT ANOTHER AREA OF HIGHER MOISTURE WILL BE MOVING INTO
THE STATE...ALTHOUGH MODEL SOUNDINGS SHOW THE INVERSION REMAINING
FAIRLY LOW. WITH BRISK TRADES IN PLACE...THE INFLUX OF ADDITIONAL
LOW LEVEL MOISTURE SHOULD BE SUFFICIENT TO INCREASE SHOWER
ACTIVITY ONCE AGAIN ACROSS WINDWARD/MAUKA AREAS...WITH SOME
SPILLOVER INTO LEEWARD.

SUNDAY THROUGH MONDAY NIGHT...RATHER STRONG HIGH PRESSURE WILL
PASS NORTH OF THE STATE...CAUSING TRADES TO INCREASE FURTHER.
ALOFT...GUIDANCE INDICATES THAT THE MID/UPPER TROUGH TO OUR
SOUTHWEST WILL SHEAR APART...WITH THE MID LEVEL TROUGH MOVING TO
THE WEST SOUTHWEST AND AWAY FROM OUR AREA...WHILE THE WEAKENING
UPPER LEVEL TROUGH DRIFTS SLOWLY TO THE EAST NORTHEAST. A DEEP DRY
LAYER WILL PERSIST ACROSS THE STATE IN THE MID LEVELS...WITH A MID
LEVEL RIDGE BUILDING JUST TO OUR NORTH. THEREFORE WOULD EXPECT
THE ONLY IMPACT TO OUR SENSIBLE WEATHER FROM THE UPPER TROUGH TO
BE PERIODS OF HIGH LEVEL CLOUDINESS. ACTIVE TRADE SHOWERS WILL
CONTINUE ACROSS WINDWARD/MAUKA...WITH SOME SPILLOVER TO LEEWARD
DUE TO THE STRENGTH OF THE TRADES.

TRADES SHOULD RETURN TO MORE MODERATE LEVELS TUESDAY THROUGH
THURSDAY...AS THE FIRST SURFACE HIGH SHIFTS FURTHER TO THE
NORTHEAST AND IS REPLACED BY A SECOND SLIGHTLY WEAKER HIGH TO OUR
NORTH. WOULD EXPECT LITTLE CHANGE IN THE WEATHER PATTERN ACROSS
HAWAII WITH SHOWERS CONTINUING MAINLY ACROSS WINDWARD SECTIONS.

&&

.AVIATION...
A TRADE WIND FLOW CONTINUES WITH EMBEDDED SHOWERS FAVORING THE
WINDWARD AND MTN AREAS. BUT IT APPEARS THIS FLOW HAS BECOME DRIER AS
THE NIGHT PROGRESSES. FIRST IS THE MARINE LAYER WHICH HAS LOWERED
FROM 10K EARLY IN THE EVENING TO THE CURRENT...BETWEEN 6 AND 7K
FEET. SECOND...RADAR RETURNS ARE NOT AS ACTIVE AS AT THE START OF
THE EVENING. THE SHOWERS HAVE TURNED LIGHTER TO PERHAPS MORE OF A
DRIZZLE. THIRD...SFC OBS FROM AROUND THE STATE SUGGEST THE LO CLD
COVER ESPECIALLY FROM THE WINDWARD STATIONS...HAS BECOME LESS.
CLOUD BASES THOUGH REMAINS AS LOW AS 2K FEET. MODELS SUGGESTS THIS
DRIER AIR MASS TO PERSISTS THROUGH ALL OF TODAY. HENCE...VFR
CONDITION RULES. ANY MVFR DUE TO CEILINGS AND VIS WILL BE BRIEF
AND ISOLATED AND FAVORING THE WINDWARD AND MTN AREAS.

LITTLE CHANGE TO THE STRENGTH OF THE TRADE WINDS WHICH WILL BE IN
THE MODERATE TO STRONG RANGE. GIVEN A DECENT INVERSION...AIRMET
TANGO FOR MOD MECHANICAL TURB WILL CONT TO WELL BEYOND 04Z SUNDAY.

&&

.MARINE...
AN 0830Z ASCAT PASS ONLY CAUGHT THE WESTERN EDGE OF THE COASTAL
WATERS...BUT DID SHOW SIGNIFICANT COVERAGE OF 23-25 KT WINDS WEST
AND SOUTH OF KAUAI. THESE WINDS ARE SEVERAL KNOTS ABOVE GUIDANCE...
BUT THIS DATA COMBINED WITH AVAILABLE LAND STATION OBS SUPPORT
EXPANDING THE SMALL CRAFT ADVISORY TO COVER ALL HAWAIIAN COASTAL
WATERS. WINDS SHOULD ONLY INCREASE FURTHER WITH TIME AS HIGH
PRESSURE BUILDS TO OUR NORTH...AND HAVE ALSO EXTENDED THE SCA
IN TIME OUT THROUGH MONDAY. WINDS MAY REACH NEAR GALE FORCE AT
TIMES SUNDAY THROUGH MONDAY IN THE ALENUIHAHA CHANNEL.

ANOTHER SMALL NORTHWEST SWELL IS EXPECTED TO ARRIVE LATER TODAY
AND LINGER THROUGH SUNDAY...BUT THE RESULTING SURF WILL REMAIN
WELL BELOW ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES. TRADE
WINDS WILL CONTINUE TO PRODUCE SHORT PERIOD CHOPPY SURF ALONG EAST
FACING SHORES...AND HIGH SURF ADVISORIES WILL PROBABLY BE NEEDED
BY SUNDAY AS THE FETCH LENGTH INCREASES DUE TO THE BUILDING HIGH
TO OUR NORTH.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST MONDAY FOR ALL HAWAIIAN
WATERS.

&&

$$

DISCUSSION/MARINE...JACOBSON
AVIATION...LAU





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