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Hawaiian Islands Synoptic Discussion and Guidance

934
FXHW60 PHFO 261346
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
346 AM HST SAT JUL 26 2014

.SYNOPSIS...
MODERATE TRADE WINDS WILL FOCUS SHOWERS MAINLY ACROSS WINDWARD
AND MAUKA AREAS. AN INCREASE IN SHOWER ACTIVITY IS EXPECTED TO
SPREAD FROM EAST TO WEST ACROSS THE STATE TODAY THROUGH SUNDAY.
TRADES MAY STRENGTHEN SOMEWHAT BY EARLY NEXT WEEK.

&&

.DISCUSSION...
A QUIET LATE JULY NIGHT IS IN PROGRESS ACROSS THE STATE. RADAR
LOOPS AND RAIN GAUGE DATA HAVE NOT SHOWN MUCH INCREASE IN WINDWARD
SHOWERS DURING THE NIGHT...WITH MOST GAUGES REPORTING EITHER ZERO
OR LIGHT RAINFALL AMOUNTS. A TYPICAL SUMMERTIME SURFACE PATTERN
PREVAILS...WITH THE NORTHEAST PACIFIC HIGH CENTERED ABOUT 1400
MILES NORTHEAST OF HONOLULU. A WEAK LOW PRESSURE AREA IS LOCATED
ABOUT 720 MILES SOUTH SOUTHEAST OF SOUTH POINT ON THE BIG ISLAND...
AND IS BEING MONITORED FOR ANY SIGNS OF DEVELOPMENT WHILE MOVING
TO THE WEST. WATER VAPOR IMAGERY DEPICTS A MID LEVEL RIDGE
EXTENDING FROM THE BIG ISLAND EASTWARD...WITH AN UPPER LEVEL
TROUGH LOCATED SEVERAL HUNDRED MILES NORTHEAST OF KAUAI. THE 12Z
SOUNDINGS SHOW INCREASED MOISTURE DURING THE PREVIOUS 12 HOURS...
WITH PWATS RANGING FROM 1.4 TO 1.5 INCHES. THE HILO SOUNDING ALSO
SHOWED A PRONOUNCED WEAKENING OF THE TRADE INVERSION NEAR 7000
FEET...WITH ANOTHER ISOTHERMAL LAYER DEVELOPING BETWEEN 10K AND
12K FEET. MIMIC-TPW IMAGERY SHOWS AN AREA OF HIGHER MOISTURE
(PWATS 1.5 TO 1.7 INCHES) APPROACHING THE STATE FROM THE EAST...
WITH THE LEADING EDGE ALMOST TO THE BIG ISLAND. MODEL AND SOUNDING
DATA SUGGEST THAT A WEAK LOW LEVEL TROUGH IS MOVING THROUGH THE
STATE AT THIS TIME...WITH WINDS IN THE 850-700 MB LAYER VEERING TO
THE SOUTHEAST AT HILO IN THE WAKE OF THE TROUGH AXIS.

TODAY THROUGH SUNDAY...THE AREA OF ENHANCED MOISTURE WILL CONTINUE
MOVING WESTWARD ACROSS THE STATE AS THE LOW LEVEL TROUGH SHIFTS
WEST OF THE AREA. DURING THIS TIME...THE UPPER LEVEL TROUGH
INITIALLY NORTHWEST OF KAUAI WILL MOVE EASTWARD...PASSING NORTH OF
THE STATE SUNDAY AND SUNDAY NIGHT. WOULD EXPECT A HIGHER/WEAKER
TRADE INVERSION...AS HAS ALREADY BEEN OBSERVED AT HILO THIS
MORNING. THE WEAKER INVERSION COMBINED WITH THE ADDITIONAL
MOISTURE MOVING THROUGH COULD PRODUCE A FEW LOCALLY HEAVIER
SHOWERS AT TIMES. FAIRLY HIGH POPS WILL PREVAIL STATEWIDE ACROSS
WINDWARD/MAUKA AREAS TONIGHT AND SUNDAY...WITH SOME OF THE SHOWERS
SPILLING OVER INTO LEEWARD AREAS. WITH SOMEWHAT VEERED LOW LEVEL
FLOW POSSIBLE...WILL NEED TO KEEP AN EYE ON WINDWARD OAHU WHERE
MORE PERSISTENT SHOWER ACTIVITY MAY TEND TO DEVELOP ALONG THE
KOOLAU RANGE.

HEADING INTO NEXT WEEK...THE NORTHEAST PACIFIC HIGH IS FORECAST
TO WEAKEN SOMEWHAT AS A COLD FRONT MOVES EASTWARD NORTH OF
30N...BUT AT THE SAME TIME A TROPICAL LOW PASSING WELL SOUTH OF
THE STATE MAY BE ABLE TO MAINTAIN OR EVEN TIGHTEN THE PRESSURE
GRADIENT LOCALLY. OVERALL WOULD EXPECT A TYPICAL TRADE WIND
PATTERN TO PREVAIL FOR MONDAY THROUGH AT LEAST TUESDAY NIGHT...
WITH PASSING SHOWERS MAINLY AFFECTING WINDWARD AND MAUKA AREAS.

TOWARD THE LATER PART OF NEXT WEEK AND INTO NEXT WEEKEND...SOME
OF THE MODEL GUIDANCE CONTINUES TO BRING ANOTHER SURGE OF DEEP
TROPICAL MOISTURE UP TOWARD THE STATE FROM THE SOUTHEAST. THIS
MOISTURE WOULD BE ASSOCIATED WITH THE REMNANTS OF CURRENT EAST
PACIFIC TROPICAL STORM GENEVIEVE. THE GUIDANCE SUITE REMAINS
RATHER INCONSISTENT WITH BOTH THE TIMING AND EXTENT OF ANY
POSSIBLE MOISTURE SURGE. THERE IS THE POTENTIAL FOR ANOTHER ROUND
OF HEAVY SHOWERS ACROSS THE STATE LATER NEXT WEEK AND INTO NEXT
WEEKEND...BUT FORECAST UNCERTAINTY REMAINS QUITE HIGH AT THIS
POINT.

&&

.AVIATION...
TEMPO MVFR CONDITIONS WILL LIKELY OCCUR THROUGH THE MORNING HOURS...
MAINLY OVER WINDWARD AND MAUKA SECTIONS...AS LOW CLOUDS AND
ACCOMPANYING SHOWERS RIDE THE TRADE WINDS. VFR WILL OTHERWISE
PREDOMINATE.

&&

.MARINE...
AN 08Z ASCAT PASS SAMPLED THE EASTERN HALF OF THE HAWAIIAN
COASTAL WATERS...FINDING SOME 20 KNOT WINDS IN THE ALENUIHAHA
CHANNEL AND SOUTH OF THE BIG ISLAND...WITH WINDS MOSTLY AROUND
15 KNOTS ELSEWHERE. EXPECT LITTLE CHANGE THROUGH TODAY AND
TONIGHT. THE NORTHEAST PACIFIC SURFACE HIGH WILL CHANGE LITTLE
IN STRENGTH WHILE MOVING SLIGHTLY FURTHER AWAY FROM THE STATE
DURING THE WEEKEND...BUT A WEAK LOW PRESSURE AREA PASSING WELL
SOUTH OF THE AREA MAY TIGHTEN THE GRADIENT ACROSS THE ISLANDS
SOMEWHAT. WINDS MAY INCREASE TO SCA LEVELS BY SUNDAY OR SUNDAY
EVENING ACROSS THE TYPICALLY WINDIER AREAS AROUND MAUI COUNTY AND
THE BIG ISLAND. SCA CONDITIONS WILL THEN REMAIN POSSIBLE IN THE
TYPICALLY WINDY AREAS THROUGH MUCH OF NEXT WEEK.

AS FOR SURF...A SMALL SSW SWELL MAY PRODUCE A MINOR BUMP ON SOUTH
FACING SHORES TONIGHT INTO SUNDAY. SHORT PERIOD TRADE WIND SWELL
WILL CONTINUE ACROSS EAST FACING SHORES...POSSIBLY INCREASING A
BIT EARLY NEXT WEEK IF TRADES STRENGTHEN. NO SIGNIFICANT NORTH
THROUGH WEST SWELLS ARE EXPECTED THROUGH THE NEXT SEVERAL DAYS.
OVERALL...SURF WILL REMAIN WELL BELOW ADVISORY LEVELS ON ALL
SHORES THROUGH THE WEEKEND AND INTO THE FIRST PART OF NEXT WEEK.

&&

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

&&

$$

DISCUSSION/MARINE...JACOBSON
AVIATION...KINEL




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