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Hawaiian Islands Synoptic Discussion and Guidance

236
FXHW60 PHFO 250212
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SUN MAY 24 2015

.SYNOPSIS...
COOL AND RELATIVELY DRY NORTHEASTERLY WINDS WILL CONTINUE THROUGH
MONDAY. THESE WINDS WILL CARRY LOW CLOUDS AND ISOLATED LIGHT SHOWERS
MAINLY TO NORTHEAST FACING SLOPES. LIGHTER WINDS ARE EXPECTED OVER
MOST OF THE STATE STARTING TUESDAY...WITH AFTERNOON SEA BREEZES AND
NIGHTTIME LAND BREEZES BECOMING MORE WIDESPREAD. CLOUDS AND ISOLATED
SHOWERS WILL BE GREATEST OVER INTERIOR AND LEEWARD AREAS DURING THE
AFTERNOON AND EVENING.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL LOW TO OUR NORTHEAST...WITH
A TROUGH EXTENDING BACK OVER THE STATE. AT THE SURFACE...A LOW
IS ALSO LOCATED NORTHEAST OF THE STATE...WITH A FRONT ASSOCIATED
WITH IT PASSING JUST SOUTHEAST OF THE BIG ISLAND. HIGH PRESSURE IS
LOCATED FAR TO THE NORTHWEST. 00Z SOUNDINGS SHOW INVERSIONS RANGING
FROM 6KFT AT LIHUE TO 8KFT AT HILO. PRECIPITABLE WATERS ARE ALSO
1.0 AND 1.2 INCHES RESPECTIVELY. LATE MORNING MIMIC TOTAL
PRECIPITABLE WATER IMAGERY SHOWS MOISTURE ASSOCIATED WITH THE FRONT
SHIFTING SOUTHEAST OF THE BIG ISLAND...WITH ANOTHER BAND OF HIGHER
MOISTURE EXTENDING ACROSS THE CENTRAL ISLANDS.

CLOUD COVER HAS BEEN PRETTY SOLID BEHIND THE FRONT...ESPECIALLY
ACROSS ALL OF KAUAI AND OAHU AS MORE WIDESPREAD MOISTURE MOVING IN
FROM THE NORTH. THE CLOUDS ARE RATHER STABLE THOUGH AND SHOWERS HAVE
BEEN ON THE LIGHT SIDE...WITH GENERALLY ONLY A FEW HUNDREDTHS OF AN
INCH OF RAINFALL DURING THE PAST 12 HOURS. WE HAVE HIT SKY COVER
PRETTY HARD OVERNIGHT...KEEPING SKIES MOSTLY CLOUDY ACROSS ALL OF
KAUAI/OAHU AND MOST REMAINING WINDWARD SECTIONS. THERE WILL LIKELY
BE A BREAK IN THE CLOUDS OVER OAHU EARLY THIS EVENING...BUT THAT
SHOULD QUICKLY FILL BACK IN. WINDS HAVE STARTED TO DIMINISH...AND
WILL CONTINUE TO WEAKEN THROUGH THE NIGHT.

THE UPPER LOW NORTHEAST OF THE STATE WILL BEGIN TO SHIFT EAST...BUT
WITH A WEAK UPPER LEVEL TROUGH CONTINUING TO REMAIN NEAR THE STATE.
AT THE SURFACE...THE FRONT SOUTHEAST OF THE BIG ISLAND WILL WEAKEN
AS THE LOW TO THE NORTHEAST SHIFTS NORTHWARDS. HOWEVER...THIS
DEPARTING LOW WILL ALSO LEAVE A SURFACE TROUGH NORTHEAST AND EAST OF
THE STATE. WINDS WILL CONTINUE TO DIMINISH OVER THE EASTERN HALF OF
THE STATE ON MONDAY AND EVERYWHERE BY TUESDAY...ALLOWING FOR LOCALLY
DRIVEN LAND AND SEA BREEZES. THE PATTERN DOES NOT APPEAR TO BE AS
PURE CONVECTIVE AS IT WAS BRIEFLY LAST WEEK WITH THE RIDGE OVERHEAD.
INSTEAD...THERE WILL BE ENOUGH OF A WEAK NORTHEASTERLY GRADIENT TO
KEEP SOME CLOUDS FOCUSED ACROSS WINDWARD SECTIONS AT NIGHT...BUT
WITH MOST BUILDUPS OCCURING DURING THE DAY ACROSS INTERIOR AND
LEEWARD SECTIONS.

THERE IS A GENERAL CONSENSUS AMONG THE EXTENDED MODELS AS TO THE
OVERALL PATTERN THROUGH THE WEEK AND INTO NEXT WEEKEND...BUT WITH
SOME DIFFERENCES WITH RESPECT TO TIMING AND STRENGTH. THE CONSENSUS
IS FOR A SURFACE TROUGH NEAR THE EASTERN HALF OF THE STATE TO
GRADUALLY MOVE WESTWARD ACROSS THE ISLANDS. THIS TROUGH WILL
MAINTAIN THE LIGHT WIND PATTERN...AND ALSO PROVIDE A FOCUS FOR
MOISTURE THAT MAY ENHANCE AFTERNOON SHOWERS. MODEL FORECAST
SOUNDINGS SHOW AN INCREASE IN MOISTURE DEPTH/INVERSION HEIGHT LATE
IN THE WEEK...IN PART DUE TO THE MOISTURE POOLED ALONG THE SURFACE
TROUGH. WITH THIS IN MIND...HAVE INCREASED AFTERNOON RAINFALL
CHANCES DURING THE LATTER HALF OF THE WEEK. BY LATE IN THE WEEKEND A
NEW SURFACE HIGH BUILDING NORTHWEST OF THE STATE SHOULD HELP TO
STRENGTHEN THE TRADE FLOW...AND RETURN US TO A MORE TYPICAL RAINFALL
PATTERN.

&&

.AVIATION...
A POST-FRONTAL NORTH-NORTHEASTERLY LOW LEVEL FLOW IS PUSHING MAINLY
STABLE STRATOCUMULUS ALONG NORTH AND NORTHEAST SLOPES WITH CLOUD
BASES A BIT HIGHER THAN NORMAL...MAINLY IN THE 4000 TO 5000 FT
RANGE. EVEN THOUGH PRECIPITABLE WATER VALUES ARE BELOW NORMAL...
ISOLATED LIGHT SHOWERS WILL BE SQUEEZED OUT OF THESE CLOUDS ALONG
WINDWARD SLOPES...AND WITH SOME RESIDUAL FRONTAL MOISTURE STILL HUNG
UP ON WINDWARD BIG ISLAND SLOPES...WE HAVE ISSUED AIRMET SIERRA FOR
MOUNTAIN OBSCURATION THERE TONIGHT. OTHERWISE...VFR CONDITIONS WILL
PREVAIL.

&&

.MARINE...
THE MORNING ASCAT PASS SHOWED WINDS HAD DROPPED OFF TO 20 KNOTS
THROUGH ALENUIHAHA CHANNEL...AND THE HIGH RESOLUTION WRF FROM UH
SHOWS WINDS CONTINUING TO DIMINISH OVERNIGHT. THEREFORE WE HAVE
DROPPED THE SMALL CRAFT ADVISORY THAT WAS IN EFFECT THROUGH THE DAY.
WITH THE CONTINUED WEAKENING AND OVERALL LIGHT GRADIENT EXPECTED...
SMALL CRAFT CONDITIONS ARE NOT ANTICIPATED THROUGH AT LEAST NEXT
WEEKEND.

AN EXTENDED PERIOD OF SOUTH SWELLS...WHICH HAVE BEEN GENERATED BY
THE FETCH ASSOCIATED WITH MULTIPLE SURFACE LOWS IN THE SOUTHERN
HEMISPHERE DURING THIS PAST WEEK...WILL MAINTAIN ELEVATED SURF ALONG
SOUTH FACING SHORES THROUGH THE COMING WEEK. SURF MAY APPROACH
ADVISORY LEVELS ALONG SOUTH FACING SHORES ON MONDAY/TUESDAY. IN
FACT...WHILE THE SWELL HEIGHT HAS REMAINED NEARLY STEADY AT BUOY
51003...THE SPECTRAL DENSITY SHOWS AN INCREASE IN LONGER-PERIOD
ENERGY. CURRENT READINGS STILL SUPPORT NEAR-ADVISORY LEVEL SURF...
HOWEVER THIS IS SOMETHING THAT BEARS WATCHING OVERNIGHT IN CASE IT
CONTINUES TO RISE.

ELSEWHERE...A SERIES OF SMALL NORTHWEST SWELLS WILL KEEP SOME SURF
GOING ALONG NORTH FACING SHORES THROUGH THE WEEKEND. A SHORT PERIOD
NORTH-NORTHEAST SWELL GENERATED BY THE LOW NORTHEAST OF THE STATE
WILL GRADUALLY DIMINISH THROUGH MID WEEK.

&&

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

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...WROE



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