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Hawaiian Islands Synoptic Discussion and Guidance

645
FXHW60 PHFO 301959
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
959 AM HST TUE JUN 30 2015

.SYNOPSIS...
A RIDGE OF HIGH PRESSURE WILL REMAIN NEAR THE ISLANDS FOR THE
NEXT SEVERAL DAYS. THE RIDGE WILL KEEP WINDS LIGHT...WITH DAYTIME
HEATING LEADING TO AFTERNOON AND EVENING CLOUDS AND LOCALIZED
SHOWERS...FOLLOWED BY CLEARING SKIES OVERNIGHT. AFTERNOON SHOWERS
MAY INCREASE IN COVERAGE AND INTENSITY THIS WEEK AS AN AREA OF
MOISTURE SOUTHEAST OF THE BIG ISLAND PUSHES UP OVER THE STATE. THE
RIDGE OF HIGH PRESSURE IS EXPECTED TO PUSH NORTH OF THE ISLANDS
OVER THE HOLIDAY WEEKEND...WHICH WILL ALLOW THE TRADES TO RETURN.

&&

.DISCUSSION...
THE 12Z SOUNDINGS ARE A BIT MISLEADING AS TO THE MESOSCALE
VARIABILITY OF THE MOISTURE FIELD OVER THE ISLANDS THIS AM.
FORTUNATELY...THE BLENDED TOTAL PRECIPITABLE WATER /TPW/ FIELD AS
WELL AS THE NOAA GPS-MET TPW SENSOR AT PHNL HELP TO PAINT A
CLEARER PICTURE OF WHAT IS GOING ON. TPW VALUES ARE NEAR NORMAL AT
BOTH LIHUE AND HILO...BUT THERE IS A NARROW RIBBON OF SIGNIFICANTLY
DRIER AIR EXTENDING SW-NE OVER THE MIDDLE ISLANDS WITH TPW RUNNING
FROM ABOUT 1.0-1.1 THERE. INTERESTINGLY THIS DRIER AIR IS
REFLECTED IN THE DEWPOINT AT PHNL AS WELL...WHICH PEAKED AT 73
LAST EVENING IS NOW DOWN TO 67. TO THE SE OF THE DRIER AIR IS AN
AREA OF MUCH DEEPER MOISTURE WITH TPW OVER 1.75 LURKING E-SE OF
THE BIG ISLAND. THIS VARIABILITY IN THE MOISTURE FIELD OVER AND
NEAR THE ISLANDS MAKES FOR TRICKY FORECASTING...AND THE MODELS ARE
NOT MUCH HELP...SHOWING FAIRLY SIGNIFICANT DIFFERENCES EVEN JUST
24 HOURS FROM NOW. MORE ON THIS IN A MOMENT.

A SUBTROPICAL RIDGE /STR/ EXTENDING SSW FROM A NEARLY STATIONARY
HIGH OVER THE NE PACIFIC WILL REMAIN NEAR THE MAIN HAWAIIAN
ISLANDS FOR THE NEXT SEVERAL DAYS. THIS WILL KEEP THE BACKGROUND
FLOW LIGHT AND GENERALLY FROM THE SE...SHIFTING EVER SO SLOWLY
BACK TOWARD ESE AND THEN E WITH TIME. IN GENERAL...A LIGHT
WIND/CONVECTIVE PATTERN IS EXPECTED EACH DAY TODAY THROUGH AT
LEAST THU...AND PROBABLY FRI. THE RELATIVE MINIMUM IN MOISTURE
TODAY MAY REDUCE AFTERNOON SHOWERS OVER OAHU AND MAUI COUNTY
COMPARED TO RECENT DAYS...BUT WITH A LITTLE BETTER CHANCE FOR
SHOWERS ON EITHER END OF THE STATE.

MOST OF THE MODELS TAKE SOME OF THE DEEPER MOISTURE JUST SE OF
THE BIG ISLAND UP TO THE NW OVER THE NEXT COUPLE OF DAYS...THOUGH
THE ECMWF HOLDS ON TO THE DRIER AIR OVER OAHU THROUGH WED...THEN
OVER KAUAI THROUGH THU. FOR NOW AM FOLLOWING THE MULTI-MODEL
ENSEMBLE MEAN WHICH IS MORE AGGRESSIVE...BUT ADJUSTMENTS WILL NEED
TO BE MADE TO WED/THU RAIN CHANCES FOR KAUAI/OAHU IF IT LOOKS
LIKE ECMWF WILL BE THE WINNER.

IN ANY CASE...THE MID LEVELS ARE WARM...WHICH SHOULD KEEP CONVECTION
FROM GETTING TOO ACTIVE STATEWIDE. THERE WILL BE SOME SHORT-LIVED
AFTERNOON DOWNPOURS IN THE AREAS WHERE THE MOISTURE WILL BE
DEEPEST...SUCH AS OVER THE BIG ISLAND AND POSSIBLY MAUI COUNTY...
BUT NOT EXPECTING ANY THUNDER AT THIS POINT. THE FLOW IS WEAK
ENOUGH TODAY AND PROBABLY WED NOT TO SHOW MUCH PREFERENCE FOR
SHOWER LOCATION...SO INTERIORS WILL HAVE THE HIGHEST RAIN CHANCES
WITH LOWEST CHANCES NEAR THE SHORELINES. LATER THIS WEEK...THE
BACKGROUND FLOW SHOULD FAVOR CARRYING SHOWERS MORE TOWARD
LEEWARD/W-NW SECTIONS OF THE ISLANDS.

THE STR WILL SHIFT FARTHER N OF THE ISLANDS AGAIN BY SAT/SUN...
WHICH SHOULD ALLOW THE TRADES TO REGAIN CONTROL OF OUR WEATHER
OVER THE HOLIDAY WEEKEND.

&&

.AVIATION...
ONSHORE SEA BREEZES HAVE ALREADY STARTED TO FILL IN ACROSS THE
STATE...AND CLOUDS ARE BEGINNING TO FORM ACROSS INTERIOR SECTIONS OF
THE ISLANDS. CONDITIONS WILL BE PRIMARILY VFR...THOUGH LOCALIZED
SHOWERS WILL BRING MVFR CEILINGS AND VISIBILITIES. THESE CLOUDS WILL
LINGER INTO THE EVENING BEFORE CLEARING OUT...AT LEAST WEST OF MAUI.
LOCALIZED MVFR CONDITIONS IN SHOWERS MAY LINGER ACROSS
EAST/SOUTHEAST SECTIONS OF MAUI/BIG ISLAND THROUGH THE NIGHT. A
SIMILAR CONVECTIVE PATTERN WITH SEA BREEZES AND INTERIOR CLOUDS IS
EXPECTED ON WEDNESDAY...THOUGH WITH AN INCREASE IN MOISTURE AND
INSTABILITY THERE WILL BE AN INCREASE IN SHOWER COVERAGE AND
INTENSITY AND SUBSEQUENT GREATER LIKELIHOOD FOR MVFR CONDITIONS.

&&

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

&&

$$
DISCUSSION...R BALLARD
AVIATION...BRAVENDER


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



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