Waterspout in Hawaiian Waters, May 4, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST MON MAY 4 2015

.SYNOPSIS...
TRADE WINDS WILL PREVAIL THROUGH THE WEEK...STRENGTHENING SOMEWHAT
INTO MID-WEEK...AND THEN DIMINISHING AGAIN BY FRIDAY. CLOUDS AND
SHOWERS WILL BE MOST ACTIVE OVER WINDWARD SLOPES...THOUGH A FEW MAY
DRIFT LEEWARD AT TIMES.

&&

.DISCUSSION...
SURFACE OBSERVATIONS AROUND THE MAIN HAWAIIAN ISLANDS REPORTED
MODERATE TRADE WINDS...THOUGH BUOY DATA AND A GOOD SCATTEROMETER
PASS FROM EARLIER IN THE EVENING SUGGESTED LOCALLY HIGHER SPEEDS
OVER THE USUALLY WINDY COASTAL WATERS. CLOUD MOTIONS UPWIND OF THE
STATE AND SOUNDING DATA ALSO SHOWED THE WINDS EXTENDING THROUGH MOST
OF THE MIXED LAYER BELOW THE INVERSION.

THE INVERSION ITSELF HAD RISEN DURING THE PAST SIX HOURS...WEAKENING
SLIGHTLY IN THE PROCESS. PRECIPITABLE WATER OVER THE STATE ALSO HAD
INCREASED AS A LOOSE CLUSTER OF SHOWERY CUMULI AND STRATOCUMULI
MOVED ASHORE FROM THE BIG ISLAND TO OAHU...AND THREATENED TO REACH
KAUAI LATER THIS MORNING. INDIVIDUAL SMALL SHOWERS ALSO WILL
CONTINUE TO ENHANCE OVER WINDWARD SLOPES THROUGH THE WEEK.

ALOFT...BROAD ZONAL RIDGING ACROSS MOST OF THE CENTRAL PACIFIC OCEAN
KEPT THE MAIN PART OF THE STORM TRACK WELL TO THE NORTH OF THE
ISLANDS...BUT SHALLOW EXTENSIONS OF PASSING TROUGHS AND RIDGES DID
REACH THE STATE. THESE WAVES...AND THEIR INFLUENCE ON THE
SUBTROPICAL RIDGE AT THE SURFACE...WILL BRING MOST OF THE CHANGES TO
LOCAL WEATHER THROUGH THE WEEK.

THE MOST RECENT GFS RUN SHOWED A NOTICEABLE SHIFT IN PHASE...BUT
CONTINUITY STILL FAVORED THE FOLLOWING SCENARIO. INTO WEDNESDAY...
CRESTING RIDGING ALOFT...AND THEN AT THE SURFACE IN REACTION...WILL
TEND TO SUPPORT GRADUAL INCREASES IN WIND SPEED. THEN...FROM
WEDNESDAY INTO FRIDAY...FOLLOWING TROUGHING ALOFT...AND A FEEBLE
FRONT ERODING THE SUBTROPICAL RIDGE AT THE SURFACE...WILL TEND TO
REDUCE WIND SPEEDS. ALTHOUGH IT MAY BE TEMPTING TO PREDICT TRENDS IN
TRADE-WIND SHOWERS SIMPLY FROM MODEL CONSENSUS PRECIPITABLE
WATER...IN PRACTICE THE CORRELATION TENDS TO BE LESS THAN IDEAL.
THUS...POPS AND RELATED FIELDS IN THE OFFICIAL FORECAST HAVE BEEN
TRENDED SLIGHTLY BACK TOWARD CLIMATOLOGY.

ADDITIONAL RUNS SHOULD BE INSTRUCTIVE...AND IF CONSENSUS AGREES WITH
THE PHASE SHIFT IN THE PASSING MID-LEVEL HEIGHTS...THEN THE OVERALL
TREND FOR THE WEEK MAY NEED REVISION. ON THE OTHER HAND...CONDITIONS
SHOULD NOT DEVIATE TOO GREATLY FROM CLIMATOLOGY EITHER WAY.

&&

.MARINE...
A SMALL CRAFT ADVISORY REMAINED IN EFFECT FOR TYPICALLY WINDIER
AREAS AROUND MAUI AND THE BIG ISLAND...AND LIKELY WILL BE EXTENDED
THROUGH MUCH OF THE WEEK. ITS AREA OF EFFECT ALSO MAY EXPAND AROUND
TUESDAY...AND THEN AGAIN FROM FRIDAY INTO SATURDAY.

THE TRADE WINDS ALSO WILL MAINTAIN ELEVATED SHORT-PERIOD SURF ALONG
EAST-FACING SHORES THROUGH THE WEEK...THOUGH HEIGHTS SHOULD REMAIN
BELOW THE ADVISORY THRESHOLD. A LONG-PERIOD SOUTH-SOUTHWEST SWELL
WILL BUILD GRADUALLY ON TUESDAY...AND THEN PEAK FROM WEDNESDAY INTO
THURSDAY...LIKELY BRINGING ADVISORY-LEVEL SURF TO SOUTH-FACING
SHORES. A TINY NORTH SWELL IS EXPECTED MONDAY THROUGH WEDNESDAY...
AND A SLIGHTLY LARGER NORTH-NORTHWEST SWELL FROM THURSDAY INTO
SATURDAY.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 PM HST TUESDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS

&&

RYSHKO