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Hawaiian Islands Synoptic Discussion and Guidance

448
FXHW60 PHFO 210152
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SAT DEC 20 2014

.SYNOPSIS...
WEAKENING SOUTHEASTERLY FLOW WILL ALLOW FOR LAND AND SEA BREEZES TO
DEVELOP THROUGH SUNDAY. A COLD FRONT APPROACHING FROM THE NORTHWEST
WILL BRING AN INCREASE IN SHOWERS AS EARLY AS SUNDAY NIGHT TO KAUAI.
THERE IS THE POTENTIAL FOR HEAVY RAIN AND A FEW THUNDERSTORMS ACROSS
THE STATE FROM MONDAY AFTERNOON THROUGH TUESDAY. STRONG NORTHERLY
WINDS WILL DEVELOP BEHIND THE FRONT.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL HIGH NORTHEAST OF THE
STATE...WITH A RIDGE EXTENDING ACROSS THE ISLANDS. A SHARP TROUGH IS
LOCATED FAR TO THE NORTHWEST...MOVING TO THE SOUTHEAST AT 30 MPH. AT
THE SURFACE...A RIDGE IS LOCATED LESS THAN 200 MILES NORTH OF
KAUAI...AND THE APPROACHING COLD FRONT IS ABOUT 600 MILES NORTHWEST
OF KAUAI. 00Z RAOBS FROM HILO AND LIHUE SHOW STABLE INVERSIONS NEAR
6KFT. PRECIPITABLE WATERS ARE 0.8-0.9 INCHES...WHICH IS DRIER THAN
NORMAL FOR THIS TIME OF YEAR. EARLY AFTERNOON MIMIC TOTAL
PRECIPITABLE IMAGERY SHOWS VALUES OF 0.8-1.0 INCHES EXTENDING
SEVERAL HUNDRED MILES UPSTREAM TO THE EAST OF THE STATE...WITH TPW
VALUES OF 1.5-1.7 INCHES POOLING ALONG THE COLD FRONT TO THE
NORTHWEST.

WITH THE SURFACE RIDGE BEING SUPPRESSED SOUTHWARD BY THE APPROACHING
FRONT...LOW-LEVEL WINDS HAVE ALREADY TURNED SOUTHEASTERLY ACROSS THE
SMALLER ISLANDS AND ONSHORE SEA BREEZES DEVELOPED DURING THE DAY.
CLOUD MOTIONS SHOW EASTERLY WINDS STILL AFFECTING THE BIG ISLAND.
WINDS ARE EXPECTED TO WEAKEN FURTHER AS THE WESTERN EXTENT OF THE
RIDGE ERODES WITH THE APPROACH OF THE COLD FRONT. SURFACE PRESSURES
ACROSS THE STATE ARE ABOUT A MILLIBAR LOWER THAN 24 HOURS AGO. THE
LOCALLY DRIVEN SEA BREEZE/LAND BREEZE PATTERN WILL CONTINUE THROUGH
SUNDAY. THE STABLE AND DRY AIRMASS WILL KEEP RAINFALL GENERALLY
LIGHT...BUT WE WILL SEE AN INCREASE IN CLOUDS AND SHOWERS ACROSS
INTERIOR AND WEST/NORTHWEST SECTIONS DURING THE LATE
MORNING/AFTERNOON.

THE UPPER LEVEL TROUGH NORTHWEST OF THE STATE WILL PROPEL THE COLD
FRONT TOWARD KAUAI BY SUNDAY NIGHT. MOST OF THE UPPER LEVEL SUPPORT
WITH THIS FIRST TROUGH WILL PASS TO OUR NORTH...AND THE FRONT IS
EXPECTED TO LOSE SOME OF ITS PUSH. HOWEVER...ANOTHER PIECE OF ENERGY
WILL DIG DOWN THE BACK SIDE OF THE TROUGH AND AMPLIFY NEAR THE
ISLANDS...REINVIGORATING THE FRONT AND DRIVING IT FARTHER SOUTHEAST.
COLD AIR ASSOCIATED WITH THIS DIGGING TROUGH WILL ALSO DESTABILIZE
THE AIRMASS OVER THE ISLANDS. 500MB TEMPERATURES DROP TO -12C TO
-14C. MOISTURE WILL BE SOMEWHAT LIMITED...WITH CROSS SECTIONS ACROSS
THE ISLANDS SHOWING DRY AIR ABOVE 700MB. HOWEVER...MODEL FORECAST
SOUNDINGS SHOW THE SUBSIDENCE INVERSION ERODING...LEADING TO
UNSTABLE PROFILES AND RELATIVELY SATURATED LOW-LEVELS. DIFFERENCES
REMAIN BETWEEN THE GFS AND ECMWF...WITH THE GFS BEING MORE
CONSISTENT THROUGH THE EXTENDED FORECAST. THE 12Z ECMWF CUTS OFF
MUCH OF THE DIGGING ENERGY SOUTHWEST OF THE STATE WHILE THE GFS
KEEPS THE TROUGH MORE PROGRESSIVE. BY MID WEEK THE ECMWF TRENDS BACK
TOWARD THE GFS SOLUTION WITH THE PLACEMENT OF A CUTOFF LOW
DEVELOPING NORTHEAST OF THE STATE. OVERALL THE PATTERN HAS
STABILIZED ENOUGH THAT WE CAN START TO ADD A BETTER DELINEATION OF
HIGHER POPS AS THE FRONT/TROUGH SWING THROUGH THE STATE. KEPT THE
BROAD BRUSH OF SLIGHT CHANCE THUNDERSTORMS FOR A ROUGHLY 24 HOUR
PERIOD GENERALLY MONDAY NIGHT/TUESDAY.

STRONG NORTH TO NORTHEASTERLY WINDS ARE EXPECTED TO DEVELOP BEHIND
THE FRONT EARLY IN THE NEW WORK WEEK. THESE WINDS WILL GRADUALLY
RELAX THROUGH THE WEEK...BUT REMAIN GENERALLY OUT OF THE
NORTH-NORTHEAST DUE TO THE PRESENCE OF THE LOW THAT IS EXPECTED TO
BE NORTHEAST OF THE STATE. BY LATE IN THE WEEK THIS LOW WILL WEAKEN
AND ALLOW HIGH PRESSURE TO BUILD NORTHWEST OF THE STATE...WHICH WILL
ALLOW WINDS TO VEER TO A MORE NORTHEASTERLY DIRECTION.

&&

.AVIATION...
WEAK SOUTHEAST FLOW IS EXPECTED ACROSS THE WHOLE OF THE STATE
THROUGH TOMORROW AS THE COLD FRONT TO OUR DISTANT NORTHWEST BEGINS
ITS APPROACH. WEAK SEA BREEZES THIS AFTERNOON WILL BE REPLACED BY
WEAK LAND BREEZES OVERNIGHT ONLY TO REDEVELOP TOMORROW AFTERNOON.
CLOUD COVER THIS AFTERNOON IS FOCUSED OVER ISLAND INTERIORS BUT
SHOULD BECOME MORE DIFFUSE AFTER SUNSET. NEXT BAND OF TRADE WIND
SHOWERS COMING OUT OF THE SOUTHEAST SHOULD REACH THE BIG ISLAND AND
MAUI LATE THIS EVENING. THE AIRMASS SURROUNDING THE ISLANDS IS
RELATIVELY DRY AND THE SUBSIDENCE INVERSION RELATIVELY LOW. ANY
PRECIPITATION NEXT 24 HOURS SHOULD BE LIGHT. ISOLATED MVFR CONDS
POSSIBLE IN WINDWARD SIDE SHOWERS OVERNIGHT OTHERWISE VFR CONDS WILL
PREVAIL ALL AREAS.

&&

.MARINE...
A LONG PERIOD NORTHWEST SWELL GENERATED BY A LARGE AND POWERFUL LOW
PRESSURE SYSTEM IS EXPECTED TO BRING WARNING LEVEL SURF TO THE
ISLANDS ON SUNDAY. A JASON ALTIMETER PASS FROM LATE THIS MORNING
JUST NORTHWEST NORTHWEST OF THE CORE OF THE HIGHEST SEAS SUPPORTS
THE CURRENT WAVEWATCH III FORECAST. WW3 SHOWS A SWELL OF 12-13 FEET
REACHING BUOY 51101 AFTER MIDNIGHT TONIGHT. THIS SWELL WILL BUILD
ACROSS THE ISLANDS ON SUNDAY...THEN LINGER INTO MONDAY BEFORE
GRADUALLY DIMINISHING. HAVE ISSUED A HIGH SURF WARNING FOR NORTH AND
WEST FACING SHORES OF THE SMALLER ISLANDS. GIVEN THE LATER TIMING
FOR THE BIG ISLAND /SWELL ARRIVING SUNDAY NIGHT/ HAVE HELD OFF ON
INCLUDING THE WEST SIDE. HOWEVER...WAVEWATCH GUIDANCE OF 6 FEET/17
SECONDS OFF THE KONA COAST WILL ALSO LEAD TO WARNING LEVEL SURF
THERE AS WELL. GIVEN THE LARGE SWELL...WE HAVE ALSO ISSUED A SMALL
CRAFT ADVISORY FOR MOST COASTAL WATERS DUE TO HAZARDOUS SEAS.

LIFEGUARD REPORTS FROM THE BIG ISLAND INDICATE HIGH SURF CONDITIONS
ALONG EAST FACING SHORES. THE HILO BUOY HAS BEEN RELATIVELY STEADY
REPORTING COMBINED SEAS OF 9-10 FEET...AND THE MAJORITY OF THE
ENERGY IS FROM ABOUT 110 DEGREES AT 10 SECONDS. WAVEWATCH GUIDANCE
AT THAT DATA POINT INDICATES THIS SWELL WILL CONTINUE AT SIMILAR
LEVELS INTO SUNDAY BEFORE GRADUALLY DIMINISHING. A HIGH SURF
ADVISORY REMAINS IN EFFECT FOR EAST FACING SHORES THROUGH SUNDAY.
GIVEN THE SWELL DIRECTION OUT OF THE EAST-SOUTHEAST AND LOWER BUOY
READINGS/SURF OBSERVATIONS ACROSS THE REMAINING ISLANDS...HAVE
CONFINED THE ADVISORY TO THE BIG ISLAND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF WARNING FROM 4 AM SUNDAY TO 6 AM HST TUESDAY FOR NORTH AND
WEST FACING SHORES OF NIIHAU KAUAI OAHU AND MOLOKAI AND FOR NORTH
FACING SHORES OF MAUI.

HIGH SURF ADVISORY UNTIL 6 PM HST SUNDAY FOR EAST FACING SHORES OF
THE BIG ISLAND.

SMALL CRAFT ADVISORY FROM 4 AM SUNDAY TO 6 AM HST TUESDAY FOR ALL
HAWAIIAN WATERS EXCEPT MAALAEA BAY.

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...BEDAL



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