| Current Conditions | ||
| Temp |  | 4.9 C |
| RH | | 20 % |
| Wind | | N 11 mph |
| Road | | Open |
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Hawaiian Islands Synoptic Discussion and Guidance 911 FXHW60 PHFO 271954 AFDHFO AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE HONOLULU HI 1000 AM HST TUE JAN 27 2015 .SYNOPSIS... WEAKENING TRADE WINDS WILL TURN SOUTHEASTERLY ON WEDNESDAY...AS A WEAK FRONT APPROACHES FROM THE WEST. THIS FRONT WILL BRING SOME SHOWERS AND LIGHT WINDS TO THE ISLANDS THURSDAY AND FRIDAY...THEN LIGHT NORTHEASTERLY TRADES WILL FILL BACK ON OVER THE WEEKEND. A STRONGER FRONT MAY APPROACH THE ISLANDS EARLY NEXT WEEK. && .DISCUSSION... WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE TO THE SOUTH AND EAST OF THE STATE. A SHORTWAVE TROUGH IS LOCATED WEST OF THE DATELINE... WHICH WILL COME INTO PLAY LATER IN THE WEEK. AT THE SURFACE...THE RIDGE IS LOCATED A COUPLE HUNDRED MILES NORTH OF KAUAI...WITH A COLD FRONT OVER 600 MILES TO THE NORTHWEST. 12Z RAOBS FROM HILO AND LIHUE SHOW INVERSIONS OF 4-6KFT AND PRECIPITABLE WATERS OF 0.8-1.0 INCHES. EARLY MORNING MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS SIMILAR VALUES UPSTREAM TO THE NORTHEAST AND EAST...WITH A NARROW BAND OF SLIGHTLY HIGHER PW VALUES POOLED ALONG THE FRONT TO THE NORTHWEST. BANDS OF HIGHER MOISTURE ARE ALSO LOCATED FAR SOUTHEAST OF THE BIG ISLAND. EVEN THOUGH SURFACE PRESSURES ACROSS THE ISLANDS HAVE RISEN AROUND 1MB IN THE PAST 24 HOURS...THE PRESSURE DIFFERENCE BETWEEN BUOY 51101 AND 51003 HAS DROPPED ABOUT 2MB OVER THE SAME PERIOD. THE SURFACE RIDGE IS WEAKENING AS IT SHIFTS SOUTHWARD OVER THE STATE... LEADING TO A WEAKENING GRADIENT AND DECREASE IN WIND SPEEDS. TRADES WILL WEAKEN AND VEER SOUTHEAST ON WEDNESDAY AS THE RIDGE MOVES SOUTH ACROSS THE STATE. THE AIRMASS IS INITIALLY DRY AND STABLE...WHICH WILL CONTINUE THROUGH WEDNESDAY. EVEN WITH ONSHORE FLOW WITH SEA BREEZES DURING THE AFTERNOON...WHICH WILL BE MORE WIDESPREAD ACROSS THE WESTERN HALF OF THE STATE...THE STABLE ATMOSPHERE WILL LIMIT RAINFALL AMOUNTS FROM AFTERNOON SHOWERS. THE FRONT CURRENTLY NORTHWEST OF THE STATE IS EXPECTED TO WEAKEN AS IT APPROACHES THE ISLANDS WEDNESDAY NIGHT. THE SHORTWAVE NEAR THE DATELINE IS EXPECTED TO AMPLIFY AND MAY EITHER LEAD TO A SECONDARY FRONT FORMING BEHIND THE FIRST...AS INDICATED IN THE 12Z GFS AND ECMWF...OR JUST A REINTENSIFICATION OF THE ORIGINAL FRONT. EITHER WAY...THIS FEATURE WILL GRADUALLY PROPEL THE FRONT SOUTHEAST ACROSS THE ISLANDS THROUGH FRIDAY. THERE IS LIMITED UPPER LEVEL SUPPORT ASSOCIATE WITH THE FRONT...AND MOISTURE WILL BE MARGINAL. FORECAST SOUNDINGS AND CROSS SECTIONS SHOW INVERSIONS RAISING/MOISTURE DEPTH INCREASING SLIGHTLY WITH THE FRONT...ALTHOUGH RAINFALL AMOUNTS ARE STILL EXPECTED TO BE RELATIVELY LIGHT. BOTH THE GFS AND ECMWF SHOW THE SURFACE BOUNDARY STALLING NEAR THE BIG ISLAND LATE FRIDAY/SATURDAY...WITH SOME ENHANCEMENT FROM THE SOUTHEAST. WITH THE LOW-LEVEL FLOW TURNING SOUTHEASTERLY AS THE RIDGE SHIFTS EASTWARD...THE EXISTING CLOUD BANDS ASSOCIATED WITH OLD FRONTS CURRENTLY SOUTHEAST OF THE BIG ISLAND WILL BE PULLED NORTHWESTWARD TOWARD THE STATE. THE 12Z MODELS SHOW THESE FEATURES ENHANCING THE MOISTURE AND LOW-LEVEL VORTICITY ASSOCIATED WITH THE OLD FRONT...AND LINGERING THIS BAND NEAR THE BIG ISLAND THROUGH THE WEEKEND. THIS FEATURE IS EXPECTED TO LIFT NORTHWARD EARLY NEXT WEEK AS THE NEXT SYSTEM APPROACHES FROM THE WEST...BUT IT MAY STILL ENHANCE MAINLY WINDWARD SHOWERS ACROSS THE BIG ISLAND AND MAUI. EARLY NEXT WEEK A STRONGER FRONT MAY APPROACH THE ISLANDS...BUT THERE REMAINS A FAIR AMOUNT OF UNCERTAINTY WITH THIS SYSTEM. THE 12Z GFS NOW DEVELOPS A LOW MUCH FARTHER SOUTH...TRACKING ALONG 30N...THAT PROPELS A FRONT SOUTHWARD ACROSS THE ISLANDS. EARLIER RUNS HAD VARYING DEGREES OF FEEDBACK THAT AFFECTED TIMING AND PLACEMENT AS WELL...BUT STILL FEATURED THE PARENT LOW MUCH FARTHER NORTH. THE 12Z ECMWF HAS A WEAKER BOUNDARY THAT COMES THROUGH WITH A SIMILAR TIMING...BUT WITH THE SURFACE WAVE TRACKING CLOSER TO 40N THAN TO 30N. ONLY ONE GFS ENSEMBLE MEMBER IS CLOSE TO THE STRENGTH OF THE OPERATIONAL RUN...WITH MOST MUCH WEAKER. THE CURRENT FORECAST FEATURES WINDS THAT ARE MUCH WEAKER THAN THE OPERATIONAL GFS...WHICH SEEMS REASONABLE GIVEN ITS OUTLIER STATUS. && .AVIATION... VFR CONDITIONS WILL PREVAIL ACROSS THE STATE TODAY. WINDWARD SECTIONS OF ALL ISLANDS MAY SEE ISOLATED MVFR CEILINGS AT TIMES THROUGHOUT THE DAY...LESS SO AROUND OAHU...BUT OCCASIONALLY OVER LANAI AT MIDDAY INTO THE AFTERNOON. WINDS LIGHTENING UP AS A FRONT APPROACHING FROM THE WEST WILL GRADUALLY WEAKEN AND PUSH THE HIGH PRESSURE RIDGE JUST NORTH OF THE STATE EASTWARD. LOOKING FOR LIGHT TRADES TODAY WITH SOME SEA BREEZES TAKING OVER AT TIMES AND LIGHT AND VARIABLE WINDS AT NIGHT WITH SOME LAND BREEZES PREVAILING IN SOME AREAS. && .MARINE... LONG PERIOD FORERUNNERS OF THE NEXT NORTHWEST SWELL PICKED UP AT BUOY 51101 EARLY THIS MORNING...WITH OVERALL SWELL HEIGHT SHOWING ONLY A GRADUAL RISE. THIS SWELL IS EXPECTED TO BRING SURF TO ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES. WE WILL HOLD OFF TO SEE HOW THE SWELL BUILDS AT THE BUOY BEFORE ISSUING THE ADVISORY FOR THE STATE...WHICH WOULD BE NEEDED AFTER MIDNIGHT GIVEN THE WAVEWATCH III TIMING. THIS SWELL IS EXPECTED TO PEAK WEDNESDAY/WEDNESDAY NIGHT...THEN DIMINISH ON THURSDAY. WITH THE INCOMING SWELL...COMBINED SEAS MAY ALSO FLIRT WITH THE 10 FOOT SMALL CRAFT ADVISORY LEVEL FOR SOME AREAS. && .HFO WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION/MARINE...BRAVENDER AVIATION...FUJII Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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