098 FXHW60 PHFO 150138 AFDHFO AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE HONOLULU HI 400 PM HST THU MAR 14 2013 .SYNOPSIS... A SURFACE RIDGE OVER THE STATE WILL KEEP WINDS LIGHT...WITH OFFSHORE LAND BREEZES AGAIN TONIGHT. A COLD FRONT WILL MOVE DOWN THE ISLAND CHAIN ON FRIDAY AND SATURDAY...INCREASING WINDS AND BRINGING A PERIOD OF HEAVIER SHOWERS. THE GREATEST CHANCE FOR SHOWERS WILL BE ACROSS MAUI AND HAWAII COUNTIES FRIDAY NIGHT AND SATURDAY. DRIER CONDITIONS ARE EXPECTED EARLY NEXT WEEK...WITH WINDS TURNING SOUTHERLY AGAIN BY MID WEEK AHEAD OF ANOTHER FRONT. && .DISCUSSION... WATER VAPOR IMAGERY SHOWS A SHEARED SHORTWAVE TROUGH NEAR OAHU... EMBEDDED WITHIN A BROAD TROUGH THAT EXTENDS NORTHEAST AND WEST OF THE AREA. A STRONG UPPER LEVEL JET EXTENDS ACROSS THE ISLANDS...WITH WINDS FROM THE 00Z SOUNDINGS UP TO 120KT AROUND 200MB. AT THE SURFACE...A WEAK RIDGE IS LOCATED OVER THE STATE...WITH A DEVELOPING COLD FRONT NEARLY 300 MILES NORTHWEST OF KAUAI. THIS FRONT IS MOVING TO THE SOUTHEAST AT LESS THAN 15 MPH...AND IS OVER THE FAR NORTHWEST OFFSHORE WATERS. 00Z SOUNDINGS FROM HILO AND LIHUE SHOW INVERSIONS AROUND 11KFT...WITH WEAKER INVERSIONS AROUND 4-6KFT. PRECIPITABLE WATERS ALSO RANGE FROM 0.8 AT LIHUE TO 1.3 INCHES AT HILO...PART OF WHICH IS DUE TO THE HIGH LEVELS CLOUDS OVER THE STATE. EARLY AFTERNOON MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS DRIER AIR NORTHWEST OF KAUAI...WITH ONLY VALUES AROUND 1 INCH ASSOCIATED WITH THE COLD FRONT FARTHER NORTHWEST. A LAYER OF HIGH CLOUDS ASSOCIATED WITH THE JET STREAM COVERS MOST OF THE STATE. AFTERNOON CONVECTION HAS BEEN STRONGEST ACROSS MAUI COUNTY AND THE BIG ISLAND DESPITE WIDESPREAD SEA BREEZES ACROSS THE OTHER ISLANDS AS WELL. THE EASTERN CONVECTION HAS BEEN ENHANCED BY THE LEADING SHORTWAVE...AND ACTIVITY ACROSS KAUAI HAS BEEN SEVERELY LIMITED IN THE SUBSIDENCE BEHIND IT. MORE WIDESPREAD LOW-LEVEL CONVERGENCE IS APPARENT ACROSS THE EASTERN ISLANDS...WITH A RATHER WIDE SWATH OF SHOWERY LOW CLOUDS APPARENT ON SATELLITE IN A BAND ABOUT 200 MILES WIDE EXTENDING SOUTH AND EAST OF OAHU. FOR TONIGHT...KAUAI WILL REMAIN ON THE DRY SIDE...ACTIVITY ACROSS OAHU WILL DIMINISH THIS EVENING...AND SHOWERS NEAR MAUI COUNTY SHOULD CONTINUE THROUGH THE NIGHT. THE ACTIVITY ACROSS THE BIG ISLAND IS A LITTLE MORE UNCERTAIN...AS THE LAND BREEZES MAY BE STRONG ENOUGH TO HELP CLEAR THEM OUT OVERNIGHT AS WAS THE CASE LAST NIGHT DESPITE THE GREATER LOW-LEVEL MOISTURE AND CONVERGENCE. A STRONGER UPPER LEVEL TROUGH FAR NORTH-NORTHWEST OF THE STATE WILL DIG SOUTH-SOUTHEASTWARD...HELPING TO STRENGTHEN THE COLD FRONT NORTHWEST OF THE STATE AND PROPEL IS TOWARD THE ISLANDS. THE COLDEST AIR/GREATEST MID-LEVEL INSTABILITY AS WELL AS THE STRONGEST DYNAMICS ASSOCIATED WITH THIS TROUGH IS EXPECTED TO REMAIN NORTH OF THE STATE. HOWEVER...FORECAST SOUNDINGS SHOW THE INVERSION ERODING ON FRIDAY. MOISTURE MAY BE A LIMITING FACTOR...AT LEAST ACROSS THE WESTERN HALF OF THE STATE. BOTH ECMWF AND GFS SHOW A GREATER MOISTURE DEPTH NEAR MAUI AND THE BIG ISLAND...AND THEREFORE BOTH PRODUCE GREATER RAINFALL AMOUNTS. THIS SEEMS REALISTIC GIVEN CURRENT IMAGERY...WITH RECENT ACTIVITY FOCUSED ACROSS THE EASTERN PORTION OF THE STATE. WILL CARRY HIGHER RAINFALL CHANCES IN THIS AREA ON SATURDAY...WITH DRIER CONDITIONS MOVING IN FROM THE WEST. THE CURRENT FORECAST ALSO HAS A MENTION OF POSSIBLE HEAVY RAIN LATE FRIDAY INTO SATURDAY...WHICH IS TIMED WELL WITH THE ERODING INVERSION. WIDESPREAD HEAVY RAIN IS NOT ANTICIPATED GIVEN THE RELATIVELY LIMITED MOISTURE WITH THE FRONT...BUT WITH NO INVERSION WE WILL LEAVE THE MENTION OF POSSIBLE HEAVY RAIN FOR MORE ISOLATED HEAVY SHOWERS. IF THE DYNAMICS ARE STRONGER OR MOISTURE IS GREATER...WE MAY NEED TO CONSIDER THUNDER POTENTIAL DURING THIS TIME FRAME...BUT FOR NOW IT DOES NOT LOOK LIKELY. THE UPPER-LEVEL TROUGH AXIS LINGERS NEAR THE STATE INTO EARLY NEXT WEEK...BUT A WEAK SURFACE RIDGE NOSES ACROSS THE ISLANDS THIS WEEKEND. WINDS VEER AROUND TO THE NORTH BEHIND THE FRONT...BRINGING DRIER LOW-LEVEL AIR. WHILE INITIALLY MODERATELY STRONG THIS WEEKEND...WINDS WEAKEN EARLY NEXT WEEK...ALLOWING FOR MORE LOCALIZED LAND AND SEA BREEZES. A NEW FRONT MAY APPROACH THE STATE DURING THE MIDDLE OF NEXT WEEK. THERE ARE SIGNIFICANT DIFFERENCES BETWEEN THE 12Z GFS AND ECMWF AT THE END OF THE PERIOD. THE GFS DIGS A TROUGH FARTHER SOUTH AND CUTS OFF A MID-LEVEL LOW SEVERAL HUNDRED MILES NORTHWEST OF THE STATE. THE ECMWF KEEPS THE SAME TROUGH CLOSER TO THE PARENT LOW...AMPLIFYING THE SYSTEM BUT KEEPING IT MUCH FARTHER NORTH. GIVEN THE OVERALL PATTERNS...IT LOOKS LIKE THE ECMWF HAS A MORE REALISTIC SOLUTION. EVEN WITH THE MORE DISTANT SYSTEM...THE ECMWF STILL SHOWS A PERIOD OF LIGHT WINDS AND INCREASING MOISTURE/CONVERGENCE AHEAD OF A FRONT THAT DOES NOT QUITE MAKE IT TO THE ISLANDS. AT THE HIGHER ELEVATIONS...A HIGH WIND WARNING REMAINS IN PLACE FOR THE BIG ISLAND SUMMITS...WITH A WIND ADVISORY FOR HALEAKALA SUMMIT. THE 12Z/18Z MODEL RUN SHOWS AN INCREASE IN WIND SPEEDS NOW... FOLLOWED BY A STRONGER INCREASE FRIDAY AFTERNOON/NIGHT. WINDS HAVE REMAINED NEAR/ABOVE THE 45 MPH WARNING THRESHOLD ON MAUNA KEA...BUT ARE BELOW THE 30 MPH ADVISORY THRESHOLD ON HALEAKALA. EVEN THOUGH WINDS ARE EXPECTED TO REMAIN BELOW THE ADVISORY LEVEL ON MAUI TONIGHT...THEY WILL LIKELY CLIMB BACK TO 30 MPH ON FRIDAY. GIVEN THE TIMING OF THE INCREASE WITHIN THE NEXT 24 HOURS...AND TO AVOID CONFUSION...WE WILL KEEP THE ADVISORY GOING BUT MENTION WINDS INCREASING ON FRIDAY. THESE STRONG WINDS ARE THEN EXPECTED TO CONTINUE THROUGH SATURDAY BEFORE WEAKENING. THERE IS ALSO THE POTENTIAL FOR WINTER WEATHER ACROSS THE BIG ISLAND SUMMITS. DEEP CONVECTION MAY REACH SUMMIT LEVEL FRIDAY AFTERNOON...BUT MORE FOCUSED SHOWERS ARE POSSIBLE FRIDAY NIGHT/SATURDAY AS THE FRONT APPROACHES. GIVEN THE LIMITED MOISTURE AND UNCERTAINTY REGARDING CONVECTION...WE WILL HOLD OFF ON ANY WINTER WEATHER HEADLINES FOR NOW. IF CONDITIONS DO DEVELOP...THEY MAY BE MORE LIKELY TO REQUIRE AN ADVISORY THAN A WARNING...SO A WATCH IS NOT ANTICIPATED. && .AVIATION... A LOW PRESSURE SYSTEM IS APPROACHING THE MAIN HAWAIIAN ISLANDS THIS EVENING WITH WEAK FLOW AHEAD OF IT. LINGERING MOISTURE OVER THE EASTERN ISLANDS WILL KEEP SHOWERS AROUND THE AREA. BY LATE FRIDAY MORNING CONVECTIVE SHOWERS WILL DEVELOP AHEAD OF THIS SYSTEM CAUSING MVFR TEMPORARILY WITH REDUCED VISIBILITY...MOUNTAIN OBSCURATIONS AND LOWER CEILINGS THROUGH THE DAY. THE JETSTREAM HAS DIPPED OVER THE MAIN HAWAIIAN ISLANDS TODAY CAUSING OCCASIONAL MODERATE TURBULENCE ABOVE FL300. AIRMET TANGO IN EFFECT. AMPLE LOW AND MID LEVEL MOISTURE ALONG WITH WEAK SEABREEZES HAVE CAUSED EASTERN SLOPES OF THE BIG ISLAND AND MAUI ISLAND TO BECOME OBSCURED AT TIMES IN CLOUDS AND SHOWERS. AIRMET SIERRA IN EFFECT. && .MARINE... A SERIES OF NORTH AND NORTHWEST SWELLS WILL AFFECT THE STATE THROUGH THE FORECAST PERIOD. A LARGER NORTHWEST SWELL IS EXPECTED TO BUILD LATE SATURDAY THROUGH SUNDAY...BRINGING SURF NEAR ADVISORY LEVELS FOR NORTH AND WEST FACING SHORES. WITH THIS SWELL...COMBINED SEAS WILL ALSO REACH SMALL CRAFT ADVISORY LEVELS FOR COASTAL WATERS EXPOSED TO THE NORTHWEST SWELL. && .HFO WATCHES/WARNINGS/ADVISORIES... HIGH WIND WARNING IN EFFECT UNTIL 6 PM HST SATURDAY FOR BIG ISLAND SUMMITS. WIND ADVISORY UNTIL 6 PM HST SATURDAY FOR HALEAKALA SUMMIT. && $$ DISCUSSION/MARINE...BRAVENDER AVIATION...EVANS/FOSTER