Heavy Rain in Hawaii, March 14 to 15, 2013

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