Flash Flood Watch 2, NWS Honolulu, HI

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
1000 PM HST TUE APR 21 2009

.UPDATE...
HAVE EXTENDED THE HIGH WIND WARNING FOR THE BIG ISLAND SUMMITS
THROUGH THE NIGHT. ALSO REMOVED MENTION OF ANY PRECIP OVERNIGHT ON
THE SMALLER ISLANDS...BUT KEPT SKY COVER HIGH OVER WINDWARD AREAS.
CANCELLED THE SMALL CRAFT ADVISORY /SCA/ FOR MAALAEA BAY DUE TO
WINDS BELOW 25 KT...BUT COMBINED SEA  NEAR 10 FEET FOR ALENUIHAHA
AND PAILOLO CHANNELS SUPPORTS SCA CONTINUATION.

&&

.SYNOPSIS...
MOISTURE ASSOCIATED WITH A WEAKENING FRONTAL BOUNDARY WILL REMAIN
NEARLY STATIONARY NEAR THE BIG ISLAND TONIGHT...FOCUSING SHOWERS
OVER WINDWARD BIG ISLAND. THE BOUNDARY WILL LINGER
ACROSS THE AREA INTO THE SECOND HALF OF THE WEEK. AN UPPER LEVEL
DISTURBANCE WILL BRING MORE WIDESPREAD RAINFALL TO THE STATE LATE
THURSDAY INTO THE WEEKEND WITH LOCALLY HEAVY RAINFALL POSSIBLE AT
TIMES. THE TRADE WINDS WILL BECOME MORE EASTERLY AND DIMINISH INTO
THE SECOND HALF OF THE WEEK.

&&

.DISCUSSION...
LOW-LEVEL MOISTURE ASSOCIATED WITH A STALLED AND WEAKENING FRONTAL
BOUNDARY/SHEAR LINE REMAINS E TO W ORIENTED ALONG 20N AT THE
LONGITUDE OF THE ISLANDS. NE FLOW NORTH OF THE BOUNDARY IS PUSHING
THIS MOISTURE UPSLOPE ALONG THE HAMAKUA COAST...WHERE SOME RAIN
GAGES RECEIVED OVER 3 INCHES OF RAIN IN THE 12 HOURS ENDING AT 5 PM.
RADAR IS CURRENTLY DETECTING MODERATE TO HEAVY RAIN NEAR CAPE
KUMUKAHI/HAWAIIAN PARADISE PARK...BUT MOST OF IT HAS REMAINED
OFFSHORE SINCE THE BEGINNING OF THE SHIFT.

FURTHER NORTH OF THE BOUNDARY...DRY LOW-LEVEL AIR IS SPREADING OVER
THE SMALLER ISLANDS FROM THE NE...DRIVING DEW POINTS DOWN TOWARD THE
UPPER 50/S. THE 00Z SOUNDING FROM PHLI HIGHLIGHTED THE DRY AND
STABLE AIR MASS NORTH OF THE FRONT...WITH PWAT DOWN TO 0.87
INCHES...AND A STURDY SUBSIDENCE INVERSION BASED NEAR 5000 FEET.
CORRESPONDING TO THIS...RAIN GAGES ON KAUAI HAVE REMAINED DRY OVER
THE PAST 6 TO 9 HOURS.

WINDS ACROSS THE BIG ISLAND SUMMITS REMAIN HIGH THIS EVENING...AND
ARE FLUCTUATING OVER A BROAD RANGE OF SPEEDS...FROM MID-RANGE
ADVISORY TO MID-RANGE WARNING. TREND IS STILL EXPECTED TO BE DOWN...
BUT IT MAY TAKE ALL NIGHT. WILL ATTEMPT TO GET AN UPDATE ON THIS OUT
BY 10 PM FOR LOCAL MEDIA OUTLETS...AS THE WARNING IS CURRENTLY SET
TO EXPIRE AT MIDNIGHT.

SHORT TERM FORECAST CALLS FOR A PRETTY DRY NIGHT OVER MOST AREAS...
SAVE WINDWARD BIG ISLAND AND ADJACENT WATERS. THE UPDATE FOR THE
WIND WARNING WILL LIKELY ALSO INCLUDE A REDUCTION IN POPS FOR THE
SMALLER ISLANDS. ON WEDNESDAY...MOISTURE ASSOCIATED WITH THE STALLED
FRONT WILL LINGER IN PLACE...BUT SHORT WAVE RIDGING MOVING IN FROM
THE WEST WILL KEEP THE ISLAND ATMOSPHERE ON THE STABLE SIDE.
LOW-LEVEL WINDS WILL FAVOR A ENE DIRECTION...AT MODERATE SPEEDS.

THE FORECAST GETS A LITTLE MORE COMPLEX AS WE HEAD INTO THE LATTER
PART OF THE WORK WEEK...WITH LATEST MODEL GUIDANCE FASTER AND MORE
PROGRESSIVE /THAN AT THIS TIME LAST NIGHT/ WITH A SHORT WAVE TROUGH
DIGGING OVER THE ISLANDS FROM THE NORTHWEST. LATEST THINKING BRINGS
A SHORT WAVE DOWN OVER THE ISLANDS THURSDAY AND THURSDAY NIGHT...
WITH A CUT-OFF LOW DEVELOPING NE OF THE ISLANDS BY FRIDAY NIGHT.
EARLIER GFS RUNS HAD THIS LOW CUTTING OFF OVER THE ISLANDS...AND THE
LATEST 00Z RUN HAS THE LOW SIGNIFICANTLY OFFSET TO THE NE...WHICH
HAS MAJOR IMPLICATIONS FOR THE FORECAST...AND THE EXISTING
HYDROLOGIC OUTLOOK. NONETHELESS...MID LEVEL TEMPERATURES STILL LOOK
COLD ENOUGH...AND WITH MOISTURE ASSOCIATED WITH THE STALLED FRONT
IN PLACE...HEAVY RAIN DEVELOPMENT IS STILL POSSIBLE. IF LATEST GFS
FORECAST PANS OUT...THEN THE HEAVY RAIN THREAT WOULD NOT LINGER INTO
THE WEEKEND...INSTEAD BEING THURSDAY NIGHT AND FRIDAY...WITH SURFACE
LOW/TROUGH DEVELOPMENT ENE OF THE ISLANDS GIVING US A RELATIVELY DRY
NE FLOW OVER THE WEEKEND. THE OLD ECMWF RUN SUPPORTS THIS SCENARIO
AS WELL...SO THE WEEKEND MAY NOT BE A WASH OUT. WE/LL HAVE TO WAIT
AND SEE WHAT LATER MODEL RUNS INDICATE.

GIVEN THE SHORT TERM UNCERTAINTY...IT SHOULD COME AS NO SURPRISE THAT
THERE IS A LARGE SPREAD IN MODEL SOLUTIONS INTO NEXT WEEK...BUT THEY
ARE POINTING TOWARD UNSTABLE CONDITIONS ALOFT TUE INTO MID WEEK.
THERE IS NOT ENOUGH CERTAINTY BEYOND THE WKND TO MAKE ANY CHANGES TO
THE LONGER TERM GRIDS RIGHT NOW.

&&

.MARINE...
A SURFACE LOW ABOUT 1200 MILES NE OF THE ISLANDS WILL DELIVER
A RELATIVELY SHORT-PERIOD NNE SWELL WHICH WILL CONTINUE TO BUILD
OVERNIGHT...PEAK ON WED AND THU...AND LOWER THEREAFTER. THE SWELL
WILL LIKELY BE LARGE ENOUGH TO EXCEED THE 8 FOOT ADVISORY THRESHOLD
ALONG E FACING SHORES WED/THU...ESPECIALLY AS THE SWELL DIRECTION
VEERS NE. WILL LIKELY BE ISSUING A HIGH SURF ADVISORY WITH THE
MORNING PACKAGE.

NE WINDS ARE CURRENTLY NEAR SCA CRITERIA /ALBEIT MARGINALLY/ IN SOME
OF THE TYPICALLY WINDIER CHANNELS AND MAALAEA BAY...BUT WINDS SHOULD
BE BELOW SCA CRITERIA BY MORNING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WARNING UNTIL 6 AM WEDNESDAY FOR BIG ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST WEDNESDAY FOR PAILOLO AND
ALENUIHAHA CHANNELS.

&&

BIRCHARD