Flash Flood Warning 18, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
315 PM HST FRI FEB 8 2008

.SYNOPSIS...WE WILL CONTINUE TO SEE A RETURN TOWARD MORE SETTLED
TRADE WIND WEATHER...AS AN UPPER TROUGH OVER THE ISLANDS SHIFTS
SOUTHWARD.

&&

.DISCUSSION...
AN UPPER LEVEL TROUGH LIES OVER THE MAIN HAWAIIAN ISLANDS THIS
AFTERNOON. THE ADDED INSTABILITY ASSOCIATED WITH THIS UPPER TROUGH
IS HELPING TO ENHANCE SHOWERS AND A FEW HEAVY THUNDERSHOWERS FROM
MOLOKAI TO THE BIG ISLAND THIS AFTERNOON. THESE CELLS HAVE POPPED UP
DURING THE DAYTIME HEATING OVER THE ISLANDS...AND SHOULD SETTLE DOWN
DURING THE EARLY EVENING.

THINGS LOOK LIKE THEY ARE IMPROVING...BUT WE ARE NOT COMPLETELY OUT
OF THE WOODS QUITE YET. THE MODELS AGREE THAT THE UPPER TROUGH AXIS
WILL CONTINUE TO PUSH SLOWLY SOUTHWARD. THE TROUGH WILL STILL BE
CLOSE ENOUGH TONIGHT TO PROVIDE SOME INSTABILITY. THE MODELS SHOW
500 MB TEMPERATURES STILL NEAR -11C OR SO NEAR THE BIG ISLAND
TONIGHT. VISIBLE SATELLITE IMAGERY SHOWS A SMALL LOW LEVEL VORTICITY
CENTER NEAR 19N 152W MOVING W AT 15 KT...ASSOCIATED WITH A WEAK
TROUGH IN THE EASTERLIES. THIS SHOULD HELP TO KEEP SOME LOCALLY
HEAVY SHOWERS GOING OVER WINDWARD BIG ISLAND AND POSSIBLY MAUI
OVERNIGHT. TO BE ON THE SAFE SIDE...WE WILL EXTEND THE FLASH FLOOD
WATCH FOR THE BIG ISLAND THROUGH TONIGHT.

OTHERWISE...IT APPEARS THAT OVER TIME...THE SYNOPTIC PATTERN IS
GOING TO CONTINUE TO TREND BACK TOWARD MORE TYPICAL TRADE WIND
WEATHER. THE MODELS SHOW MID AND UPPER LEVEL RIDGING BUILDING OVER
THE ISLANDS FROM THE NW STARTING THIS WEEKEND...AS THE TROUGH OVER
US PUSHES TO THE SE. THERE WILL CONTINUE TO BE AN UPPER TROUGH TO
THE EAST OF THE ISLANDS MUCH OF NEXT WEEK. THIS USUALLY GIVES SOME
MINOR ENHANCEMENT TO INCOMING TRADE SHOWERS...BUT THE RIDGING THAT
IS EXPECTED TO BE OVER THE ISLANDS WILL KEEP US FROM SEEING
EXCESSIVE RAIN OR THUNDER. BOTH THE GFS AND THE WRF-NMM SHOW A
FAIRLY SIGNIFICANT DROP IN THE PRECIPITABLE WATER NEAR THE ISLANDS
OVER THE NEXT COUPLE DAYS...THOUGH THE GFS IS A LITTLE BIT SLOWER TO
DRY THINGS OUT. A SERIES OF FAIRLY WEAK HIGH PRESSURE AREAS PASSING
NORTH OF THE ISLANDS SHOULD KEEP LIGHT TO MODERATE TRADES BLOWING.
BY TUE THE MODELS ARE SHOWING VERY DRY AIR AT 700 MB ENCOMPASSING
MOST OF THE AREA...SO MUCH LIGHTER AND FEWER SHOWERS ARE EXPECTED.
ALSO...BEST CHANCES FOR SHOWERS WILL SHIFT BACK TOWARD THEIR MORE
TYPICAL WINDWARD/MAUKA SECTIONS.

&&

.MARINE...RECENT RUNS OF THE WAVEWATCH III HAVE NOT TAKEN THE SWELL
DOWN AS FAST AS PREVIOUSLY INDICATED...AND IN FACT SUGGEST THAT HIGH
SURF WILL CONTINUE THROUGH SATURDAY AS THE MID-AFTERNOON SURF
DISCUSSION INDICATES. THE WAIMEA BAY BUOY IS STILL SHOWING 7 FEET
AT 13-15 SECONDS.

THE GFS AND ECMWF MODELS ARE IN VERY GOOD AGREEMENT WITH THEIR
HANDLING OF NW PACIFIC SYSTEMS THAT WILL BE SENDING SWELLS AT OUR
NORTH AND WEST FACING SHORES THIS WEEK. THE FIRST WILL BE FROM A
LONG FETCH AND BROAD SWATH OF 30 TO 40 KT WESTERLIES TO THE SOUTH OF
A PAIR OF LOWS SOUTH OF THE ALEUTIANS THIS WEEKEND. ANGULAR
SPREADING WILL ALLOW SOME OF THE ENERGY TO REACH US AND WWIII SHOWS
THIS SWELL SHOULD ARRIVE LATER MONDAY WITH SURF HEIGHTS CLIMBING
BACK SOLIDLY INTO THE ADVISORY CRITERIA BY MONDAY NIGHT. GFS/ECWMF
AGREE ON A DEEPER AND SOMEWHAT CLOSER 955-960 LOW PASSING ACROSS THE
DATELINE NEAR 40N ON MONDAY...WHICH WOULD SEND A MORE POTENT WNW
SWELL OUR WAY WED NIGHT AND THU.

WE HAVE ADJUSTED THE WINDS DOWNWARD JUST A BIT ON SUNDAY TO REMOVE
25 KT WINDS FROM THE COASTAL WATERS. THE GFS SEEMS TO BE OVERDOING
THE SURFACE PRESSURE GRADIENT OVER THE ISLANDS DUE TO SPURIOUS LOW
PRESSURE TO THE SOUTH OF THE STATE...SO WENT MORE TOWARD THE ECMWF
SOLUTION. HOWEVER...A SMALL HIGH PASSING TO OUR N ON MON MAY BOOST
THE TRADES TO SMALL CRAFT ADVISORY LEVELS IN THE ALENUIHAHA CHANNEL
OR AROUND SOUTH POINT.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH WILL BE EXTENDED UNTIL 4 AM SAT FOR MAUI COUNTY
AND THE BIG ISLAND.

HIGH SURF ADVISORY WILL BE EXTENDED THROUGH 4 PM SAT FOR N AND W
SHORES OF NIIHAU KAUAI AND OAHU...AND N FACING SHORES OF MOLOKAI
AND MAUI.

&&

R BALLARD