Flash Flood Warning 31, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST FRI FEB 23 2007

.SYNOPSIS...WET TRADES WILL CONTINUE THROUGH THE WEEKEND AND INTO
NEXT WEEK...THOUGH WIND SPEEDS MAY FLUCTUATE A BIT. THE HEAVIEST
SHOWERS ARE LIKELY IN THE UNSTABLE AIRMASS PREVAILING FROM THIS
EVENING THROUGH SUNDAY.

&&

.DISCUSSION...WATER VAPOR IMAGES SHOWED A BAGGY AND SHALLOW UPPER
LOW OVER THE MAIN HAWAIIAN ISLANDS WITHIN A LARGER UPPER RIDGE.
ALTHOUGH THE LOW DID NOT APPEAR TO BE PARTICULARLY IMPRESSIVE
DYNAMICALLY...TEMPERATURES AT 500 MILLIBARS IN THE RANGE OF -12 TO
-15 CELSIUS NO DOUBT CONTRIBUTED TO THE DEVELOPMENT OF TOWERING
CUMULI FROM SHOWERS BLOWING IN ON THE TRADES. SIMILARLY...AN AREA OF
LOW-LEVEL VORTICITY APPROACHING THE ARCHIPELAGO FROM THE EAST
PROBABLY WILL ENHANCE SHOWERS NOT ONLY WITHIN IMMEDIATE REACH OF ITS
CLOUDS...BUT ACROSS MOST WINDWARD AREAS OF THE STATE. SLIGHTLY
WEAKER TRADES PERMITTED THE SHOWERS TO LINGER RATHER LONGER THAN
THEY DID YESTERDAY...RESULTING IN A LESS UNIFORM DISTRIBUTION OF
RAINFALL...WITH FEWER GAUGES REPORTING GENERALLY LARGER AMOUNTS OVER
THE PAST SIX HOURS. OVERALL...ACCUMULATION WAS HIGHEST ON OAHU AND
KAUAI FOLLOWED BY THE OTHER ISLANDS...AND SOME OF THE TOWERING
CUMULI DRIFTED SIGNIFICANTLY LEEWARD. WHILE THEY WERE NO LONGER AS
GUSTY AS EARLIER IN THE WEEK...MODERATE TRADES PERSISTED.

GUIDANCE SUGGESTS THAT THE LOW OVERHEAD WILL EXPAND GRADUALLY AS IT
BECOMES MORE SHALLOW OVER THE NEXT TWO DAYS...EFFECTIVELY RESULTING
IN LITTLE CHANGE. THE COLD AIR ALOFT MAY PERMIT THUNDERSTORMS OVER
THIS WEEKEND...BUT SHEAR BETWEEN THE TRADES AND UPPER-LEVEL WIND
PROBABLY WILL PROVE TOO STRONG OVER LAND...SO THIS POSSIBILITY IS
LIMITED TO THE COASTAL FORECAST. ON THE OTHER HAND...ATMOSPHERIC
MOISTURE BANKING UP ALONG AND OVER THE BIG ISLAND SUMMITS...TOGETHER
WITH THE VERY COLD TEMPERATURES ALOFT...MAY DEPOSIT SOME FREEZING
RAIN OR RIME AT HIGH ELEVATIONS AGAIN TONIGHT AND INTO SATURDAY
MORNING. THE COLD AIR WILL MODERATE THROUGH NEXT WEEK...EVENTUALLY
REDUCING ITS EFFECT ON THE ISLANDS. A DEEPER UPPER LOW APPROACHING
THE AREA FROM THE NORTHWEST OVER THE WEEKEND IS NOT EXPECTED TO
REACH QUITE FAR ENOUGH SOUTHEAST TO AFFECT THE STATE MUCH. THE
SURFACE RIDGE TO THE NORTHEAST OF THE ISLANDS WILL PERSIST THROUGH
MOST OF THE FORECAST PERIOD...THEN RECEDE A BIT BY THE END OF NEXT
WEEK. THUS...THE TRADES WILL CHANGE LITTLE AT LEAST THROUGH NEXT
THURSDAY.

&&

.MARINE...NO SIGNIFICANT SWELLS ARE EXPECTED DURING THE NEXT SEVERAL
DAYS. QUIKSCAT SATELLITE WIND DATA AND SURFACE OBSERVATIONS
INDICATED TRADE WINDS NEAR 25 KT IN THE CHANNELS AND SOUTH OF THE
BIG ISLAND...REQUIRING A SMALL CRAFT ADVISORY FOR THOSE AREAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY THROUGH 6 PM HST SATURDAY FOR THE KAUAI...
KAIWI...PAILOLO...AND ALENUIHAHA CHANNELS...AND WATERS SOUTHEAST OF
THE BIG ISLAND.

WINTER WEATHER ADVISORY FROM 6 PM HST THIS EVENING THROUGH 6 AM HST
SATURDAY FOR THE BIG ISLAND SUMMITS AND BIG ISLAND INTERIOR.

&&

RYSHKO