Flash Flooding and High Surf in Hawaii, December 10 to 13, 2011

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE HONOLULU HI
1045 AM HST SAT DEC 10 2011

.UPDATE...
QUICK UPDATE TO ADD WIND ADVISORY FOR MOUNT HALEAKALA...SINCE WINDS
HAVE INCREASED DUE TO TIGHTENING OF THE PRESSURE GRADIENT ABOVE MAUI
TODAY. FOR NOW...WILL ASSUME THESE BREEZY CONDITIONS MAY IMPROVE
AFTER MIDNIGHT HST.

&&

.SYNOPSIS...
SURFACE HIGH PRESSURE FAR NORTH OF THE AREA WILL MAINTAIN TRADE
WINDS THROUGH NEXT WEEK. INSTABILITY DUE TO AN UPPER LEVEL TROUGH
AND ABUNDANT LOW-LEVEL MOISTURE MOVING IN FROM THE EAST OF THE
ISLANDS WILL MAINTAIN WET WEATHER...WITH LOCALLY HEAVY RAINFALL
POSSIBLE ACROSS MOST OF THE ALOHA STATE INTO EARLY NEXT WEEK.

&&

.DISCUSSION...
A NEARLY STATIONARY SURFACE RIDGE EXTENDS SOUTHWEST FROM A 1029 MB
HIGH NEAR 37N 154W THROUGH A POINT ABOUT 715 MILES NORTH NORTHWEST
OF KAUAI. A NEARLY NORTH TO SOUTH ORIENTED SURFACE TROUGH LOCATED
ABOUT 215 MILES EAST OF HILO IS PROPAGATING SLOWLY WESTWARD. THE
PRESSURE GRADIENT OVER THE HAWAIIAN ISLANDS...WHICH ARE NEARLY
SANDWICHED BETWEEN THESE SURFACE FEATURES...IS MAINTAINING FRESH
TRADES ACROSS THE ISLANDS THIS MORNING. DUE TO THE PRESENCE OF THE
TROUGH APPROACHING FROM THE EAST...THE WIND DIRECTION IS ALSO
GRADUALLY BACKING TO A MORE NORTHEASTERLY AZIMUTH.

ALOFT...WATER VAPOR IMAGERY AND SATELLITE DERIVED UPPER LEVEL WINDS
INDICATE MID- AND UPPER-LEVEL TROUGHING OVER THE ISLANDS...WHICH IS
PRODUCING ATMOSPHERIC INSTABILITY ACROSS THE STATE. SATELLITE
IMAGERY AND RADAR REFLECTIVITY DATA INDICATE CUMULUS CLOUDS WITH
EMBEDDED SCATTERED CONTINUE TO BE CARRIED OVER THE ISLANDS IN THE
LOW-LEVEL NORTHEASTERLY TRADE WIND FLOW. OVER THE EASTERNMOST
ISLANDS...THE WESTERN EDGE OF A 300 MILE WIDE AREA OF OVERCAST
LAYERED CLOUDS WITH EMBEDDED SHOWERS AND ISOLATED THUNDERSTORMS IS
LOCATED ABOUT JUST EAST OF MAUI AND THE BIG ISLAND. THIS WESTERN
EDGE HAS BEEN SPREADING WESTWARD AT ABOUT 20 MPH. THIS FEATURE WILL
CONTINUE TO MOVE OVER THE ISLANDS...AND BRING CLOUDY SKIES AND
FREQUENT SHOWERS THROUGH THE DAY TODAY. SOME OF THESE SHOWERS MAY BE
LOCALLY HEAVY.

AS THE SURFACE TROUGH APPROACHES THE STATE LATER TONIGHT...THE
NORTHEASTERLY SURFACE WINDS OVER THE ISLANDS WILL CONTINUE TO
STRENGTHEN. THEN AS THE TROUGH MOVES OVER THE ISLANDS ON SUNDAY...
THE WINDS WILL WEAKEN AND VEER TO AN EAST SOUTHEAST DIRECTION.
SURFACE HIGH PRESSURE NORTH OF THE ISLANDS WILL MAINTAIN GENTLE TO
MODERATE TRADES DURING THE FIRST HALF OF THE NEW WORK WEEK.

BASED ON THE LATEST OUTPUT FROM THE FORECAST MODELS...THE MID- AND
UPPER-LEVEL TROUGHING WILL REMAIN OVER THE ISLANDS THROUGH WEDNESDAY
MORNING. AT THE SAME TIME...ABUNDANT LOW-LEVEL MOISTURE WILL LIKELY
CONTINUE TO ADVECT OVER THE ISLANDS FROM THE EAST. THEREFORE...THE
POSSIBILITY OF LOCALLY HEAVY SHOWERS WILL CONTINUE THROUGH WEDNESDAY
MORNING. THUNDERSTORMS ARE ALSO MENTIONED IN THE FORECAST STARTING
TONIGHT AS THE ATMOSPHERE BECOMES INCREASINGLY UNSTABLE. WE WILL
HAVE TO DECIDE LATER TODAY WHETHER OR NOT THE THREAT OF FLASH
FLOODING IS GREAT ENOUGH TO REQUIRE A FLASH FLOOD WATCH FOR
PARTS...OR ALL...OF THE STATE STARTING TONIGHT OR SUNDAY.

A SURFACE RIDGE WILL BUILD NORTHWEST OF THE ISLANDS ON WEDNESDAY...
WHILE THE UPPER-LEVEL TROUGH WILL SHIFT SOUTHEASTWARD AWAY FROM THE
AREA. AS A RESULT...THE ATMOSPHERE WILL STABILIZE AND THE THREAT OF
HEAVY SHOWERS AND THUNDERSTORMS WILL GRADUALLY DIMINISH.

RECENT RUNS OF THE GFS AND ECMWF MODELS CONTINUE TO INDICATE A FRONT
MAY PUSH DOWN OVER THE ISLANDS FROM THE NORTHWEST FRIDAY. IF SO...A
STRONG HIGH WILL BUILD IN BEHIND THE FRONT AND BRING A RETURN OF
BREEZY TRADE WINDS NEXT WEEKEND.

&&

.AVIATION...
VFR CONDITIONS ARE EXPECTED...BUT LOCALLY HEAVY SHOWERS WILL PRODUCE
MVFR CIG AND VIS. AIRMET TANGO IS IN EFFECT FOR LOW LEVEL TURB S
THRU W OF MT ON KAUAI...OAHU...MOLOKAI...LANAI AND MAUI. INCREASING
CLOUDS AND SHOWERS REQUIRES AIRMET SIERRA FOR MT OBSC FOR WINDWARD
MAUI...MOLOKAI THE BIG ISLAND.

&&

.MARINE...
THE CURRENT NORTHWEST SWELL WILL CONTINUE TO GRADUALLY DIMINISH...
DROPPING BELOW THE HIGH SURF ADVISORY THRESHOLD LATER TODAY...SEE
THE LATEST HIGH SURF ADVISORY FOR DETAILS. THIS SWELL WILL CONTINUE
TO GRADUALLY FADE AWAY THROUGH MONDAY. A NORTHWEST SWELL ARRIVING ON
TUESDAY WILL PEAK WELL BELOW THE HIGH SURF ADVISORY CRITERIA ALONG
NORTH AND WEST FACING SHORES ON WEDNESDAY...WHILE ANOTHER NORTHWEST
SWELL ARRIVING WEDNESDAY NIGHT WILL PEAK BELOW THE ADVISORY
THRESHOLD ON THURSDAY. A LARGER NORTHWEST SWELL IS EXPECTED TO
ARRIVE ON FRIDAY...WHICH MAY PRODUCE SURF NEAR THE HIGH SURF
ADVISORY THRESHOLD.

FRESH TO STRONG EAST NORTHEAST TO NORTHEAST WINDS ALONG WITH LARGE
COMBINED SEAS DUE TO THE NORTHWEST SWELL AND LOCALLY GENERATED TRADE
WIND WAVES WILL PRODUCE SMALL CRAFT ADVISORY...SCA...CONDITIONS FOR
ALL HAWAIIAN WATERS THROUGH THIS AFTERNOON. THE SCA WILL LIKELY BE
DISCONTINUED FOR SOME AREAS...SUCH AS THE WATERS SOUTHEAST OF THE
BIG ISLAND...AS THE NORTHWEST SWELL CONTINUES TO DECLINE THIS
EVENING.

HAVE ALREADY INTRODUCED THUNDERSTORMS OVER PARTS OF THE WINDWARD
MAUI AND BIG ISLAND COASTAL WATERS FOR TODAY. THE DEEP CONVECTION
MAY SPREAD TO ADDITIONAL HAWAIIAN WATERS TONIGHT. IF SO...WE MAY
ISSUE A MARINE WEATHER STATEMENT LATER TODAY TO DESCRIBE POTENTIAL
HAZARDS TO MARINERS...SUCH AS REDUCED VISIBILITIES IN LOCALLY HEAVY
DOWNPOURS...STRONG GUSTY WINDS...LOCALLY ROUGH SEAS AND FREQUENT
LIGHTENING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY THROUGH 600 PM HST THIS EVENING FOR NORTH AND
WEST FACING SHORES OF NIIHAU...KAUAI AND OAHU...NORTH FACING SHORES
OF MOLOKAI AND LANAI...AND WEST FACING SHORES OF THE BIG ISLAND.

SMALL CRAFT ADVISORY THROUGH 600 PM HST THIS EVENING FOR THE WATERS
SOUTH OF THE BIG ISLAND.

SMALL CRAFT ADVISORY THROUGH 600 AM HST SUNDAY MORNING FOR ALL
HAWAIIAN WATERS...EXCEPT THE WATERS SOUTH OF THE BIG ISLAND.

WIND ADVISORY THROUGH MIDNIGHT HST FOR MOUNT HALEAKALA SUMMIT.

&&

DISCUSSION/MARINE...HOUSTON
AVIATION...CRAIG