Drought in Hawaii, October 18 to 31, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST TUE OCT 18 2011

.SYNOPSIS...
LIGHT WINDS WILL BECOME SOUTHEASTERLY BY WEDNESDAY. RAINFALL WILL
BE LIGHT. TRADE WINDS WITH MODEST WINDWARD SHOWERS WILL RETURN
GRADUALLY FROM WEDNESDAY INTO EARLY SATURDAY...BEFORE DIMINISHING
AGAIN LATE IN THE WEEKEND.

&&

.DISCUSSION...
WINDS REMAINED LIGHT ACROSS THE MAIN HAWAIIAN ISLANDS. RAINFALL
ACROSS THE STATE GENERALLY WAS FOCUSED WINDWARD...DESPITE THE WEAK
OROGRAPHIC FORCING. THE INTENSE BURST OF SHOWERS OVER THE KOOLAU
RANGE ON OAHU EARLIER IN THE EVENING DID NOT REPEAT...BUT RADAR DATA
CONTINUED TO SHOW MORE SHOWER ACTIVITY OVER WINDWARD OAHU THAN MIGHT
BE EXPECTED FROM THE SYNOPTIC SITUATION...SO THE FORECAST THERE
STILL CALLS FOR RAINY CONDITIONS THROUGH THE REST OF THIS MORNING.

GUIDANCE GENERALLY AGREED THAT FROM TODAY INTO WEDNESDAY...A
MID-LEVEL TROUGH DEVELOPING TO THE NORTHWEST OF THE STATE WILL ERODE
THE WEAK RIDGE ALOFT AS A FRONT APPROACHES CLOSE ENOUGH TO THE
ISLANDS TO TURN THE PREVAILING WINDS SOUTHEASTERLY. WHILE IN SUCH
SITUATIONS CONSIDERABLE WATER VAPOR CAN ADVECT IN FROM THE DEEP
TROPICS...POTENTIALLY LEADING TO INTENSE AIR-MASS CONVECTION...THIS
PARTICULAR CASE LOOKS DRIER THAN USUAL...WITH BOTH SATELLITE
MEASUREMENTS AND MODEL PREDICTIONS OF PRECIPITABLE WATER BELOW
CLIMATOLOGY. ALSO...WHILE PROLONGED SOUTHEAST WINDS CAN BRING
VOLCANIC EMISSIONS FROM KILAUEA TO THE SMALLER ISLANDS...THE
SOUTHEAST WINDS ARE NOT EXPECTED TO BE STRONG NOR PERSISTENT ENOUGH
IN THIS SITUATION.

FROM WEDNESDAY INTO FRIDAY...THE NEW FRONT FROM THE NORTHWEST WILL
STAGNATE WITHOUT REACHING KAUAI...AS THE SUBTROPICAL RIDGE REBUILDS
AND SHIFTS NORTH AGAIN. THUS...THE FORECAST TRENDS TOWARD MORE
TYPICAL TRADE-WIND WEATHER. WIND SPEEDS WILL PEAK AT MODERATE LEVELS
BY FRIDAY EVENING. SHOWER ACTIVITY WILL INCREASE MODESTLY WINDWARD
AS OROGRAPHIC FORCING RESUMES.

ALOFT...THE PREVAILING RIDGE WILL RESIST THE TROUGH ENCROACHING FROM
THE NORTHWEST...THOUGH WITH SOME DIFFICULTY. MODELS STILL FAIL TO
AGREE WITH RESPECT TO HOW CLOSELY THEY PERMIT THE UPPER TROUGH TO
APPROACH THE ISLANDS. THE 06 UTC GFS RUN NOW EDGED CLOSER TO ECMWF
PREDICTIONS THAT THE COLD POOL WILL STAY TOO FAR FROM KAUAI TO
AFFECT THE WEATHER MUCH THERE. GIVEN THE PAST HISTORY OF THE
MODELS...WHICH FAVORED A CLOSER APPROACH AT VARIOUS TIMES...A SLIGHT
CHANCE OF HEAVY SHOWERS HAS BEEN ADDED TO THE WEDNESDAY FORECAST FOR
KAUAI COUNTY. LOW PRECIPITABLE WATER PROBABLY WILL LIMIT
CONVECTION...THOUGH MODEL CONSENSUS HAS TRENDED TOWARD MOISTER
CONDITIONS OVER THE NORTHWEST END OF THE STATE FROM WEDNESDAY NIGHT
ON. IT DOES SEEM EXCESSIVE TO INTRODUCE THUNDERSTORMS WITH SO MUCH
UNCERTAINTY STILL PRESENT...BUT THE SLIGHT CHANCE OF HEAVY RAIN
SHOULD BE SEEN AS A REMINDER OF A LOW-PROBABILITY...BUT POTENTIALLY
HIGH-IMPACT...EVENT. THIS PART OF THE FORECAST WILL REQUIRE
CONTINUED ATTENTION.

BY LATE IN THE WEEKEND...GUIDANCE PREDICTS ANOTHER FRONT APPROACHING
FROM THE NORTH TO SMASH THROUGH THE SUBTROPICAL RIDGE AND TURN
PREVAILING FLOW INTO WEAK SOUTHEASTERLIES AGAIN. THUS...TRUE TRADES
MAY LAST ONLY FROM THURSDAY INTO SATURDAY.

&&

.MARINE...CONDITIONS ARE EXPECTED TO REMAIN WELL BELOW SMALL CRAFT
ADVISORY LEVELS THROUGH THE WEEK. HOWEVER...WINDS AND SEAS WILL
CONTINUE TO BE ENHANCED IN THE USUAL WINDY AREAS.

NORTHEAST SWELL WILL BRING HIGH SURF TO EAST-FACING SHORES THROUGH
THE NEXT 12 HOURS...BUT WILL DIMINISH THIS EVENING.

LONG-PERIOD NORTHWEST SWELL WILL ARRIVE SUNDAY...AFFECTING
NORTH-FACING SHORES.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 PM HST THIS EVENING FOR KAUAI
WINDWARD-OAHU KOOLAU-OLOMANA-MOLOKAI WINDWARD-MAUI WINDWARD WEST-
WINDWARD HALEAKALA-BIG ISLAND NORTH AND EAST.

&&

RYSHKO