Heavy Rain in Hawaii, December 19 to 22, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
422 PM HST SAT DEC 19 2015

.SYNOPSIS...
HIGH PRESSURE NORTH-NORTHEAST OF THE STATE WILL MAINTAIN STRONG
TRADE WINDS THROUGH THE WEEKEND. THIS HIGH WILL GRADUALLY SHIFT
EAST AND WEAKEN THROUGH THE NEW WEEK AS A FRONT APPROACHES FROM
THE NORTHWEST. THIS FRONT WILL STALL AND DISSIPATE WELL NORTH OF
THE AREA...CAUSING TRADES DIMINISH A BIT BY THE SECOND HALF OF
THE WEEK. CLOUDS AND SHOWERS WILL BE GREATEST ACROSS WINDWARD AND
MAUKA AREAS...WITH THE STRONG WINDS SPREADING SOME OF THE SHOWERS
TO LEEWARD SECTIONS OF THE SMALLER ISLANDS AS WELL.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN EAST TO WEST ORIENTED UPPER LEVEL
TROUGH PUSHING SOUTHWARD ACROSS THE STATE. A SHORTWAVE IS LOCATED
ON THE BACK SIDE OF THE TROUGH...ABOUT 300 MILES NORTHEAST OF
HONOLULU AND MOVING TO THE WEST-SOUTHWEST AT 30 MPH. AT THE
SURFACE...HIGH PRESSURE IS LOCATED NORTH-NORTHEAST OF THE STATE
WITH A TIGHT PRESSURE GRADIENT ACROSS THE ISLANDS. A FRONT EXTENDS
EAST-NORTHEAST FROM THE STATE TOWARD CALIFORNIA. WHILE THE FRONT
ITSELF HAS WEAKENED THIS FAR WEST...IT LEFT BEHIND A CLOUD BAND
THAT IS BEING PUSHED ACROSS THE ISLANDS BY THE TRADES. 00Z
SOUNDINGS FROM LIHUE AND HILO SHOW WEAK INVERSION HEIGHTS OF
7KFT AND 9KFT AND PRECIPITABLE WATERS OF 1.13 AND 1.26 INCHES
RESPECTIVELY. THE GPSMET SENSOR ON KAUAI HAS BEEN DRIER THAN THE
PAST TWO RAOBS...AND SHOWS PW VALUES JUST UNDER 1 INCH. EARLY
AFTERNOON BLENDED TOTAL PRECIPITABLE WATER IMAGERY SHOWS THE WEDGE
OF DRIER AIR ACROSS THE NORTHERN ISLANDS...WITH A 150 MILE WIDE
BAND OF HIGHER VALUES UP TO 1.6 INCHES JUST NORTH AND NORTHEAST
OF KAUAI.

THE COMBINATION OF MOISTURE POOLED ALONG THE OLD FRONT AND UPPER
LEVEL SUPPORT FROM THE SHORTWAVE PASSING NORTH OF THE STATE HAVE
COMBINED TO PRODUCE ISOLATED THUNDERSTORMS OVER THE WINDWARD
COASTAL AND OFFSHORE WATERS NORTH OF KAUAI AND OAHU. THERE HAS
BEEN AN OVERALL WEAKENING TREND IN THE DEEPER CONVECTION OVER THE
PAST FEW HOURS...WHICH SUPPORTS THE DOWNWARD TREND SEEN IN THE
THUNDERSTORM GUIDANCE FROM THE HIGH RESOLUTION WRF-ARW DURING THE
AFTERNOON/EVENING. HOWEVER...SINCE THE SHORTWAVE IS STILL UPSTREAM
AND CONVECTION CONTINUES TO PERCOLATE NEARBY...FEEL IT IS PRUDENT
TO KEEP A SLIGHT CHANCE OF THUNDER WITH THIS MOISTURE BAND THROUGH
THE EVENING. THIS WILL AFFECT MAINLY KAUAI/OAHU AND WINDWARD
WATERS. BEHIND THE SHORTWAVE CONDITIONS WILL START TO
STABILIZE...AND MODELS SHOW THE MOISTURE BAND SHEARING OUT FURTHER
OVERNIGHT...AT LEAST ACROSS THE SMALLER ISLANDS.

THE DRYING TREND IN PRECIPITABLE WATER SEEN AT HILO IS IN LINE
WITH THE DIMINISHING TREND WE HAVE SEEN IN SHOWER COVERAGE ACROSS
MAUI/BIG ISLAND OVER THE COURSE OF THE AFTERNOON. THIS WILL BE A
BRIEF LULL IN ACTIVITY...AS BLENDED TPW IMAGERY SHOWS ONLY ABOUT
A 100 MILE WIDE AREA OF LOWER MOISTURE VALUES AHEAD OF THE OLD
FRONTAL BAND MENTIONED ABOVE. 18Z NAM AND GFS SHOW THE LOW-LEVEL
MOISTURE ASSOCIATED WITH THIS BAND REACHING MAUI EARLY THIS
EVENING AND THE BIG ISLAND BY THE OVERNIGHT PERIOD. RECENT MODEL
RUNS SHOW THIS MOISTURE GETTING HUNG UP NEAR MAUI/BIG ISLAND...AND
CONTINUING TO ENHANCE TRADE WIND SHOWERS ACROSS THE EASTERN HALF
OF THE STATE FOR THE NEXT FEW DAYS.

THE HIGH NORTH-NORTH OF THE STATE WILL MAINTAIN A STRONG PRESSURE
GRADIENT ACROSS THE ISLANDS. FOR TODAY OBSERVATION SITES ON
KAHOOLAWE WERE THE ONLY ONES THAT WERE CONSISTENTLY REPORTING
BOTH SUSTAINED WINDS AND GUSTS TO ADVISORY LEVELS...THOUGH THERE
WERE A NUMBER OF OTHER LOCATIONS ACROSS THE STATE THAT WERE JUST
BELOW ADVISORY CRITERIA GUSTING INTO THE 40S. HIGH RESOLUTION
GUIDANCE SHOWS SIMILAR WINDS FOR TOMORROW...WITH ONE DIFFERENCE
BEING THAT THERE WILL BE SUBSIDENCE AND A DEVELOPING INVERSION AS
THE UPPER LEVEL TROUGH PASSES SOUTH OF THE STATE AND A BRIEF
PERIOD OF RIDGING BUILDS IN BEHIND IT. WITH A STRENGTHENING
INVERSION TO HELP ACCELERATE WINDS OVER THE ISLAND TERRAIN...WE
SHOULD SEE MORE ADVISORY-LEVEL REPORTS ON SUNDAY. THE CURRENT
ADVISORY RUNS THROUGH SUNDAY FOR ALL LOCATIONS EXCEPT THE BIG
ISLAND SUMMITS. THE SUMMITS WILL ALSO BE PRETTY WINDY...THOUGH IT
DOES NOT APPEAR AS THOUGH THEY WILL REACH THE HIGHER WIND
ADVISORY CRITERIA FOR THE UPPER ELEVATIONS.

THE FRONTAL MOISTURE BAND MENTIONED ABOVE IS EXPECTED TO STALL
NEAR MAUI/BIG ISLAND ON SUNDAY...ENHANCING WINDWARD SHOWERS THERE
THROUGH THE FIRST PART OF THE NEW WEEK. ELSEWHERE WE SHOULD SEE
MORE OF A TYPICAL TRADE WIND WEATHER PATTERN. EVEN WITH MOST OF
THE RAINFALL OCCURRING ACROSS WINDWARD SECTIONS...THE STRONG WINDS
WILL CONTINUE TO SPREAD SHOWERS LEEWARD AS WELL. BY NEXT WEEKEND
BOTH THE 12Z GFS/ECMWF SHOW A NEW HIGH BUILDING NORTH OF THE
STATE...WHICH COULD BRING ANOTHER PERIOD OF STRONGER TRADE WINDS.

&&

.AVIATION...
BREEZY TRADE WINDS WILL CONTINUE INTO MONDAY DUE TO A STRONG SURFACE
HIGH PASSING TO THE NORTH. AIRMET TANGO IS IN EFFECT FOR STRONG
SUSTAINED WINDS AND MODERATE LEEWARD TURBULENCE...WHICH WILL LIKELY
CONTINUE THROUGH THE SHORT TERM.

SHOWERS USHERED IN BY THE TRADES ARE PROVIDING SOME MTN OBSC AND
REDUCED VISIBILITY AT TIMES FOR THE WINDWARD SIDES OF THE ISLANDS.
AIRMET SIERRA FOR TEMPO MTN OBSC IS IN EFFECT FOR WINDWARD SIDES OF
KAUAI AND OAHU FOR THIS EVENING...AND MAY SPREAD SOUTHWARD TO THE
WINDWARD SECTIONS OF OTHER ISLANDS LATER TONIGHT. OTHERWISE...EXPECT
BRIEF / ISOLATED MTN OBSC.

&&

.MARINE...
HIGH PRESSURE NORTH-NORTHEAST OF THE STATE WILL MAINTAIN AT LEAST
SMALL CRAFT ADVISORY WINDS ACROSS ALL THE COASTAL WATERS...WITH
GALES THROUGH THE TYPICALLY WINDIER AREAS NEAR MAUI AND THE BIG
ISLAND. IN ADDITION...A LARGE TRADE SWELL ASSOCIATED WITH THIS
STRONG HIGH WILL CONTINUE TO PRODUCE ADVISORY LEVEL SURF ALONG
EAST FACING SHORES. CONDITIONS WILL IMPROVE BY THE SECOND HALF OF
NEXT WEEK...BUT LOCALIZED SMALL CRAFT CONDITIONS AND EAST SIDE
SURF NEAR ADVISORY LEVELS ARE STILL EXPECTED.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY UNTIL 6 PM HST SUNDAY FOR NIIHAU-KAUAI-OAHU-
MOLOKAI-LANAI-KAHOOLAWE-MAUI-KONA-SOUTH BIG ISLAND-BIG ISLAND
NORTH AND EAST-KOHALA-BIG ISLAND INTERIOR.

HIGH SURF ADVISORY UNTIL 6 PM HST MONDAY FOR NIIHAU-KAUAI
WINDWARD-OAHU KOOLAU-OLOMANA-MOLOKAI WINDWARD-MAUI WINDWARD WEST-
WINDWARD HALEAKALA-SOUTH BIG ISLAND-BIG ISLAND NORTH AND EAST.

GALE WARNING UNTIL 6 PM HST MONDAY FOR ALENUIHAHA CHANNEL.

GALE WARNING UNTIL 6 PM HST SUNDAY FOR PAILOLO CHANNEL-BIG
ISLAND LEEWARD WATERS-BIG ISLAND SOUTHEAST WATERS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST MONDAY FOR KAUAI NORTHWEST
WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-KAUAI CHANNEL-
OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI CHANNEL-MAUI COUNTY
WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS-MAALAEA BAY- BIG ISLAND
WINDWARD WATERS.

&&

DISCUSSION/MARINE...BRAVENDER
AVIATION...FOSTER