Small Craft Advisory 15, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
1000 AM HST WED MAR 2 2011

.SYNOPSIS...
A HIGH PRESSURE RIDGE TO THE NORTH WILL MAINTAIN GENTLE TO MODERATE
TRADE WINDS ACROSS THE ISLANDS TODAY...WITH LIGHT SHOWERS AFFECTING
MAINLY WINDWARD AND MAUKA SECTIONS. LOW PRESSURE DEVELOPING WEST OF
THE STATE WILL BRING UNSETTLED WEATHER TO THE ISLANDS IN THE NEXT
FEW DAYS. A COLD FRONT ASSOCIATED WITH THE LOW IS EXPECTED TO REACH
KAUAI FRIDAY MORNING. THE FRONT WILL WEAKEN AS IT MOVES DOWN THE
ISLAND CHAIN THROUGH SATURDAY.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS A NEARLY ZONAL UPPER LEVEL JET NORTH OF
THE STATE...WITH A SHORTWAVE TROUGH NEAR 22N175E. AT THE SURFACE...
A WEAKENING FRONT IS LOCATED SOUTH AND EAST OF THE BIG ISLAND...WITH
A SURFACE RIDGE ABOUT 400 MILES NORTH OF THE STATE. A TROUGH IS
LOCATED ABOUT 800 MILES WEST OF KAUAI. 12Z RAOBS SHOW INVERSIONS
NEAR 5-6KFT AND PRECIPITABLE WATERS OF 1.0-1.1 INCHES. THE EARLY
MORNING AMSU/SSMI BLENDED TOTAL PRECIPITABLE WATER IMAGERY SHOWS
DRIER AIR NORTH OF THE STATE...BUT PW VALUES OF 1.2-1.4 INCHES JUST
TO THE SOUTH. HIGHER VALUES NEAR 2.0 INCHES ARE POOLED ALONG THE
SURFACE TROUGH TO THE WEST OF THE STATE.

GENTLE TO MODERATE TRADE WINDS WILL LINGER THROUGH TODAY BEFORE
STARTING TO VEER AHEAD OF THE DEVELOPING SYSTEM. WINDS WILL BE LIGHT
ENOUGH THAT THEY MAY ALLOW SEA BREEZES TO DEVELOP ACROSS SHELTERED
LEEWARD AREAS. ABOVE THE SURFACE...THE VAD WIND PROFILE FROM THE
MOLOKAI RADAR AND ACARS SOUNDINGS INTO HONOLULU SHOW WINDS ABOVE
6KFT /ABOVE THE INVERSION/ HAVE ALREADY VEERED TO THE SOUTHWEST.
CLOUDS ARE GREATEST IN COVERAGE ACROSS THE EASTERN HALF OF THE
STATE...BUT ARE PRIMARILY STABLE. RADAR RETURNS REMAIN LIGHT AND
RAIN GAGES SHOW ONLY A FEW LOCATIONS THAT RECEIVED RAINFALL OVER THE
PAST 12 HOURS. THE CURRENT FORECAST HAS RELATIVELY LOW POPS FOR
TODAY...BUT MUCH HIGHER CLOUD COVER GIVE THE MIDDLE AND HIGH LEVEL
CLOUDS PASSING OVER THE STATE.

THE SHORTWAVE TROUGH WELL WEST OF THE STATE WILL MOVE EAST AND
SHARPEN. THIS TROUGH WILL PASS OVER THE SURFACE TROUGH WEST OF THE
STATE...CAUSING LOW PRESSURE TO DEVELOP ALONG IT BEGINNING TONIGHT.
ACROSS THE ISLANDS...SURFACE WINDS WILL VEER TO THE SOUTHEAST AND
SOUTH BY MORNING. THE DEEPER LAYER SOUTHWESTERLY FLOW WILL BRING
MOISTURE NORTHWARD ACROSS THE ISLANDS. AN AREA OF CLOUDS CAN BE SEEN
SOUTH THROUGH WEST OF THE BIG ISLAND...IN THE AREA OF THE HIGHER
PRECIPITABLE WATER MENTIONED ABOVE. CONTINUED MOISTURE ADVECTION
WILL HELP TO MAINTAIN THE CLOUDS AND LEAD TO LIGHT SHOWERS TONIGHT.
MODEL POPS ARE PRETTY HIGH...ESPECIALLY ACROSS KAUAI AND OAHU CLOSER
TO THE STRONGEST ISENTROPIC LIFT. DO NOT EXPECT HEAVY RAIN WITH THIS
FIRST ROUND...BUT WIDESPREAD LIGHT RAIN IS POSSIBLE.

THE 12Z GFS AND ECMWF HAVE COME INTO MUCH BETTER AGREEMENT OVER THE
LAST 24 HOURS WITH RESPECT TO THE EVOLUTION OF THIS SYSTEM. BOTH
MODELS KEEP THE UPPER TROUGH OPEN AND PROGRESSIVE. THEY TAKE THE
SURFACE LOW NORTH OF KAUAI THURSDAY NIGHT...THEN TRACK IT FAR
NORTHEAST OF THE STATE THROUGH FRIDAY. THIS SEEMS MORE REALISTIC
THAN EARLIER SOLUTIONS...SUCH AS THE GFS...WHICH CUT OFF THE UPPER
TROUGH NORTH OF THE STATE AND KEPT THE LOW MUCH CLOSER AND SLOWER.
THE CURRENT TIMING WOULD HAVE THE FRONT APPROACHING KAUAI ON
FRIDAY...THEN STALLING AND WEAKENING BETWEEN OAHU AND MAUI COUNTY
FRIDAY NIGHT/SATURDAY. SHOWERS ARE EXPECTED TO DEVELOP AHEAD OF THE
FRONT...WITH BOTH MODELS HINTING AT A BREAK BETWEEN THE RAIN TONIGHT
AND THE HEAVIER SHOWERS WITH THE FRONT. PW VALUES ARE FORECAST TO
CLIMB NEAR 1.8 INCHES...WHICH IS WELL ABOVE NORMAL FOR THIS TIME OF
YEAR. WE DO NOT ANTICIPATE MUCH IN THE WAY OF THUNDER WITH THIS
SYSTEM. IT DEVELOPS AT A RATHER LOW LATITUDE...AND DOES NOT HAVE A
VERY COLD CORE OF AIR ALOFT. HOWEVER...BECAUSE OF THE MOISTURE AND
INSTABILITY THAT IS PRESENT...HAVE MAINTAINED A MENTION OF HEAVY
RAIN. STRONG SOUTH TO SOUTHWEST WINDS ARE ALSO EXPECTED
THURSDAY/FRIDAY...MAINLY ACROSS THE WESTERN HALF OF THE STATE. DO
NOT ANTICIPATE ANY WIND ADVISORIES AT THIS POINT...MAINLY DUE TO THE
WEAKER SOLUTION THAT APPEARS AS THOUGH IT WILL PAN OUT. HOWEVER...IF
THE LOW THAT DEVELOPS IS STRONGER...AREAS ALONG THE NORTH AND
NORTHEAST SECTIONS OF THE ISLANDS MAY NEED AN ADVISORY DUE TO
DOWNSLOPE ACCELERATIONS.

THE FRONT WILL WEAKEN INTO A TROUGH...WHICH WILL LINGER OVER THE
CENTRAL ISLANDS INTO SATURDAY. AS THE TROUGH WEAKENS...HIGH PRESSURE
BUILDING NORTH OF THE STATE WILL ASSERT ITS INFLUENCE AND BRING A
RETURN TO TRADE WINDS ON SUNDAY. LOW-LEVEL MOISTURE LEFT BEHIND BY
THE FRONT WILL BE PICKED UP BY THE TRADES AND WILL HELP TO ENHANCE
SHOWERS ACROSS WINDWARD AND MAUKA SECTIONS INTO EARLY NEXT WEEK.

&&

.MARINE...
WINDS WILL REMAIN BELOW SMALL CRAFT ADVISORY LEVELS THROUGH
TONIGHT...BUT WILL BEGIN TO STRENGTHEN AS THEY VEER AROUND TO THE
SOUTH ON THURSDAY. SOUTHERLY WINDS WILL INCREASE...AND REACH
SMALL CRAFT ADVISORY LEVELS OF 25 KNOTS OR GREATER FOR THE WESTERN
HALF OF THE COASTAL WATERS GENERALLY THURSDAY NIGHT-FRIDAY. COMBINED
WITH A BUILDING NORTHWEST SWELL...WIND WAVES WILL LEAD TO COMBINED
SEAS ABOVE THE 10 FEET ADVISORY THRESHOLD AS EARLY AS THURSDAY.

&&

.AVIATION...
LOW LEVEL TRADE WIND FLOW WILL PRODUCE MVFR CIGS OVER TYPICAL
WINDWARD SLOPES TODAY WITH ONLY ISOL SHRA. LOW LEVEL TRADE WINDS
WILL BE WEAKENING...LEADING TO LOCALIZED SEA BREEZES AND ISOL MVFR
CIGS OVER LEEWARD AREAS. BKN HIGH CLOUDS WILL CONTINUE TO STREAM
OVER THE STATE DUE TO UPPER LEVEL TROUGHING FAR W OF THE ISLANDS.

THE LOW LEVEL FLOW WILL VEER SOUTHEASTERLY OVERNIGHT...LIKELY
BRINGING INCREASED MOISTURE AND MVFR CIGS OVER KAUAI AND OAHU FROM
THE S LATE TONIGHT...WITH ISOL MVFR VIS IN SHRA.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

PUBLIC/MARINE...BRAVENDER
AVIATION...WROE