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Hawaiian Islands Synoptic Discussion and Guidance

040
FXHW60 PHFO 211339
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
339 AM HST MON APR 21 2014

.SYNOPSIS...
STRONG HIGH PRESSURE TO THE NORTH OF THE STATE WILL CREATE LOCALLY
WINDY TRADES TODAY. THE STRONG TRADES WILL CONTINUE TO BLOW PASSING
WINDWARD AND MAUKA SHOWERS OVER INTO LEEWARD AREAS AT TIMES TODAY
INTO TONIGHT. THE TRADES WILL GRADUALLY EASE TUESDAY THROUGH
WEDNESDAY AS HIGH PRESSURE MOVES FURTHER AWAY FROM THE STATE.
DRIER AIR AND FEWER SHOWERS ARE EXPECTED FROM TUESDAY THROUGH THE
REST OF THE WORK WEEK.

&&

.DISCUSSION...
RADAR LOOPS SHOW AN ACTIVE SHOWER PATTERN CONTINUING ACROSS
WINDWARD PORTIONS OF THE STATE DURING THE NIGHT...WITH SOME
SHOWER ACTIVITY SPILLING OVER INTO LEEWARD SECTIONS OF OAHU...
KAUAI AND MOLOKAI. RAIN GAUGES REPORT LIGHT TO LOCALLY MODERATE
ACCUMULATIONS AS THE SHOWERS ARE MOVING QUICKLY WITHIN RATHER
BRISK TRADE WIND FLOW. THE 12Z SOUNDINGS SHOW WEAK INVERSIONS
BASED FROM AROUND 6500 FEET AT LIHUE UP TO 9000 FEET AT HILO. AS
HAS BEEN THE CASE FOR THE PAST COUPLE OF DAYS...ABUNDANT MID/HIGH
LEVEL CLOUDINESS CONTINUES TO STREAM NORTHWARD ACROSS THE
STATE...TO THE EAST OF A STATIONARY MID/UPPER LEVEL TROUGH
CENTERED SEVERAL HUNDRED MILES TO THE SOUTHWEST. AT THE
SURFACE...A 1032 MB HIGH IS CENTERED ABOUT 1050 MILES DUE NORTH OF
THE BIG ISLAND...AND CONTINUES TO MOVE EASTWARD.

THE MAIN FORECAST ISSUE REMAINS GUSTY AND LOCALLY ADVISORY LEVEL
WINDS ACROSS SOME DOWNSLOPE AND EXPOSED HEADLAND/RIDGE AREAS. IN
ADDITION TO KAMUELA WHICH HAS REPORTED SUSTAINED WINDS OF 30 MPH
OR GREATER OFF AND ON FOR THE PAST 12 HOURS OR SO...A FEW OTHER
MESONET SITES ON OAHU...LANAI AND KAHOOLAWE HAVE REPORTED PEAK
WIND GUSTS AT OR JUST BELOW 50 MPH AS WELL. MODEL GUIDANCE...
INCLUDING THE HIGH RESOLUTION UNIVERSITY OF HAWAII WRF MODEL...
DOES STILL INDICATE BOUNDARY LAYER WINDS PEAKING TODAY...WITH 925
MB WINDS REACHING 35 KNOTS OR GREATER IN SOME AREAS. MODEL
SOUNDINGS ALSO SHOW SOME LOWERING AND STRENGTHENING OF THE
INVERSION DURING THIS TIME...ALTHOUGH THIS SEEMS SOMEWHAT
QUESTIONABLE GIVEN THE WEAK INVERSIONS OBSERVED BY THE 12Z
SOUNDINGS. WILL LEAVE THE WIND ADVISORY IN PLACE FOR TODAY WITH
NO COVERAGE CHANGES...ALTHOUGH IT SEEMS LIKELY THAT THE STRONGEST
WIND GUSTS OF 50 MPH OR GREATER WILL REMAIN FAIRLY ISOLATED.
IT SHOULD BE REITERATED THAT THE STRONGEST WINDS WILL NOT
AFFECT ENTIRE FORECAST ZONES...BUT INSTEAD ONLY SPECIFIC FAVORED
AREAS IMMEDIATELY DOWNSLOPE OF THE SMALLER MOUNTAIN RANGES AND
OVER SOME RIDGES AND HEADLANDS.

THE TRADES SHOULD GRADUALLY RELAX TO MORE MODERATE LEVELS TUESDAY
THROUGH WEDNESDAY AS THE SURFACE HIGH MOVES FURTHER NORTHEAST AND
AWAY FROM THE STATE. ANOTHER WEAKER HIGH WILL BUILD TO THE NORTH
OF OUR AREA WEDNESDAY THROUGH THURSDAY...BUT WOULD NOT EXPECT ANY
SIGNIFICANT WIND PROBLEMS. A STATIC PATTERN WILL THEN DEVELOP
ACROSS THE CENTRAL PACIFIC FOR THE LATE WEEK INTO THE WEEKEND...
WITH AN EAST-WEST HIGH PRESSURE RIDGE NORTH OF THE STATE AND
MODERATE TRADES PREVAILING ACROSS HAWAII.

AS FOR RAIN CHANCES...THE AREA OF HIGHER MOISTURE CURRENTLY ACROSS
HAWAII IS FORECAST TO SHIFT WEST OF THE STATE BY LATE TONIGHT
OR TUESDAY MORNING...WITH A DRIER AIRMASS THEN PREVAILING THROUGH
THE REST OF THE WEEK. THEREFORE WOULD EXPECT ANOTHER 24 HOURS OR
SO OF A MORE ACTIVE WINDWARD/MAUKA SHOWER PATTERN WITH SOME
LEEWARD SPILLOVER.. FOLLOWED BY MORE LIMITED SHOWER COVERAGE
FOCUSING ON WINDWARD AND MAUKA AREAS THEREAFTER.

&&

.AVIATION...
AIRMET TANGO WILL CONTINUE TODAY FOR THE MODERATE TO STRONG TRADES
ACROSS THE REGION...PRODUCING MECHANICAL TURBULENCE TO THE LEE OF
THE MOUNTAINS. AN UPPER LEVEL AIRMET CONTINUES ACROSS THE ISLANDS
DUE TO MODERATE TO STRONG SPEED AND DIRECTIONAL WIND SHEAR. THE
UPPER LEVEL CONDITIONS ARE EXPECTED TO IMPROVE AS THE DAY
PROGRESSES.

CLOUDS AND SHOWERS BEING CARRIED IN ON THE TRADES WILL BRING MVFR
AND ISOLATED IFR CONDITIONS TO WINDWARD AND MOUNTAIN AREAS. AIRMET
SIERRA HAS BEEN POSTED FOR MOUNTAIN OBSCURATION IN THESE AREAS.
IMPROVED CONDITIONS ARE POSSIBLE LATER TODAY.

&&

.MARINE...
THE SMALL CRAFT ADVISORY HAS BEEN EXTENDED THROUGH 6 AM TUESDAY
MORNING FOR ALL HAWAIIAN COASTAL WATERS. THE OVERNIGHT ASCAT PASS
MISSED THE COASTAL WATERS...BUT AVAILABLE LAND OBS AND MODEL DATA
INCLUDING THE UH-WRF SUGGEST THAT WINDS ARE LIKELY AROUND 25 KNOTS
ACROSS AT LEAST PARTS OF MOST ZONES. WINDS ARE FORECAST TO PEAK
JUST BELOW 30 KNOTS IN THE ALENUIHAHA CHANNEL TODAY. WINDS SHOULD
SLOWLY SUBSIDE TUESDAY THROUGH WEDNESDAY.

THE LATEST WAVEWATCH DATA CONTINUES TO SHOW BUILDING SHORT PERIOD
EASTERLY SWELL FOR TODAY...WHICH MAKES SENSE AS THE FETCH LENGTH
INTO HAWAII INCREASES DUE TO THE MOVEMENT OF THE STRONG HIGH TO
OUR NORTH. A HIGH SURF ADVISORY FOR EAST FACING SHORES REMAINS IN
EFFECT THROUGH TUESDAY. NO PROBLEMS THROUGH THE WEEK FOR THE OTHER
SHORES...WITH SMALL NORTHWEST SWELL THROUGH TUESDAY. THE NEXT
MODERATE NORTHWEST SWELL WILL ARRIVE WEDNESDAY AND CONTINUE
THROUGH FRIDAY...BUT THE RESULTING SURF ON NORTH/WEST FACING
SHORES WILL REMAIN WELL BELOW ADVISORY LEVELS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY UNTIL 6 PM HST THIS EVENING FOR WAIANAE COAST-OAHU
KOOLAU-OLOMANA-WAIANAE MOUNTAINS-LANAI-KAHOOLAWE-MAUI WINDWARD
WEST-MAUI LEEWARD WEST-MAUI CENTRAL VALLEY-SOUTH BIG ISLAND-BIG
ISLAND NORTH AND EAST-KOHALA-BIG ISLAND INTERIOR.

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

SMALL CRAFT ADVISORY UNTIL 6 AM HST TUESDAY FOR ALL HAWAIIAN
WATERS.

&&

$$

JACOBSON





Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office