Area Forecast Discussion
Issued by NWS Honolulu, HI

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139
FXHW60 PHFO 210634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Mon Feb 20 2017

.SYNOPSIS...
Breezy trade winds can be expected through the workweek as a
series of high pressure areas build north of the state. Clouds
and showers will favor windward and mauka areas, with showers
spilling over into leeward locations from time to time. Winds
will diminish over the weekend as an area of low pressure
develops northwest of the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1029 mb high is centered around 775
miles north-northeast of Honolulu, with breezy to locally windy
trades in place across the island chain. Infrared satellite
imagery shows mostly cloudy conditions across windward areas, with
partly cloudy skies in most leeward locales. Meanwhile radar
imagery and rain gauge observations indicate only a few very
light showers or some mist moving into windward areas, with rain
free conditions over leeward locales. Main short term concern
revolves around the breezy to windy conditions through mid week.

Tonight through Wednesday,
A series of high pressure areas will build north of the island
chain through the period, maintaining breezy to locally windy
trades through the period. A dry airmass with precipitable water
(PW) values generally at or below 1 inch will keep shower activity
light and limited primarily to windward and mauka areas through
tonight. A bit of an increase in windward and mauka showers is
then expected Tuesday through Wednesday as an upper level trough
moves overhead. This will elevate inversion heights, allowing
some light showers to push into leeward areas from time to time.
Additionally, due to very cold temperatures aloft and some mid-
level moisture, thunderstorms with snow, sleet, and freezing rain
may develop over the Big Island summits each afternoon.

Winds have dropped below advisory levels this evening and they are
expected to remain below advisory thresholds through the night.
Therefore, the Wind Advisory has been cancelled for Lanai,
Kahoolawe and for portions of the Big Island. Winds are expected
to increase again Tuesday afternoon through Wednesday as a front
well north of the state stalls out, and a re-enforcing area of
high pressure builds north of the islands. A Wind Advisory may
once again be needed for some of the typically windier areas
during this time frame.

Wednesday night through next Monday,
High pressure north of the state will keep moderate to breezy
trade winds in place through Friday, with the trades diminishing
over the weekend as a stacked area of low pressure (surface and
aloft) develops to the northwest of the island chain. A dry
airmass will remain in place across the state for the tail end of
the work week and through the weekend, with PW values remaining
at or below 1 inch. As a result, showers should remain limited
primarily to windward and mauka areas through the period. There
may be an increase in showers by early next week as some tropical
moisture begins to work its way northward from the Intertropical
Convergence Zone (ITCZ). Confidence remains low regarding the
exact details at this time given continued disagreement amongst
the reliable model guidance.

&&

.AVIATION...
VFR conditions will prevail across the main Hawaiian Islands
overnight as a rather stable airmass remains over the area.
Any showers that do materialize will be light in intensity with
no restriction to vis and brief MVFR ceilings. These showers will
favor the windward and mountain areas of the larger islands. The
base of the low level inversion ranged from 57 hundred feet at LIH
to 75 hundred feet at ITO.

For the second day in a row, a thunderstorm (ts) formed over the
Big Island summit. The ITO profile suggest an unstable layer there
to 27k feet. Models shows a bit more instability with this layer for
Tuesday so scattered ts will be part of the summit`s forecast for
Tuesday afternoon.

At the lower levels, a stagnant strong high pressure system
sitting north of the islands will maintain breezy and gusty trade
winds well through Tuesday thereby maintaining AIRMET Tango for
low level turb below 8k feet.

&&

.MARINE...
Strong high pressure passing by around 675 nm north-northeast of
Honolulu will keep strong trade winds in place through Wednesday.
The trades have diminished below Gale force across the typically
windy waters around Maui and the Big Island, so the Gale Warning
has been cancelled. As a result, a Small Craft Advisory (SCA) is
in effect for all waters through Wednesday. Moderate to Breezy
trade winds will continue through the remainder of the work week,
with the SCA likely needing to be extended in time for portions of
the marine area. The trades are then expected to diminish over the
weekend.

Large combined seas comprised of a mix of north-northwest swell
and wind waves are producing high surf across north facing shores
and hazardous seas contributing to the SCA. Combined seas at the
NOAA buoys to the northwest of the islands are down to 7 to 8 ft
at 11 to 12 seconds, while larger seas mainly passing just east of
the islands are down to 12 to 13 ft at 13 seconds at buoy 51000.
Based on the NOAA buoy data, expect the surf to continue
declining tonight, falling below advisory levels by the morning
forecast package between 3 and 4 AM, with a steady decline then
expected through the day on Tuesday.

A pair of long period swells from distant sources will produce
somewhat elevated surf along north and west facing shores later
this week, while rough, short period trade wind seas maintain surf
near to just below advisory levels along east facing shores
through Thursday. The first northwest swell is expected to arrive
on Wednesday and peak in the double overhead range Wednesday night
and Thursday. The second northwest swell will likely be larger and
may lead to advisory level surf Friday night and Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala-Big Island
North and East.

Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian
waters-

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Lau



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