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Hawaiian Islands Synoptic Discussion and Guidance

061
FXHW60 PHFO 270704
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
904 PM HST Sun Feb 26 2017

.SYNOPSIS...
East to east-southeast winds will strengthen slightly on Monday
before weakening again on Tuesday. High clouds, currently
blanketing Big Island skies, will thicken over the remainder of
the island chain on Monday, and remain in place through Wednesday.
A surface low developing in the vicinity of the islands will
result in wet and unsettled weather from Tuesday through
Wednesday, with the potential for locally heavy downpours and a
chance of thunderstorms. The surface low will eventually move
north of the area on Thursday, and island weather will become more
settled, with light winds expected from Thursday into next
weekend.

&&

.DISCUSSION...
The rounded western end of a weak northeast to southwest oriented
surface ridge appears to be about 350 miles north of Lihue early
this evening. A weak front is evident west of this feature, or
about 360 miles northwest of Lihue. The low-level flow across the
island chain is light to moderate from the east to east-southeast,
with the strongest surface winds (trades) near Maui and the Big
Island.

Loops of water vapor imagery indicate the axis of a broad
middle and upper tropospheric trough is west of the area. There
appears to be a short-wave trough moving through this larger
trough west of Kauai. This feature, along with the larger trough,
is producing some atmospheric instability across the region early
this evening. As a result, a large swath of broken to overcast
layered clouds continues to blanket Big Island skies. Showers are
embedded within these layered clouds over the windward Big Island,
as well as southwest and southeast of that island. We recently
updated the forecast to add the possibility of isolated showers
and a slight chance of thunderstorms for the leeward and interior
sections of the Big Island. Elsewhere, localized heavy showers are
currently falling from convective clouds southwest of Kauai and
Niihau.

Other than adding the possibility of showers/thunder over parts
of the typically dry sections of the Big Island, little
significant change is expected in the current weather pattern
tonight. The layered clouds will likely persist over the eastern
end of the state, while a relatively dry trade wind flow persists
elsewhere. Showers are forecast to increase over windward portions
of the Big Island and Maui later tonight, and forecast POPs are
higher there. Local land breezes developing over leeward and
interior sections over most of the smaller islands will likely
limit showers there, except the trade winds may deliver some brief
windward showers.

Trade winds, while remaining from an east to east-southeast
direction, are expected to gradually increase starting early
Monday as the front northwest of Kauai moves to the northwest away
from the area, and the ridge to our north builds. Locally breezy
low-level east to east-southeast flow is expected to persist from
Monday into Tuesday.

The forecast guidance continues to depict the upper level trough
northwest of Kauai will deepen through Tuesday before it
eventually propagates over the islands from late Tuesday through
Wednesday. Increased atmospheric instability and enhanced
moisture will also spread over the islands. This will likely
result in thickening high clouds over most of the state starting
on Monday. With a deep layer of mid- and upper-level moisture over
the state starting as early as Monday, this may combine with cold
temperatures aloft to support wintry weather over the summits of
the Big Island. Therefore, a Winter Storm Watch has been issued
for Monday night through Tuesday night for the Big Island summits.

As the sharp upper level trough aloft passes over the islands, a
weak surface low may develop somewhere in the vicinity of the
islands on Wednesday. This type of cyclogenesis, if it occurs near
the islands, will likely produce a period of unsettled weather.
However, since there is still a great deal of uncertainty about
where and when such a low might develop, the specifics of the
weather that will occur remain uncertain. Based on the latest
forecast, the best chance for wet weather appears to be from
Tuesday through Wednesday, with the potential for locally heavy
downpours and thunderstorms. The current confidence and timing do
not warrant issuing a Flash Flood Watch this evening. We will
continue to monitor the guidance to determine if we might need to
issue such a watch on Monday.

Lighter winds are expected to return starting Wednesday. At the
same time, a trend toward drier weather is expected starting
Wednesday night/Thursday as the trough aloft and its associated
surface low move north away from the area, and a surface ridge
develops over the islands. This period of light winds and mostly
dry weather is expected to persist through next weekend. Note that
the flow around the ridge may advect volcanic haze, or vog, from
sources on the Big Island to the smaller islands. This will
likely result in widespread hazy conditions across most of the
state from Friday into next weekend.

&&

.AVIATION...
An upper level trough moving across the Hawaiian Islands
interacts with a jet stream over Maui and the Big Island to
producing extensive middle and upper level cloudiness along with
some enhanced showers and isolated thunderstorms. Expect shower
activity to increase over the next 24 hours as the disturbance
slowly drifts across the state.

AIRMET Tango for moderate turbulence between FL200 and FL350 over
Maui and the Big Island continues this evening with turbulence
diminishing around 10Z as the jet maximum moves east of the
island chain.

AIRMET Zulu continues this evening for light rime icing
associated with the jet stream clouds over just the Big Island.

No AIRMETs are in effect for mountain obscurations at this time,
but the potential threat for this will be across the Big Island
summits as the mid and high clouds move into interior sections
later this evening.

&&

.MARINE...
The current west-northwest (300-320 deg) swell that peaked early
Saturday will continue to ease into the early portion of the week.
Surf along exposed north and west facing shores will remain below
high surf advisory levels through this time, including the west side
of Big Island from Kona to South Point. A small reinforcement out of
the west-northwest (300 deg) associated with a recent gale over the
far northwest Pacific is forecast to fill in Tuesday through mid
week.

A moderate northeast swell (030 deg) associated with a recent gale
that was centered around 1000 nautical miles northeast of the
islands will fill in across the local area by daybreak Monday and
continue through Tuesday night before easing. This swell will peak
through the day Monday and will generate near advisory-level surf
along east facing shores.

Moderate to strong breezes out of the east-southeast direction will
continue through Monday night, which has resulted in a Small
Craft Advisory for the Pailolo and Alenuihaha Channels and the
waters south of the Big Island. Some uncertainty remains in the
forecast Tuesday through mid week, with regard to the local winds,
as the models continue to depict a developing area of low pressure
or trough over/near the state lifting northward. Plenty of
moisture and instability associated with this feature and an upper
trough will keep the shower/thunderstorm chances in the forecast
through the first half of the week.

For the extended, the latest deterministic and ensemble guidance all
depict a gale to storm-force low developing a few hundred miles west-
northwest of Midway Wednesday through Thursday, then tracking toward
the north-northeast across the Date Line Thursday. Despite some
model differences shown with this evolving feature, a large west-
northwest swell associated with it will become a possibility across
the local waters over the upcoming weekend that may be large
enough to generate advisory-level surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Monday evening through late Tuesday
night for Big Island Summits.

Small Craft Advisory from 4 AM Monday to 6 AM HST Tuesday for
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...Gibbs

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