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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 271412

Area Forecast Discussion
National Weather Service Honolulu HI
412 AM HST Mon Feb 27 2017

East to east-southeast winds will strengthen slightly later
today before weakening late Tuesday and Wednesday. High clouds
are expected to spread over portions of the island chain later
today and tonight, and then remain in place through Wednesday. A
surface low developing in the vicinity of the islands will result
in wet and unsettled weather from late tonight and 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 by Thursday, and island weather will become more
settled, with light winds and hazy conditions expected from
Thursday through Friday.


The rounded western end of a northeast to southwest oriented
surface ridge appears to be about 415 miles north of Lihue early
this morning. A weakening frontal boundary is evident to the west
of this feature, or about 450 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 indicates a broad upper level
trough remains west of the area. The close proximity of this
feature is producing some atmospheric instability across the
region early this morning. As a result, patches of cirrus from
thunderstorms west and southwest of the islands are streaming over
parts of the state this morning. Low clouds and showers are also
over portions of the windward Big Island and Maui, as well as the
adjacent coastal waters, according to radar reflectivity data.

The low-level flow, while remaining from an east to east-southeast
direction, is expected to gradually increase later today as the
frontal boundary northwest of Kauai continues to move
northwestward away from the area, and the ridge to our north
builds. Therefore, locally breezy low-level east to east-southeast
flow is expected to persist from later today into Tuesday.

The forecast guidance continues to depict the upper level trough
northwest of Kauai deepening through Tuesday afternoon before it
eventually propagates eastward over the islands from Tuesday
night through Wednesday. Increased atmospheric instability and
abundant deep moisture will simultaneously spread over the
islands. This will likely result in thickening layered clouds
covering most of the state starting later today or tonight. Also,
with deep layer moisture over the Big Island later today and
tonight combined with cold temperatures aloft, wintry weather may
develop over the summits of the Big Island. Therefore, a Winter
Storm Watch is in effect from tonight through Tuesday night for
the Big Island summits.

As the sharp upper level trough aloft passes over the islands, a
weak surface low appears likely to develop somewhere in the
vicinity of the islands late Tuesday or Wednesday. This type of
cyclogenesis, if it occurs near the islands, will likely produce
a period of unsettled weather. However, since there is still some
uncertainty about where and when this low might develop, the
specifics of the weather that will occur remain somewhat
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 early this morning. We will continue to monitor the guidance
to determine if we might need to issue such a watch later today or

Lighter winds are expected starting late Tuesday or 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 in the vicinity of the islands. This period of light
winds and mostly dry weather is expected to persist through
Friday. Note that the flow around the ridge will likely advect
volcanic haze, or vog, from sources on the Big Island to the
smaller islands. This is expected to result in widespread hazy
conditions across most of the state later this week.

Finally, the long-range forecast guidance continues to show a new
front reaching the western islands next weekend. There are
differences in the timing of this front and its associated weather
in the models, so details about the conditions on Saturday and
Sunday remain uncertain at this time. It does appear that the
winds will remain relatively light, with an increase in moisture
in the form of low clouds and showers, especially over the western
end of the state.


A deep mid/upper level trough west of the state will move slowly
eastward today and tonight. It appears that the best deep-layered
moisture and forcing will remain south of the state today, but
will begin to spread northward across the state tonight. Therefore
expect scattered showers through today with a few locally heavier
downpours possible. Mainly VFR conditions will prevail, although
windward sections of Maui and the Big Island will have a good
chance for MVFR conditions early this morning.

Showers are expected to become more widespread starting this
evening, with isolated thunderstorms possible as well. There will
be increasing chances for more widespread MVFR and localized IFR
conditions tonight.

AIRMET SIERRA for mountain obscuration is currently posted for
windward slopes of East Maui. Mountain obscuration will remain
possible across windward Maui and the Big Island today, becoming
more likely across other parts of the state tonight.

The earlier mention of tempo light icing in AIRMET ZULU has been
dropped as layered deep cloudiness has shifted east of the Big
Island for now. Icing may become a concern once again later today
or this evening when thicker high clouds could spread back over
the state.


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 continue to fill in across the local area today and
remain in place through Tuesday night before easing. This swell
will peak through the day today and generate near advisory- level
surf along east facing shores.

Moderate to strong breezes out of the east-southeast direction will
continue through tonight, 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.


Winter Storm Watch from this evening through late Tuesday night
for Big Island Summits.

Small Craft Advisory until 6 AM HST Tuesday for Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman