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Hawaiian Islands Synoptic Discussion and Guidance

767
FXHW60 PHFO 231419
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
419 AM HST Thu Mar 23 2017

.SYNOPSIS...
Expect mainly dry trade wind weather through early Friday, with
brief trade showers mainly along windward facing slopes,
especially over the eastern end of the island chain. Trade wind
speeds are forecast to strengthen again starting late Friday, with
a slight increase in trade showers expected mainly over windward
sections this weekend.

&&

.DISCUSSION...
The western end of a surface ridge appears to be about 260 miles
north of Lihue based on an ASCAT pass from late Wednesday evening.
A surface front is also located about 525 miles north of Lihue.
The close proximity of the front has weakened the surface ridge,
and this has caused the pressure gradient to become relatively
slack across the islands early this morning. As a result, trade
wind speeds have continued to diminish overnight, especially over
the western end of the state, where background surface winds are
from an east-southeast direction. This weaker flow also allowed
local land breeze circulations to develop over the leeward and
interior sections of many of the islands.

Aloft, water vapor imagery continues to show a middle tropospheric
ridge near the Big Island, which is maintaining relatively stable
atmospheric conditions across the islands. Elsewhere, the axis of
an upper tropospheric short-wave trough is evident about 415 miles
west of Lihue. This feature is moving rapidly toward the east-
northeast. The main influence this trough aloft appears to be
having on island weather are thin high clouds, mainly cirrus,
which are streaming rapidly toward the east-northeast across the
western islands this morning. These translucent clouds will
partially block sunshine over some areas in a few hours after
sunrise.

The cold front north of the region is forecast to continue
pushing eastward. This will keep the western end of the weak
surface ridge close to the islands, with east to east-southeast
surface winds. Note that the weaker background flow will likely
allow local afternoon sea breezes and nighttime land breezes to
develop over leeward and interior sections of the individual
islands, especially on Kauai and Oahu, through tonight.

The early morning sounding from Hilo shows moistening overnight
compared with Wednesday afternoon, since the most recent
precipitable water (PW) value increased to 1.27 inches.
Elsewhere, the Lihue sounding still showed relatively dry
conditions, with a PW value of 1.05 inches. In addition, the low-
level trade wind inversion has increased to around 8 thousand
feet at Hilo, while it remains below 6 thousand feet at Lihue.
The latest forecast guidance continues to indicate additional
low-level moisture will be advected over the eastern islands
today. Satellite derived PW values of 1.3 inches extend a few
hundred miles upstream from the Big Island. Therefore, expect the
low-level flow to bring additional low clouds and showers from
the east-southeast toward the Big Island, and possibly Maui, from
this morning into Friday.

The middle tropospheric ridge will likely erode near the eastern
end of the state during the next couple of days. This is due in
part to the short-wave trough moving by just to the north of the
islands during the next 18 to 30 hours. Once this feature is
northeast of the Big Island, the models suggest a sharp upper
tropospheric trough will develop east and northeast of the Big
Island starting later tonight or Friday, and persist into this
weekend. This may allow some additional increase in inversion
heights, especially near the windward Big Island this weekend.

High pressure will gradually build north of the area after the
front dissipates, resulting in an increase in trade wind speeds.
The trades will also likely be from a more typical east-northeast
direction starting late Friday, and continuing through this
weekend. In addition, as the inversion heights gain altitude this
weekend, expect an increase in trade wind showers, particularly
along windward facing slopes of the eastern islands. The models
also show a new high moving to a position far north of the islands
late Saturday. The pressure gradient south of this feature will
maintain locally breezy trade winds at least into Monday as it
continues moving toward the east-northeast. There may be some
fluctuations in trade speeds starting around Monday night or
Tuesday. In addition, trade showers may also increase over some
windward sections from Tuesday into the middle of next week.

&&

.AVIATION...
A cold front passing well north of the state will weaken the high
pressure ridge, decreasing the strength of the trade winds and
keeping the wind direction more east to southeasterly. Cloud
remnants from an old frontal boundary to our east will drift into
eastern sections of the Hawaiian Islands with isolated to
scattered MVFR ceilings and a few showers in the forecast. Expect
mostly VFR conditions elsewhere.

No AIRMETs are in effect at this time.

&&

.MARINE...
A surface high is currently located far northeast of the area
with a surface ridge trailing west to about 200NM north of Kauai.
The high will move east and the ridge will weaken over the next
couple of days as two fronts pass to the north of the state.
Winds are expected to remain below Small Craft Advisory (SCA)
speeds through Friday.

The surface ridge will strengthen again north of the area starting
Friday. Winds are expected to reach SCA strength by Saturday and may
reach near-gale strength around the Big Island and Maui over the
weekend as a new high builds north of the area. Surf along east
facing shores will increase a bit again as well, with the
strengthening trade winds.

A storm east of Japan will produce a very large west-northwest
swell reaching the islands Saturday. Surf from this swell will be
well above the advisory threshold through early next week and may
reach warning levels, most likely on Kauai and Niihau, where surf
heights will be largest. Those islands will block some of the
swell, so surf heights will be somewhat lower on the rest of the
smaller islands. As this swell subsides next Tuesday, a new
moderate northwest swell (from around 320 deg)is expected to
arrive.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...TS

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