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WindWSW 34 mph
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Hawaiian Islands Synoptic Discussion and Guidance

739
FXHW60 PHFO 261341
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Sun Feb 26 2017

.SYNOPSIS...
A surface ridge to the north will maintain gentle to moderate trade
winds through Monday. Rainfall will remain light through Monday
morning but high clouds associated with a jet stream aloft will
blanket Maui County and the Big Island. A trough aloft is deepening
northwest of the islands. The trough will move over the islands and
pull moist air up from the south. There will be a chance of heavy
showers starting Monday afternoon, with thunderstorms and flooding
rains possible Tuesday through Wednesday. A ridge will build over
the islands late next week bringing light winds and drier, but hazy
weather.

&&

.DISCUSSION...
A surface ridge north of the area will maintain gentle to moderate
trade winds over the islands through Monday. The atmosphere is
rather stable and showers are quite sparse upstream of the islands.
Radars show some showers over the leeward waters, but these showers
are remaining offshore. A band of thick high clouds associated with
a strong jet stream covers most of the area. The edge of the clouds
has just cleared Kauai, and should clear Oahu later this morning,
but clouds will continue to blanket Maui County and the Big Island
today.

A trough aloft will deepen northwest of Kauai over the next few days
and make the atmosphere unstable. The trough will also help pull
moist air up over the islands from the south. This enhanced moisture
may reach the Big Island and windward Maui by Monday afternoon,
where heavy showers are in the forecast along with increasing
chances for snow. Some of this moisture may get caught up in the
easterly trade winds and fuel heavy shower development along
windward slopes across the rest of the islands Monday night.

The ECMWF and GFS models show a strong jet stream diving into the
trough aloft. The trough will deepen further, with a low possibly
developing Tuesday night and moving over the islands Wednesday.
Precipitable water will increase as tropical moisture moves over the
islands from the south and heavy rains may produce flash flooding.
It will not get especially cold aloft, and the atmosphere will not
get too unstable, so that will limit the chances for strong
thunderstorms. The freezing level is not expected to be especially
low, but winter storm conditions with strong winds and significant
snowfall are still likely on the summits of Mauna Kea and Mauna Loa.
The weather will turn drier and more stable starting Wednesday night
as the trough aloft moves off to the northeast.

A surface ridge building over the islands during the second half of
next week will keep winds light. The atmosphere will stabilize as
the trough northwest of the area lifts north. Afternoon clouds may
continue, but rainfall is expected to become light. The light winds
may allow vog to spread over the smaller islands through the second
half of the week.

&&

.AVIATION...
A strong jet stream will produce moderate turbulence aloft over the
area today. AIRMET TANGO is in effect for these areas. We received
one report of severe turbulence southwest of Oahu. SIGMET ROMEO is
effect for the area from Kauai to Maui county for Severe Turbulence.
The jet and associated turbulence are forecast to shift east. A
broad band of high, cirrus, clouds associated with the jet blankets
the area from just east of Kauai. These clouds may thicken enough
over the next couple of days to start producing moderate icing.

VFR conditions will prevail for the next 24 hours, but isolated MVFR
ceilings and mountain obscuration are possible over northeast
sections of the islands this morning and tonight and over interior
ares this afternoon.

&&

.MARINE...
The current northwest swell will hold (around 6 ft at 13-14 seconds)
into this evening before dropping further over the next couple of
days. Surf from this swell will remain below High Surf Advisory
(HSA) levels. 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.

The latest surface analysis showed two compact gales centered
northeast and north-northwest of the islands. A tight pressure
gradient between these lows and a 1048 mb high centered to their
north just south of Kodiak was producing strong to gale-force winds
located on the north and northwest sides of the lows, per the last
couple of ASCAT passes. The latest wave model guidance is
initializing well with these synoptic features and depicts a small
to moderate north-northeast swell (030 deg) in the local area from
the gale to the northeast of the islands starting tonight, peaking
Monday, then subsiding through Tuesday night. This swell will
produce surf near the HSA threshold of 8 feet for exposed east
facing shores around the peak of the swell.

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 north-northeast
across the Date Line Thursday. A large west-northwest swell
associated with this low is possible across the local waters next
weekend.

A ridge north of the area will maintain light to fresh breezes out
of the east-southeast through Monday night with winds and seas below
Small Craft Advisory (SCA) levels. The wind forecast becomes more
uncertain Tuesday through mid week. The global models show a low or
trough forming over the area then lifting northward. Plenty of
moisture and instability associated with this feature will lead to
increasing chances of showers and thunderstorms.

&&

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

&&

$$
Donaldson

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