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Hawaiian Islands Synoptic Discussion and Guidance

115
FXHW60 PHFO 250131
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 PM HST Tue Apr 24 2018

.SYNOPSIS...
Strong high pressure northeast of the state will maintain breezy
trades and a stable regime of mainly windward and mauka rainfall
through tonight. Trades will gradually weaken on Wednesday and
Thursday, then shift out of the southeast on Thursday night ahead
of a front. The front will affect the western end of the state on
Friday then move to the eastern end of the island chain on
Saturday, likely bringing wet and unsettled weather to portions of
the state.

&&

.DISCUSSION...
A typical breezy spring trade wind pattern remains in place. A
1036 mb surface high located 1,300 miles north-northeast of the
state is maintaining a breezy and gusty trade wind flow. Ridging
aloft is producing stable conditions. Afternoon soundings and
recent aircraft data show an inversion between 4,500 and 6,500
ft. Precipitable water (PW) ranges from near April normal at 1.0
to 1.1 inches in most areas to upwards of 1.5 inches in the
random pockets of moisture observed in the trade wind flow. One
such area of moisture has been affecting windward areas from the
Big Island to Oahu today, bringing up to 1.50 inches of rainfall
to the wetter windward slopes and generally less than one third of
an inch to most windward areas.

Only minor changes are expected during the next couple of days.
The breezy trades will hold through tonight then gradually decline
Wednesday and Thursday as the surface high to the northeast
weakens. The mid level ridge will weaken but remain strong enough
to maintain somewhat stable conditions, with the inversion slowly
weakening and rising as an upper level trough moves in from the
northwest. The current pocket of moisture in the trade wind flow
should clear the state by evening, and the GFS and ECMWF are
showing another area of low level moisture moving in on Thursday
along with increasing high clouds.

A marked shift in the weather pattern will begin Thursday night as
a deep low forms roughly 800 miles north of the state and
generates a cold front about 250 miles north and northwest of
Kauai. Trades will rapidly decrease and shift out of the southeast
as the front develops and advances toward the islands. The GFS and
ECMWF show the area of low level moisture lingering along windward
areas, and as the atmosphere destabilizes, we will have to watch
out for the potential of heavy showers, mainly over the western
half of the island chain.

The front will likely affect the western half of the state on
Friday and Friday night. The GFS and ECMWF depict this feature as
somewhat poorly defined initially, then merging with a surface
trough forming in the pocket of low level moisture pooled near
the islands. The supporting upper level low will likely be closest
to the state late Friday, and the GFS and ECMWF depict the
islands near the entrance region of a forming jet stream aloft.
This points to the potential for localized heavy rainfall along
the eastward-moving front, mainly from Kauai to Molokai during
this time.

There is high potential for wet and unsettled weather over
portions of the state this weekend, though uncertainty remains
high on the details. The GFS and ECMWF have yet to show a
consistent depiction of the front during this time, though the
latest runs suggest that the feature will stall near the Big
Island late Saturday. PW values in excess of 1.75 inches are
expected along the stalled front as deep southeasterly flow draws
up deep moisture from the tropics, and flooding rainfall is
possible for the eastern end of the state. To the west of the
front, a drier and more stable northerly flow will fill in. And
while uncertainty in the frontal position remains, Kauai and Oahu
have the greatest chance of experiencing the drier conditions as
early as Saturday.

Heading into next week and the start of May, a return to typical
trade wind weather is unlikely. An upper level trough will remain
parked over the region, and a surface low should meander several
hundred miles north of the state. As a result, winds should
remain somewhat light.

&&

.AVIATION...
high far north of the area will maintain locally strong ENE
trade winds over the main Hawaiian islands. These winds are
pushing an area of showery low clouds over the islands. As of
0130 UTC, the leading edge of the shower area was over Kauai and
the trailing edge was over Maui. AIRMET SIERRA is in effect for
the islands from the Maui to Kauai. We may be able to drop the
AIRMET soon for Maui.

AIRMET Tango remains in effect on the leeward side of the
mountains on all islands due to the strong winds.

&&

.MARINE...
A 1036 mb surface high to the NNE of the islands is driving
fresh to strong trade winds over the area, and a Small Craft
Advisory (SCA) is posted for all Hawaiian waters through tonight.
As the high gradually weakens and moves S and E Wednesday through
Thursday respectively, trade winds will gradually diminish, and
the SCA will diminish in area to include the windier zones around
Maui and the Big Island through Wednesday night. A low developing
N of the area will lead to rapidly diminishing wind speeds
Thursday, with winds and seas dropping below SCA criteria
statewide. The developing low is expected to send a front down the
island chain this weekend, and post-frontal N winds may reach SCA
criteria in some zones. The expectation is that the nearby low
will send a sloppy NNW swell toward the islands this weekend into
next week, with associated seas rising above 10 feet in most
zones.

A diminishing NW swell will continue to lower through Wednesday,
with a moderate long-period NNW swell expected to arrive on
Thursday, and peak Friday with peak surf heights below advisory
levels. As discussed above, a low is expected to develop N of the
islands late in the week, and remain in place until early next
week. The amount of swell/surf that arrives in the islands will
depend on the evolution of the low and associated fetch, and
current indications are that a fairly significant swell will
produce high-end advisory-level surf along exposed N and W facing
shores, with the peak of the swell around Sunday. Uncertainty
remains high, and the forecast will likely need tweaking as
details become clearer. Meanwhile, the weather pattern in the S
hemisphere has become conducive for S swells to arrive in the
islands, and a series of relatively small SW to S swells is
expected over the next week or so. Finally, rough and choppy surf
along E facing shores is just below advisory level. Little
significant change in surf heights is expected in the short term,
with seas and surf diminishing by the end of the week as trade
winds diminish.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.

&&

$$

Discussion...Wroe
Aviation...Donaldson
Marine...Birchard

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

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