Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 242013
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1013 AM 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 Friday 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 somewhat typical spring trade wind pattern is in place. A 1034
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, as overnight soundings and recent
aircraft data show an inversion between 4,500 and 6,000 ft.
Precipitable water (PW) ranges between 0.9 to 1.1 inches on the
soundings, which is just below April normal, and random pockets
of moisture observed in the trade wind flow contain PW values
slightly above April normal. One such area of moisture is
affecting windward areas from the Big Island to Oahu this morning,
bringing up to a half inch of rainfall to the wettest windward
slopes in the last three hours and less than one tenth of an inch
to most windward areas during this time.

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 although the atmosphere does not appear to be
destabilizing rapidly, 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, and given ongoing model differences,
the greatest probability for this rainfall looks to be from Kauai
to Molokai during this time.

Uncertainty increases heading into the weekend, though there is
high potential for wet and unsettled weather over portions of the
state. 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 most likely the
eastern half 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 drier conditions as early as Saturday.

&&

.AVIATION...
Fresh to strong trade winds will carry in an area of low clouds
and showers to the islands today. The clouds and showers will
focus along windward and mountain areas from the Big Island to
Oahu, with some showers tracking to leeward sides of the smaller
islands at times. Brief mountain obscurations will be likely
across windward areas today, with AIRMET Sierra for tempo mountain
obscurations likely.

AIRMET Tango is in effect through today over and west of the
mountains on all islands due to the strong trade winds.

&&

.MARINE...
A Small Craft Advisory is currently posted through tonight for
all Hawaiian Coastal Waters due to strong high pressure far north
of the area. Winds are expected to decrease a bit Wednesday and
Thursday as the high weakens.

The current small northwest swell will continue to lower today
through Wednesday. The first in a series of north northwest swells
is expected to arrive on Thursday, peak on Friday, then lower
gradually on Saturday. A rather strong low pressure system is
forecast to develop north of the area this weekend. Surf heights
later this weekend and on into early next week will be highly
dependent on the location of the low center and the fetch
orientation aimed at the islands. Current guidance suggests
advisory level surf from this low but confidence in surf heights
remains rather low since much will depend on the strength of the
low and the fetch orientation.

Easterly trade winds will continue to produce short period choppy
surf along east facing shores through Thursday with a downward
trend expected later in the week and on through the weekend.
Small, mainly background south swells are expected through
Saturday with a slightly larger south swell expected Sunday and
Monday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian
waters.

&&

$$

Discussion...Wroe
Aviation...Wroe
Marine...Burke



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