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fxhw60 phfo 190157 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
357 PM HST Wed Sep 18 2019

Synopsis...
easterly trade winds will prevail over the next few days thanks
to high pressure far north of the area. Clouds and passing light
showers will tend to favor windward and Mauka areas. A wetter
weather pattern is expected beginning Friday and continuing through
the weekend as a moist and unstable airmass moves in from the east.
A more typical trade winds weather pattern is expected to return
early next week.

&&

Discussion...
surface high pressure is centered far to the north northeast of
the area while an area of low pressure is centered several hundred
miles to the west of Kauai. Radar and satellite imagery indicates
that the low level wind flow across the area is slightly south of
due east due to the lows proximity to the state. The airmass is
rather stable across the area due to a ridge aloft. Light passing
showers can be expected across some windward and Mauka areas
overnight but most locations will be generally dry. Some thin high
clouds may also pass over the area from time to time due to deep
convection west of the state.

The surface high will drift slowly eastward over the next couple
of days while the low to our west moves off to the northwest. The
low level wind flow will become more easterly as the low pulls
away from the area. Wind speeds will generally be in the moderate
range. The ridge aloft will remain over the area through Thursday
so shower activity will be highly dependent on incoming moisture
availability and primarily fall over windward and Mauka areas.

A change to this weather pattern is expected as we head into the
weekend. An upper low is forecast to move down over the area from
the northeast while high pressure will remain far to the north
northeast of the area. The upper low will destabilize the airmass
across the area. In the meantime, moisture associated with a weak
area of low pressure currently centered several hundred miles
southeast of the state will move up over the area. Rainfall
chances will increase beginning Friday over the east end of the
state, then gradually spread westward over the weekend. The
airmass may become unstable enough to allow for some thunderstorm
development to occur especially during the afternoon and evening
hours. Locally heavy rainfall is also a possibility across some
areas. Winds will also become lighter due to lows proximity to the
state.

By Monday, the upper low is forecast to move off to the west of
Kauai which will allow for a more stable airmass to move overhead.
Trade winds are also expected to pick back up to moderate levels.
Showers will then favor windward and Mauka areas at that time.

&&

Aviation...
cloud cover has filled in along the lower slopes and coasts of Big
Island this afternoon, but rainfall has remained light and fairly
scattered in coverage. Flow remains out of the east-southeast but
should become gradually more easterly through tomorrow as a low
west of the island chain moves away. This should also help to
limit showers for tomorrow and VFR conditions will prevail.

No airmets are currently in effect.

&&

Marine...
fresh to locally strong, east to east-southeast winds will continue
due to high pressure to the northeast and a tropical disturbance
moving northward to the west of the state, but shift back to a
more typical east to northeast direction Thursday through Friday.
A Small Craft Advisory remains in effect for the typical windy
areas around Maui and The Big Island through Thursday night.
Guidance shows a return of light to moderate southeast flow late
Saturday through Sunday due to an approaching trough from the
east-southeast, which would support localized land and sea breeze
conditions near the coasts.

A series of small swells from the southwest and south are expected
through the rest of this week. The largest of these looks to be
towards the end of the week as a long-period south-southwest swell
and short-period west-southwest swell combine across the area. This
will likely boost surf along south and west facing shores late
Thursday through Friday. A small northwest swell is possible late
in the weekend into early next week. A small swell from tropical
cyclone Kiko should fill in Thursday along exposed east facing
shores before dropping over the weekend for only a slight boost in
surf heights. Surf heights are expected to be below advisory
levels with all of these swells.

&&

Hfo watches/warnings/advisories...
Small Craft Advisory until 6 am HST Friday for Pailolo Channel-
Alenuihaha Channel-Big Island leeward waters-Big Island
southeast waters.

&&

$$

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