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fxhw60 phfo 181421 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
421 am HST Wed Sep 18 2019

Synopsis...
a brief period of more stable weather conditions remains in the
forecast through Thursday. More unstable wet weather will spread
from east to west across the state from Friday through Sunday as
a deepening upper level low approaches the islands from the
northeast and deep tropical moisture move in from the south. A
high pressure ridge will build back in over the region from Monday
to Tuesday with weather slowly trending back toward more stable
and drier conditions through the first half of next week.

&&

Discussion...
this mornings radar imagery shows isolated to scattered showers
drifting through the east to southeast wind flow. The satellite
picture continues to show a high pressure ridge far north of the
region, a narrow upper level trough (tutt) just north of the
island chain, and low pressure systems roughly 500 and 700 miles
southwest of Kauai. Cirrus clouds forming under a subtropical jet
stream are streaming across the state from southwest to northeast.
Upper air balloon soundings from Lihue and Hilo show a stable
trade wind inversion height around 6000 to 7000 feet. This
more stable inversion height will tend to cap vertical cloud
development and limit shower activity.

Large scale surface winds will continue to flow from east to west
today with a slight southeasterly direction in some areas. These
wind directions are modified by the position of the high pressure
ridge over the region interacting with the relatively large low
pressure system southwest of Kauai. Wind directions will become
more east to northeast on Thursday, as this low pressure system
drifts northward away from the islands, and the high pressure
ridge builds back in over the state. Expect a brief period of more
stable weather conditions through Thursday night with moderate
trade winds.

The weather forecast this weekend will trend towards wet, humid
and unstable conditions as the large scale pattern gets
complicated. On Friday, an upper level low will deepen and
approach the state from the northeast, drifting over the eastern
islands by Friday afternoon. This cold core low will add
instability to the forecast along with increasing clouds and
showers from Friday into Saturday. A low pressure trough in
tropical convergence zone approaches the islands from the south
on Saturday bringing up a plume of deeper tropical moisture into
the state.

The latest medium range weather model solutions for this weekend
show the tropical moisture plume spreading out across the state on
Saturday afternoon as the upper low drifts slowly westward across
the island chain. Precipitable water (pw) levels within the
moisture plume range form 2.0 to 2.5 inches. Winds will diminish
on Sunday as a surface trough under the upper low weakens surface
pressure gradients. Numerous showers are forecast this weekend,
some of these showers will be locally heavy at times, and a few
thunderstorms are possible. Changes to the morning forecast grids
include increasing precipitation chances from east to west on
Saturday with the highest chances for rainfall added to the Sunday
forecast period. Confidence in these wet weather trends remain
fair at this point in the forecast cycle. However, more specific
location and timing impacts for each island may evolve over time
as the forecast depends greatly upon the final strength and track
of these two low pressure systems. Stay tuned for more details on
this event as the forecast time period grows shorter.

In the extended range forecast starting next week Monday, models
are indicating the upper low will begin to weaken and drift
northward away form the state. Higher moisture levels and
instability will keep enhanced showers in the forecast for one
more day. By Tuesday the long range model guidance shows a high
pressure ridge building back in across the eastern half of the
state with more stable trade winds returning. Enhanced showers
with higher amounts of tropical moisture will linger over Kauai
and Oahu on Tuesday with drier and more stable trends spreading
statewide from next week Tuesday into Wednesday.

&&

Aviation...
pretty much a trade wind type situation for the main Hawaiian
islands today and tonight, and a dry one at that, meaning VFR
conditions prevailing. Any showers that comes in will affect
mainly the windward and mountain areas of the larger islands.
The Lee side of The Big Island will be going through its diurnal
cycle with Morning Sun, followed by afternoon/evening clouds,
plus a shower or two. Brief MVFR ceilings and vis are expected
with these showers. The trade winds are blowing at moderate to
locally strong, but it is weak enough not warrant airmet Tango. A
broad area of low pressure southwest of here, between the islands
and the dateline and high pressure to the NNE, will maintain a
slight south component to the trade wind flow.

&&

Marine...
fresh to locally strong east to east-southeast winds will
continue today driven by high pressure to the northeast and a
tropical disturbance passing west of Hawaii. A shift back to a
more typical east to northeast direction is expected Thursday
through Friday. A Small Craft Advisory is now 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 potential short-period west-southwest swell combine across
the area, boosting surf along south and west facing shores late
Thursday into Friday. A small northwest swell is possible late in
the weekend into early next week. Surf heights are expected to be
below advisory levels with these swells. A small swell from
tropical cyclone Kiko around Thursday along exposed east facing
shores before dropping over the weekend.

&&

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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