Current Conditions
Temp0.4 C
RH96 %
WindSW 15 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

588
FXHW60 PHFO 190144
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
344 PM HST Thu Oct 18 2018

.SYNOPSIS...
Gentle to locally moderate trade winds will persist through the
next couple of days. Showers will remain focused across windward
slopes, while instability near the Big Island will likely trigger
leeward and interior showers and a few thunderstorms through this
afternoon. An upper level trough slowly passing overhead from
west to east on Friday through Sunday will produce unstable
conditions and bring a risk of thunderstorms to much of the
island chain. Trade winds will rebuild on Sunday and early next
week, though unstable conditions may linger over portions of the
state.

&&

.DISCUSSION...
A gentle to locally moderate trade wind flow remains in place,
along with moderately unstable conditions around the Big Island.
A slow-moving front sitting less than 500 miles north of Kauai
has weakened the local pressure gradient and resulting trade
winds. A nearly stationary mid- to upper-level trough remains
overhead, sending high clouds over the eastern half of the state
and producing some instability. While low level ridging continues
to be strong enough to maintain modest stability over the western
half of the state, a stubborn pocket of moisture with precipitable
water values of 1.75 to 2 inches is contributing to instability
around the Big Island. This will continue fuel a diminishing
amount of showers and thunderstorms over interior Big Island
through the remainder of the afternoon. The rest of the state will
experience mainly windward showers through the night.

On Friday, trades will remain somewhat weak, and unstable
conditions will develop over much of the state. A shortwave aloft,
currently located about 500 miles northwest of Kauai, will drop
into the mid- to upper-level trough already in place over the
islands. As the shortwave trough passes over the western end of
the state, 500 mb temps will approach -10C, but a lack of deep
moisture will likely inhibit thunderstorms from Kauai to Maui.
These islands will see continued windward rainfall with some
afternoon showers over leeward terrain. Lingering moisture around
the Big Island should trigger another round of afternoon
thunderstorms.

Gentle to moderate trades and unstable conditions will hold over
most the state Friday night and Saturday. The latest runs of the
GFS and ECMWF are showing some differences in the handling of the
mid- to upper- level trough. The GFS keeps the feature moving
eastward, while the ECMWF suggests that it will stall over the
central islands on Saturday. Expect unstable conditions to
persist. The trades will be strong enough to produce showers over
windward slopes. Thunderstorms could pop up anywhere during this
time, mainly over interior terrain during the day and windward
terrain at any time. With 0-6 km wind shear climbing to around 30
kt and ample instability, strong thunderstorms will be possible
over central and eastern portions of the island chain.

Building trade winds and decreased instability are expected on
Sunday. The GFS and ECMWF continue to differ on the upper level
features, so there is some uncertainty. The GFS pushes the now
cut-off mid- to upper-level low east of the Big Island, while the
ECMWF drifts the gradually weakening feature just southwest of
the state. The GFS solution would suggest lingering instability
over the Big Island and typical trade wind weather elsewhere. The
ECMWF solution points toward decreasing chances of heavy showers
and thunderstorms over most islands. For now, the forecast leans
heavily toward the GFS.

By Monday, trade winds will strengthen to locally breezy levels.
Uncertainty in the location of the low aloft remains, with the
ECMWF pointing toward a wetter and less stable pattern. Both the
GFS and ECMWF hint at deeper tropical moisture moving up from the
southeast Tuesday and Wednesday and potentially affecting the
southern end of the island chain.

&&

.AVIATION...
Light winds, combined with some instability over the Big Island,
have produced a few towering cumulus and thunderstorms this
afternoon. IFR conditions are possible in and near thunderstorms,
with some improvement expected after sunset. AIRMET SIERRA for
mountain obscuration was expanded to include all of the Big
Island this afternoon with the cloud buildup, but expect that to
be trimmed back, and possibly eliminated overnight.

Clouds have also increased over Maui, so AIRMET SIERRA for
mountain obscuration was also expanded this afternoon to over the
whole island. Conditions should improve over Maui overnight.

Winds will continue to weaken over the next 24 hours, and
thunderstorms are possible again tomorrow afternoon on the Big
Island. With the lighter winds, we could see additional clouds and
showers over all the islands tomorrow afternoon, which could
prompt additional AIRMETs for mountain obscuration. Otherwise, VFR
conditions are expected.

&&

.MARINE...
Trade winds are forecast to remain weak through Friday night.
The trades may gradually strengthen from Saturday into next week.
The current forecast does not have Small Craft Advisory (SCA)
conditions over any of the coastal waters or channels through
Sunday night. SCA conditions may develop across the typically
windy waters adjacent to the islands of Maui County and the Big
Island early next week.

Unstable atmospheric conditions are forecast to develop over
parts of the state beginning Friday night. This may cause locally
heavy downpours, and allow thunderstorms to form across some
areas this weekend. Mariners should monitor future weather
forecasts as this situation evolves.

The current north northwest swell will subside through this
weekend. The weak trades will cause surf to slowly decrease along
most east facing shores through Friday. However, some east facing
shores with a northerly exposure may continue to see small
breakers from the north northwest swell energy during the next
couple of days.

A new long period south swell is expected to Friday and peak this
weekend. Based on the latest guidance, surf will likely reach the
advisory criteria along south facing shores starting Saturday.
There is also a possibility surf may approach the High Surf
Warning threshold along south facing shores this weekend, before
the swell begins to gradually decline early next week.

See the latest Collaborative Nearshore Swell and Wind Forecast for
Oahu (SRDHFO), which was updated Wednesday afternoon, for more
details on swells and surf.

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...M Ballard
MARINE...Foster

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