Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 150640
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Thu Dec 14 2017

.SYNOPSIS...
Cool and windy conditions will remain through early Friday
morning, before winds begin to gradually weaken from Friday into
the weekend. An upper level low will bring unsettled weather to
portions of the state from Friday through this weekend. As a
result, rain chances will increase, with the possibility of
locally heavy downpours and isolated thunderstorms. Widespread
rainfall is possible early next week as an upper-level trough
pushes toward the islands from the northwest.

&&

.DISCUSSION...
A 1031 mb surface high is near 40N 142W, or about 1600 miles
northeast of Honolulu. A surface ridge extends southwest from
this high through a point approximately 600 miles north of Kauai.
Elsewhere, a north to south oriented surface trough, which is
shifting west at about 10 mph, is evident about 360 miles east of
Hilo. The tight pressure gradient between these features is
maintaining strong north to northeast winds across the state
early this evening. A wind advisory remains in effect for most
areas, including the Big island Summits, through early Friday
morning. Dew points remain below normal for Hawaii, so these
windy conditions continue to make sensible temperatures feel
cool.

Aloft, middle- and upper-tropospheric lows are located nearly 325
miles northeast of Hilo. The surface trough described above is a
reflection of these features, which are propagating slowly toward
the west-southwest. The upper level troughing does not appear to
be having any significant impact on the local weather in the
islands, since the afternoon soundings continued to show strong
low-level inversions near 6 thousand feet. Satellite imagery and
radar reflectivity data show mainly broken low clouds with
embedded isolated to locally scattered showers moving down from
the north and northeast toward the windward and north facing sides
of most islands.

Significant changes will occur in weather conditions across the
islands starting Friday. The upper level lows, as well as the
surface trough, will continue to approach the islands. As a
result, the north to northeast winds will gradually weaken from
Friday morning through Friday night. The wind advisory will likely
be cancelled for all lower elevations by daybreak Friday, but we
will monitor observations on the Big Island Summits to determine
if windy conditions will persist there.

The upper lows are expected to reach a position just northeast of
the islands on Saturday, and then slowly pull away toward the
northeast on Sunday. By Saturday, with the surface trough in the
vicinity of the islands, the background flow will become light and
variable across most areas. This will allow the development of
local afternoon sea breezes and nighttime land breezes. By Sunday,
the surface trough is forecast to be located west of the state,
with southerly winds gradually spreading from east to west across
the area.

The cooler temperatures aloft associated with the middle and
upper-level lows will be close enough to start destabilizing the
atmosphere Friday, especially over the eastern end of the island
chain. In addition, increasing moisture will move in to the
islands as the surface trough approaches the state. This will
likely usher in an extended period of unsettled weather across the
region, with increasing chances for rainfall, and the possibility
of locally heavy downpours and isolated thunderstorms. In
addition, there is a chance of snow on the Big Island Summits.

The current forecast continues to favor the eastern half of the
state for the heavier rainfall. There is a chance of thunderstorms
Friday and Saturday afternoons due to daytime heating and local
sea breezes, in addition to the instability aloft. Low clouds may
clear out over many areas at night due to the downslope flow.

Widespread showers are becoming a distinct possibility early next
week as an upper-level trough digs down toward the islands from
the northwest, and deep tropical moisture pushes up from the
southeast.

&&

.AVIATION...
A strong and gusty north-northeast flow will persist tonight and
then gradually decrease by Friday evening as a developing cutoff
low drifts eastward toward the state. The low level flow remains
rather stable over the western end of the island chain, with some
enhanced low level moisture affecting the eastern half of the
state. Enhanced low level moisture across the eastern half of the
island chain will produce MVFR ceilings over northeast slopes of
the Big Island and Maui tonight. The upper level low will drift
over the islands Friday afternoon enhancing showers across the
islands with increasing coverage for MVFR in SHRA and isolated
thunderstorms are possible.

Widespread, moderate low level turbulence will occur over and
downwind of island terrain tonight. AIRMET Tango for moderate low
level turbulence remains in effect for all islands and will likely
be extended into Friday.

&&

.MARINE...
Northwest swell continues to slowly decline this evening. The
swell remains high enough to produce advisory-level surf on north
and west facing shores of the smaller islands, and on north facing
shores of the Big Island, so the High Surf Advisory remains in
effect through 6 AM Friday morning. Depending on how quickly the
swell subsides, this advisory may need to be extended into Friday.
Will make that decision with the early morning forecast package.

Strong northeast winds are expected to produce advisory level surf
on east facing shores Friday, and we expect to issue a High Surf
Advisory for those areas early Friday morning. These winds will
drop off quickly late Friday night and Saturday, but the residual
swell will cause surf along east facing shores to subside more
slowly.

The Thursday morning ASCAT pass found large areas of 25 knot winds
across the coastal waters, with 30 knot winds in the Alenuihaha
Channel. A marginal Gale Warning remains in effect for the
Alenuihaha Channel overnight, with Small Craft Advisories (SCA)
posted elsewhere. We have extended the SCA for most zones through
Friday afternoon due to expected wind and sea conditions. The
Gale Warning will likely be replaced with a SCA early Friday
morning. Winds and seas will gradually subside Friday into the
weekend, with conditions expected to be below SCA criteria for all
waters Saturday through Monday.

Heavy showers and isolated thunderstorms will be possible across
eastern sections of the coastal waters late tonight through the
weekend due to a nearby upper level trough.

See the Collaborative Nearshore Swell and Wind Forecast for
details on expected swell and resulting surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Friday for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-Big
Island.

High Surf Advisory until 6 AM HST Friday for Niihau, Kauai,
Oahu, and Molokai, and the north facing shores of Maui and the
Big Island.

Small Craft Advisory until 6 PM HST Friday 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-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Leeward
Waters.

Gale Warning until 6 AM HST Friday for the Alenuihaha Channel.

Small Craft Advisory until 6 AM HST Friday for Big Island
Southeast Waters.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...Jacobson



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