Winter Storm Warning 1, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Tue Feb 12 2019

.SYNOPSIS...
Strengthening northeasterly trade winds will bring increasingly
showery weather beginning tonight, and continuing through at least
Friday. Clouds and showers will focus over windward areas, but the
strong trade winds will blow some showers to leeward areas of the
smaller islands. Unsettled weather may persist into the weekend as a
weak area of low pressure develops near the islands.

&&

.DISCUSSION...
Strong high pressure far N of the islands will become the dominant
low-level weather feature dominating island weather the next several
days, as low pressure to the distant NE continues to eject toward
the NE. With the proximity of the low, and an E-W oriented trough E
of the Big Island, the low-level flow is out of NNE over Kauai, and
out of the NNW near the Big Island. Satellite shows scattered low
clouds moving over the islands in this flow, and radar shows more
showers than one might otherwise expect - mainly affecting N and E
facing slopes and coasts.

An area of showery clouds about 150 to 300 miles N of the islands
will move in overnight, bringing showery conditions. In the upper
atmosphere, several waves of cold air will once again sweep over the
islands from the N, leading to unsettled weather as the island
atmosphere will become quite unstable. These potent short wave
troughs may lead to the development of another closed low aloft near
the islands by the weekend, with ECMWF guidance more aggressively
developing a low as compared to the GFS. Regardless, very cold
temperature aloft, combined with an increase in moisture moving in
from the N, will bring what is expected to be bouts of winter
weather to the summits on Maui and the Big Island. While one round
is expected overnight and early Wednesday, a more significant shot
of cold air aloft around Thursday will likely bring a better shot of
wintry weather. A Winter Storm Watch has been issued in anticipation
of these developments.

With differences in the way the models are handling the evolution
aloft, the weekend forecast remains quite uncertain. Recent
deterministic runs of the ECMWF and GFS continue to indicate a
surface low developing near the islands, but with significant
differences as to where. The wind and sensible weather realized in
the islands will be highly dependent on the evolution and track of
the low. ECMWF guidance is indicating that the deep-layer low will
move over the islands, thereby killing the winds, while the GFS has
the low further E, thereby bringing strong and gusty NE winds, at
least to parts of the chain. The official forecast currently leans
heavily toward the GFS solution.

&&

.AVIATION...
A northerly flow will produce isolated MVFR conditions in SHRA along
north facing terrain from Kauai to Molokai today. Meanwhile, a band
of low level moisture will affect north facing slopes of Maui and
the Big Island, where AIRMETs for mountain obscuration are in place.
MVFR ceilings and VIS in SHRA will diminish over Maui by noon, while
these conditions will develop over windward Big Island and persist
into mid afternoon. Another band with deeper moisture and
accompanied by greater instability will move in from the north
tonight and likely bring another round of mountain obscuration to
north facing slopes from Kauai to Oahu. The band will reach windward
Big Island near day break on Wednesday as it clears Kauai. Although
some showers will affect leeward areas, VFR conditions will prevail.

&&

.MARINE...
Winds and seas will be trending down today as the storm-force low
responsible for the weather this past weekend lifts northeastward
and away from the local area late this morning. Guidance is
initializing well with the current pattern, which depicts a strong
pressure gradient setting up between the low and a strong high
pressure system far north of the region. A large fetch with strong-
to near gale-force northeast winds focused at the islands within the
020-050 deg directional bands will drive a large northeast swell
toward the area beginning tonight. Seas will respond and build
tonight keeping small craft conditions in place for most waters,
which will likely continue through the second half of the week. The
small craft advisory continues through Thursday.

Winds will remain out of the north-northeast through the day today
at moderate to fresh levels, then shift out of the northeast and
increase into the fresh to strong category tonight through
Wednesday. Small craft winds will return over the channels and
typically windier marine areas between the islands through Friday.
Forecast confidence quickly lowers over the upcoming weekend due to
significant model differences. All depict another area of low
pressure developing near the eastern end of the state, but the
positions of this feature vary significantly.

Surf along exposed north and east facing shores will lower through
the afternoon, then quickly rise tonight as the new northeast swell
previously discussed builds across the waters. A High Surf Warning
has been issued this morning for east facing shores of Kauai, Oahu,
Molokai, Maui and Big Island in anticipation of the surf building
tonight and continuing through Thursday. A High Surf Advisory is
also posted for north facing shores of the aforementioned islands.

Impacts associated with this large northeast swell will include
significant beach erosion along exposed coasts due to the long
duration of the event. A combination of the large northeast swell
and fresh to strong onshore winds will likely correspond to overwash
along vulnerable coastal sections of the islands around the high
tide cycles each day early Wednesday through Friday.

Surf along west facing shores will trend down through the week with
mainly small long-period northwest swells expected through Friday. A
moderate west-northwest swell will be possible over the upcoming
weekend that may bring surf up to around advisory levels for north
and west facing shores by Sunday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

High Surf Advisory until 6 PM HST Thursday for North facing shores
of Kauai Oahu Molokai Maui and the Big Island.

Winter Storm Watch from Wednesday morning through late Thursday
night for Big Island Summits.

Winter Storm Watch from late tonight through late Thursday night for
Haleakala Summit.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha
Channel-Big Island Windward Waters.

&&

SYNOPSIS/DISCUSSION...Birchard
AVIATION...Wroe
MARINE...Foster