Winter Storm Warning 1, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Mon Feb 5 2018

.SYNOPSIS...
A surface front near Maui will move slowly east through Tuesday,
while a trough in the upper atmosphere will keep the island
atmosphere unstable through Wednesday. Drier and more settled
weather is expected from Thursday through next weekend as trade
winds return.

&&

.DISCUSSION...
A surface front between Oahu and Maui is combining with a sharp
trough aloft over the area, and heavy showers and thunderstorms are
impacting Maui county and the Big Island as a result. Albeit fairly
weak, a post-frontal NW to N flow is spreading over Kauai and Oahu,
but instability provided by the trough aloft is fueling pockets of
moderate to briefly heavy showers near and over these islands. Deep
moisture spreading over the Big Island from the W prompted the
recent issuance of a Winter Weather Advisory for the Big Island
Summits, while the trough aloft is also supporting strong winds over
the Summits, where a Wind Advisory is in effect.

Forecast models indicate the surface front will slide slowly E
overnight, moving E of the Big Island on Tuesday. While most of the
associated deep moisture is expected to move E of the islands as
this occurs, an impulse aloft is expected to sharpen the trough
aloft over the area Tuesday and Tuesday night, supporting a very
unstable island atmosphere. A light and variable wind flow statewide
is expected to favor a N direction on Kauai and Oahu as high
pressure begins to nose in from the NW, and these light winds will
likely allow sea breezes to converge over the islands on Tuesday and
possibly again on Wednesday, potentially leading to localized
downpours over the islands. Therefore, a Flash Flood Watch (FFA) for
all islands remains posted, but will be reevaluated overnight.

A very weak and shallow front moving through Wednesday night and
Thursday will finally usher a cool, dry and stable air mass over the
islands. This occurs as the surface high to the NW passes N of the
islands and mid-level temperatures warm significantly in response to
a developing ridge aloft. N winds on Thursday will veer to NE
Friday, with light to moderate trade winds generally expected next
weekend.

&&

.AVIATION...
A weakening cold front has nearly stalled over the center the state.
Southerly flow in advance of the front will continue to spread thick
clouds, showers, and isolated thunderstorms across Maui County and
much of the Big Island. Northerly flow behind the front has spread
low topped clouds and scattered light showers across Kauai and Oahu.
MVFR and areas of IFR have been observed with the heaviest showers.
Expect some of this activity to spread to the Big Island overnight.

AIRMET SIERRA continues for mountain obscuration across Maui Molokai
Lanai and the Big Island. Conditions are expected to persist
overnight.

SIGMET XRAY continues for thunderstorms over most of the area east
of Oahu. Anticipate this SIGMET remaining up through the early
morning hours.

Icing remains a possibility in the upper levels, even outside of
thunderstorms. Mention of the possibility of icing continues in
AIRMET ZULU across the islands and adjacent coastal waters east of
Oahu.

&&

.MARINE...
A slowly dissipating front over the islands is keeping a weak
pressure gradient and light winds over most waters. The front will
likely weaken to a surface trough and shift slowly E over the next
couple of days. Light winds are expected to persist in most of the
local marine areas through Wednesday, though some moderate
northerlies are expected near Kauai Wednesday. High pressure will
build NW of the islands Wednesday night, increasing northerlies near
Kauai and Oahu. Fresh trades will then overspread the state Thursday
and Thursday night. The trades may reach Small Craft Advisory levels
in the typically windier waters by Friday.

Moist and unstable air associated with the front and a strong trough
aloft will maintain the chance for locally higher winds and seas,
reduced visibility in localized downpours, and cloud to surface
lightning for the next couple of days.

Guidance from the Wave Watch III model continues to indicate a large
northwest swell spreading across the state late Thursday through
Friday. The swell should peak near warning levels for N and W facing
shores, and will likely result in a SCA due to elevated seas across
exposed waters Thursday night and Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Wednesday afternoon for all islands.

Winter Weather Advisory until 6 PM HST Tuesday for Big Island
Summits.

Wind Advisory until 6 AM HST Wednesday for Big Island Summits.

&&

DISCUSSION...Birchard
AVIATION...Bedal
MARINE...Morrison

Special Weather Statement
National Weather Service Honolulu HI
908 PM HST MON FEB 5 2018

HIZ023-025>027-060830-
Big Island North and East-Kohala-Kona-Big Island Interior-
908 PM HST MON FEB 5 2018

...STRONG THUNDERSTORMS AND GUSTY WINDS APPROACHING THE BIG ISLAND
FROM THE WEST...

At 905 PM HST, radar indicated a line of strong thunderstorms over
waters to the northwest and west of the Big Island, moving east at
15 to 20 mph. These storms extended from the Alenuihaha Channel to
about 20 miles west of Honaunau. As these storms move over the Big
Island, wind gusts of 50 to 55 mph are possible, as well as frequent
dangerous lightning, and torrential downpours reducing visibility to
near zero.

Locations impacted include...
Kailua-Kona, Captain Cook, Waikoloa Village, Kapaau, Hawi, Honaunau,
Kahaluu-Keauhou, Kealakekua, Puuanahulu, Kainaliu, Honalo, Pololu
Valley, Kohala Ranch, Kalaoa, Halaula, Holualoa, Puako, Kamuela,
Pohakuloa Training Area and Mahukona.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

LAT...LON 1936 15589 1959 15597 1965 15603 1975 15606
      1978 15605 1986 15593 1998 15583 2003 15582
      2016 15591 2023 15591 2027 15587 2025 15577
      2018 15569 1920 15571 1917 15591
TIME...MOT...LOC 0707Z 276DEG 19KT 1916 15635 2030 15635

Birchard