Flash Flood and High Wind in Hawaii, February 5 to 11, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
428 AM HST Wed Feb 5 2020

.SYNOPSIS...
A cold front moving into Kauai this morning from the northwest
will sweep through the islands reaching the Big Island by late
Thursday. Expect showers to develop along the main frontal band,
becoming heavy at times, with isolated thunderstorms. Several days
of northerly to northwesterly winds will blow in behind the front
bringing much colder air into the state. Unstable moisture
embedded in this northerly wind pattern will likely produce
scattered showers mainly along north facing slopes of each island
through the weekend. Unsettled weather with periods of showers may
persist into the first half of next week due to a stationary
upper level low lingering just north to northeast of the island
chain.

&&

.DISCUSSION...
Radar imagery from Kauai indicates shower bands along the cold
front are moving into Niihau and western Kauai this morning. Some
of these showers will become heavy at times with the possibility
for isolated thunderstorms. Wet weather will then move eastward
down the island chain reaching Oahu later this afternoon, Maui
County later this evening through Thursday morning, and then
finally getting to the Big Island by Thursday afternoon and
evening. Cold temperature aloft and deep moisture will likely
produce snowfall over the Big Island summits on Thursday and
Friday, and a Winter Storm Watch may be required during this time
period. However, we decided to wait another forecast cycle to
issue this watch as our confidence in snowfall amounts remains
only fair at this time due to precipitation amount differences in
run to run weather model solutions.

Additionally, an extended period of very strong winds are
forecast over the Big Island summits with wind speeds likely
exceeding High Wind Warning thresholds from Thursday into the
weekend. A High Wind Watch was issued with the morning forecast
grid package for the Big Island summits with high wind impacts
starting by Thursday morning.

Cold northerly winds moving in behind the front as a cut off low
develops north of the islands. Statewide temperatures will drop
by 4 to 6 degrees statewide for the next several days with high
temperatures forecast around the mid 70's and low temperatures
hovering between the low to mid 60's for low elevation stations.
This cut off upper level low will deepen and stall just north to
northeast of the Hawaiian Islands, producing several days of cold
northerly to northwesterly winds. Upper troughs rotating around
the low are forecast to produce periods of unsettled weather
lasting into early next week. Enhanced clouds and showers for this
time period will be focused mainly along north facing slopes and
mountain areas of all islands with less shower activity forecast
along southern facing slopes.

In the extended range forecast for next week, we continue to see
large differences between global model guidance due to changes in
the location and intensity of this persistent cut off low. We
continue to forecast scattered showers for northern slopes of each
island for now. However, changes to the wind and precipitation
forecast are likely for the first half of next week as the long
range guidance adjusts the position of the low center relative to
the islands. Stay tuned.

&&

.AVIATION...
VFR conditions and land breezes have prevailed over most areas
this early morning. A front approaching the area from the
northwest is just about to reach Kauai as of this writing.

As the front treks across the state today through Thursday night
or Friday, moderate to occasionally heavy rainfall is expected to
generate widespread MVFR, and possible IFR, conditions. A slight
chance for thunderstorms will also be possible. Strengthening
north winds will fill in behind the front.

&&

.MARINE...
A front expected to move down the island chain today through
Friday will bring wind and weather changes across the area. The
front will be accompanied by brief heavy showers and a slight
chance of thunderstorms, especially over the northern offshore
waters. Strong northerly winds will follow the front along with
some much cooler air. A Small Craft Advisory (SCA) has been issued
for western and central waters through Friday, but may need to be
extended and expanded later. Over the weekend, winds may shift to
northwesterlies while remaining breezy before weakening slightly
on Sunday.

A series of large swells will impact the islands through the
weekend. An expected west-northwest swell will slowly build
through tonight and peak Thursday, producing advisory level surf
across north and west facing shores of the smaller islands. This
west- northwest swell will be reinforced by a northwest swell that
will peak Thursday. High Surf Advisories will likely need to be
issued later today.

A gale low is forecast to form some 600 nm north of the state
Thursday and generate a large north swell which will reach the
islands Friday afternoon through Saturday, reinforcing the High
Surf Advisory for north facing shores. We will be monitoring these
swells closely as they may reach warning levels late in the week.
Aside from the High Surf Advisory, the aforementioned SCA will
also cover the elevated seas greater than 10 feet due to these
swells.

A northeast swell will maintain moderate surf along east facing
shores through Thursday. The large north swell Friday and Saturday
will also refract some energy into exposed east facing shores,
possibly requiring a High Surf Advisory there as well. South shore
surf will remain small through the forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Thursday morning through late Saturday
night for Big Island Summits.

Small Craft Advisory from 6 AM Thursday to 6 PM HST Friday for
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Friday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel.

&&

DISCUSSION...Bohlin
AVIATION...Kinel
MARINE...Powell