Flash Flooding and Heavy Rain in Hawaii, February 13 to 15, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST Tue Feb 12 2019

.SYNOPSIS...
Strengthening northeasterly trade winds will bring increasingly
showery weather beginning tonight, with showery periods 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 kona low develops near the islands.

&&

.DISCUSSION...
Cold air moving in from the north this evening behind a weak
frontal boundary will produce enhanced shower activity statewide.
This frontal cloud band shows up on infrared satellite imagery
this evening as a west to east oriented band of convection
drifting into Kauai and Oahu. A strong upper level trough will
sweep in behind the front late tonight through Wednesday helping
to spread shower activity southeastward across the state, keeping
wet weather in the forecast, and possibly producing isolated
thunderstorms through Thursday morning. Breezy to windy trade
winds at the surface will focus much of the shower activity over
mountain areas and along the windward north through east sections
of all islands. Due to strong instability and forcing from the
upper level trough some of these showers will remain strong enough
to move into southern and western leeward areas.

Temperatures will remain on the cold side across the Hawaiian
Islands for the next several days. These temperatures will remain
cold enough for periods of freezing rain and snow showers across
the summits of Haleakala on Maui where a Winter Storm Warning
will start later tonight through Friday morning. Additional
wintery weather may also impact the summits of Maunakea and
Maunaloa on the Big Island this week and a Winter Storm Watch
remains in effect from Wednesday morning through Thursday night.
After the initial shot of winter weather on Wednesday expect
additional rounds possibly on Thursday and Friday. Avoid
traveling to the summits if possible due to icy wintery road
conditions and low visibilities.

Cold air aloft will settle in over the state on Wednesday and
Thursday causing a kona low to form over the island chain. The
latest models are showing this kona low will move in a counter
clockwise track over the islands through the weekend with upper
troughs rotating around the low center like spokes in a wheel.
Periods of enhanced showers will develop over each island as the
upper troughs rotate through the state. The upper low will likely
be strong enough to cause a surface low to form over the islands
significantly changing the surface wind pattern.

Expect continued cool and unsettled showery weather for most
islands lingering into the weekend. The movement on this kona low
three to five days out remains challenging and therefore
confidence for medium range weather impacts for each island is
only fair at this time. Heavy rainfall remains possible for some
areas this weekend with more details later on as the time period
gets closer. Keep up to date with the latest weather forecast
on this system, weather impacts for each island will likely
evolve over time as models improve their solutions on the track
and intensity of this cold core low.

&&

.AVIATION...
A band of deep moisture will move in from the north tonight and
likely bring another round of mountain obscuration to north facing
slopes from Kauai to Maui. The band will reach windward Big
Island near day break Wednesday as it is clearing Kauai. Although
some showers will affect leeward areas at times, VFR conditions
will prevail. An upper level trough dropping over the state will
enhance instability that may trigger isolated heavy showers and
thunderstorms within the band.

AIRMETs for mountain obscuration have been issued for Kauai and
Oahu, and will likely include Molokai and Maui later tonight.
Also, north-northeast winds are expected to increase behind the
shower band moving through tonight and Wednesday. Winds may become
strong enough to produce moderate low level turbulence to the
south of island terrain.

&&

.MARINE...
Satellite imagery showed a cloud band associated with an old
frontal boundary dropping south into the area this evening
bringing increasing rain chances and a surge of strong north-
northeast winds. Recent ASCAT pass and offshore buoy observations
confirmed this showing pockets of 25 to 30 kt winds trailing this
boundary. This boundary marks the leading edge of a large fetch of
strong to gale force northeast winds that has setup across the
northeast Pacific between low pressure several hundred nautical
miles northeast of the islands and strong high pressure centered
far north of the state.

Seas have already responded at the offshore buoy north-northeast of
Maui and are nearing 18 ft this evening, which is already a few feet
above predicted levels. A combination of the increasing winds and
building seas will translate to small craft advisory conditions
across exposed waters through the overnight hours. Expect this trend
to continue through the rest of the week. Additional marine zones,
such as Maalaea Bay and waters around South Point, will need to be
included in the advisory as the winds transition to northeast and
increase later tonight.

Surf along exposed north and east facing shores will quickly rise
once again overnight as the new northeast swell previously discussed
builds across the waters. Warning-level surf will result for east
facing shores with advisory levels continuing for north facing
exposures. This will be a long-duration event that will likely
continue through the second half of the week. At this point, a peak
appears likely Wednesday, then slowly lowering to advisory levels
for east facing shores by Friday.

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 by Sunday.

Forecast confidence quickly lowers over the upcoming weekend due to
significant model differences. All depict another area of low
pressure developing nearby/over the islands, but the positions of
this feature vary significantly from cycle to cycle.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

High Surf Advisory until 6 PM HST Thursday for Oahu North Shore-
Molokai Leeward-Maui Central Valley.

Winter Storm Watch from 6 AM HST Wednesday through late Thursday
night for Big Island Summits.

Winter Storm Warning from midnight tonight to 6 AM HST Friday
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.

&&

DISCUSSION...Bohlin
AVIATION...Kinel
MARINE...Gibbs