National Weather Service Raw Text Product

189 
FXHW60 PHFO 082025 
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1025 AM HST Fri Feb 8 2019

.SYNOPSIS...
A powerful area of low pressure developing to the north of the
islands will move toward us this weekend. This will send large, 
destructive surf, and locally damaging winds, to parts of the
state this weekend. Unsettled weather may persist into the middle
of next week as the low lingers in the island vicinity.

&&

.DISCUSSION...
A relatively cool, dry, and locally breezy northerly flow is
affecting the main Hawaiian Islands this morning, in response to a
surface low analyzed about 450 miles to the NE of Honolulu. Despite
the rather dry, cool air, the airmass is unstable enough to be 
bringing some showers to primarily north facing slopes, especially
the north shore on Kauai and the Hamakua Coast on the Big Island.

A complex and very unusual evolution of low pressure to the N and
NE of the state will bring significant impacts to some parts of 
the state over the next few days. Models agree that a new low will
develop to the N of the existing one, with rapid cyclogenesis 
occurring far to the N of the islands tonight and Sat. Starting 
Sat and continuing into Sat night, the powerful storm-force low is
expected to drop due S rapidly toward the islands. By Sun the low
is forecast to gradually weaken, but will only be passing about 
200 mi to the NE of Kauai and Oahu as it likely begins a slow 
eastward track. 

The models have thankfully been fairly consistent over the past 
several runs. They bring background oceanic flow of 25-30 kt or 
so in two major pushes. The first one will be from the N tonight. 
The second push will be from the NW and W Sat night and Sun, and 
looks to be stronger than the first push. As usual, the island 
terrain will enhance and accelerate the flow in some areas. High 
resolution guidance suggests 40 to 45 kt gusts will be possible 
tonight for some areas tonight. This will be especially true for
parts of Kauai and Oahu, but probably not limited to those areas.
After a very minor lull during the day on Sat, the closest 
approach of the low may bring localized 45 to 55 kt gusts late 
Saturday night into Sunday, most likely for Kauai but very 
possible for other downslope and exposed areas. 

A big caveat here is that if the low were to get closer than the 
guidance has been suggesting, this would bring even higher wind 
speeds to more areas. We will be keeping a close eye on this and 
regardless, it's almost a certainty we will be adding more wind 
headlines for the weekend time frame in the coming hours.

Winds at the Big Island summits have been decreasing as a belt of
strong mid-level winds shifts south away from the Big Island. 
However, even stronger winds are likely to begin spreading in as 
the wind field around the mid level reflection of the powerful low
begins affecting the summits. 

Very cold air aloft will make the airmass more unstable, so
although moisture is limited, we expect an increase in showers
later today and tonight. These will affect mainly N facing shores,
with the highest amounts along the N shore of Kauai and the 
Hamakua Coast on the Big Island. Some of these could be briefly
heavy. Much drier, air is expected to quickly overspread the 
state on Sat, with sea level dewpoints getting down into the mid
to upper 40s. This drier air should limit the rain chances into 
Sat evening, but could increase the gustiness of the wind. Another
band of moisture and strong mid-level height falls pushes in
Saturday night which should be accompanied by bands of chilly, 
squally showers, with possibly even a few thundershowers, is 
expected to sweep down across the islands late Sat night into Sun.

Uncertainty in the forecast increases following the weekend 
storm, with the GFS and ECMWF continuing to display some 
differences in the synoptic pattern evolution. Despite the 
differences in the movement and track of the low to the north of 
the islands, they both indicate that unsettled weather will hold 
in place through much of the upcoming week. As a result, will keep
showers in the forecast even for the typically drier areas 
through the period. Trade winds don't appear like they'll return 
anytime soon, with the models showing a general west-northwest 
wind direction at moderate to locally strong speeds continuing 
through much of the new week.

&&

.AVIATION...
A deepening low to the north of Hawaii will act to tighten the 
surface pressure gradient over the next 24-hours. Expect 
increasing north winds at the local airports, especially this 
evening into tonight. Strong and gusty northerly winds and
moderate or greater mountain wave turbulence will continue to 
affect the airports on Saturday.

Through this evening limited moisture will allow for ISOL -SHRA
along the windward sides of the islands. By tonight, SCT SHRA 
are forecast to fill in over the entire region. This may create 
MVFR conditions at times, especially due to mountain obsc and
lowered vis in the heavier showers. 

No AIRMETS are currents in effect, but likely needed after 06Z 
due to sustained winds over 30 KT near Kauai and surrounding 
waters. 

&&

.MARINE...
A deepening low pressure system is forecast to develop far north 
of the region starting tonight, and then move rapidly south 
toward the state this weekend. This feature is expected to bring 
unprecedented impacts, with very hazardous, and potentially life- 
threatening, conditions spreading across all Hawaiian Waters from
this weekend into early next week. If experienced mariners are 
considering venturing away from protected waters this weekend, 
please prepare not only for very strong sustained winds and 
treacherous seas, but for the likelihood of extreme squally 
weather conditions. If in doubt, DO NOT GO OUT! 

Currently, increasing northwest to north winds are spreading down
the island chain from west to east. Gale force winds are expected
to develop over the waters around Kauai County this afternoon and
a Gale Warning has been issued for these areas. The strongest 
winds are expected to be between Kauai and Niihau as the winds are
accelerated between the two islands. Note that across the 
northern Hawaiian Offshore Waters beyond 40 nm from the Hawaiian 
Islands, winds will be near Gale force through Saturday, and then 
possibly increase to storm force (at least 50 knots!) by Saturday 
night. Besides the Gale Warning for the far western coastal 
waters, a Small Craft Advisory (SCA) has been issued for rest of 
the Hawaiian Waters for a combination of increasing seas and 
strengthening winds. 

A new large northwest swell will continue to build today along 
most north and west facing shores of the smaller islands. 
A High Surf Warning is in effect for increasing surf along north 
and west facing shores of Kauai, Niihau, Oahu and Molokai, and for
north facing shores of Maui. Increasing surf may also require a 
HSA for west facing shores of the Big Island starting tonight or 
Saturday. In addition, surf may eventually reach the HSA criteria 
along north facing shores of the Big Island by late Saturday or 
Sunday.

By late Saturday and Sunday, the model guidance continues to 
insist that the increasing winds and captured fetch associated 
with storm-force low pressure system will cause extremely large 
and disorganized waves to spread across the coastal waters. The 
combined seas will become extremely high due to a combination of 
the longer period swell, and the huge wind waves generated by near
gale to gale-force winds over the coastal waters. 

There is also a strong potential for extreme harbor surges in
north and west facing harbors, as well as significant coastal 
inundation and beach erosion along north and west facing shores of
most islands this weekend. This event may bring unprecedented 
coastal flooding to north and west facing shores, so anyone with 
vulnerable ocean front property needs to complete their 
preparations for this event as soon as possible.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 2 PM this afternoon to 6 AM HST Saturday for 
Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui Windward West-
Maui Leeward West-Maui Central Valley-Windward Haleakala-Leeward 
Haleakala.

High Surf Warning until 6 AM HST Monday for N and W facing shores
of Niihau Kauai Oahu Molokai, and N facing shores of Maui.

High Wind Warning until 6 PM HST this evening for Big Island 
Summits.

High Wind Watch from Saturday morning through Monday afternoon 
for Big Island Summits.

Small Craft Advisory until 6 AM HST Monday for Oahu Windward 
Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward 
Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward 
Waters-Big Island Southeast Waters.

Small Craft Advisory until 2 PM HST this afternoon for Kauai 
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel.

Gale Warning from 2 PM this afternoon to 6 PM HST Saturday for 
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward 
Waters-Kauai Channel.

&&

$$
R Ballard/JT/Kino