High Surf in Hawaii, March 8 to 10, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Wed Mar 8 2017

.SYNOPSIS...
Unsettled weather, with heavy rain and thunderstorms, will be
possible through Thursday as strong upper-level disturbances pass
over the island chain. For today, heavy rain is most likely along a
stalled band of moisture currently near Maui and the Big Island.
This band of moisture is then expected to slide northward tonight
and Thursday, potentially shifting the threat of heavy rain to Kauai
and Oahu. Gradually improving conditions are expected on Friday,
with a more settled weather pattern expected for the weekend, with
light winds bringing humid conditions.

&&

.DISCUSSION...
Latest weather analysis features a surface high far NW of the
islands, a NE-SW oriented surface boundary near Maui and the Big
Island, and a longwave trough aloft W of the area. A NW-SE gradient
in dew point temperatures across the area is about as sharp as it
ever gets, with dew points at Buoy 51001 in the upper 40s, while
near and E of the Big Island, dew points are in the upper 60s. The
high to the NW is supplying the cool and dry N to NE winds that are
in place from Molokai westward, with moisture pooled along what used
to be a NE-SW oriented frontal boundary near Maui and the Big
Island. Overnight soundings show a weak low-level inversion over the
Big Island, with significant CAPE above, while the Kauai sounding
indicates a relatively dry and stable air mass in place.

The short term forecast calls for light to moderate NE winds to
prevail over most of the state today, supplied by the high to the
NW. A light E to SE flow near the Big Island is related to the flow
around a surface high to the distant NE, and the low-level
convergence between these two highs is acting to pool moisture along
the stalled boundary described above. Weather will resemble a fairly
typical trade wind day from Kauai to Molokai, while other portions
of Maui county and the Big Island remain under the gun for heavy
showers and thunderstorms, where light trade winds may become
interrupted by winds associated with heavy showers/thunderstorms.

As one shortwave trough aloft that triggered thunderstorms and
localized flooding on Tuesday exits to the NE, new shortwave energy
is expected to dive into the longwave trough later today/tonight.
Water vapor imagery shows a shortwave near 22N171W that may be
beginnings of this wave sharpening. The sharpening shortwave is
expected to trigger heavy showers and thunderstorms along the
stalled surface boundary later today and tonight, with another
shortwave doing the same on Thursday. This is expected to induce the
sharpening of a surface trough along the stalled boundary near Maui,
with the trough axis moving N toward Kauai and Oahu before gradually
moving NE away from the area. The surface trough will provide a low-
level for focus for heavy shower development, but there remains
considerable uncertainty as to where the surface trough axis will
develop/move, and when/where heavy showers and thunderstorms
develop. A Flash Flood Watch remains in effect statewide for the
ongoing threat of localized downpours, and a Winter Storm Watch
remains in effect for the Big Island summits starting tonight due to
the cold air aloft leading to the potential for wintry weather. The
passage of the shortwave troughs is also bringing strong winds to
the Big Island summits, where a High Wind Warning is in effect.

The upper level trough will gradually weaken Thursday night and
Friday, and conditions will be gradually improving, but the Flash
Flood Watch may need to be extended to cover the threat of unsettled
weather on Thursday. Friday looks to feature light and variable
winds which may allow afternoon sea breezes to drive interior and
upslope clouds and showers, with lingering instability. A surface
ridge is forecast to develop over the islands this weekend and
Monday, keeping winds light, as mid-level height rises lead to a
more stable atmosphere. According to the GFS, a weak front is
possible around Tuesday and Wednesday, with trade winds prevailing
after its passage. However, ECMWF guidance indicates no frontal
passage and light trade winds, with the official forecast blending
the wind solutions, with no frontal passage indicated.

&&

.AVIATION...
A stalled and dissipating frontal band over Maui and the Big Island
will bring increasing showers to the eastern half of the state
through the afternoon. For Maui and Hawaii Counties expect periods
of MVFR conditions today with isolated IFR with ceilings below 1000
FT AGL over mountain interior sections. Isolated thunderstorms
remain in the forecast. Enhanced shower activity continues through
Thursday.

For Oahu and Kauai a high pressure ridge will build into the
islands. Expect mostly VFR conditions today with isolated MVFR
possible in showers. Additional showers will develop on Thursday.

No AIRMETs in effect.

&&

.MARINE...
The fetch associated with the circulation around a surface low with
gale force winds located far north-northwest of the state has sent a
large northwest swell (330-340 deg) toward the islands.
This swell has already arrived at buoys 51001 / 51101 located
northwest of Kauai. Based on the most recent observations from these
buoys, this swell appears to be at least 2 ft larger than the model
guidance suggested. Therefore, we have issued a High Surf Warning
starting late this morning and continuing through tonight for
exposed north and west facing shores of the smaller islands. In
addition, a High Surf Advisory has also been issued for rising surf
along west facing shores of Oahu, and starting this evening along
north facing shores of the Big Island. This northwest swell is
expected to gradually subside on Thursday, but will likely continue
to produce advisory level surf over most north and west facing
shores. This swell will continue to diminish from Friday into this
weekend.

As the large northwest swell spreads down the island chain today, it
will produce seas of 10 ft or greater over exposed waters.
Therefore, a Small Craft Advisory (SCA) has been issued starting
this morning over the coastal waters surrounding Kauai and the
waters north of Oahu. As this swell continues to spread down the
island chain, additional waters have been added to the SCA starting
this evening. The elevated seas due to this northwest swell will
likely affect many of the same the coastal waters on Thursday, when
the swell is forecast to start to gradually subside. No additional
large swells are in the forecast through this weekend.

The background surface winds over the waters surrounding the
Hawaiian Islands will remain relatively light through this weekend.
However, unsettled weather conditions with locally heavy downpours
and isolated thunderstorms remain in the forecast through Thursday
due to the combination of a surface trough in the vicinity of the
central islands, an upper level trough moving over the region and
enhanced moisture. Mariners should be aware that any thunderstorms
they encounter will be capable of producing locally strong wind
gusts, frequent lightening, and reduced visibilities during the next
couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 AM HST Thursday for all Hawaiian
islands.

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

Winter Storm Watch from 6 PM HST today through Thursday afternoon
for Big Island Summits.

High Surf Warning from 10 AM this morning to 6 AM HST Thursday for
north and west facing shores of Niihau Kauai Molokai and for north
facing shores of Oahu and Maui.

High Surf Advisory from 10 AM this morning to 6 AM HST Thursday for
west facing shores of Oahu.

High Surf Advisory from 6 PM this evening to 6 AM HST Thursday for
north facing shores of the Big Island.

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

Winter Storm Watch from this evening through Thursday afternoon for
Big Island Summits.

Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday for
Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory from 8 AM this morning to 6 AM HST Thursday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters.

&&

Public...Birchard
Marine...Houston
Aviation...Bohlin