High Surf in Hawaii, November 10 to 11, 2017

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
400 AM HST Fri Nov 10 2017

.SYNOPSIS...
Weak high pressure north of the islands will maintain gentle to
locally breezy trade winds through early next week. A mid-level
trough approaching from the northwest will allow passing showers to
be enhanced, especially for windward and mauka sections over the
weekend. Lingering instability over the islands from the nearby
trough aloft will keep the trades wetter than normal through the
first half of next week.

&&

.DISCUSSION...
As expected, quite a bit of low clouds and showers are passing
through the islands early this morning, though rainfall amounts
since last night do not add up that much. Mount Waialeale reported
the most in the past six hours, with slightly over half an inch in
the past six hours. There are still patches of low clouds just east
of the islands, and these clouds will provide more showers to the
area, especially windward and mountain sections. Both early morning
Lihue and Hilo soundings continue to depict a typical trade wind
weather profile, with Hilo sounding a little drier and more stable
while Lihue sounding has a little more low level moisture and
slightly more unstable. Weak high pressure north of the state will
continue to bring gentle to locally breezy trade winds to the
islands through the weekend. Therefore, trade wind weather will
prevail today, with a bit more passing low clouds and showers
affecting mainly windward and mountain areas in the morning.

Forecast models continue to be in good agreement with a mid-level
trough digging over the state from the NW in the next few days. The
cold air aloft associated the trough will allow the airmass in the
island vicinity to become more unstable, with enhanced showers
developing. Meanwhile, an increase of low level moisture under the
trade wind flow is also expected for the islands over the weekend.
In all, expect the shower coverage as well as intensity to increase
for most windward and mauka sections over the weekend, most notably
near the Big Island. There could also be localized, briefly heavy
afternoon showers over upslope leeward locales on the Big Island.

The mid-level trough axis is expected to shift a little farther E
early next week, with a slight decrease in low level moisture in the
island vicinity. However, there will still be some lingering
instability due to the nearby broad mid-level trough. Look for
wetter than normal trade wind weather during the first half of next
week.

&&

.AVIATION...
The locally breezy low-level trade wind flow will persist across the
main Hawaiian Islands through tonight. Low clouds and showers
carried by the trades will remain focused along northeast facing
slopes and coastlines. A band of increased cloud cover with embedded
scattered showers is in the vicinity of the islands early this
morning. This will result in localized MVFR conditions with lower
ceilings or reduced visibilities due to windward clouds and showers,
especially this morning. Conditions may improve over some windward
sections later this morning, or this afternoon, as this band shifts
slowly westward.

AIRMET Tango remains in effect due to the potential for moderate
low-level turbulence leeward of the mountains on all islands. We
will be monitoring observations to determine if this AIRMET needs to
be extended into tonight. In addition, AIRMET Sierra is in effect
for mountain obscuration on Kauai, Oahu, Molokai, Maui and the Big
Island. This AIRMET may be required through this morning, but
conditions are forecast to improve over most windward terrain by
this afternoon.

&&

.MARINE...
Moderate trade winds accelerated by island terrain are sufficiently
strong to warrant a Small Craft Advisory (SCA) in the Alenuihaha
Channel through Saturday. Moderate trade winds are then expected to
persist through Tuesday, with the potential for increased seas and
winds prompting a SCA issuance for more zones by the middle of next
week.

A High Surf Advisory for E facing shores of most islands has been
expanded to include N facing shores, based on observations from Buoy
51000 to the NE, and from nearshore PacIOOS buoys. The incoming
swell is nearly due N (arriving from 010-020 degrees), and thus surf
along N facing shores will be larger than that observed along E
facing shores. Surf will likely drop below advisory levels by
Saturday as the swell gradually diminishes through the weekend.
Other potential swell sources include a moderate WNW swell, and
another NNE swell, both around Tue/Wed. Trade winds may strengthen
near/upstream of the islands around the middle of next week, and
combine with the NNE swell to bring high surf to E facing shores
again.

A Marine Weather Statement continues to highlight the potential for
moderate harbor surges due to the long-period NNE swell, primarily
in Kahului and Hilo harbors.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 600 PM this evening for north facing shores
of Niihau and north and east facing shores of Kauai Oahu Molokai
Maui and the Big Island of Hawaii.

Small Craft Advisory until 600 PM Saturday for the Alenuihaha
Channel.

&&

Birchard/Houston/Hui