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Hawaiian Islands Synoptic Discussion and Guidance

702
FXHW60 PHFO 191335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Sun Nov 19 2017

.SYNOPSIS...
Cool northerly winds will be locally breezy today before diminishing
slightly Monday and Tuesday. Low clouds streaming toward the islands
will focus along north facing slopes and coasts, while also dropping
a few showers. Trade winds will return on Wednesday and may become
quite strong for the Thanksgiving holiday weekend, with windward
showers occasionally spreading leeward.

&&

.DISCUSSION...
A deep-layer complex cutoff low continues to meander around 700
miles NNE of the islands, with a strong surface high centered about
1500 miles to the NNW. The gradient between these two features is
producing light to moderate NW to N winds over the islands this
morning, while also advecting cool and relatively dry air over the
area. An organized frontal cloud band associated with the low to the
NNE is no longer discernible on latest satellite imagery. Instead,
diffuse bands of showery and stable low clouds are developing in the
cyclonic flow around the low, and advecting toward the islands from
the N. Broken to overcast high clouds are obscuring low cloud
features near Maui and the Big Island, while radar detects widely
scattered showers tracking SSE. The gradient associated with the low
aloft is generating strong winds over the summits of the Big Island
(winds gusted to near 100 mph overnight), where a High Wind Warning
has been extended through tonight.

The deep-layer cutoff low will linger to the N of the islands into
Monday before it gradually moves away to the NE later Monday through
Wednesday. This will keep winds elevated over the Big Island summits
through tonight, with decreasing winds expected Monday as an
associated short wave slides E of the area. At the surface, NW to N
winds will continue to keep a cool air mass in place, while also
sending a varying amount of low clouds toward the islands. These
clouds will likely produce a few showers along N facing slopes and
coasts, but as the low-level winds continue to advect dry air over
the area, these showers are not expected to amount to very much,
especially on the SE end of the island chain. High clouds are
expected to move E of all islands later today. Low-level winds are
expected to be locally breezy today before diminishing slightly
Monday and Tuesday as the low-level gradient loosens in response to
the gradually departing low.

Winds will gradually shift toward a NE to ENE trade wind direction
on Wednesday then potentially become quite strong on Thanksgiving as
the low departs and strong high pressure builds to the N of the
area. This high is expected to support strong trades into the
holiday weekend. A trough aloft may make for a showery trade wind
weather regime with windward showers occasionally spreading leeward
on the strong winds.

&&

.AVIATION...
Unstable and diffuse cloud bands will continue to move through the
islands today. These bands of showers will produce brief periods of
MVFR conditions and scattered showers. The overall shower activity
for the smaller islands will become more isolated this afternoon,
while scattered showers and cloud ceilings will linger over the Big
Island through Sunday evening.

No AIRMETs in effect and none are expected.

&&

.MARINE...
A slow moving intense low pressure system located far NE of the
islands will be the primary weather feature impacting the islands
over the next day or so. Northerly winds blowing across the islands
will remain below Small Craft Advisory (SCA) levels through Tuesday.
The low departs the area thereafter and will be replaced by a strong
high pressure system. This will allow moderate to strong NE trades
to spread across the main Hawaiian Islands. This process is expected
to take place between Tuesday night and Thursday. Trades may reach
Gale Warning criteria for a couple of the channels such as the
Alenuihaha Channel. An SCA will fill in the rest of the coastal
waters. Waters will become dangerously very rough and choppy as well.

This intense low has generated a large open swell a couple of days
ago, targeting the islands. The north swell will quickly spread down
the island chain this morning. It is expected to peak late this
afternoon or this evening before fading through Tuesday.

A High Surf Warning has been posted for Kauai and Niihau, and a High
Surf Advisory for Oahu, Molokai, Maui and the Big Island.
Both the warning and advisory are good through 6 pm HST Monday.
This swell will bring seas in excess of 10 ft to exposed waters, so
an SCA has also been posted for many of the marine zones.

As one swell departs, another one is expected to arrive. Models are
predicting a storm low will form in the Gulf of Alaska later tonight
and Monday. This low will generate a large ocean swell toward the
islands, slated to impact the north facing shores on Wednesday. This
swell will have about the same magnitude as the current one,
although its period will be slightly longer, at around 15 seconds,
as compared to the current 10 to 13 seconds. Never- the- less, near
or at warning level surf is likely for Thanksgiving before subsiding
into the weekend.

In response to recent activity in the far South Pacific, small surf
is expected to continue along the south facing shores of the islands
through next week.

See the latest collaborative nearshore and swell forecast for Oahu
for more details on the surf at: www.prh.noaa.gov/hnl/pages/SRF.php

&&


.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Monday for North facing shores of
Niihau and Kauai.

High Surf Advisory until 6 PM HST Monday for North facing shores of
Oahu, Molokai, Maui and the Big Island.

High Wind Warning until 6 AM HST Monday for Big Island Summits.

Small Craft Advisory until 6 PM HST Monday for all waters except
Maalaea Bay and waters south and west of the Big Island.

&&

$$

Public...Birchard
Aviation...Bohlin
Marine...Lau

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office