High Surf in Hawaii, November 6 to 12, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Sun Nov 6 2016

.SYNOPSIS...
A remnant front will bring a band of clouds and showers across
the smaller islands on Monday before dissipating near the Big
Island on Tuesday. A surface ridge setting up closer to the state
will lead to a light wind pattern by the middle of the week, which
will continue into next weekend.

&&

.DISCUSSION...
Water vapor imagery shows a mid and upper level ridge just north
of the state, extending from a high far to the west-northwest. At
the surface, a cold front is located several hundred miles north-
northwest of the state. A front dissipated closer to the state,
leaving behind a cloud band over the northern offshore waters. 12z
soundings show inversions around 5kft and precipitable waters
around 1.0 inches, which is about one standard deviation below
normal for this time of year. MIMIC total precipitable water
imagery from this morning shows similar values across the area,
with pockets of drier air mixed in as well. A band of slightly
higher values near 1.25 inches is located over the northern
offshore waters, with values over 1.5 inches pooled along the
front farther to the north-northwest.

A narrow band of low clouds extends northeast from Oahu, with a
couple other bands offshore to the northeast of Maui and southeast
of the Big Island. Rainfall was limited overnight, with only a
handful of gages on Oahu and the Big Island picking up a couple
hundredths over the past 12 hours. The pressure gradient across
the state is weak, with a col just to the north and a surface
ridge farther to the northeast. Land breezes developed at many
locations overnight, and we will see some localized sea breezes
today. High-resolution models show an increasing north-northeast
wind potentially affecting us late this afternoon, which may
focus afternoon clouds across leeward and southern sections of the
islands.

The increasing north-northeast winds late this afternoon/tonight
precede the main cold front. Patchy moisture left behind by the
dissipated front over the northern offshore waters may bring some
showers tonight, but the main moisture associated with the front
should reach Kauai early Monday morning. Recent models have been
trending slightly slower bringing the front down the island
chain, now stalling it near Maui/Big Island later Monday night.
The atmosphere remains capped by an inversion, though forecast
soundings/cross sections show inversion heights/moisture depth
increasing by a few thousand feet. While deeper convection is not
anticipated, low-levels become more saturated and we will see an
increase in shower coverage and overall rainfall chances.

As the front dissipates near Maui/Big Island on Tuesday, it will
leave a cloud band that may linger into early Wednesday. High
pressure building in behind the front, which helped to drive it
southward, is expected to stall and weaken as well, leaving a
surface ridge over the northern offshore waters. This ridge is
expected to shift southward across the state during the middle of
the week as another front approaches from the northwest. This will
lead to a dry convective pattern, with onshore sea breezes and
offshore land breezes but little in the way of rainfall due to the
dry and stable conditions. There are still differences in the
extended, although the GFS and ECMWF have both been trending
farther north with the next front Thursday/Friday. While specifics
are hard to pin down, there is better agreement that we will see a
continued break from trade winds through the weekend and a
potential transition to a more showery pattern.

&&

.AVIATION...
Light ENE trade winds today will back to the NNE by tonight, with
strengthening NE winds expected Monday as high pressure builds to
the NW of the area. While VFR conditions will prevail into
tonight, the light winds will deliver a few clouds to windward
slopes and coasts, with ISOL MVFR VIS/CIG in -SHRA expected. No
AIRMETs are in effect, and none are expected into tonight.

The building high will push a weakening and relatively shallow
frontal boundary over the islands from the N beginning early
Monday, leading to an increase in showery low clouds over N
through E facing slopes and coasts, and potentially warranting an
AIRMET for mountain obscuration. Increasing low-level NE winds on
Monday will likely warrant the issuance of AIRMET TANGO for
moderate turbulence S through W of island terrain.

&&

.MARINE...
The current NNW swell will gradually diminish through tonight. A
very large NNW swell arriving Monday is expected to produce
warning level surf along the north and west facing shores of most
islands for Monday and Tuesday. This swell will gradually subside
from late Tuesday into Wednesday before a large NNW swell arrives
late Wednesday night and continues into Friday.

Trade wind speeds weakened overnight, and there are no Small
Craft Advisories (SCA) currently in effect. A SCA will be
necessary for most waters Monday as seas increase due to the NNW
swell and strengthening northeasterly trade winds associated with
a frontal band.

The large NNW swell will impact harbors exposed to northwest and
north swells with the potential for harbor surges and large
breaking waves at harbor entrances.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

Discussion...Bravender
Aviation...Birchard
Marine...Morrison