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Hawaiian Islands Synoptic Discussion and Guidance

722
FXHW60 PHFO 132010
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST Tue Nov 13 2018

.SYNOPSIS...
Locally breezy trade winds will deliver passing low clouds and a
few brief windward showers into tonight. Winds will weaken later
tonight and Wednesday, with mostly dry conditions prevailing.
Trade winds will strengthen again on Thursday and Friday, and
remain breezy into the weekend, likely bringing increased windward
showers. Rainfall may become more widespread over the weekend as
an upper-level low develops near the islands. Periods of high
clouds are expected the next couple of days, potentially becoming
quite thick at times.

&&

.DISCUSSION...
A ridge of high pressure is about 500 miles N of Honolulu, roughly
along 28-29N. This is providing gentle to locally breezy trade
winds across the state this morning. The morning soundings showed
the airmass is relatively dry, with a strong capping inversion
around 7500 ft. A relatively steep lapse rate below the inversion
and lingering convergence in the trades appears to be enough to
yield some small areas of moderate to locally heavy showers near
windward Kauai.

The showers near Kauai should diminish and push WNW of the islands
later today, with relatively quiet weather expected statewide this
afternoon. The ridge to the north will slowly weaken into tonight,
with gentle trades trending toward land breezes overnight. The background
flow will turn SE from Oahu to the Big Island, which could be
enough to bring a few clouds and a shower or two to the usual
windward sides overnight. Otherwise, dry air and land breezes will
yield mainly clear skies for a lot of the state by Wednesday
morning, and anticipating lowering sky cover for interior and
leeward areas with the afternoon package.

A col passing north of the islands on Wed will leave us in a light
pressure gradient. Relatively dry air with PWs at or below 1" and
warm mid-level temperatures should allow for a dry, rather hot
convective day with lots of sunshine and only limited afternoon
clouds over island interiors. Can't completely rule out a shower
or two but the vast majority will not see rainfall.

A fast-moving high pressure area over the NW Pacific will slide
toward the SE, and trades will gradually start to fill in as
early as Wed night. The high will slow down as it approaches the
area N of the islands late in the week, keeping breezy to locally
windy trades blowing into the weekend. At the same time, however,
a seasonably strong mid-level trough will be digging SE over the
islands, cutting off to an upper low just west of the islands by
the weekend. The models show increasingly cold air aloft helping
to enhance increasing moisture riding in on the trades, which
should make for a rather wet end of the week, especially for
windward and mauka sections, with at least some possibility for
localized downpours and heavy rainfall from about Friday into the
weekend. Current grids do not yet include heavy rainfall mention,
but will need to take a closer look at this as time goes on. The
weather should improve by Mon as the upper trough weakens and
shifts E of the islands.


&&

.AVIATION...
A typical trade wind pattern will persist today, with east-northeast
flow pushing low-topped stratocumulus clouds up against the
windward slopes and coasts. Isolated MVFR conditions are expected
in passing showers. Otherwise, VFR conditions will prevail.

High pressure to the northeast will continue to weaken and shift
eastward through tonight. The air mass over the Big Island will
become gradually less stable, allowing clouds and isolated
moderate showers to build over interior and leeward Big Island
late this afternoon and early evening.

&&

.MARINE...
A front will approach from the northwest today and the ridge north
of the islands will shift south and weaken. Trade winds will weaken
and wind speeds are expected to drop below Small Craft Advisory
(SCA) levels this evening. Winds will become light and variable
tomorrow. The front will dissipate Thursday as a new high builds
northwest of the area. Trade winds will strengthen, and a SCA
will likely be needed by Friday for the waters around the Big
Island and Maui.

As of 900 AM, a northwest swell had reached 9 and a half feet at
buoys 51001, 180 nm NW of Kauai. This swell should be big enough
to produce High Surf Advisory (HSA) level surf along northwest
facing shores of the smaller islands starting this afternoon and
continuing through tomorrow. As this swell subsides Thursday
night, a slightly bigger northwest swell will likely boost surf up
over the HSA threshold again. A smaller north swell will help
reinforce this decaying northwest swell over the weekend.

As trade winds strengthen late this week surf will build along
east facing shores, but will remain below the advisory threshold.
Surf will remain small along south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday for N and W facing
shores of Niihau and Kauai and N facing shores of Oahu, Molokai,
and Maui.

Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$
R Ballard/TS/Donaldson

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