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Hawaiian Islands Synoptic Discussion and Guidance

261
FXHW60 PHFO 140206
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
406 PM HST Fri Dec 13 2019

.SYNOPSIS...
Surface high pressure passing north of the islands tonight will
keep breezy to locally windy trades blowing tonight. The trades
will ease some over the weekend as the high moves away. The
trades will bring rainfall to mostly windward and mauka locales,
with a few showers briefly reaching leeward spots. The high level
jet stream will bring increasing high clouds later today into
tonight. A dry, stable airmass will take control of our weather
this weekend and the first half of next week, with few showers and
considerable sunshine. The next high passing far to the north
will return locally windy trades Monday and Tuesday.

&&

.DISCUSSION...
A surface trough is moving away to the west of the islands.
Meanwhile, high pressure passing about 800 mi to the N of
Honolulu is bringing locally windy conditions to the islands this
afternoon. This high will continue to the east over the next
couple of days, so trade winds will trend down just a bit over the
weekend.

An mid-to-upper level trough to the west of the islands has been
making the local airmass more unstable over the last few days and
enhancing trade wind showers. This trough will continue to weaken
as we head into the weekend, leading to fewer and less intense
trade showers. High level moisture riding in on the subtropical
jet will continue to bring high clouds at times, and these will
increase tonight into Saturday before pushing away later Saturday
as the jet shifts to the SE of the islands.

Trade wind showers will decrease further early next week, as
decreasing moisture and warm mid-level temperatures overspread
the islands. Locally, precipitable water may drop below 1 inch
with 700 mb temperatures near double-digits. A strong mid-level
anticyclone centered just to our N will make for generally nice
pre-holiday weather with lots of sunshine and relatively few
showers. Rainfall amounts will be light, and should favor
windward and mauka sections during the nights and mornings.
Showers will likely increase again starting about Thursday as mid-
level heights fall near the islands and cooler temperatures aloft
start to make the airmass a little more unstable again.

The surface high to the N will continue eastward into the weekend,
so trade wind speeds on Saturday will be a notch below today's
levels. Winds will even be a notch lower on Sunday as a col
between two highs passes N of the islands. The next surface high
will pass well N of the islands on Monday, returning breezy to
locally windy trade wind speeds for Mon and Tue. The winds should
ease Wed, but may increase again on Thu as the next in the series
of highs passing to the N approaches our longitude.

&&

.AVIATION...
An area of enhanced moisture will continue to reside over the
islands late this afternoon in association with a dissipated
front. Showers will remain over mainly interior/windward areas of
Oahu and Kauai as well as windward areas of Molokai, Maui and the
Big Island. Brief MVFR conditions and mountain obscuration may
occur due to the passing showers.

Surface high pressure to the northeast of the state will allow
for moderate to occasionally breezy easterly trades to persist
through the period. AIRMET TANGO will remain in effect for areas
below 9000 feet south thru west of the mountains on all islands
due to tempo moderate turbulence.

&&

.MARINE...
The current northwest swell will continue to subside and the High
Surf Advisory has been cancelled for all islands. Surf tonight
will hold just below the advisory level along north and west
facing shores, then lower to moderate levels Saturday. A short
period north- northeast swell will keep surf elevated along
exposed east facing shores, mainly on Kauai and Oahu, tonight but
should stay just below the advisory levels.

A reinforcing large northwest swell is forecast to build late
Sunday into Monday and peak Monday night, possibly near warning
levels. This swell should slowly decline Tuesday into Wednesday,
but will likely be reinforced by another large northwest swell
Wednesday night into Thursday.

High pressure north of the state will keep moderate to locally
strong trade winds over the area through Saturday. The combination
of the current northwest swell and the strong trade winds is
resulting in a Small Craft Advisory for all coastal waters, except
the Maui leeward waters. Winds will begin to trend down slightly,
likely only keeping small craft winds through the typically
windier areas around Maui County and the Big Island Saturday.
Seas will also lower as the northwest swell subsides.

A slight decline in trade winds and period without Small Craft
conditions is possible Sunday as the current high moves east and
a new high builds north. Trade winds should return to moderate to
locally strong speeds by early next week and seas will build
again with the reinforcing northwest swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Saturday for all Hawaiian
Coastal Waters except Maui Leeward Waters.

&&

$$

DISCUSSION...R Ballard
AVIATION...Thomas
MARINE...EATON

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

Current Conditions
Temp		1.1 C
RH		17 %
Wind		NNE 21 mph
Road		Closed