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Hawaiian Islands Synoptic Discussion and Guidance

470
FXHW60 PHFO 191349
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Mon Nov 19 2018

.SYNOPSIS...
Breezy trade winds will continue through middle of the week,
delivering low clouds and showers mainly to windward areas,
especially during nights and mornings. High clouds over the southern
islands will gradually clear late today. Lighter winds are expected
for the second half of the week, with increased moisture potentially
leading to greater shower coverage.

&&

.DISCUSSION...
Breezy trade winds continue across the state early this morning,
driven by a strong high far north-northeast of the area. There is
quite an extensive field of low clouds along windward waters of the
islands as well as east of the area. These low clouds, carried by
the winds, will reach the area at times today, providing some
showers. Much of the showers will affect mainly windward areas,
though winds are strong enough to carry some low clouds and showers
to the lee areas of the smaller islands. Both early morning Lihue
and Hilo soundings revealed a drier and more stable airmass, thus
some of the low clouds will probably dissipate during the day. These
passing showers will provide measurable precipitation, though the
breezy trades will keep these showers moving at a good pace,
limiting the rainfall amounts for any location. Some high clouds are
still being drawn over the southern islands from the convection
south of the area, as a weakening upper level trough lingers in the
vicinity. Do expect the high clouds to clear the islands late today.

The aforementioned high will provide breezy trades to the islands
through early half of the week. Furthermore, as this high advances
southeast in the next few days, even though it is expected to weak
slightly, pressure gradient will tighten a bit across the area by
Tuesday night and Wednesday. Hence, expect the already breezy trades
to become locally strong during that time frame. Moisture associated
with a frontal boundary currently south of the high is expected to
advance south, reaching the islands by Wednesday. This will possibly
lead to an increase in trade showers for the area. The high will
continue to weaken and move further east toward the end of the week,
while a trough develops northwest of the islands. Latest GFS model
run still indicates a series of fronts may pass north of the islands
over the weekend into early next week. This will result in lighter
winds across the area from Thursday through early next week. There
are still differences between models upon how strong these
transitory systems passing north of the area, with GFS hinting
variable and somewhat light winds for the state while latest ECMWF
keeps light trades over the area. GFS is also a little more moist,
while ECMWF is rather dry. Regardless, looks like lighter winds are
in store for the islands toward end of the week into early next week.

&&

.AVIATION...
A strong high pressure system will track across the Central Pacific
basin for the next several days, maintaining breezy trade winds
across the Hawaiian Islands through Wednesday. Expect isolated to
scattered showers favoring windward and mountain areas. An upper
level subtropical jet over the eastern half of the state continues
to produce high clouds over the eastern islands this morning. The
upper level jet stream will diminish later today.

AIRMET Tango remains in effect for moderate turbulence across all
islands. Breezy trade winds will cause low level mountain wave
turbulence over and immediately south through west of mountains on
all islands from SFC-080 for the next few days. The upper level jet
stream will also produce moderate turbulence aloft between FL200-350
through at least tonight, diminishing through the day today.

&&

.MARINE...
Fresh to strong trade winds are forecast to continue through
Tuesday. The overnight ASCAT pass found areas of 25 kt winds south
of Oahu and Kauai, and high-resolution model guidance suggests that
similar winds will continue around those islands today and tonight,
so we have expanded the Small Craft Advisory to include some
additional marine zones. The SCA also remains in effect for the
typically windy zones around Maui County and the Big Island through
Tuesday. Surf along east facing shores will remain rough and choppy
over the next couple of days due to the winds, however surf heights
should remain below advisory levels as the upstream fetch will not
likely be long enough.

Trade wind speeds (and east shore surf) will increase further
beginning Tuesday night as the high to the north strengthens and
digs closer to the island chain. Winds will likely be strong to near
gale over exposed coastal waters, potentially reaching gale force
over the Alenuihaha Channel Tuesday night and Wednesday. SCA will
likely be expanded into additional coastal zones during that time.
Later shifts may also need to issue a Gale Watch for the Alenuihaha
Channel. East shore surf could also potentially approach or reach
advisory levels at the peak of the wind speeds on Wednesday. Winds
will taper off and veer east-southeast late in the work week as
the high shift northeast of the state in response to an approaching
trough from the northwest.

A moderate northwest swell will arrive late today, peaking tonight
into Tuesday, then fading through midweek. Surf should remain below
the advisory levels along north and west facing shores with this
swell. Small surf will continue along south facing shores with
mainly background southern Pacific pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Tuesday for Kauai Leeward
Waters-Kauai Channel-Oahu Leeward Waters-Kaiwi Channel-Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-
Big Island Southeast Waters.

&&

$$

DISCUSSION...Hui
AVIATION...Bohlin
MARINE...Jacobson

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



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