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Hawaiian Islands Synoptic Discussion and Guidance

281
FXHW60 PHFO 191423 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
423 AM HST Fri Oct 19 2018

.UPDATE...
The zones were updated to include thunderstorms for the 2 Big
Island summits this afternoon.

&&

.SYNOPSIS...
A rather light trade wind flow will persist through today before
a gradual strengthening takes hold tonight through Saturday. A
strong upper level disturbance will bring some unsettled weather
to most of the islands over the weekend, but especially targeting
the Big Island. This unsettled trade wind regime may linger into
next week.

&&

.DISCUSSION...
The Kona area of the Big Island had an active evening with heavy
showers and thunderstorms, resulting in rainfall totals of up to
1.16 inches recorded at Kona Airport. Some of the blow-offs
spilled over to the summits of Mauna Kea and Mauna Loa with a
light dusting of snow at the higher elevations.

No real change to the reasoning behind the forecast. After the
overnight showers and thunderstorms in the Kailua-Kona area, we
suspect the weather will get more active especially over the Big
Island this afternoon and evening. Thus, we dished out a Flash
Flood Watch for the Big Island that is good through 4 pm HST
Saturday afternoon.

The approaching upper level disturbance will deepen the already
establish upper trough over the islands. The deepened upper trough
will moving eastward tonight starting from Kauai. In doing so,
the weather will be getting rather active area-wide with heavy
showers and a outside chance of a thunderstorm just about every
where across the main Hawaiian Islands. The upper trough pinches
off an upper low over the Big Island Saturday night and stays
there through Monday. This is per the latest 06Z GFS run, which
is now siding with the ECMWF (EC) solution. The new EC run still
maintain its stance of an upper low over the Big Island through
Monday before moving westward to Kauai by Wednesday. The GFS
outlook is much friendly too. The upper low/trough continues to
linger around the Big Island or the eastern part of the island
chain, including Maui, through at least Wednesday.

To probably make things look more bleak, the trades could be
carrying some tropical moisture to the islands toward the middle
of next week with the Big Island getting the brunt of the weather.

Currently, the smaller islands are in O.K. shape with some minor
trade wind showers. Multilayered clouds cover all the islands but
Kauai. The layered clouds are also passing through the Big Island
from the southwest, grazing the very top of the two summits at
13k feet in the process. There are a couple of thunderstorms, one
located just beyond the coastal waters some 50 mile out from the
Kona coast. A couple of thunderstorms are noted 30 miles east of
Cape Kumukahi of the Big Island. Radar is just beginning to pick
up higher tops from showers elsewhere outside the Big Island
premise, a 25k foot top 20 miles south of Hawaii Kai Oahu.

&&

.AVIATION...
Light trade winds will hold through the day and into tonight.
There will also be periods of daytime sea breezes and nighttime
land breezes because of the weak overall flow. Brief MVFR ceilings
and/or visibilities will mainly affect windward and mauka
sections around the island chain, especially the Big Island where
remnant moisture from an old front lingers. Too, statewide
afternoon interior and leeward showers are possible when seas
breezes become established. And a slight chance of thunderstorms
will exist around the Island of Hawaii as well.

No AIRMETs are currently in effect, though mountain obscurations
are possible with afternoon cloud buildups and showers.

&&

.MARINE...
The arrival of a significant long-period S swell is anticipated
today and tonight, with surf along S facing shores of all islands
building to advisory levels by Saturday. A High Surf Advisory has
been issued, with the swell expected to remain elevated for several
days. Peak surf heights with this long-lived swell could approach
the south shore High Surf Warning threshold (15 feet) over the
weekend, and buoy observations will be monitored to fine tune the
forecast. Elsewhere, surf heights will remain below advisory
levels for at least the next several days, although a new short-
period NNW swell is expected today. See the Oahu Surf Discussion
(SRDHFO) for additional details.

Light to locally moderate trade winds are expected through tonight,
with a gradual increase in NE trade winds Saturday as high pressure
builds NW of the islands. Winds should remain below Small Craft
Advisory (SCA) speeds through Sunday, with moderate to locally fresh
trade winds potentially requiring a SCA for the windier zones
early to mid next week. Uncertainty with this portion of the
forecast is due to forecast models increasing the low-level
pressure gradient as a tropical disturbance passes S of the
islands.

A persistent upper-level trough over the area will develop a cut-
off low this weekend, maintaining unstable conditions that will
bring the threat of locally heavy showers and/or thunderstorms,
especially over Big Island waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 PM this evening to 6 PM HST Monday for
Niihau-Kauai Windward-Kauai Leeward-Oahu South Shore-Waianae
Coast-Molokai-Lanai Makai-Kahoolawe-Maui Leeward West-Maui
Central Valley-Windward Haleakala-Leeward Haleakala-Kona-South
Big Island-Big Island North and East-Kohala.

Flash Flood Watch through Saturday afternoon for Big Island.

&&

$$
Lau/Kinel/Birchard

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