Current Conditions
Temp-0.1 C
RH98 %
WindSW 37 mph
RoadClosed
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Hawaiian Islands Synoptic Discussion and Guidance

785
FXHW60 PHFO 190635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Thu Oct 18 2018

.SYNOPSIS...
A gentle to locally moderate trade winds will persist through the
next couple of days. An unstable air mass may settle over the
islands over the weekend as an upper level disturbance slowly
moves through the area. This will result in the potential for a
thunderstorm across most of the island chain beginning Friday
night. Trade winds will rebuild on Sunday and early next week,
though unstable conditions may still linger over portions of the
state.

&&

.DISCUSSION...
The Big Island had another active afternoon, more so than past
couple of days. Today's activity with scattered showers and a few
thunderstorms was not only occurred on the windward side near the
town of Volcano, bu the entire length of the Kau district, and on
the lee side of the island south of the junction at Saddle Road.
This weather on the Big Island could be attributed to lingering
moisture from a front that had passed through a few days ago, an
unstable air mass, and daytime heating. These ingredients will
once again act together Friday afternoon, with additional help
from an approaching upper level disturbance. So, more widespread
activity is possible.

The precipitable water value (PW) is highest over the Big Island
at 2 inches, and the GFS solution maintain this value to until
Saturday night, followed by a drop to 1.65 inches on Sunday. As
for the other islands, PW increases from a low of 1.25 inches to
between 1.30 and 1.50 inches tomorrow afternoon, which is pretty
seasonable for this time of the year. The GFS based relative
humidity cross-section of the island chain, suggest a low level
inversion is still active across the smaller islands but not so
for the Big Island. The GFS has this inversion at 8 to 10k feet
over Kauai and Maui, and 6 to 8k over Molokai and Oahu. The wind
forecast calls for a weak trade wind flow, that will be weak
enough, to support a daytime sea breeze, onshore flow for all of
the smaller islands on Friday, but especially Kauai.

Putting all these elements together, we can expect some scattered
interior/leeward showers for all islands but particularly Kauai
and Maui. The low level inversion over Oahu and Molokai will limit
these convective showers. This GFS solution has the low level
inversion weakening Friday afternoon across the smaller islands.
The upper level disturbance will be moving down the island chain
on Saturday with the height of the moisture ranging from 12k feet
over Kauai, sloping to 20k feet over Maui with the highest RH
below 8k feet. The low level inversion remains weak over the
smaller islands through Sunday, with moisture extending to between
12k and 18k between Kauai and Maui. Also, the northeast trades
are expected to gain strength to moderate speed on Friday
night/Saturday, with further strengthening to locally strong by
early Monday morning. So, what we may be encountering, is some
elevated/enhanced trade showers Saturday through at least Sunday
night. The RH profile over Big Island on Sunday suggest the
unstable air mass to continue there. But may be some good news to
come.

The new GFS and the old ECMWF (EC) solutions differs thereafter,
with the old EC stalling the upper trough over Maui County,
and pushing it westward through Tuesday, while the GFS is more
progressive and nudging the upper trough east of the Big Island
Sunday night/early Monday morning, and bringing in a mid level
ridge over the islands. If the GFS holds true, it will finally
put an end to a period of unsettled weather for the Big Island.

The current forecast calls for a slight chance of a thunderstorm
for the smaller islands starting Friday night. Given the
assessment above, it is too close a call and will continue to be a
part of the forecast package.

&&

.AVIATION...
Light to occasionally moderate trade winds will hold through
Friday. As has been the case, brief to TEMPO 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. And a slight chance of thunderstorms
will exist Friday afternoon for the Island of Hawaii as well.

As per weather radar, the showers appear to have diminished over
the windward areas of Maui and the Big Island, where there is
currently an AIRMET Sierra for mountain obscuration. This AIRMET
will be lowered at the next update at 10z or sooner should the
conditions hold.

&&

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

Light to locally moderate trade winds are expected into Friday,
with a gradual increase in NE trade winds Friday night and
Saturday as high pressure builds to the NW of the islands. Winds
should remain below Small Craft Advisory (SCA) speeds through
Saturday into 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.
&&

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

&&

$$

Lau/Kinel/Birchard

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