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Temp7.5 C
RH19 %
WindNE 6 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

270
FXHW60 PHFO 221413 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
413 AM HST Sat Sep 22 2018

.SYNOPSIS...
High pressure to the north and ridging aloft will maintain a
stable trade wind flow through Sunday with clouds and showers
favoring the windward and mountain areas. A surface low forming
northwest of Kauai in the next 24 hours will cause the winds to
blow out of the southeast and south starting Sunday. This flow
will bring in not only some humid weather but the potential for
some unsettled weather to the western end of the island chain that
may last through the first half of the new week.

&&

.DISCUSSION...
The trade winds are pretty much back, although there is slight
south component over Oahu and Kauai. This flow will be weakening
slightly as the day progresses. In the way of showers, radar is
pretty quiet, only a few specks around, favoring mainly the
windward and mountain areas. Earlier in the evening, there were a
few showers laid out across the leeward waters, mainly in the form
of plumes coming off Molokai, Lanai, and the Big Island. Most of
these showers have dissipated, but there is the potential that
they will form again later today, and a shower or two may easily
stray onshore. A northeast to southwest oriented band of showers
poses a threat for especially the Puna district of the Big Island.
The GFS solution suggest that most of the showers from the band
will pass east and south of the island. We do have a stable air
mass, thanks to an upper level ridge, which created a low level
inversion of between 5 and 7k feet area-wide. This inversion will
help limit these showers through at least Sunday.

The latest GFS run still maintain its stance with the wet and
unsettled weather commencing over Kauai on Monday afternoon or
Monday night. The latest ECMWF run, meanwhile, still maintain its
slower solution of onset not until Tuesday night. The differences
the models have will be sorted out in the next 24 to 48 hours,
but we will be monitoring the situation closely. Other than the
rain, the humidity will rise under the south and southeast winds.
Dew points are expected to rise into the mid-70s by Tuesday.

Currently, the source of the upcoming unsettled weather, an upper
low, is located roughly 770 miles NW of Kauai with a trough
extending southwestward. At the surface, is an inverted trough, a
reflecting of the trough aloft, is located 400 miles west of
Kauai. An extensive broken to overcast layer of mid and high clouds,
stretched eastward from the trough to the main Hawaiian Islands.
The GFS thins out part of this overcast layer over the eastern
half of the island chain later today. Thunderstorms are noted
some 200 miles west of Kauai. For the second half of the week,
we anticipate strengthening high pressure across the area with the
trough being nudged away from the islands resulting in the return
of the trade winds.

&&

.AVIATION...
High pressure northeast of the state will keep a light to moderate
trade wind flow in place through tonight. Low clouds and light
showers will favor windward and mauka areas, with some brief MVFR
cigs/vsbys possible here mainly during the early morning and overnight
hours. Predominantly VFR conditions are expected at the TAF sites
through 23/12Z.

No AIRMETs are in effect and none are expected today.


&&

.MARINE...
High pressure northeast of the state will keep a moderate to
locally strong trade wind flow in place through today. As a
result, a Small Craft Advisory (SCA) remains in place for the
typically windy waters around Maui County and the Big Island
through this afternoon. An area of low pressure several hundred
miles northwest of Kauai will gradually move east over the
weekend, allowing the winds to weaken and shift to the southeast
Sunday through middle of next week.

Surf will remain below advisory levels along all shores through the
middle of next week. Rough, short-period surf will remain around
the summer average along east facing shores through today, then
decline Sunday into early next week. A north-northwest is
expected late next week, which may bring near advisory level surf
along north facing shores. Background south swells will keep surf
below average along south facing shores through middle of next
week. A small southwest swell is possible late next week.


&&

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

&&

$$

H Lau/Jelsema/Kino

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