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Hawaiian Islands Synoptic Discussion and Guidance

735
FXHW60 PHFO 210713
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
913 PM HST Sun Aug 20 2017

.SYNOPSIS...
Light to moderate trade winds combined with increasing moisture will
support increasing rainfall chances and humid conditions through
Tuesday over the islands. Additionally, a few thunderstorms will
remain possible through this time due to an upper disturbance over
the region. Drier and windier conditions will return Wednesday
through the second half of the week as the ridge strengthens to our
north and the upper disturbance shifts away from the region.

&&

.DISCUSSION...
Heavy showers and thunderstorms that developed over the Big Island
today have diminished this evening. Peak rainfall accumulations over
the past six hours have reached the 2 to 4 inch range from this
activity over portions of the Hilo, Puna, and Kona districts.
Additionally, sleet and snowfall were reported on the summits near
the Visitor Information Station earlier this evening. The latest
forecast has been updated to reflect the latest trends and
observations over the Big Island. Elsewhere, the forecast remains
on track with no additional updates anticipated this evening.

The short-term model guidance has initialized well with the current
pattern and indicates warm and humid conditions with light to
moderate trade winds continuing through Tuesday. Model
precipitable water (PW) output reflects this and depicts an
increasing trend across the islands with values reaching the
1.7-2" mark by Tuesday (above average for this time of year). A
combination of this increasing moisture and upper height falls
associated with a broad upper low drifting southwest across the
region will support a few thunderstorms. The best instability will
remain over the smaller northwest islands, where decent mid-level
lapse rates and cooler upper-level temperatures will be.
Localized flooding will remain a concern each day, mainly where
the heavy showers become focused. Although the bulk of the
rainfall coverage will focus over windward and mauka areas, some
of this activity will setup over the leeward areas through the
afternoon and early evening periods where sea breezes are
expected.

For the extended period, the GFS and ECMWF both depict breezy trades
and drier conditions returning across the state beginning Wednesday
as the upper low shifts away from the region and high pressure
builds north of the state. Model PWs support this and dip back down
to below average for this time of year (near 1").

&&

.AVIATION...
Surface high pressure north of the area will maintain moderate
east northeast surface winds across the area. The east northeast
winds will carry very moist air over the area. A low aloft 240 nm
north of Kauai is forecast to move south southwest over the next
24 hours, then move off to the northwest. The low aloft will make
the atmosphere unstable.

AIRMET SIERRA is currently in effect for MTN OBSC on the Big
Island. Conditions are improving there and it may be possible to
cancel the AIRMET. Later tonight, AIRMETs for MTN OBSC may be
needed for the other islands, especially for northeast sections.
Thunderstorms are expected overnight, with the greatest chance
over waters around Kauai. Thunderstorms may develop again tomorrow
overland as the islands heat up. VFR conditions prevail this
evening, but TEMPO MVFR and ISOL IFR conditions are expected
overnight, with the greatest change on Kauai.

&&

.MARINE...
Winds and seas are expected to remain in the light to moderate
range through Tuesday, then strengthen into the fresh to locally
strong category Wednesday through the remainder of the week.
Increasing shower coverage will remain in the forecast through
Tuesday as an upper trough drifts southwestward over the region.
This upper trough will also support a slight chance of
thunderstorms through Tuesday. Drier conditions will return
Wednesday through the second half of the week.

Surf will remain generally small all around. Small background
long-period pulses from the south-southwest (210-220 deg) and
south-southeast (130-160 deg) will keep the surf from being flat
along south facing shores this week. A slightly larger south-
southwest (220 deg) swell will be possible this coming weekend due
to a gale over the Tasman Sea.

Surf along east facing shore will remain generally small through
the first half of the week, then build through the second half as
stronger trades return. Additionally, a long-period swell
associated with tropical activity across the eastern Pacific
will be possible Wednesday through Friday, especially along the
exposed beaches of the Big Island.

Water levels continue to run around a foot above predicted, which
led to a preliminary record being set at the Honolulu station
today (3.28 ft above MLLW). This measurement today exceeds the
peak water levels measured through the summer months. These high
water levels are in response to a combination of an oceanic eddy
moving through from east to west and the regular King Tide cycles.
Water levels will begin to slowly lower each day through the
week. Please refer to the Special Weather Statement for more
details on impacts associated with these higher water levels.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Donaldson
MARINE...Powell

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