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Hawaiian Islands Synoptic Discussion and Guidance

457
FXHW60 PHFO 180625
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Thu Oct 17 2019

.SYNOPSIS...
High pressure north-northeast of the state will keep moderate to
locally breezy trade winds in place through Saturday, with showers
favoring windward and mauka areas and a stray shower reaching
leeward locales from time to time. The trades will ease briefly
Saturday night and Sunday, with an increase in shower activity
expected Sunday night and Monday and the trades strengthen once
again. Typical trade wind weather featuring windward and mauka
showers and a stray leeward spillover will then return Tuesday
through the middle of next week.

&&

.DISCUSSION...
Currently at the surface, a trough of low pressure is located
around 650 miles west of Kauai, while a 1028 mb high is centered
around 1000 miles north-northeast of Honolulu. The resulting
gradient is producing moderate trade winds across the island
chain this evening. Infrared satellite imagery shows partly to
mostly cloudy skies in windward areas, with partly cloudy
conditions in most leeward locales. Radar imagery shows scattered
showers moving into windward areas, with a few showers reaching
leeward communities at times as well. Main short term concerns
revolve around trade wind trends and rain chances during the next
few days.

High pressure north-northeast of the state will keep moderate to
locally breezy trade winds in place through Saturday. A weakening
cold front will approach from the northwest Saturday night and
Sunday, easing the trade winds briefly and shifting them around to
more of an east-southeasterly direction. As the front dissipates
well northwest of the islands and high pressure strengthens to the
northeast, we should see the trades increase again Monday and
Tuesday, before easing again during the middle of next week as a
new front approaches from the northwest.

A fairly dry trade wind pattern should prevail through the
weekend, with showers favoring windward and mauka areas and a
stray shower reaching leeward sections from time to time. We will
likely see an increase in shower activity Sunday night and Monday
as a slightly higher precipitable water airmass moves in and has
some speed convergence to work with under the strengthening trade
wind regime. A fairly typical trade wind pattern should then
follow Tuesday through the middle of next week, with mainly
windward and mauka showers and the occasional leeward spillover.

&&

.AVIATION...
Moderate to locally breezy trade wind flow will reestablish
itself across the islands by Friday afternoon. Clouds and
scattered showers will become focused across windward slopes and
coasts by morning. Leeward coasts will experience partial clearing
overnight. Cumulus buildups are not expected to redevelop over
the interiors of the smaller islands, but afternoon sea breezes
across leeward Big Island will promote inland showers again on
Friday.

No AIRMETs are in effect or anticipated at this time. Brief MVFR
ceilings and visibilities are to be expected in showers, otherwise
VFR conditions will prevail all areas.

&&

.MARINE...
Fresh to strong easterly trade winds associated with high pressure
to the north-northeast of the state will continue into Saturday.
Winds are forecast to shift out of the east-southeast late Saturday
and Saturday night as a front passes far to the north. If this
scenario materializes, light and variable winds will become a
possibility as early as Saturday night over and around the Kauai
and Oahu waters. For the eastern end of the island chain, moderate
to fresh east-southeast winds are expected through this time.
This pattern may hold into early next week before the flow shifts
back out of a more typical eastern trade wind direction.

Surf along north facing shores will hold steady through Friday due
to a mix of a west-northwest swell and a northerly swell moving
through. Observations at the nearshore PacIOOS buoys at Waimea and
Pauwela were running higher (around a foot) than predicted this
evening, mainly due to the northerly source. The offshore buoys to
the northwest have peaked and should begin to slowly trend down late
tonight through Friday. This will translate to a downward trend in
surf over the weekend. A small long-period north-northwest swell
expected Saturday should be enough to keep the surf from going flat
up north at the spots that favor this direction. No other long-
period sources are anticipated Sunday through early next week.

For the long range, guidance does depict a gale low developing near
the Kurils Saturday night into Sunday, then tracking eastward over
the far northwest Pacific toward the Date Line Sunday into Monday.
If this evolves as predicted, a northwest swell associated with it
will become a possibility locally late next week.

Surf along south facing shores may trend up over the weekend as long-
period south swell fills in from a recent storm-force low that
developed far southeast of New Zealand last weekend. This swell
will peak over the weekend, then fade into early next week.

Surf along east facing shores will remain rough each day due to
the gusty onshore trades. Conditions may improve for the east
facing shores of Oahu and Kauai by Sunday as the trades relax
locally.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bedal
MARINE...Gibbs

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



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