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Hawaiian Islands Synoptic Discussion and Guidance

023
FXHW60 PHFO 232003
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Mon Apr 23 2018

.SYNOPSIS...
High pressure passing north of the islands today will bring
locally windy trades over the state through Tuesday. Areas of
moisture riding in on the trades will bring increased clouds and
showers to some windward and mauka areas at times this week. Trade
wind speeds will gradually decline during the latter half of the
week as low pressure develops to our north. A cold front will
start to push down the chain later Friday and Friday night, and
may stall with wet weather over the islands this weekend.

&&

.DISCUSSION...
A surface high will pass about 900 mi N of the islands today,
boosting trade winds to locally windy speeds through Tue. The high
will stall roughly 1200 mi NNE of the islands through midweek,
far enough away to allow the trades to moderate just a bit. A
surface low will develop roughly 850 mi to the N of the islands
Thu night, disrupting the trades and leading to land breezes. The
low will sharpen a cold front near Kauai on Friday. Unfortunately,
the models have been trending much slower with this front over
the last day or so (a trend started by the 18z GFS run on Sunday),
now showing it hanging up near Maui and the Big Island on Sat and
leaving it there through the weekend. The brand new 12z ECMWF run
is also in this camp. This will probably allow locally breezy N
or NW winds to overspread the northern main islands, with lighter
winds elsewhere through the remainder of the weekend.

Trade wind rainfall will increase a bit today as an area of
clouds and showers moves across Maui and the Big Island. Another
area of clouds and showers riding in on the trades will push
across the islands from E to W late tonight and Tue, followed by
yet another for Thu before the low level flow begins to veer ahead
of the next front.

A slower front stalling over the islands has significant impacts
for the weekend forecast. For now, the models are trending toward
a rather ominous solution with very deep tropical moisture
(precipitable water in excess of 2.1 inches) intersecting the
frontal boundary. There will be more than sufficient cold air
aloft and a low level focusing mechanism to generate excessive
rainfall and heavy thunderstorms. The latest guidance suggests the
highest risk extends from about Oahu to the Big Island. However,
model placement and trends this far out (despite good agreement)
are often suspect, and it's very possible that the models could
switch gears once again, with the front in a completely different
place by the weekend. It seems prudent for the afternoon package
to raise PoPs for the southern half of the chain, and continue to
carefully monitor guidance trends.

&&

.AVIATION...
Expect mainly VFR conditions through the forecast period with
scattered showers along north and east slopes of all islands.
Moderate to strong trade winds will produce low level mechanical
turbulence over and downstream of the mountains. An increase in
windward showers is expected tonight as enhanced moisture pushes
into the windward slopes.

&&

.MARINE...
Fresh to strong trade winds will pick up slightly tonight and hold
into Tuesday as strong high pressure currently far north of the
state moves off to the northeast. A Small Craft Advisory (SCA)
currently covers most waters from Oahu to the Big Island and
expands to all Hawaiian waters tonight and Tuesday. Winds will
gradually ease on Wednesday and Thursday as the ridge north of the
state is weakened. The SCA will likely be trimmed back to the
usual windy areas around the Big Island and Maui, then be dropped
completely on Friday as a front moves over the state from the
northwest. Uncertainty in the position of the front means low
confidence in the wind forecast for the weekend.

A pair of moderate northwest swells are due during the next
several days, with considerable uncertainty for the weekend. The
current swell is peaking at 4 to 6 ft at 14 to 15 seconds
according to NOAA and PacIOOS buoys. This swell will drop to 3 to
4 ft at 12 seconds on Tuesday, then fade on Wednesday. A
similarly sized swell will build on Thursday and peak on Friday.
Uncertainty increases in the swell forecast for the weekend. The
GFS and ECMWF are showing a gale low forming north of the state
between 30N and 40N, but the models lack agreement in the exact
position and strength of the low. There is the potential for a
large north-northwest swell to produce surf well above advisory
levels.

Rough, moderate surf will hold along east facing shores during the
next few days. The Mokapu and Hilo PacIOOS buoys currently show
short-period, trade wind generated seas of 6 to 7 ft at 7 to 9
seconds. Seas should increase slightly tonight and Tuesday, but
resulting surf along east facing shores should remain below the
advisory level. East shore surf will gradually decline Thursday
and Friday.

Small to occasionally moderate surf is expected along south
facing shores this week. Inconsistent southwest swell will persist
during the next few of days. A moderate south-southwest swell is
expected this weekend into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Wednesday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST
Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai
Leeward Waters-Kauai Channel-Oahu Windward Waters-Big Island
Windward Waters.

&&

$$
R Ballard/Morrison/Wroe

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



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