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Hawaiian Islands Synoptic Discussion and Guidance

707
FXHW60 PHFO 211330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Sat Apr 21 2018

.SYNOPSIS...
Breezy trade wind weather is expected over the weekend and into
next week, with clouds and showers focused over windward areas.
Some of these showers will blow over to leeward locations on the
smaller islands, while the Kona side of the Big Island will see
afternoon clouds and showers. Trades will strengthen to locally
windy levels next week.

&&

.DISCUSSION...
A 1032 mb high near 40N135W, far northeast of the main Hawaiian
islands is driving locally strong trade winds across the state
this morning. Radar shows scant shower activity upwind of the
islands, but satellite loop shows broken to overcast middle and
high clouds lingering across Maui county and the Big Island.
Overnight soundings showed PW falling below 1.3 inches with
increasing stability.

Models show the high pressure to the north will keep trade winds
on the breezy side through the middle of next week, gradually
gaining strength and spreading westward over the weekend and into
next week. Clouds and showers brought in by the trades will favor
windward and mauka areas, but leeward areas may get wet from time
to time as well as winds increase. Models show the layered mid and
upper level clouds may linger across the eastern half of the
state into next week, possibly drifting south of the islands
Tuesday. We have increased sky cover in the gridded database to
account for this. Clearer weather is then expected through the
rest of the work week. GFS shows a new low forming north of the
state next weekend, disrupting trade flow. However, ECMWF develops
this low much farther north with far less trade flow disruption.
We will follow the ECMWF scenario for now.

&&

.AVIATION...
A ridge of high pressure to the north of the state will keep a
moderate to breezy trade wind flow in place through tonight.
Low clouds and showers will favor windward and mauka areas, with
occasional MVFR cigs/vsbys expected. A few showers will also spill
leeward from time to time, but predominantly VFR conditions should
prevail.

AIRMET Sierra is in effect for mountain obscuration over windward
sections of Maui and the Big Island. Conditions will likely
remain in place through much of the day. The AIRMET may need to be
expanded to include Molokai and Oahu later this morning, and Kauai
this afternoon.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. These conditions
are expected to continue through tonight. AIRMET Tango also
remains in effect for moderate upper level turbulence between
FL250 and FL450. These conditions will likely continue through
most of the weekend.

&&

.MARINE...
A surface high northeast of the area will move northeast through
the weekend. This high continues to produce locally strong trade
winds over Hawaiian waters. A Small Craft Advisory (SCA) continues
for strong trade winds in the windier areas near Maui and the Big
Island. A new high building northwest of the area will cause the
trade winds to strengthen further this weekend. The SCA will
likely be expanded starting tonight for waters around Lanai,
Molokai and Oahu.

Surf is expected to remain below High Surf Advisory (HSA) thresholds
along all shores through this weekend. Surf along east facing
shores will remain rough due to the trade winds, but heights will
remain moderate. There will be some increase in surf heights
along east facing shores next week as trade winds strengthen.

A small northwest swell will peak today. A slightly larger swell
is expected to arrive Sunday evening and peak Monday. Another
moderate swell is possible next Thursday.

Surf along south facing shores will remain small through early
next week. Lows in the southern hemisphere may strengthen and
shift east enough to produce some bigger surf along south facing
shores by the end of next week.

See the Collaborative Surf Discussion for more information.

&&

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

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Maui County Leeward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Jelsema
MARINE...Donaldson

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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