Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 221945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Sun Apr 22 2018

.SYNOPSIS...
Quiet trade wind weather is expected to finish out the weekend
with plenty of sunshine for most areas. High pressure passing
north of the islands on Monday will give us a bit of an uptick in
trade wind speeds. Areas of moisture riding in on the trades will
give increased clouds and showers to some windward and mauka
areas on Tuesday, and again later Wednesday into Wednesday night.
Trade wind speeds will gradually decline during the latter half of
the week as low pressure develops to our north. A cold front may
push down the chain next weekend.

&&

.DISCUSSION...
Quiet trade wind weather with a good deal of sunshine is expected
for most places today, in stark contrast to some other weekends
over the last several weeks. A 1032 mb surface high far to the
northwest will pass about 1100 miles N of the islands Mon, giving
a modest uptick in trade wind speeds. The high will then stall
well to the NE of the islands through midweek, with trade wind
speeds relaxing back to locally breezy levels. The models begin to
diverge a bit Thu night with the placement of a low developing to
the N of the islands, though the new 12z ECMWF is closer to the
12z GFS solution than previous runs were. This continues to
suggest a one day period of light winds with diurnal island
circulations taking over for late Thursday night into Friday. The
low will push a late season cold front down the chain Friday night
into Saturday, with cool dry northerlies for next weekend as it
looks right now.

We are within a relatively dry zone as yesterday`s trade wind
shower area departs to the west. Not even expecting a lot of
windward and mauka showers today and tonight, though it`s
difficult to rule them out completely. The high clouds from the
subtropical jet appear to have thinned quite a bit, and now are
expected to continue to diminish. A little more low-level trade
wind moisture returns on Mon, with the models showing a more
robust push of trade wind moisture arriving on Tue. This will be
followed by yet another later Wed into Wed night. None of these
areas of trade wind clouds and showers will be accompanied by
significant upper level support so only minor ups and downs in the
shower coverage are expected. The same is true for the cold front
next weekend, which is likely to have a band of clouds and
showers with it, but should not be too heavy.

&&

.AVIATION...
Pretty much fair weather across the main Hawaiian Islands this
morning. AIRMET Sierra for mountain obscuration was lowered a
little while ago after visual satellite imagery reveal minor
cloudiness hugging the mountain areas of all the larger islands.
This is despite a low ceiling reported by a few windward stations.
Perhaps windward Kauai has the most clouds, but there aren`t much
showers with these clouds. Not much trade showers actually, all
around, as a pocket of a drier air mass has descended on the
islands. This drier pattern should persist into the early evening
hours, followed by a generous increase in activity after midnight.
But, the area with the most significant change today will likely
be the Kona and Kau slopes, where daytime heating, will lead to
mostly cloudy skies and a couple of showers this afternoon and
evening. A stray shower may linger along the Kona coast well into
the nighttime hours. Max top of the trade wind clouds is expected
to be around 9k feet.

High above the island sky is a layer of lumpy looking cirrus
moving thru the area from west to east, especially east of Kauai.

A 1029 mb surface high, located about a thousand miles NW of Kauai
will be moving E 30 kt, which puts it directly N of the islands
this time Monday morning. This will beef up the trades a little
more area wide tonight and into Monday. Thus, AIRMET Tango will
continue pretty much for at least the next 24 hours.

&&

.MARINE...
Fresh to strong trades are forecast to hold through much of the
upcoming week as high pressure remains established to the north.
A Small Craft Advisory (SCA) is currently in place for most
marine areas from Oahu to the Big Island through tonight. The
advisory for the typically windier locations from Maui to the Big
Island has been extended through Tuesday. Other marine zones may
become added Monday through Tuesday as the high strengthens and a
moderate northwest swell builds down the island chain boosting
seas up to around the 10 ft mark.

Rough surf is forecast to persist through the week along east facing
shores due to fresh to strong trades holding in place locally and
just upstream of the area. The best chance for advisory-level surf
will come Tuesday through Thursday as the trades increase locally
and expand farther upstream. Although confidence remains low this
far out in the forecast, there could be a break in the trades by
Thursday/Friday as a front approaches and moves into the area.

A new northwest (320 deg) swell is expected to begin slowly
filling in by this evening, peak Monday, then slowly ease through
midweek. This source is from a potent system that reached storm-
force category Friday as it crossed the date line just south of
the Aleutians between 40N and 50N (around 2000 miles northwest of
the state). Surf is forecast to peak on Monday at heights below
advisory levels along north and west facing shores.

A reinforcing moderate northwest (330 deg) swell is forecast to fill
in Thursday, peak Thursday night into Friday, then ease into the
weekend. This reinforcement will be from a developing storm that is
currently near the Kamchatka Peninsula and forecast to track east
while skirting the Aleutian Islands Sunday through Wednesday.
Surf associated with this feature should remain below advisory
levels along north and west facing shores.

A reinforcing southwest (220 deg) swell is forecast to fill in
Monday and hold through midweek due to recent activity across the
southern Pacific/Tasman Sea. Small to moderate surf will result
along south facing shores each day.

&&

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

Small Craft Advisory until 6 AM HST Monday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters.

&&

$$
R Ballard/Lau/Foster



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