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Hawaiian Islands Synoptic Discussion and Guidance

913
FXHW60 PHFO 230146
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Sun Apr 22 2018

.SYNOPSIS...
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 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 likely push
down the chain Friday night and Saturday, followed by drier,
chilly northerlies.

&&

.DISCUSSION...
A surface high roughly 1000 mi to the NW of Kauai will continue to
push E, passing N of the islands Mon morning. Trades are already
responding to the approaching high and somewhat drier airmass over
the islands today, and should be even a little bit stronger
tomorrow and Tue at high-end breezy/locally windy levels. The high
will then stall well to the NE of the islands through midweek,
with trade wind speeds relaxing back to locally breezy levels.

Starting Thu night, the models show low pressure developing
somewhere to the N of the islands. There is considerable
disagreement among the models as to where this happens, which has
some bearing on local weather. Most of the guidance shows trades
dying off and giving way to land breezes Thu night, and this seems
reasonable. Most of the guidance is showing a clean-sweep cold
frontal passage down the chain later Friday into Saturday, but the
18z GFS drastically slowed the front down and taps into deep
moisture from the S. When compared to the GFS ensemble members and
other global models, this solution is a significant outlier and
will be discounted for now. We will maintain the idea of cooler,
drier northerlies becoming trades once again next weekend.

The mainly dry trades today will return to more typical shower
coverage starting Monday. A minor area of clouds and showers
about 300 mi E of the Big Island and Maui will move in tomorrow
morning, affecting mainly the southern main islands. Another band
of clouds and showers will ride in on the trades late Mon night
and Tue morning for Oahu to the Big Island, and yet another Wed
into Thu. Assuming the aforementioned 18z GFS is wrong about what
happens with the front, it too should not be accompanied by a lot
of rainfall Fri night into Sat.

&&

.AVIATION...
VFR will be the prevailing flying condition for the upcoming
overnight hours, and this may extend further into Monday as well.
This pocket of somewhat drier air mass, supported by a healthy
inversion at around 7k feet, as per LIH sounding, may loose a
slight edge later tonight as nocturnal cooling will cause a
slight uptick in showers, but it will help limit these trade
showers to brief periods of MVFR ceilings and vis, especially
the windward and mountain areas. Any increase in these clouds and
showers, will give way to plenty of sunshine, especially across
the leeward areas, by mid morning. Expect max top of these trade
wind clouds/showers at 9k feet.

A layer of high cirrus clouds will be slow to clear overnight,
but considerable thinning are expected Monday morning.

A 1029 mb surface high, located about a thousand miles NW of Kauai
will be moving E 30 kt, which puts it N of the islands this time
Monday afternoon. 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
this week as high pressure remains north of the area. A Small
Craft Advisory (SCA) is currently in place for most marine areas
from Oahu to the Big Island through Monday. The SCA will expand to
all coastal waters Monday night due to a boost in wind speeds as
the high reaches it's closest proximity to the islands, as well
as a new northwest swell that is expected to peak on Monday. The
breezy winds and northwest swell will increase seas around the 10
foot 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 Monday night through Wednesday morning as the wind
speeds peak. There could be a break in the trades by
Thursday/Friday as a front approaches and moves into the area.

The aforementioned northwest 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 last 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 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 Monday 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/Lau/Foster

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

Current Conditions
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RH		23 %
Wind		NE 13 mph
Road		Open (4x4)