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Hawaiian Islands Synoptic Discussion and Guidance

620
FXHW60 PHFO 121321
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
321 AM HST Sun Jul 12 2020

.SYNOPSIS...
Moderate to locally breezy trades will continue through Monday,
with an increase in trade showers expected beginning Monday night
as an upper level low nears the state. A more active shower
pattern will hold through the middle of the week as the trade
winds ease into the light to moderate range. A more typical trade
wind shower regime will return for the end of the week, with
lighter trades holding through Friday, then becoming breezy next
weekend.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high centered around 1300
miles north-northeast of Honolulu, is driving moderate to locally
breezy trade winds across the island chain early this morning.
Meanwhile, an upper level low is evident in water vapor imagery
600 northeast of the state. Infrared satellite imagery shows a
combination of high and low clouds resulting in partly cloudy
skies across much of the state, with mostly cloudy conditions
affecting portions of the Big Island. Radar imagery shows
scattered showers moving into windward areas, with a stray shower
spilling leeward from time to time. Main short term focus
revolves around trade wind trends and rain chances during the
next few days.

High pressure north of the state will keep moderate to locally
breezy trade winds in place through Monday. The trades will ease
into the light to moderate range Tuesday through Friday as a
couple surface troughs move by north of the islands and weaken the
gradient locally. This will likely lead to some localized sea
breezes over the more sheltered leeward areas each afternoon.
Breezy trades then appear to return next weekend as strong high
pressure re-establishes itself north of the state.

Fairly typical trade wind weather will prevail today, with
showers favoring windward and mauka areas and a stray shower
spilling leeward from time to time. A slow moving upper level low
north of the islands should begin to elevate inversion heights
beginning tonight, bringing an increase in trade wind showers.
The trade showers are expected to remain more active Monday
through Wednesday, with showers most common in windward areas
during the night and early morning hours, although the higher
inversions should allow a few more showers to reach leeward areas
as well. Additionally, in the weakened trade wind pattern Tuesday
and Wednesday, localized sea breezes will aid in convective shower
development each afternoon over leeward and interior areas. We
could see a decrease in shower coverage Wednesday night and
Thursday as the upper level low departs to the west and upper
ridging begins to build in from the east. Shower coverage appears
to increase again Thursday night and Friday in advance of the
strengthening trades, with more typical trade wind weather
returning next weekend.

&&

.AVIATION...
A high pressure center north of the Hawaiian Islands will keep
moderate to locally breezy trade winds in the forecast through
Monday. An upper level disturbance along with breezy trade winds
will keep passing showers focused mainly over windward and
mountain areas for the next two days. Increasing shower activity
may bring periods of MVFR conditions mainly over windward areas
and mountain slopes.

AIRMET Sierra remains in effect for Tempo Mountain Obscuration
along the north and east slopes of the Big Island this morning.
Low cloud ceilings will diminish by late morning.

AIRMET Tango remains in effect for mechanical turbulence to the
lee of the islands. Locally breezy trades will continue through
Monday keeping this AIRMET in effect.

&&

.MARINE...
Fresh to strong trade winds will continue through Monday, then trend
down by midweek as a weakness in the ridge to the north-northwest
develops and the gradient loosens over the region. Winds may trend
down enough for land and sea breeze conditions over some leeward
coastal areas where the trades usually hold, Wednesday into the
second half of the week. Seas will respond and lower through this
time.

Surf along east facing shores will remain rough into Monday, then
gradually ease as the winds lower Tuesday. A medium period east
swell from Tropical Storm Cristina over the eastern Pacific is
forecast to fill in from east to west Tuesday through midweek.
This swell combined with the short period trade wind energy will
be enough to bring the surf back up headed into the second half of
the week.

Surf along south facing shores will remain small each day with
mainly a mix of south to southeast swell and the typical background
long-period south to south-southwest swells moving through. A
slightly larger south-southwest Tasman swell will be possible
next weekend.

Surf along north facing shores will remain at the summertime average
each day - exception being at the typical locations exposed to trade
wind energy through Monday.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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