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Hawaiian Islands Synoptic Discussion and Guidance

468
FXHW60 PHFO 120641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Sat Jul 11 2020

.SYNOPSIS...
Moderate to 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 next 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 1225
miles north of Honolulu, is driving moderate to breezy trade winds
across the island chain this evening. Meanwhile, an upper level
low is evident in water vapor imagery 700 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 in windward sections of Maui and
most 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
to light to moderate levels 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 tonight and Sunday,
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 Sunday night, 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 mountain
slopes.

AIRMET Sierra was issued for Tempo Mountain Obscuration along the
north and east slopes of Maui and the Big Island this evening.
Conditions over Maui will improve after midnight tonight, with
low cloud ceilings and showers lingering over the north and east
slopes of the Big Island though the early morning hours.

AIRMET Tango remains in effect for mechanical turbulence to the
lee of the islands. With locally breezy trades expected to
continue, this AIRMET will likely continue through the weekend.

&&

.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.

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

&&

.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