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Hawaiian Islands Synoptic Discussion and Guidance

722
FXHW60 PHFO 040629
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
829 PM HST Wed Mar 3 2021

.SYNOPSIS...
Fairly typical trade wind weather will persist through Thursday,
with showers favoring windward and mauka areas, particularly at
night and into the early morning hours. A surface trough will
develop in the vicinity of the islands Thursday night and Friday,
then slowly shift westward Friday night. This will bring wet
trade wind conditions particularly to windward sections of the
smaller islands. As the trough shifts westward over the weekend,
more typical trade wind weather will return, with the trades
rebounding back to breezy levels. The trades will strengthen
further Monday through the middle of next week, with windy and wet
conditions expected.

&&

.DISCUSSION...
Currently at the surface, a 1037 mb high is centered around 1400
miles northwest of Honolulu, and is driving moderate to breezy
trades across the island chain this evening. Infrared satellite
imagery shows a band of low clouds moving through the smaller
islands, with mainly partly cloudy conditions over the Big Island.
Radar imagery shows scattered to numerous showers moving into
windward sections of Kauai and Oahu, with a few showers spreading
leeward from time to time.Across the Big Island a few isolated
showers are present in both windward and leeward areas. Main short
term concern revolves rain chances during the next couple days.

High pressure northwest of the islands will weaken slightly as it
settles slowly southeastward during the next couple days.
Meanwhile, a weak surface trough will develop along the tail end
of an old front near or just north of the islands Thursday night
and Friday. This trough will keep the tightest gradient north of
the state, and maintain trade winds generally at moderate
moderate levels through Friday. As the trough shifts westward
Friday night and Saturday, a new strong 1040 mb high will build
north of the state. This high will then slowly settle
southeastward while maintaining its strength of around 1040 mb
through the middle of next week. As a result, we should see breezy
conditions over the weekend, with windy conditions (possibly
advisory level winds) developing Monday through the middle of next
week.

As for the remaining weather details, the band of clouds and
showers moving through the smaller islands this evening, will
gradually sink southward into the Big Island and increase shower
coverage here later tonight. Meanwhile, we should see a decrease
in shower coverage from north to south across the smaller islands
after midnight. Fairly dry trade wind weather is then expected on
Thursday, although windward sections of Kauai will likely see more
shower coverage than the remainder of the state. Quite a bit of
high cloud cover will be overspreading the islands as well due to
an upper level trough digging to the northwest of the island
chain.

Moisture will begin to pool along a weak surface trough developing
over or just north of the islands Thursday night and Friday, with
this trough then sliding slowly westward Friday night. The moisture
associated with the surface trough combined with added
instability from a trough aloft, will lead to an increase in
shower coverage and intensity across the islands during this time,
with the potential for some locally heavy rainfall over windward
areas. The latest 00Z GFS model cycle suggests that the surface
trough and deepest moisture may slide by just to the north of the
state, while the 12Z ECMWF suggests a weaker surface trough which
keeps the deepest moisture targeting the smaller islands,
particularly Kauai and Oahu. Will stick with the current plan for
now which resembles the ECMWF solution, but if the new 00Z ECMWF
model cycle follows the latest 00Z GFS trend, we may need to
adjust rain chances downward slightly. Considerable cloudiness
will prevail through the period due to a combination of high and
low clouds.

Once the surface trough shifts west of the state, we should see a
return of more typical trade wind weather over the weekend.
Showers will favor windward and mauka areas, particularly at
night and during the early morning hours, with a few showers
spreading into leeward communities from time to time. Models
suggest that deeper moisture may arrive Monday through the middle
of next week, resulting in wet trade wind conditions across the
island chain. Additionally, moisture may extend high enough to
allow for a thunderstorm or two to develop over the Big Island
each afternoon Saturday through next Wednesday.

&&

.AVIATION...
At the surface, and area of high pressure remains rooted northwest
of the state allowing for breezy northeast trade winds to continue
to flow over the main Hawaiian Islands. However, speeds are
expected to weaken a bit over the next day or two before
increasing once again heading into the weekend. Aloft, a strong
250mb jet remains in place across the southern half of the state this
evening but is forecast to exit to the east overnight tonight.
Model data suggests that moderate turbulence will continue to be a
possibility throughout the overnight hours but should wane early
Thursday morning. For now, AIRMET Tango remains in effect for
tempo moderate turbulence between FL220 and FL300 across the
state, as well as for tempo moderate low level turbulence on the
leeward side of mountains.

Latest satellite and radar imagery this evening show a band of
showers currently moving across the smaller islands. Periodic showers
and brief MVFR conditions will continue for the next couple of
hours until this band of moisture diminishes as it moves southwest.
For the rest of the night and throughout the day on Thursday,
showers will continue to ride in on the trade wind flow and impact
mainly north through east sections of the islands, though most
showers should remain light with only brief periods of MVFR
ceilings/visibility.

&&

.MARINE...
A Small Craft Advisory (SCA) remains in effect for all zones
except Big Island Windward and Maui County Leeward waters through
tonight. Winds and seas will slowly diminish as the high to our
north weakens, and the SCA may be scaled back tomorrow as strong
trade winds and elevated seas persist only over northwest waters.
SCA conditions may return for most waters Friday due to building
seas, and extend over the weekend as the high is reinforced and
strong trades become more widespread.

Surf heights remains below High Surf Advisory (HSA) threshold
across all shores this evening. Surf along east facing shores
will continue to subside through tomorrow. A bump in east
shoreline surf will occur late Friday night into the weekend as
trade wind swell increases and an arriving north swell wrap fills
in. A northwest swell will continue to fill tonight, peaking at
or just below HSA threshold along north and west-facing shores
late tomorrow into Friday. A secondary, reinforcing north swell
will arrive Friday night and, in addition to the returning trade
wind swell wrap, may boost north facing shore surf heights to, or
slightly above, HSA heights Friday into Saturday and slowly
diminish Sunday. A small, long- period southwest swell will fill
in tomorrow, providing a small bump in south shore surf, and
slowly decline through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Vaughan
MARINE...Powell

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

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