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Temp5.6 C
RH61 %
WindSW 11 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

559
FXHW60 PHFO 211303
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
303 AM HST Wed Apr 21 2021

.SYNOPSIS...

High clouds will gradually decrease today. Leeward sea breezes
will support inland clouds and a few showers this afternoon.
Breezy trades and more typical trade wind weather will return by
the end of the week.

&&

.DISCUSSION...

Overcast mid and high clouds are thinning and slowly pushing
southward as an upper wave shifts east of the area. Showers
remain confined to the adjacent waters of Kauai and southeast
sections of Big Island. Light winds and land breezes will maintain
dry conditions over the smaller islands through early this
morning. Mid-level subsidence associated with a building ridge
from the west will bring an increase in stability today. However,
the existing ribbon of deep moisture over the area will still
hold in place through at least the lowest 10kft. As a result,
afternoon leeward sea breezes should have an easy time causing
afternoon cloud build-ups and a even few showers over inland
areas. The same could be said for Thursday, although the gradual
increase in trades reduces confidence in sea breeze development.
So, showers may instead manifest in more typical trade wind
fashion by Thursday afternoon, especially for Oahu through Big
Island.

Locally breezy trades will return by week's end as high pressure
establishes over the north-central pacific. There is strong model
consensus that weak shortwave energy currently near 170E/35N will
shear toward the islands along the eastern periphery of the
building ridge, eventually cutting off near or directly over the
area this weekend. The resulting reduction in stability will
support a wetter trade wind pattern for the weekend. Low
confidence with respect to the to the evolution of this closed low
results in higher than normal forecast uncertainty by early next
week.

&&

.AVIATION...

A trough northwest of the islands continues to spread layered
clouds across the whole of the state. As of 2am HST, radar
indicated only isolated light showers within the layered clouds.
These conditions are expected to persist throughout the day today.
Mostly clear skies developing across the smaller islands late
this evening.

On the Big Island, weak southeasterly flow will steer bands of
low topped clouds and isolated showers towards east facing
slopes and coasts this morning. In the afternoon and early
evening hours, light sea breezes will focus shower activity over
the island's interior.

SIGMET Xray is in effect for severe turbulence above FL320
associated with a digging upper level trough. The band of moderate
to severe upper level turbulence currently affects all the
smaller islands and will spread across the Big Island within the
next few hours. Conditions should improve from the northwest later
this morning.

&&

.MARINE...

A trough northwest of Kauai will gradually dissipate through
Friday. In doing so, light trade winds are expected to spread
westward from Maui and the Big Island today. A surface high is
forecast to pass north of the islands Thursday through Friday,
resulting in a boost in the trade winds with speeds reaching
locally strong for especially the sensitive nearshore areas of
Maui County and waters south of the Big Island. This breezy trades
will persist through Sunday followed by a drop off Sunday night
into Monday. A Small Craft Advisory for those nearshore waters
will be hoisted as early as tonight.

A small to moderate northwest swell arriving today will lead to a
bump up to the north and west facing shores. This swell will max
out Thursday followed by a gradual decline through Friday. A small
to moderate long period west-northwest swell is slated to arrive
next week Monday night and peak Tuesday, followed by a slow
decline. A continuous stream of small but long period swell from
the south and southwest will maintain small surf to the south
facing shores throughout the forecast period. The return of the
trade wind means the surf along the east facing shores will be on
the rise between Friday and Saturday when the trades turn breezy.
Expect surf to build to small to moderate heights. Surf along the
east facing shores will be on the downtrend Monday as the trade
winds weaken.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...JVC
AVIATION...Bedal
MARINE...H Lau

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