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Hawaiian Islands Synoptic Discussion and Guidance

379
FXHW60 PHFO 271935
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
935 AM HST Sun Sep 27 2020

.SYNOPSIS...
Moderate trade are expected today, with weakening winds this week.
Moisture riding in on the trades will increase shower potential
tonight through tomorrow morning, and some of the showers could
reach leeward areas. Winds are expected to weaken by Tuesday with
land and sea breezes expected to close out the week. Expect more
clouds and showers during the afternoon and evening hours with the
light wind regime.

&&

.DISCUSSION...
High pressure far to the northeast, with a ridge north of the
islands, will continue to drive moderate trade winds across the
islands today, with some downward trend expected over the next
couple of days. The high is expected to lift northward late
Monday and Monday night, weakening the winds over the islands.
Winds are expected to weaken enough by Tuesday for land and sea
breezes to become established, and those light winds are expected
to linger through the end of the week.

Several features will factor into rain potential this week.
Satellite shows a batch of moisture upstream of the islands, which
is expected to ride in on the trades later today and tonight. Will
likely be boosting PoPs for some areas a little big based on the
latest blend of models, but no substantial changes. In addition
will be boosting some of the QPF values, but again no significant
changes. The next area of concern is a batch of moisture about 600
miles east-northeast of the Big Island. This batch of moisture is
expected to move over the southern islands Wednesday and Thursday
when we are expecting the light winds. After that, the global
models show some additional moisture rising over the southern
islands from the south. None of these features are currently
expected to bring any significant rainfall totals. Will be
monitoring these features throughout the week to see how the
models trend, and how the higher resolution models deal with them
as time gets closer.

An upper level low west of Kauai is expected to linger for another
day, and then move to the west away from the islands. There were
been some thunderstorms associated with this system during the
pre-dawn hours about 575 miles west of Kauai, but these have died
out over the last few hours.

&&

.AVIATION...
Ceilings and brief showers will develop over interior areas this
afternoon as light to moderate trades give way to leeward sea
breezes. Windward locations will remain on the dry side of normal
through this afternoon with just passing light showers capable of
producing very brief MVFR.

Deeper moisture will then spread across the Islands tonight
bringing the potential for MVFR showers to windward and mauka
slopes and lighter activity to lee areas. The greatest probability
of prevailing MVFR warranting issuance of AIRMET Sierra will be
after 12z/2am HST over Windward Maui and Big Island.

No AIRMETs are currently in effect.

&&

.MARINE...
Trade winds will weaken today and remain gentle through most of
the week in response to a series of fronts passing north of the
area.

Surf along south facing shores will peak near advisory levels today
as a long-period south southwest swell moves through the islands.
Some exposed breaks along west and east facing shorelines will
see some surf from this swell. A gradual downward trend is expected
tonight and Monday. Small south swells will remain through the
rest of the week.

Surf along east facing shores will remain small through today. A
small easterly swell from what was tropical cyclone Lowell may
bring an uptick in surf heights Monday and Tuesday.

Surf along north facing shores will remain small through the beginning
of the week. The next swell is expected Wednesday/Thursday, with surf
around advisory levels, while another swell is possible on Friday.



&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Foster

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



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