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Hawaiian Islands Synoptic Discussion and Guidance

631
FXHW60 PHFO 190653
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
853 PM HST Mon Mar 18 2024

.SYNOPSIS...
Moderate easterly trades will continue through the first half of
the week, with clouds and light showers favoring windward and
mauka locations during the nights and mornings. A return of breezy
easterly trades and increasing rainfall chances is possible for the
second half of the week, as an upper disturbance moves into the
area.

&&

.DISCUSSION...
Guidance remains in good agreement, indicating dry and stable
conditions continuing through the first half of the week as upper
heights rise over the region. This will create a more suppressed
environment with precipitable water values remaining around an
inch or less and a strong subsidence inversion hovering around
5-6 kft. The best chances for a few, mostly light, showers will
be over typical windward and mauka locations overnight through the
early mornings with moderate easterly trades continuing. Any
leeward showers are more likely to occur in the afternoons, where
localized sea breezes form. Accumulations will be minimal with the
stable conditions in place.

A gradual strengthening and destabilization of the trade wind
flow is possible through the second half of the week. There is
higher confidence that trades will likely increase as a deeper
surface high sets up far north of the state, potentially resulting
in breezy conditions by Friday. An upper-level trough will slowly
pass over the region during this time, bringing 500 mb
temperatures as low as -15C Wednesday through Friday. While there
could be some increase in shower coverage in response to the
added instability, a mid-level ridge may be able to maintain
stability and lower the chances of locally heavy rainfall.
Guidance shows a pulse of upper-level energy dipping southward on
the backside of this upper trough Friday through Saturday,
potentially closing off an upper low over the eastern end of the
state. If this were to materialize, these conditions anticipated
through the second half of the week could linger through next
weekend.


&&

.AVIATION...
Moderate easterly trades will persist, with clouds and showers
favoring windward and mauka overnight through the morning hours.
Outside of a brief shower moving through leading to MVFR
CIGS/SHRA, VFR conditions will prevail.

&&

.MARINE...
Trades have eased back to more moderate magnitudes as a result of
high pressure settling in north of the coastal waters. Little
change is expected through Wednesday when another high develops
farther north of the region. This high will build in Thursday and
Friday and tighten the regional pressure gradient enough to
strengthen trades back to fresh to locally strong levels into the
weekend.

A few overlapping northwest swells are scheduled to arrive this
week. The latest moderate size, long period northwest (310-320
degree) swell peaked surf to near or slightly under High Surf
Advisory (HSA) thresholds along most north and west-facing Kauai-
to-Maui shores early this morning. This swell has decline through
the afternoon and into Tuesday to then be followed by a slightly
larger northwest (310-330 degree) swell building in Tuesday night.
As a result, the arrival of this high end moderate size, long
period swell will likely produce mid week advisory level north and
west-facing shore surf. North and west shore surf will fall below
HSA heights Thursday. A moderate size northwest (310 degree)
swell is due next weekend. A weak pulse of early season, south
southwest (200 degree) swell has been passing through the past day
or so. Expect this inconsistent swell to peak south-facing shore
surf at slightly above March averages through tonight, then surf
will gradually decline with the fading swell through Wednesday.
East shore surf will remain small through the middle of the week.
There will be a slight pick up in chop later this week in response
to strengthened trades.

&&

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

&&
$$

DISCUSSION...Bohlin
AVIATION...Gibbs
MARINE...Blood

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