Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 181933
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
933 AM 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...
Current visible satellite and radar imagery show mostly clear
skies across the state, with only scattered clouds and a light
shower or two over windward slopes. Early morning soundings from
Hilo and Lihue show a very stable and dry atmosphere, with a low
temperature inversion around 4500-5500 ft, and precipitable water
values of only 1.04" and 0.85", respectively.

Guidance remains in good agreement on dry and stable conditions
continuing through the first half of the week as mid and upper
heights rise over the region. This will create a more suppressed
environment with precipitable water values 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
windward and mauka locations during the nights and early mornings
with moderate easterly trades continuing. Any leeward showers are
more likely to occur in the afternoons where localized sea breezes
form. Rainfall 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. Model consistency
provides greater 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, persistent ridging around 700 mb may be able to
maintain some semblance of stability and limit the chances of
locally heavy rainfall. However, guidance is in decent agreement
in the upper trough closing off into a low over the far eastern
portion of the state by Friday evening and moving back westward to
the south of the islands. Meanwhile, a surface high northwest of
the state will strengthen as it moves eastward through the second
half of this week. If this collective pattern materializes, the
already breezy trade wind pattern could become significantly
wetter (depending how close the upper low stays to the islands)
with stronger winds through the weekend and into early next week.

&&

.AVIATION...
Light to moderate east-northeast trade winds will persist through
midweek. In this pattern, clouds and showers will favor windward
and mauka areas, especially during the late night and morning hours.
Overall, shower activity will be limited in coverage and intensity
due to a shallow layer of moisture being trapped underneath a
strongly stable air mass overhead.

Widespread low clouds are still impacting windward Kauai and the
Big Island this morning, but bases have lifted and VFR conditions
are prevailing, so AIRMET Sierra has been cancelled. Tonight,
with the slight uptick in low clouds and showers, AIRMET Sierra
may once again be needed for tempo mountain obscuration across
windward portions of the Big Island...and perhaps along other
windward areas. Otherwise and elsewhere, VFR conditions will
continue to prevail.

&&

.MARINE...
Moderate to locally fresh trade winds will persist through Thursday
as weak high pressure to the north slowly drifts east. A new high
will build north of the state on Thursday and Friday, as trades
will increase to fresh to locally strong levels. A Small Craft
Advisory (SCA) will be needed for at least the typically windy
waters around the Big Island and Maui. The stronger trades look to
hold into the weekend.

Several overlapping northwest swells are due this week. The current
moderate size, long period northwest (310-320 degree) swell will
slowly decline through Tuesday. A slightly larger reinforcement
from the northwest (310-330 degree) will build in Tuesday night. A
High Surf Advisory (HSA) is expected with this swell Wednesday
into Thursday for exposed north and west facing shores. The swell
will decline later Thursday and Friday with another moderate long
period northwest swell expected this weekend.

South facing shores will remain slightly elevated today but will
gradually decline with the fading southwest swell through Wednesday.
Short period trade wind swell will be below seasonal average through
Thursday and will likely pick up Friday and next weekend as trade
winds increase.

&&

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

&&

$$

DISCUSSION...TS
AVIATION...Farris
MARINE...Shigesato


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