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Hawaiian Islands Synoptic Discussion and Guidance

961
FXHW60 PHFO 190205
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
405 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...
Current visible satellite imagery shows scattered to broken low
clouds built up over the windward slopes, as well as most leeward
areas where local sea breezes have set up in the moderate trade
wind flow. Little in the way of cloud cover is being observed
upstream at this time. The afternoon soundings from Hilo and
Lihue continue to show a very stable and dry atmosphere, with
temperature inversions between 4500-6000 ft, and precipitable
water values of only 1.01" and 0.80", respectively. This is
reflected in the overall lack of showers indicated on radar
imagery, aside from a few light sprinkles being carried into
windward areas of Kauai, Maui, and the Big Island. Any rainfall
has been light, with only a few hundredths of an inch reported.

Guidance remains in good agreement on stable and relatively dry
conditions continuing for the next couple of days 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 expected through the second half of the week. Model consistency
is good, giving 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
enough stability to 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 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.

Tonight, with a 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 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.

Surf along north and west facing shores will slowly decline
through Tuesday as the current northwest (310-320 degree) swell
fades. A slightly larger reinforcement from the northwest
(310-330 degree) will build in Tuesday night with long period
forerunners expected Tuesday afternoon. 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 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 through the weekend as trade winds increase.

&&

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

&&

$$

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

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