Heavy Rain in Hawaii, May 13 to 14, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
944 AM HST Wed May 13 2020

.SYNOPSIS...
A light wind regime will hold in place today through Friday, with
daytime sea breezes and overnight land breezes common. Showers
will favor interior and mauka areas during the afternoon and
evening, and areas near the coast at night. Trade winds are
expected to return late Friday or Friday night, and continue
through early next week. Fairly dry conditions are expected Friday
night through the weekend, with a few passing shower affecting
mainly windward areas. We should see an increase in shower
coverage Sunday night into early next week as an old front pushes
southward through the islands chain.

&&

.DISCUSSION...
The main weather concerns over the next several days will be the
threat of brief heavy rainfall this afternoon. As the land heats
up today, sea breezes will develop and will lead to afternoon
clouds and showers over interior and mauka areas. The showers will
be enhanced by a short-wave trough moving through the area today.
The 12Z sounding from Lihue shows the weakening of the inversion with
precipitable water values of 1.40 inches. Although the showers
will be spotty this afternoon, the slow-moving nature of them
could cause some minor flooding impacts at select areas.

Additional impacts from the short-wave trough will be gusty winds
at the Big Island summits. Winds have increased overnight, and
observations from the Mauna Kea weather center is showing wind
gusts of around 60 mph this morning. Winds are expected to remain
gusty through this evening. The current Wind Advisory for the Big
Island summits remains in effect through 6 pm today. Winds are
expected to drop below advisory thresholds by Thursday.

Overall a similar weather pattern is expected on Thursday with
overnight land breezes and day time sea breezes. The one thing
that does change, is the atmosphere will be slightly more stable
due to the upper level trough moving away from the area and a
surface ridge moving near the state. This should help limit the
intensity of the showers during the afternoon and evening hours.

High pressure rebuilding north of the area late Friday will bring
the return of moderate trades late Friday afternoon. Fair trade
wind weather is expected over the weekend with some scattered
showers over windward and mauka areas. Latest model guidance is
showing remnant moisture from an old frontal boundary approaching
the state Sunday night. This will likely bring an increase of
trade wind showers Sunday night into early next week.

&&

.AVIATION...
A ridge north of the area will maintain gentle low-level easterly
flow over the main Hawaiian Islands through Thursday morning. This
easterly flow is producing MVFR ceilings over the east half of
Kauai and AIRMET Sierra for MTN OBSC is in effect for that area.
Conditions on Kauai have been improving, and we are likely to
cancel the AIRMET by noon today.

The prevailing flow will be light enough to allow daytime sea
breezes and nighttime land breezes to develop. The sea breezes
will produce broken to overcast low clouds over interior areas of
the islands during the afternoon and evening with scattered
showers. MVFR ceilings may become extensive enough to require
AIRMET Sierra for MTN OBSC on some islands

The nighttime land breezes will clear the clouds over most areas
of the islands. Showery low clouds will persist over water tonight
and could affect some coastal areas.

&&

.MARINE...
The overnight surface analysis shows the ridge to the north of the
islands. Light trade winds today are expected to weaken further,
particularly over the waters near Oahu and Kauai. This could lead
to some land and sea breezes along the coasts. Slightly stronger
winds are expected near Maui and the Big Island. Little change in
the winds are expected into Friday.

A new high building north of the islands this weekend will allow
trade winds to rebound, with moderate to fresh winds returning.
Will be monitoring future runs of the models, but its possible for
some of the typical windier areas near Maui and the Big Island to
flirt with Small Craft Advisory (SCA) levels winds through the
weekend, but a more prolonged period is more likely early next
week.

The current small, long-period, west-northwest swell will
continue to bring elevated surf to the north facing shores today,
and is expected to fade tomorrow. A new northwest swell is
expected to arrive late on Friday, bringing advisory level surf to
the north and west facing shores by Friday night. The swell is
expected to hold through Saturday, before easing Sunday through
Monday.

A long-period, south-southwest swell is expect to arrive later
today, boosting surf along south facing shores late today through
tomorrow. This swell is expected to linger through the weekend,
with surf heights nearing advisory levels by Friday. The swell
is expected to become more southerly as the event wears on. Another
small, long-period south swell will be possible by next
Tuesday/Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Big Island Summits.

&&

DISCUSSION...Kino
AVIATION...Donaldson
MARINE...M Ballard