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Hawaiian Islands Synoptic Discussion and Guidance

029
FXHW60 PHFO 201418
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
418 AM HST Tue Aug 20 2019

.SYNOPSIS...
A strengthening ridge north of the islands will maintain a mainly
moderate trade wind regime through the upcoming weekend. Under
this flow, showers riding in with the trades will favor the windward
and mountains areas.

&&

.DISCUSSION...
A nice brief batch of trade wind showers roll through earlier in
the evening, but the frequency of showers appear to have dropped
off in the past couple of hours. Cloudiness upwind of the smaller
islands, from Molokai westward, doesn't look all that bad,
scattered to broken in coverage. Do expect a few more passing
showers to occur the rest of this morning. The afternoon looks
pretty dry, although not as dry compared to the past few days.
Windward Maui is getting some spill- over clouds from windward Big
Island, where it is mostly cloudy with scattered showers. So
windward Maui will be getting a few more showers than the other
smaller islands today. More shower- bearing clouds lies upwind of
the Big Island, so a cloudy and showery day is instore for the
windward districts today. These clouds will be spreading to the
Kau and leeward areas of the island this afternoon along with
scattered showers. No thunderstorms for the higher elevations,
though, as a mid-level ridge is expect to cap the moisture to
below 13k feet.

These clouds and showers affecting the Big Island and parts of
Maui are associated with a surface trough located 200 miles east-
southeast of the Big Island. It is moving southwest, and the trough
is slated to dissipate later this afternoon/evening while passing
well south of the Big Island. Therefore, we expect some of these
clouds and showers to thin out across windward Big Island this
evening, returning to the norms of a trade wind shower pattern.

A boost in the trades to locally strong is still on track, where
it should be in place by this evening. This boost could be brief,
lasting through Thursday, before trending slightly lighter for 18
to 24 hours between Thursday night and Friday or Friday night.
Never-the-less, locally strong trades will return by Saturday, and
continue through the weekend. This softening of the trades is due
in part of a front moving into our northern offshore waters Thursday
into Friday. The remnants of the front, which doesn't reach the
islands, is pushed westward, away from the islands after Friday,
as the surface high strengthens far north of the islands. But, the
latest GFS and ECMWF solutions point to an upper low dropping down
from the northeast to about 600 miles northwest of Kauai over the
upcoming weekend. This upper feature could be close enough to
favor an uptick in trade showers for the area. The warm days
though, will continue with highs near or at 90 degrees.

&&

.AVIATION...
A ridge of surface high to the north of the state will maintain
moderate east-northeast trades to the area today. Meanwhile, a
surface trough passing south of the islands will lead to a boost
in the strength of trade winds due to tighter pressure gradient
across the areas.

Expect most shower activity to occur along the windward side of
the islands today, with brief MVFR conditions at times, due to
ceilings and lowered visibility.

No AIRMETs are currently in effect, though windward areas,
especially Big Island may see enough cloud coverage to warrant
mountain obscuration. The anticipated strengthening in trade winds
may also result in AIRMET TANGO being posted later today due to
possible increasing low turbulence on the immediate lee areas of
mountains.

&&

.MARINE...
A trough of low pressure sliding by to the south of the state and
a new high building to the northwest, will gradually strengthen
the trade winds over the next couple days. The trades are expected
to reach Small Craft Advisory (SCA) thresholds tonight across the
typically windier waters around Maui and the Big Island, with
winds holding at these levels through Thursday. A weak front
approaching the area from the north late this week is expected to
cause a slight disruption in the pressure gradient over the
region, easing the trade winds below SCA levels late Thursday
through Friday. The trades are expected to strengthen to moderate
and breezy levels once again over the weekend, with SCAs likely
being required for the windier marine zones.

Surf along south facing shores will build through the day today,
peak tonight, then gradually lower Wednesday through Thursday.
Long-period energy has arrived early this morning, with the
latest observations as of 3 AM from the Barbers Point and Lanai
buoys indicating a SSW swell of 3-4 feet/20 seconds. This should
translate to solid advisory level surf later along south facing
shores today through Wednesday night. As a result, a High Surf
Advisory (HSA) remains in effect through 6 AM Thursday along
south facing shores of all islands. We will continue to
monitor observations closely as the swell builds, in case a High
Surf Warning might be required during the peak of this event late
today and tonight. There is the potential that advisory level surf
could hold into the day Thursday.

Surf along north facing shores of the smaller islands may
increase slightly late Wednesday through Friday as small northwest
swells spread down the island chain. Small, choppy surf may trend
up slightly along east facing shores this weekend as the trades
strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai
Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central
Valley-Leeward Haleakala-Kona-South Big Island-Big Island North
and East.

Small Craft Advisory from 6 PM this evening to 6 PM HST Thursday
for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.


&&

$$


H Lau/Hui/Jelsema

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