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Hawaiian Islands Synoptic Discussion and Guidance

217
FXHW60 PHFO 201339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Sun Oct 20 2019

.SYNOPSIS...
Lighter winds are expected through tonight, with some afternoon
sea breezes possible over Kauai and Oahu. Showers will continue to
favor windward and mauka areas, but increased moisture over Kauai
and Oahu could lead to more showers across these islands through
tonight. Moderate trades are expected to return Monday and
Tuesday.

&&

.DISCUSSION...
Have made some changes to the forecast through tonight to account
for higher precipitable water (PW) values that are forecast to
reach Kauai and Oahu. Have also weakened winds a bit over Oahu
this afternoon and overnight.

The ridge to the north of the islands is being pushed southward by
a front to the northwest of the islands. Some southeasterly winds
are already making it into the islands, and expect more of that
today as the ridge moves closer to the region. That being said,
the latest high resolution models show winds remaining a bit
stronger over areas near Maui County and the Big Island today than
originally thought.

Satellite derived PW continues to show increased moisture moving
into the waters to the southwest of the islands. An isolated
thunderstorm was noted south of the Kauai coastal waters within
the last hour, and have added the slight chance of thunderstorms
to some of the nearby coastal waters for today. The morning
soundings at Lihue and Hilo had PW values of 1.52 and 1.68 inches
respectively. Normal for October is around 1.4. Models show PW
will increase over Kauai and Oahu to between 1.7 and 2.0 inches,
which is above normal. Near normal 500 mb temperatures are
expected, but as the nearby thunderstorms show, with enough PW,
thunderstorms aren't out of the question. For now will hold off on
bringing any mention of thunderstorms over the islands, but will
be monitoring the situation.

With the increasing moisture, have boosted PoPs over Kauai and
Oahu, and the adjacent waters through tonight. There is
disagreement with the various models as to how much of a chance
there was for rain, but ultimately decided to trend things up
through tonight.

The front is expected to pass by to the north over the next 24 to
36 hours, with the ridge retreating to the north as it lifts to
the north. This will allow the winds to become more easterly and
increase Monday and Tuesday. As this happens, we'll see more
typical trade wind showers.

Another front passing to the north could weaken winds during the
second half of the week. At this time, there is no increasing in
PW during that time, so the main question will be if winds go
light enough to support local land and sea breezes.

&&

.AVIATION...
Moderate easterly trade winds will gradually shift out of the
east-southeast direction through the day today. Lightest winds are
expected over the northwest islands with this pattern, which may
be light enough for sea breezes to develop over portions of Kauai
and Oahu this afternoon. Although the bulk of the clouds and
showers will continue to favor windward and mountain locations,
afternoon showers associated with sea breezes will be possible
over interior areas of Kauai and Oahu.

No AIRMETs are in effect or anticipated at this time. Brief MVFR
ceilings and visibilities are possible in and around passing
showers. Otherwise, VFR conditions will prevail for all areas.

&&

.MARINE...
A surface ridge extending southwestward from a high to the
distant northeast will move south and weaken today as a front
approaches from the northwest. Although trade winds will diminish
and veer to the east-southeast near Kauai and Oahu in response,
moderate to locally fresh easterly trade winds elsewhere will
continue to be sufficiently strong to warrant a Small Craft
Advisory (SCA) for waters around the Big Island today. Increased
moisture associated with a disturbance passing south of the area
will bring a slight chance of thunderstorms to waters south and
west of Kauai today.

The front will stall well north of the islands tonight and
dissipate Monday, allowing the ridge to move north and strengthen,
likely leading to an expansion of the SCA to include other
typically windy zones by Tuesday. Another front approaching late
in the week may once again lead to diminishing winds.

Surf is expected to remain below advisory levels along all shores
through the upcoming week. A small north-northwest swell will
diminish further today into Monday, with a new northwest swell is
possible Thursday into next weekend. A new long-period south
swell has moved in over the last 12-24 hours, with this swell
expected to linger into Monday before diminishing. Trade winds
will push short-period wind waves onto east facing shores for at
least the next couple of days, trending down later in the week.

See the Collaborative Nearshore Swell and Wind Forecast (SRDHFO)
for more details on surf and swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$


DISCUSSION...M Ballard
AVIATION...Gibbs
MARINE...Birchard

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