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Hawaiian Islands Synoptic Discussion and Guidance

037
FXHW60 PHFO 211337
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Mon Oct 21 2019

.SYNOPSIS...
Trade winds are expected to return today and tomorrow, strengthening
to moderate levels. Expect showers to be focused over windward
and mauka areas. The trades will back off a bit during the mid to
later part of the week.

&&

.DISCUSSION...
Have made a variety of changes to the forecast. Little change in
the winds for the next day or so, but then have generally trended
the trade winds down, in line with the general model consensus
over the last several runs. Have also turned the winds a bit more
southeasterly for the end of the week. Have also adjusted the PoPs
for much of the week, with an increase in PoPs over the southern
end of the state for the end of the week and into the weekend, and
a slightly drier pattern for several other areas.

High pressure to the northeast has a ridge to the north of the
islands. A weakening front north of the ridge is expected to
further weaken and continue to move to the east, allowing the
ridge to lift northward today. This should allow a more easterly
trade wind flow to spread across the islands, with winds trending
upwards. Satellite derived precipitable water (PW) shows an area
of increased moisture southeast of the Big Island. Satellite shows
plenty of clouds in this area, and the models suggest an increase
in showers will brush the Big Island later today and tonight.

Previously, the forecast reflected moderate, easterly trades
persisting through much of the week, however the last few runs of
the global runs have shown a lighter wind flow pattern, with some
periods of east-southeasterly to southeasterly flow. Have trended
the forecast towards this pattern, and have also modified the PoP
areas as a result.

The ridge to the north of the islands would be shifted southward
again next weekend, bringing southeasterly winds to the region.
Model PW shows another batch of 1.7+ inches reaching the Big
Island Friday night through Saturday night. The forecast has been
updated to reflect an upward trend in PoPs for the Big Island and
waters nearby during this time. Meanwhile, the southeasterly wind
flow alters some of the areas with PoPs so have made those
adjustments as well.

&&

.AVIATION...
Trade winds will re-strengthen today, focusing most of the clouds
and shower activity on the windward side of the islands for the
next few days. Brief MVFR conditions are possible in passing
showers, but VFR conditions are expected to prevail, with no
AIRMETs expected through tonight.

&&

.MARINE...
Since the most current winds remain at borderline Small Craft
Advisory (SCA) speeds, the SCA continues through this afternoon
for waters south of the Big Island and Alenuihaha Channel.

A front has been stalling well north of the islands and will
dissipate later today, which will result in the nearby ridge
moving north and strengthening. This will boost winds slightly
across the area and will likely lead to an expansion of the SCA to
include other typically windy zones by tonight into Tuesday.
Another front approaching late in the week may once again lead to
diminishing winds.

Small background northwest swells will persist over the next
several days along north facing shores. Further out, the latest
model guidance is indicating surf rising late Wednesday through
Thursday and peaking Friday along north facing shores. There are
still some uncertainties with the swell heights as the swell
makes it to local shores. However, with the typical low bias of
these models, the isles may see surf heights near High Surf
Advisory levels along north and west facing shores later this
week.

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. Surf along south facing shores will remain small
through the week with mainly background swells out of the SW,S,
and SE.

&&

.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...TS
MARINE...Kinel

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



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