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Temp8.1 C
RH8 %
WindNE 16 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

110
FXHW60 PHFO 201922
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
922 AM HST Sun Sep 20 2020

.SYNOPSIS...
A bit of an increase in shower activity is expected through
tonight, mostly over windward areas, and mostly overnight and int
the early morning hours. Drier air is expected Monday through
midweek, which will limit shower coverage. Moderate trade winds
are more likely to be locally breezy Wednesday and Thursday, with
a bit of a downward trend next weekend.

&&

.DISCUSSION...
High pressure to the north of the islands will continue to bring
moderate trades to the region. The high is expected to be
disrupted by a front passing north of the islands during the first
part of the work week, with a new high settling in behind the
front. The new high will help to boost winds a little during the
second half of the week, but then look for winds to weak The
front will introduce additional moisture upstream of the islands
which will likely be carried in on the trades during the second
half of the week.

Water vapor imagery shows a mid to upper level low about 450 miles
west-southwest of Oahu. On the east side of the associated
trough, southwest upper level winds are bringing some high clouds
to Kauai and Oahu, however most of the high clouds are remaining
further south of the islands. The presence of the upper low could
be contributing to some of the shower activity over Kauai this
morning.

The global models are in good agreement with an upper level
trough/low to the Northwest of the islands for the next 24 to 36
hours. They are also in good agreement with the development of a
closed upper low north- northwest of the islands along the upper
trough Tuesday night. The GFS and ECWMF keep the low/trough
further northwest during the second half of the week, but bring it
towards the islands as we head into the weekend. That is when the
models diverge in their handling of the upper low, so will have
to keep an eye on that over the next few days to see how the
models handle it. Not expecting any significant weather from this
upper low, but it could help to increase shower activity depending
on how close it comes to the islands.

No significant changes are expected to the grids for today, with
things largely on track. Will likely be tweaking some of the PoPs
and associated fields by the afternoon package.

&&

.AVIATION...
A high far north of the area will maintain moderate to locally
strong east winds over the area. The low level flow will push
some clouds and showers over east sections, especially through the
morning. These showers will be most active over Kauai. Sea
breezes will bring some clouds and showers to the southwest
section of the Big Island of Hawaii during the afternoon and
evening.

AIRMET Sierra remains in effect for mountain obscuration on Kauai.

&&

.MARINE...
Moderate to locally strong trades will continue through tonight,
then slightly weaken to fresh speeds as the high to our northeast
weakens. Another high will build north of the state towards the
middle of the week and will likely bring a slight increase of wind
speeds during the second half of the week. A Small Craft Advisory
remains in effect through late tonight for the typically windy
areas around Maui County and the Big Island.

The current south-southwest swell will begin to decline tonight,
but a new smaller southwest swell should begin to fill in on
Monday. This should help keep surf heights near the summer average
(3 to 5 feet) during the first half of this week. Another small
south-southwest swell is possible Friday into Saturday.

Along north facing shores, we will continue to see small northerly
swells over the next few days. A moderate long-period north-
northwest swell will begin to fill in on Wednesday and peak
Wednesday night into Thursday for north and west facing exposures.
Surf along east facing shores will remain small through much of
the week, but northeast facing exposures should see some moderate
surf from the north-northwest swell during the middle of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


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

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