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Hawaiian Islands Synoptic Discussion and Guidance

097
FXHW60 PHFO 150719
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
919 PM HST Sat Sep 14 2019

.SYNOPSIS...
A high will persist far north of the islands. A low aloft is
making the atmosphere unstable and weakening the trade winds near
Oahu and Kauai. Over the next couple of days the low will move off
to the west. Trade winds will strengthen and the atmosphere will
become drier and more stable.

&&

.DISCUSSION...
The 8 PM surface analysis shows a 1031 mb high far north of the
islands near 42N159W. Satellite water vapor loops show a low aloft
550 miles north of Oahu near 29N158W. The low is inducing a
surface trough between the islands and the high weakening the
trade winds around Oahu and Kauai. The low is also making the
atmosphere unstable and allowing locally heavy showers to form.

The high north of the area is forecast to remain nearly stationary
through Monday before moving off to the west. The low aloft is
forecast to move away to the west. As the low moves away, the
atmosphere will become more stable. The trade winds will
strengthen and drier air will spread over the area from the east.
Locally heavy rain is possible through tomorrow afternoon. After
that, trade winds will focus showers over mainly windward areas,
with the showers most active nights and mornings.

&&

.AVIATION...
Trade winds are still relatively light from the east/southeast
and should stay light through at least a good portion of Sunday. A
lower-level trough still lies between the islands and a high
north of Hawaii. As the trough pushes to the north, expect an
increase in wind speeds as we get into Monday.

The heavier rain showers from Saturday afternoon have dissipated
this evening. Expect some windward showers to continue through
the overnight hours, expect some windward showers. For Sunday, we
may have a repeat of what we saw on Saturday afternoon with
leeward showers possible which could be locally heavy at times.
The showers may not be as heavy as they were on Saturday but still
could lead to MVFR conditions over interior sections of the
islands.

AIRMET Sierra is currently in effect for Oahu for mountain
obscuration but we should be able to drop that once the lingering
showers on the windward side diminish tonight.

&&

MARINE...
A surface trough in the vicinity of the Kauai Channel continues
to move west this evening. Although the easterly winds are
generally light west of Molokai, winds are moderate in strength
across Maui County and off the Hamakua coast of the Big Island.
Some high resolution wind models are suggesting that there will
be a boost in the winds on Sunday over some of these waters
of Maui County and the Big Island. We will be evaluating this
information during the course of the evening, but a Small Craft
Advisory (SCA) is likely to be issue with the 6 am HST marine
package. Moderate to locally strong trades are expected to linger
though most of the new work week.

Surf along south facing shores has peaked and will now gradually
subside. But the new week points to a series of modestly small
swells from the southwest, south, and southeast. It commences with
a prolong southwest swell on Tuesday that will last through most
of the week. The southeast swell is slated to arrive Wednesday,
then a south swell follows in on Friday.

Small surf is expected along north, east and west facing shores
through the new week. The boost in the easterly winds will
slightly raise the surf along east facing shores. Then, expect a
small east swell with a moderate period from Tropical Cyclone Kiko
to reach our shores as early as Wednesday. Latest model guidance
is showing a swell of 2 to 3 feet with periods of 11 to 13 seconds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Donaldson
AVIATION...Chevalier
MARINE...Lau

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



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