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Hawaiian Islands Synoptic Discussion and Guidance

599
FXHW60 PHFO 061937
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
937 AM HST Mon Apr 6 2020

.SYNOPSIS...
Light winds will persist through the week. A weakening front
moving towards the islands through Tuesday will bring some
locally heavy downpours and a few thunderstorms. Scattered
showers will continue across through the remainder of the week,
as the front slowly dissipates over the central and eastern
islands.

&&

.DISCUSSION...
No changes to the forecast this morning. Wind reports across the
islands show light winds prevailing and that is expected to
continue for much of the week. The lightning networks this morning
have detected thunderstorms near Kauai, in the Kauai Channel and
near Molokai over the last couple of hours. More thunderstorms are
possible across the region today, and again tomorrow during the
day.

A low pressure system about 1000 miles north-northwest of Kauai
will be moving to the northeast over the next 24 hours or so. The
overnight surface analysis showed a ridge over the islands, with a
trough to the northwest of Kauai, and a cold front about 500 miles
northwest of Kauai. The ridge is helping to keep winds light over
the islands, while an upper level trough associated with the
surface trough and front are helping to destabilize the
atmosphere. This combination has led to the thunderstorm and
locally heavy rain that has occurred over and near the islands in
the daytime heating. A drier airmass combined with warmer
temperatures aloft moving in Tuesday night will limit any
thunderstorm development by Wednesday. However the light winds
will persist, which will lead to cloud and shower development over
the islands, mainly during the day during the second half of the
week.

The low mentioned above will continue to move to the northeast
through the week, keeping the ridge over or south of the islands,
resulting in the lighter winds. Another low forms Thursday to the
northwest of the islands and moves to the east before lifting to
the north Friday night. Another cold front associated with this
low looks to approach the islands next weekend. At this time, the
models show upper level dynamics will not be conducive to heavy
rain or thunderstorms, but of course we will continue to monitor
the situation with future model runs.

&&

.AVIATION...
VFR conditions prevail at all TAF sites but occasional MVFR
conditions can be expected at times especially from Maui County
westward, as bands of showers move up from the south. A moist and
unstable airmass across the area will allow for a slight chance of
thunderstorms through this evening.

AIRMET Sierra remains in effect for tempo mountain obscuration
for Kauai, Molokai, Lanai, and Maui. This AIRMET may be expanded
or cancelled across some areas as band of showers move up from the
south.

AIRMET Tango remains in effect for tempo moderate turbulence
between FL230-340 across the area. This AIRMET may be discontinued
later today or tonight.

&&

.MARINE...
A weakening trough near Kauai and a front northwest will slowly
make its way through the Hawaiian waters over the next few days,
then stall near the eastern waters during the second half of the
week. This will lead to a light to moderate wind regime with a
southerly flow expected ahead of the front and a westerly flow
behind it.

Latest satellite imagery is showing a broad area of convergent
southerly flow near Kauai with isolated thunderstorms developing
over the northern waters and in the Kauai Channel. As a upper
level short-wave passes through the area, we should see an increase
in showers and some thunderstorms over the area through Tuesday.
With seas and swells expected to stay below Small Craft Advisory
thresholds for much of this week, the main threat to mariners
will be thunderstorm development.

A small long-period northwest swell will fill in today and peak
tonight around 4 to 5 feet. This swell will be followed by a
slightly larger northwest swell arriving late Tuesday, which is
expected to peak Tuesday night or early Wednesday below advisory
level for north and west facing shores. A series of small south
to southeast swells are expected this week, with surf expected to
reach the summer average by Tuesday. Surf along east facing shores
will remain small this week due to the lack of trades.

&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...Burke
MARINE...TS

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



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