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Hawaiian Islands Synoptic Discussion and Guidance

178
FXHW60 PHFO 090200
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Wed Apr 8 2020

.SYNOPSIS...
Light and variable winds will persist through Friday, with clouds
and showers over the islands most likely in the afternoon and
evening. A slight increase in west winds is expected Friday into
Saturday as a front approaches from the northwest. The front will
bring a wind shift to the north, and a few showers, as it moves down
the island chain Saturday night and Sunday. After the front passes,
trade winds are expected into next week, delivering primarily
windward clouds and showers.

&&

.DISCUSSION...
Although the trade wind-supporting high is in a position close to
its climatological norm far NE of the islands, the associated
surface ridge is suppressed S and E of the Big Island as deep-layer
low pressure prevails N of the area. With the ridge in this
location, light W to NW winds prevail over the area, allowing island-
scale land and sea breezes to develop. This afternoon's sea breezes
prompted a few showers over the Big Island's Kau slopes, and over
the windward slopes of Haleakala on Maui. Additionally, convergence
associated with a weak front near Oahu is supporting an area of
broken to overcast low clouds from Oahu to Maui, with a few embedded
showers.

The forecast anticipates the ridge to remain suppressed until the
weekend, with a weakly-forced, light wind pattern continuing. On
Saturday, another weak front is expected to move over the islands,
and potentially stall, as it dissipates. Following high pressure on
Sunday and Monday will bring a return of the trade winds as the
ridge moves N of the area. Moisture associated with this front could
potentially fuel increased windward showers during the early to
middle part of next week, with long range guidance depicting
prevailing trade winds.

In the meantime, the weak frontal boundary near Oahu will move SE
down the island chain through Thursday, acting as a weak focus for
cloud and shower development. However, overnight land breezes may
act to keep shower coverage suppressed over land, with afternoon sea
breezes driving a few showers on Thursday. On Thursday night, mostly
dry conditions are expected as the frontal boundary dissipates. A
slight increase in W winds on Friday will likely remain mostly dry.
Showers that develop are not expected to be heavy or widespread, and
will be just as (or more) likely to affect leeward areas as windward
areas.

&&

.AVIATION...
VFR conditions prevail at all TAF sites and these conditions are
expected to continue through the day and on through the evening
hours. There may some brief MVFR conditions across portions of the
area as moisture associated with a weakening frontal boundary near
Oahu.

No AIRMET's are currently in effect and none are expected through
the evening and on through the overnight hours.

&&

.MARINE...
Low pressure centered far north of the state will maintain mainly
light to gentle winds over the coastal waters through Friday with
some increase due Saturday. A barely identifiable front passing Oahu
this afternoon will move eastward and dissipate later tonight.
Gentle westerly winds will dominate over most areas, with southerly
winds prevailing near the Big Island. Another shallow front is
expected to move down parts of the island chain Saturday night and
Sunday, producing a brief period of northerly winds. High pressure
following the front will bring a return to trade winds late Sunday
or Monday. Trades should persist through the middle of next week as
a surface ridge holds just north of the state.

The low north of the state will continue to send pulses of small to
moderate, mainly north-northwest swell into early next week. A
longer period northwest swell currently mixed in will fade tonight,
and the next significant pulse of north-northwest swell will peak at
around 6 to 7 ft 12 seconds on Friday before easing during the
weekend. Another pulse of north-northwest swell is expected early
next week, potentially followed by a larger northwest swell late
Tuesday.

Surf along south and east facing shores will remain rather small
through much of the week. Mainly background south to south-southeast
swells will affect south facing shores, though a slightly larger
south-southwest pulse is possible this weekend. With trades
disrupted locally and upstream of the islands, surf along east
facing shores will be well below the April normal through the
weekend. East shore surf is expected to build early next week as
trades return.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Burke
MARINE...Wroe

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



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