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Hawaiian Islands Synoptic Discussion and Guidance

795
FXHW60 PHFO 091933
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
933 AM HST Thu Apr 9 2020

.SYNOPSIS...
Light and variable winds will persist through Friday, with clouds
and showers over the islands most likely in the afternoon and
evening. An 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. Light to
moderate trade winds will begin to return late Sunday and
continue through much of next week.

&&

.DISCUSSION...
Currently at the surface, high pressure systems are located far to
the west and northeast of the islands, with an area of low
pressure well to the north of the state. Closer to home, a
weakening frontal boundary remains over the central islands. The
resulting pressure gradient remains quite weak, with land breezes
beginning the transition over to sea breezes. Visible satellite
imagery shows partly cloudy conditions in most areas, although
there area a few areas where cloud cover is more prevalent. Radar
imagery shows a few light showers across the state, with most
areas rain free at the moment. Main short term concern revolves
around rain chances the next few days.

The weak frontal boundary will slide slowly eastward today and
tonight, then dissipate by Friday. This will keep a light wind
regime in place, featuring daytime sea breezes and overnight land
breezes. A new cold front will approach from the northwest Friday
through Saturday, bringing an increase in west to southwest winds.
The front will slide southeastward across the island chain
Saturday night and Sunday, with moderate north winds filling in
behind the front Saturday night and shifting around to
northeasterly trades on Sunday. High pressure will slide by to the
north of the islands Sunday night and Monday, with light to
moderate trades prevailing. The trades are expected to ease again
Monday night through Tuesday night as surface troughing develops
over the islands. Moderate trades are then expected to return
Wednesday and continue through late in the week.

As for weather details, a sea/land breeze pattern is expected to
continue through Friday, with showers favoring interior mauka
areas during the afternoon/evening and locations near the coast at
night and during the morning hours. An approaching shortwave
trough may de-stabilize things enough for a thunderstorm over the
Big Island Friday afternoon, so will mention a slight chance of
thunderstorms here. Shower coverage should increase a bit Friday
night and Saturday in the low level convergent flow ahead of the
approaching front, with assistance from shortwave energy moving
overhead. Showers will favor south and southwest facing slopes at
night and during the morning hours and interior/mauka locations
during the afternoon. A few showers will accompany the front as it
slides southeastward through the islands Saturday night and
Sunday, but no significant rainfall is expected. We should begin
to see an increase in trade wind showers Sunday night and Monday,
favoring the typical windward areas. Shower coverage and intensity
may then increase Monday night through Tuesday night as surface
troughing develops over the islands and some shortwave energy
moves overhead. A more typical trade wind pattern should then
return by the middle and latter part of next week.

&&

.AVIATION...
Cloud cover over Maui County continues to thin out and showers
that had been impacting portions of the Big Island early this
morning have fizzled out. Currently, an area of heavier showers
sits about 100 miles east of the Big Island, but things are quiet
elsewhere. However, we will see an uptick in afternoon clouds and
showers for interior areas thanks to the afternoon sea breezes. Tempo
MVFR conditions are possible with any heavier showers that do
develop, but expect VFR to prevail.

Land breezes will take over again tonight and clouds and showers
will dissipate. No AIRMETs are currently in effect and none are
anticipated.

&&

.MARINE...
An eastward moving and dissipating frontal boundary lies near
Maui while the west end of a ridge of high pressure lies just
southeast of the Big Island. The overall wind flow is light west-
northwest west of the boundary with slightly stronger south and
southwest winds out ahead of the boundary, mainly over the waters
around the Big Island. Expect generally light winds west of the
boundary today with slightly stronger winds east of the boundary.
Sea breezes will develop nearshore across most areas today with
land breezes developing nearshore tonight.

Winds will remain light Friday and Friday night, favoring a south
to southwest direction, as another weak front approaches the
area from the northwest. Low level winds will become more
westerly on Saturday as the front nears the state. Guidance
indicates a weak frontal boundary will pass the area Saturday
night and Sunday with north to northeast winds filling in behind
it. An area of high pressure will be passing by north of the area
Sunday night through Monday night with moderate trade winds
returning.

A small, short period northwest swell will continue to fill in
today, peak on Friday, then lower gradually Saturday and Sunday. A
series of very small, short period north swells will move through
the area Saturday through Tuesday. A larger northwest swell is
expected to arrive Tuesday night, peak Wednesday and Wednesday
night, then lower gradually on Thursday. Surf heights may approach
low end advisory levels during the peak of this swell. A series
of small south swells will be moving through the area through the
weekend and on through the middle of next week. Peak surf heights
will likely be on Saturday when the largest of these swells
arrive. Due to the lack of trade winds, surf will remain small
along east facing shores on into early next week. With the
expected uptick in trades around Tuesday and Wednesday, surf
heights are expected to gradually pick up at that time.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Dye
MARINE...Burke

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



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