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Hawaiian Islands Synoptic Discussion and Guidance

148
FXHW60 PHFO 270137
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Wed Feb 26 2020

.SYNOPSIS...
A stable and breezy trade wind flow will continue to focus modest
rainfall along windward slopes through Thursday. High pressure
will strengthen northeast of the state on Friday, possibly
boosting trades further through the weekend. A low aloft will
drift overhead during this time, leading to an increase in
showers, some possibly heavy with a slight chance of
thunderstorms. Wet conditions may persist into early next week as
trades decline and potentially shift out of the southeast.

&&

.DISCUSSION...
A stable, breezy, and gusty trade wind flow continues. A surface
ridge far north of Kauai is driving the breezy trades, and a
broad ridge aloft is maintaining stable conditions with a solid
low level inversion near 6500 ft. Precipitable water below
seasonal normal at an inch or less, leading to shallow showers
within the trade wind flow. These showers dropped modest rainfall
of less than 1/4 of an inch across windward slopes today, while
the few showers that briefly spread to leeward areas produced
little to no accumulation. Dew points are holding in the upper 50s
to lower 60s, which is maintaining a somewhat cool feel under the
breezy trades.

Expect little change through tomorrow. The surface ridge far
north of the state will break down as another, strengthening high
moves in from the western Pacific. Trade winds should be more or
less unchanged during this transition. The ridge aloft will erode
as a mid to upper level low far east of the state deepens, but
stable conditions will hold with modest rainfall remaining focused
over windward slopes.

On Friday through the weekend, there will be a possibility of
stronger trade winds. The above mentioned surface high will
strengthen to around 1040 mb as it becomes parked about 1300 to
1500 miles north-northeast of the state. A high of this strength
and at that position would normally ensure that trades would ramp
up significantly. However, there is some uncertainty, as the mid
to upper level low currently east of the state approaches on
Friday and drifts overhead during the weekend. This feature will
likely weaken the local surface pressure gradient, keeping the
strongest trade winds to the northeast of the islands. At this
time, it still appears that we will see an increase in trades,
possibly requiring a Wind Advisory.

There is greater confidence that there will be an increase in
shower activity late Friday through the weekend. As the mid to
upper level low moves overhead, precipitable water will not change
substantially, but very cold temperatures aloft will produce
unstable conditions that will likely generate spotty showers, some
briefly heavy, and perhaps a few thunderstorms. Rainfall will
continue to be focused along windward slopes, but the gusty trades
will easily push fast-moving, though briefly heavy, showers
across leeward areas. In addition, winter weather will be
possible on the high summits, and freezing levels could drop low
enough to affect Haleakala.

Uncertainty remains rather high heading into next week. The GFS
and ECMWF show the mid level low dissipating and lifting
northward on Monday, while an associated surface trough is pulled
over the islands from the east. Trades should drop and may shift
out of the southeast if the trough is strong enough. Moisture
convergence on the east side of the trough could lead to another
period of wet weather for some windward areas, but uncertainty
remains too high to provide any details.

&&

.AVIATION...
A strong surface high northeast of the Hawaiian Islands will
maintain locally strong northeast to east trade winds around the
main islands. AIRMET TANGO remains in effect for TEMPO MOD TURB
below 8000 feet south thru west of the mountains of all the
islands.

VFR conditions will prevail, but the trade winds will push broken
clouds and isolated showers over windward areas of the islands
producing isolated MVFR ceilings and showers.

&&

.MARINE...
Strong to near gale force trade winds are expected to prevail
into Sunday, with wind speeds gradually increasing as a surface
high strengthens to near 1040 mb N and NE of the islands. Winds
are expected to be strongest from Friday night into Sunday.
Forecast uncertainty increases thereafter as a potent mid-level
low develops over the area, and induces a surface trough near the
islands. When and where the surface trough forms, and where it
moves, will have profound affects on wind speed and direction from
late Sunday into next week. The forecast currently anticipates a
weak trough developing just E of the islands on Sunday, then
moving W and sharpening somewhat Monday, with winds responding by
veering to the SE and weakening. The destabilizing low aloft will
bring the potential for thunderstorms from Friday into next week.

With trade winds increasing near and upstream of the islands,
combined seas will build, and a Small Craft Advisory (SCA) is in
effect for all zones through the weekend. Surf along E facing
shores will also increase over the next several days, with surf
rising to advisory-levels by Saturday. Latest guidance places a
belt of enhanced trade winds immediately upstream of the islands
late in the weekend as the surface trough develops, leading to a
surge in mid-period ENE swell/wind waves Sunday/Monday, with
warning level surf along E facing shores a distinct possibility.

A High Surf Advisory (HSA) for N and W facing shores has been
dropped, but another long-period WNW swell arriving Thursday into
Friday will push surf heights back up close to advisory levels
along exposed shores. Pending the swell's arrival at upstream
buoys, a new HSA will likely be issued on Thursday. This swell
will gradually diminish through the weekend, with no other
significant NW swells in the forecast through the middle of next
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Donaldson
MARINE...Birchard

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



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