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Hawaiian Islands Synoptic Discussion and Guidance

317
FXHW60 PHFO 220119
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
319 PM HST Sun Jul 21 2019

.SYNOPSIS...
A trough of low pressure will pass by a few hundred miles to the
south of the islands during the next couple days, bringing an
increase in clouds and showers from southeast to northwest across
the island chain tonight through Tuesday. Drier weather and
decreasing cloud cover is expected by Wednesday as the trough of
low pressure exits to the east and high pressure builds to the
north. Moderate to locally breezy trades and typical windward and
mauka showers with the occasional leeward spillover will then
prevail into next weekend.

&&

.DISCUSSION...
Currently at the surface, a trough of low pressure is located
around 320 miles southwest of South Point on the Big Island,
while a 1028 mb high is centered around 1225 miles north of
Honolulu. The resulting gradient across the island chain is
producing moderate to locally breezy trade winds this afternoon.
Visible satellite imagery shows partly cloudy skies in most
areas, with bit more extensive cloud cover over some island
interiors, particularly the Big Island and Kauai. Radar imagery
shows a few light showers moving into windward areas, with a few
pop up convective showers also over leeward sections of the Big
Island. Main short term concerns revolve around the potential for
heavy rainfall and thunderstorms early in the work week.

Tonight through Tuesday,
Model solutions remain in fairly good agreement showing the
trough of low pressure moving off to the west-northwest over the
next couple days, passing by a few hundred miles to the south of
the island chain. Meanwhile, the ridge of high pressure well north
of the islands will shift eastward. The resulting gradient will
keep a general moderate to locally breezy trade wind flow in place
across the island chain, with perhaps a subtle easing Monday
night and Tuesday.

The latest guidance continues to depict a plume of deep tropical
moisture associated with the trough of low pressure passing very
close to the island chain, with precipitable water values within
the plume exceeding 2 inches. The big question continues to be
how far north this deep moisture will get. The latest GFS and
ECWMF solutions keep the highest precipitable water values just
south of the state. This aligns very well with the latest UW-
CIMSS total precipitable water imagery. As a result, the
potential for heavy rainfall in association with this trough of
low pressure appears like it will remain just to the south of the
islands. Overall, we should see showers increase across the
island chain beginning tonight across the Big Island, and
spreading northwestward to the smaller islands Monday and Monday
night. Conditions should then begin to dry out from east to west
across the island chain Monday night and Tuesday. With the trough
passing by to the south of the state, we can expect a considerable
amount of cloud cover across the islands Monday through Tuesday
as well. The potential for thunder over the islands appears to be
decreasing, with the highest chances for thunderstorms expected on
the Big Island during the afternoon and evening hours Monday,
where a slight chance of thunderstorms remains in the forecast.

Tuesday night through Saturday,
The GFS and ECMWF are in good agreement showing the trough of low
pressure continuing to shift west-northwestward and away from the
state, while high pressure builds to the north of the island
chain. As a result, we should see a return of more typical trade
wind weather featuring windward and mauka showers and the
occasional leeward spillover. Trade wind speeds should hold in the
moderate to locally breezy range through the period.

&&

.AVIATION...
Strong high pressure north of the state will continue to generate
locally strong east winds over the area over the next couple of days.
AIRMET TANGO remains in effect for moderate turbulence below 8,000ft
and will likely continue through Monday.

Scattered to broken low clouds and light showers will continue to
be carried over east sections of the islands in the trade wind flow.
Isolated MVFR ceilings are possible over these areas, especially in
the overnight to early morning hours and for the Big Island and Kauai.
Light sea breezes will continue this afternoon and into the early
evening across leeward portions of the Big Island. This will result
in an increase in showers, as well as localized heavy downpours and
a slight chance of thunderstorms over the island interior and
leeward slopes, diminishing overnight. A disturbance south of the
Big Island will bring a chance of thunderstorms to the waters
south and west of the Big Island as well, beginning tonight.

&&

.MARINE...
Fresh to locally strong easterly trades will continue through the
week as high pressure remains north of the area. Stronger winds
will remain across the typically windier locations from Maui
County to the Big Island due to terrain accelerations. The current
Small Craft Advisory through 6 PM Monday will likely be extended
into the week.

Rough surf will continue along east facing shores due to the short
period trade wind swell. A slight downward trend is expected
through Tuesday as upstream trades relax. A slight uptick is
expected once again Wednesday into Thursday as upstream trades
increase.

Surf along south facing shores will remain small through tonight,
then trend up this week due to a recent system moving across the
Tasman Sea. Heights should remain below advisory levels as this
southwest swell moves through and peaks Monday and Tuesday. By
mid-week, a combination of small south swells will be enough to
keep surf from going flat along southern shores. A south-
southeast long-period swell associated with a compact gale will
be possible Friday. Surf will rise along exposed shores but
looks to remain below advisory levels.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...TS
MARINE...Morrison

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



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