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Hawaiian Islands Synoptic Discussion and Guidance

045
FXHW60 PHFO 202002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Mon May 20 2019

.SYNOPSIS...
Gentle to moderate east southeast winds expected through Friday which
will allow daytime sea breezes and nighttime land breezes to set
up over the islands. Clouds and showers will build each day across
island interior sections and then clear at night. Expect warm and
humid conditions to continue as well. Trade winds are forecast to
return over the Memorial Day weekend.

&&

.DISCUSSION...
A 1027 mb surface high is positioned about 1150 miles northeast
of Kauai while an approaching front is about 800 miles northwest
of of Kauai. The two features will bring gentle to moderate east
southeast wind flow across the state. Because the wind direction
will line up parallel to the island chain, most areas will be
blocked from the wind flow allowing sea breezes along with
interior clouds and showers to set up later today. Winds will then
shift to land breezes tonight with cloud cover and showers
decreasing over the islands. Exceptions to this will be along the
Hana and Hamakua coastlines which are generally more exposed to
east southeast winds. Both Hilo and Hana may see stronger than
typical winds from the topographical effects.

The diurnal weather pattern will persist over through much of the
work week with mostly sunny mornings, cloudy afternoons and
clearing at night. Conditions will feel warmer and more humid
under this light wind regime. Rain amounts should be on the light
side, but can't rule out some moderate intensity showers
developing over the island interior sections each afternoon.

An upper level trough will gradually shift closer to the islands
to the west over the next several days, but should not destabilize
our weather. However, we can expect some passing cirrus clouds
moving across the area as the subtropical jet along the eastern
flank of the trough moves on top of the island chain. The front is
expected to stall just west of Kauai Tuesday night and then
gradually shift back to northwest during the middle and ending of
the work week. Long range guidance shows trade winds slowly
building in over the Memorial Day weekend as the front weakens and
dissipates.

&&

.AVIATION...
Expect a mainly VFR day across the area. A ridge of high pressure
N of Kauai will move S today, weakening the trade winds and causing
them to veer to the SE over the smaller islands.

Shower activity is quite light across the islands this morning and
is expected to remain so through this afternoon. High cirrus clouds
continue to stream across the state today as well, ahead of a front
far NW of Kauai. With the trades weakening, expect some cloud build-
ups and possible a light shower or two over leeward areas of each
island this afternoon. The coverage of the clouds and showers is
not expected to be extensive to warrant AIRMET Sierra for mountain
obscuration.

Although the low-level winds are weakening, there is enough vertical
speed shear at the mid to high levels, leading to the potential
of tempo moderate turbulence between FL220 and FL300. The turbulence
is expected to dissipate this afternoon.

&&

.MARINE...
High pressure to the northeast of the state will continue to move
to the east as a low moves north of the islands. The low and its
associated cold front will remain north of the islands, but will
ease winds and shift them to the east to southeast near the
islands through tomorrow. The low will move to the north as we
head into the middle of the week, stalling the front to the north
of the islands. This will allow the surface ridge to build back
over the area during the second half of the week, allowing winds
to strengthen and become more easterly.

Surf is expected to remain below advisory levels through the week
for all shores. A series of small southwest and south swells will
produce small to moderate surf along south facing shores through
the middle of the week. The Memorial Day weekend could advisory
level surf with the arrival of a larger long-period south-
southwest swell.

Small to moderate sized swells will impact the north and west
facing shores into the holiday weekend. Surf along east facing
shores will remain small through the middle of the week. As winds
increase during the second half of the week, we can expect a boost
to surf along east facing shores this weekend.

&&

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

&&

$$

DISCUSSION...Foster
AVIATION...TS
MARINE...M Ballard

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

Current Conditions
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RH		15 %
Wind		ENE 12 mph
Road		Open (4x4)