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Hawaiian Islands Synoptic Discussion and Guidance

074
FXHW60 PHFO 201321
AFDHFO

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

.SYNOPSIS...
Trade winds will become light southeasterlies today with as a
cold front approaches the state from the northwest. This front
will stall north of the islands with light and variable winds in
the forecast through Friday. Clouds and scattered showers will
favor windward mountains and interior sections of each island
mainly in the afternoon and early evening time periods. Moderate
trade winds are forecast to return just in time for the Memorial
Day weekend.

&&

.DISCUSSION...
High level icy cirrus clouds, riding the subtropical jet stream,
are moving across the state from southwest to northeast this
morning. Fairly stable bands of cumulus and stratocumulus clouds
are upstream of the islands drifting in on the southeasterly winds
this morning.

A 1025 MB high center far northeast of the Hawaiian Islands will
slowly drift eastward today as a cold front approaching the
state from the northwest breaks down the ridge north of the
islands. Decreasing pressure gradients as the front stalls north
of the islands will cause winds to veer southeasterly today as we
transition to light and a light and variable wind pattern.
A weak upper level disturbance will also pass through the western
islands today, producing a slight enhancement in clouds and
showers mainly over Kauai this afternoon and evening.

Light southeasterly winds today will allow local scale sea
breezes to develop along sheltered leeward slopes as the islands
heat up relative to the ocean. A light easterly background wind
flow from Tuesday through Friday will allow a stronger land and
sea breeze circulation to dominate island weather conditions
through the end of the work week. Weather conditions will feel
hotter and more humid than normal under this light wind pattern.
Clouds and scattered showers will form along windward mountain
ranges, interior sections, and sea breeze convergence boundaries;
favoring the afternoon and early evening hours. Medium range
weather model solutions remain in good agreement with this light
and variable wind pattern through Friday.

Trade winds are expected to return just in time for the Memorial
Day weekend as the stalled front lifts north allowing the ridge
to slowly build back in over the islands. Subtle differences
between the American (GFS) model and the European (ECMWF) model
remain keeping our confidence for returning trades in the fair
category this far out. We continue to lean towards the GFS model
and climatology for the long range Memorial Day wind forecast.
Clouds and showers will then favor more typical windward and mauka
areas.

&&

.AVIATION...
Expect a mainly VFR day across the main Hawaiian Islands area. A
ridge of high pressure located 420 nm N of Kauai will be moving S
throughout today, weakening the trade winds and causing it to veer
to the SE over the western islands.

The area of low clouds and showers has moved on, clearing Kauai in
the process, and thereby lowering AIRMET Sierra for the mountains
north through east of Kauai. Radar returns paint a very loose
field of showers upwind of the islands. There is a concentrated
area of showers but it is located some 45 nm north of Oahu and
will likely miss Kauai. Tops of the showers area between 7 and 10k
feet. Expect brief periods of MVFR ceilings and vis with these
clouds and showers, as they cross the mountain ranges. The coverage
of the clouds and showers is not expected to be extensive to
warrant AIRMET Sierra for mountain obscuration. There is a broken
to overcast layer of cirrus clouds above fl250, covering the
islands.The cirrus should be thinning out later today as the jet
stream begins to move away from the islands.

Although the low level winds are down, there is enough vertical
speed shear at the mid to high levels, lending to the potential of
tempo moderate turbulence between fl220 and Fl300. Satellite
water vapor imagery show transverse banding with the cirrus over
the islands, which is a sign of clear air turbulence. The
turbulence is expected to dissipate by 22z.

&&

.MARINE...
Winds have dropped below advisory levels across the typically
windy waters around Maui and the Big Island early this morning,
therefore the Small Craft Advisory (SCA) has been cancelled. The
winds will continue to ease and shift around to the east-
southeast today through Tuesday, as a cold front approaches from
the northwest. The front will then stall northwest of the area
during the middle and latter part of the week, while high pressure
strengthens northeast of the island chain. This will bring an
increase in the east to east-southeasterly winds Wednesday through
Friday.

A series of small southwest and south swells will produce small
to moderate surf along south facing shores through the middle of
the week. Currently, we have a small background southwest swell
from the Tasman Sea. A reinforcing long-period south-southwest
swell is expected to build Tuesday into Wednesday. Further out, a
larger long-period south-southwest swell is expected to arrive
during the Memorial Day weekend, with surf possibly reaching
advisory levels along south facing shores.

The current small northwest swell will continue to decline
today, with only small surf expected along north facing shores
through Thursday. A new small to moderate sized northwest swell
is expected to arrive Friday and linger through much of the
Memorial Day weekend. Surf along east facing shores will remain
small through the middle of the week. A developing fetch of
moderate trades upstream of the islands could bring a boost to
east shore surf later in the week through the Memorial Day
weekend.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Lau
MARINE...Jelsema

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