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Hawaiian Islands Synoptic Discussion and Guidance

150
FXHW60 PHFO 190203
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 PM HST Sat May 18 2019

.SYNOPSIS...
High pressure to the northeast will drive moderate to locally
fresh trade winds through Sunday. Scattered showers to ride in
with the trades, affecting mainly the windward and mountain areas
of the smaller islands through Sunday. Frequency of these showers
will be highest during the night and morning. A change in the
weather is on tap for Monday, with warm and humid conditions
settling in as the winds turn east southeast and daytime sea
breezes and nighttime land breezes take over. Afternoon clouds and
showers will be followed by clearing during the nighttime through
much of next week.

&&

.DISCUSSION...
A 1024 mb high is centered 1000 miles northeast of the Hawaii
tracking eastward. The high is driving moderate to locally fresh
trade winds across the islands. Visible satellite and radar show
a few low clouds and showers over and around the smaller islands
late this afternoon. A midlevel inversion is providing stable
conditions across the area. 00Z soundings at Hilo and Lihue show a
midlevel inversion around 6000 feet with precipitable water
values of 1.03 inches at Hilo and 1.31 inches at Lihue. Trade
winds have gradually been sliding the area of moisture across the
smaller islands westward through the day, while the Big Island
continues to be drier than the rest of the state.

Anticipating a relatively dry and pleasant evening, with some of
the aforementioned clouds and showers filling back in overnight
into Sunday morning. Guidance shows windward sides of Kauai and
Oahu getting the most rain tonight with amounts up to a half inch
possible. Leeward sides, particularly the upper slopes, will see
passing showers at times. Mostly sunny to partly cloudy weather is
expected on Sunday, with showers filling in again at night. Windward
Big Island may see an uptick in showers as well Sunday night.

A gradual change in the weather is expected beginning on Monday as
a cold front approaches from the northwest. The winds are forecast
to weaken a bit and veer to the east southeast. Aside from areas
such as Hilo and Hana on Maui which are exposed to the southeast
flow, daytime sea breezes and nighttime land breezes should take
over for most of the state Monday possibly until around Thursday.
A moderate uptick in showers is expected ahead of the front with
the focus over windward and southeast slopes at night, shifting to
leeward and interior sections during the day as the sea breezes
set in. Temperatures will also be on the warm side during this
period along with elevated dew points. Latest guidance shows the
front stalling and weakening just northwest of Kauai around the
middle of next week, with high pressure slowly pushing the front
back to the west Thursday and Friday.


&&

.AVIATION...
A 1024 MB surface high to the north of Hawaii will allow for
light to moderate easterly trade winds to persist through this
afternoon and into the evening. This flow pattern will act to
concentrate most of the cloud cover and shower activity along the
windward coast and mauka slopes. MVFR conditions could occur
periodically tonight due to mountain obscuration from bands of
thick low clouds that advect onshore. Otherwise, VFR conditions
expected elsewhere.

No AIRMETs are current in effect.


&&

.MARINE...
High pressure far north of the state will maintain moderate to
locally strong trade winds through the weekend. As a result, a
Small Craft Advisory (SCA) is in effect for the typically windier
waters adjacent to Maui County and the Big Island, and will
likely continue through Sunday night. The winds will then ease and
veer to the east-southeast early next week as the high moves
northeast of the area ahead of a front approaching from the
northwest.

A series of small southwest and south swells will produce small
to moderate surf along south facing shores through the middle of
next week. A low pressure system currently developing southeast
of New Zealand is expected to strengthen tonight into Sunday and
will likely produce a long-period south-southwest swell for next
weekend. If this storm materializes as expected, surf may
approach the High Surf Advisory criteria Memorial Day weekend.

A small northwest swell is starting to fill in this afternoon and
will continue to fill in overnight tonight. This small northwest
swell is expected to peak on Sunday and lower into early next
week. Small, choppy surf along east facing shores may get a
slight boost on Sunday due to the stronger trade winds. Another
increase around mid-week is possible due to a developing fetch of
moderate trades east of the islands.


&&

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


&&

$$


DISCUSSION...Foster
AVIATION...Thomas
MARINE...TS

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



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