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Hawaiian Islands Synoptic Discussion and Guidance

364
FXHW60 PHFO 200155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Fri Oct 19 2018

.SYNOPSIS...
A weak trade wind flow will persist through the night. A
disturbance aloft passing over the western end of the state
tonight will trigger spotty showers and thunderstorms from Kauai
to Maui, while deeper moisture in place over the Big Island will
fuel more extensive shower and thunderstorm activity. The
disturbance aloft will slowly pass over the state Saturday and
Sunday as trade winds gradually increase. Heavy showers and a few
thunderstorms will be possible over most of the state on Saturday
and will likely be confined to the Big Island on Sunday. Stronger
trade winds will focus clouds and showers over windward slopes
next week, though portions of the state could experience an
increase in rainfall mid week.

&&

.DISCUSSION...
A continued weak and increasingly unstable trade wind flow is
in place. Trades will remain in the gentle to locally moderate
range, thanks to a dissipating front 450 miles north of Kauai that
is weakening the local pressure gradient. A mid- to upper-level
trough that has been overhead continues to send a shield of high
clouds over Maui and the Big Island and is being reinforced by a
shortwave aloft that is currently moving over Kauai. The
atmosphere remains moist and unstable over the Big Island, and the
absence of an inversion and precipitable water (PW) values of
around 2 inches have led to fairly extensive showers and
thunderstorms today. Moisture decreases significantly toward the
western half of the island chain, where PW values near seasonal
normal have allowed only spotty sea-breeze-driven heavy showers
from Kauai to Maui this afternoon.

The unstable trade wind flow will persist tonight and Saturday
for much of the state. Unstable conditions will be greatest along
and east of the mid- to upper-level trough, which will creep
eastward from Kauai to Maui, and building high pressure to the
northwest of the state will drive a gradual increase in the trade
winds. The deepest moisture and threat of heavy showers and
thunderstorms will remain near the Big Island, where a Flash Flood
Watch will be in effect. However, heavy showers and thunderstorms
will remain a threat for all islands tonight, with the threat
diminishing for Kauai on Saturday then Oahu Saturday night.

Building trade winds and increasing stability are expected on
Sunday. The GFS and ECMWF still differ on the evolution of the
mid- to upper-level trough, so there is some uncertainty. The GFS
continues to push the feature east of the state Sunday night,
which would lead to a stable trade wind flow with mainly windward
rainfall Sunday night. The ECMWF has been trending toward the GFS
solution, but it lags the weakening trough near the Big Island
for another day. The forecast favors the GFS, with the limited
thunderstorm threat on the Big Island ending on Sunday evening
and a typical trade wind flow expected late.

Moderate to breezy trade winds will likely settle over the
islands on Monday and Tuesday. The next feature of interest is an
area of disturbed weather in the tropics, currently located about
1,000 miles southeast of the Big Island. This feature could brush
the state with enhanced rainfall sometime Tuesday through
Thursday, with higher chances of rainfall to the south.

&&

.AVIATION...
Expect relatively light winds into the overnight hours. Winds
will be light enough for daytime sea breezes/night land breezes.

There will be isolated MVFR conditions in windward and mauka areas
this afternoon and evening. The windward mauka MVFR conditions
will be more likely through the day Saturday with heavier showers
and possible thunderstorms with the upper level trough dragging
slowly over the island chain. The Big Island will have the most
rain with possible thunderstorms creating widespread MVFR
conditions.

No AIRMETs are currently in effect. Mountain obscurations are
more likely Saturday into Sunday, especially for the Big Island.

&&

.MARINE...
A significant, long period south swell will build tonight and
maintain into early next week. A High Surf Advisory is now in
effect for all south facing shores. Surf heights will build to
advisory levels during the overnight hours. Peak surf heights with
this long-lived swell could approach the south shore High Surf
Warning threshold (15 feet) over the weekend, and buoy
observations will be monitored to fine tune the forecast. Near
shore buoys exposed to the south swell show the longer period
wave energy increasing.

Elsewhere, surf heights will remain below advisory levels for at
least the next several days, A moderate short period north northwest
swell is expected gradually drip through Saturday. See the Oahu
Surf Discussion (SRDHFO) updated today for additional details.

Light to locally moderate trade winds are expected through tonight,
with a gradual increase in northeast trades Saturday as high
pressure builds northwest of the islands. Winds should remain
below Small Craft Advisory (SCA) speeds through Sunday, with
moderate to locally fresh trade winds potentially requiring a SCA
for the windier zones early to mid next week. Uncertainty with
this portion of the forecast is due to forecast models increasing
the low-level pressure gradient as a tropical disturbance passes
south of the islands.

A persistent upper-level trough over the area will develop a cut-
off low this weekend, maintaining unstable conditions that will
bring the threat of locally heavy showers and/or thunderstorms,
especially over Big Island waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 PM this evening through Monday afternoon
south facing shores of all islands.

Flash Flood Watch through Saturday afternoon for Big Island.

&&

$$

DISCUSSION...Wroe
AVIATION...Chevalier
MARINE...Foster

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