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Hawaiian Islands Synoptic Discussion and Guidance

957
FXHW60 PHFO 192002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Fri Oct 19 2018

.SYNOPSIS...
A weak trade wind flow will lead to localized afternoon sea
breezes today. A disturbance aloft reaching the islands today will
trigger spotty afternoon showers, some heavy from Kauai to Maui,
while deeper moisture in place over the Big Island will fuel
widespread showers and thunderstorms this afternoon. The
disturbance aloft will slowly pass over the state tonight through
the weekend as trade winds gradually increase. Heavy showers and a
few thunderstorms will be possible, especially across the Big
Island. 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
expected today. Trades will remain in the gentle to locally
moderate range today, thanks to a stalled front 500 miles north of
Kauai that is weakening the local pressure gradient. A mid- to
upper-level trough that has been overhead will be reinforced by a
shortwave aloft that is currently approaching Kauai. The
atmosphere remains moist and unstable over the Big Island, where
the overnight sounding showed no inversion and precipitable water
(PW) values of around 2 inches, and the upper tough is maintaining
a shield of high clouds over the eastern half of the island
chain. Moisture decreases significantly toward the western half of
the island chain, where PW values are near seasonal normal. Expect
sea breezes to trigger spotty showers, some briefly heavy, over
interior sections and terrain from Kauai to Maui this afternoon.
More extensive shower and thunderstorm development is forecast
this afternoon on the Big Island, where a Flash Flood Watch is in
effect.

The unstable trade wind flow will persist through at least
Saturday for much of the state. Unstable conditions will persist
along and east of the mid- to upper- level trough, which will
creep eastward, and building high pressure to the northwest of
the state will drive a gradual increase in the trade winds. The
deepest moisture will remain near the Big Island and perhaps Maui
County. This will place the greatest chances for heavy showers and
thunderstorms over the eastern half of the state, though Kauai
and Oahu will have some threat of thunderstorms tonight and early
Saturday. With the trades on the rise, windward areas will have
the highest chances of rainfall. Leeward areas are expected to
see some convection in the afternoon, especially on the Big Island
and Maui.

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 today into the overnight hours.
Winds will be light enough to set up daytime sea breeze/night
land breeze scenario. With the pressure gradient gradually
tightening from through the day Saturday expect increasing trade
winds.

There will be isolated MVFR conditions in windward and mauka areas
today. 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, though mountain obscurations
are possible this afternoon with showers and low ceilings.
Mountain obscurations are more likely Saturday into Sunday,
especially for the Big Island.

&&

.MARINE...
The arrival of a significant long-period S swell is anticipated
today and tonight, with surf along S facing shores of all islands
building to advisory levels by Saturday. A High Surf Advisory has
been issued, with the swell expected to remain elevated for several
days. 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 are showing
the longer period wave energy increasing this morning.

Elsewhere, surf heights will remain below advisory levels for at
least the next several days, although a new short-period NNW
swell is expected today. See the Oahu Surf Discussion (SRDHFO) for
additional details.

Light to locally moderate trade winds are expected through tonight,
with a gradual increase in NE trade winds Saturday as high pressure
builds NW 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 S 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



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