Flash Flooding and Heavy Rain in Hawaii, November 3 to 6, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
820 PM HST Fri Nov 2 2018

.SYNOPSIS...
High pressure northeast of the islands will support breezy trade
winds through the weekend. The trade winds will focus clouds and
showers over windward slopes with isolated to scattered showers
developing over leeward areas, while a low aloft will bring
unstable conditions to the region. This added instability will
result in some windward areas receiving periods of frequent
soaking showers, especially for the Big Island and portions of
Maui County where isolated thunderstorms are also possible. Drier
weather and lighter trade winds are expected by the middle of
next week with winds becoming more light and variable ahead of an
approaching cold frontal trough.

&&

.DISCUSSION...
There are two main weather features to discuss tonight, an upper
level cut off low moving in from the northeast, and the breezy
trade winds from the surface high far to the northeast of the
Hawaiian Islands. These two features will combine to produce a wet
trade wind environment across the island chain for the next
several days. Expect wet trades across all islands through Tuesday
with higher rainfall trends across the eastern half of the state.

The upper level low is roughly 250 miles northeast of the Hilo
this evening moving quickly towards the Big Island. Weather models
remain in good agreement with the track of this system as the
center of the low settles in just northeast of Maui and Hawaii
Counties by early Saturday morning. The cold core of this system
will create an unstable environment for areas under this low
pressure system. Enough instability will bring chances for
thunderstorms over the eastern islands with some convective
snowfall possible over the slopes of Maunakea and Maunaloa this
weekend.

Meanwhile, a 1032 MB surface high far to the northeast of the
state will continue to produce moderate to breezy trade winds
across the islands through next week Monday. As the trade winds
pass under this upper level low, clouds and showers will become
enhanced by the added instability and lift. These unstable trade
wind shower bands will then move westward across the windward
slopes of each island, producing high chances for rainfall over
the next several days. While frequent rainfall remains in the
forecast for windward slopes, some of the more unstable showers
will be strong enough to affect leeward areas.

On Monday and Tuesday, an upper level short wave trough passing
far to the north of the islands will kick this low northward
gradually reducing the unstable influence on island weather as the
low center weakens and moves away from the state. Expect a gradual
decrease in shower activity from Monday to Tuesday as trade winds
slowly weaken in response to a cold frontal trough moving into
the Central Pacific basin.

Global model solutions diverge from Wednesday onward with the
movement of a shallow cold front into the islands. Stay tuned for
more information on this next weather system as the time period
gets closer.

&&

.AVIATION...
High pressure northeast of the state will keep a moderate to
locally breezy trade wind flow in place through Saturday. Clouds
and showers will favor windward and mauka areas, but will also
spread leeward due to the strength of the trades. An unstable
airmass over the eastern end of the state may even result in the
development of a thunderstorm or two, mainly during the afternoon
and evening hours on Saturday.

AIRMET Sierra is in effect for mountain obscuration for windward
sections of Molokai, Maui and the Big Island. The AIRMET may need
to be expanded to include windward sections of Oahu and Kauai
later tonight.

AIRMET Tango is in effect for moderate low level turbulence over
and downwind of the terrain of all islands. These conditions are
expected to remain in place through Saturday.

&&

.MARINE...
High pressure northeast of Hawaii will continue to drive moderate
to breezy trades through the weekend, then weaken slightly by
Sunday into next week. A Small Craft Advisory (SCA) remains in
effect for the typical windier coastal zones around Maui County
and the Big Island through Saturday night. Winds in these areas
should fall below SCA criteria by Sunday.

An upper level low centered 200 miles northeast of Hilo this
evening will approach the state and bring unstable weather with a
slight chance of thunderstorms for the eastern coastal waters
tonight through Sunday morning.

No major swells are expected through the next week. The current
northwest swell will gradually lower into Saturday. Another small
northwest swell will arrive Sunday and continue into the first
half of next week followed by another small reinforcement during
the second half of the week. Small background south swell will
continue through next week. Breezy trade winds will produce small
to moderate surf along east facing shores into early next week.

&&

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

&&

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Kino