Heavy Rain in Hawaii, December 20 to 21, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
944 AM HST Mon Dec 20 2021

.SYNOPSIS...
A rather windy and wet trade wind weather pattern is expected
over the next several days as high pressure builds in to the
north. Moisture embedded in the trades combined with instability
in the island vicinity will provide for passing showers over
windward and mauka areas with the trade winds sufficiently strong
to blow some showers over to leeward areas at times. Winds are
expected to drop off slight as we head into the holiday weekend as
low pressure approaches from the west.

&&

.DISCUSSION...
A 1044 mb surface high is centered far north of the state. This
is producing breezy to locally windy trade winds across the area.
Aloft, a broad upper trough is centered far northeast of the area
with its southwest periphery lying near the Big Island. Also, a
small upper low is centered a few hundred miles west of the state.
These two combined features are providing for instability across
the area. Daytime heating combined with the instability will allow
for the possible development of thunderstorms over portions of
the Big Island this afternoon and evening. Elsewhere, moisture
embedded in the trades will cause passing showers over windward
and mauka areas with the trades sufficiently strong enough to blow
some brief passing showers over to some leeward locations.

Satellite imagery shows an east to west oriented frontal boundary
just north of the state. Current timing has this band reaching
Kauai and possibly Oahu by late this afternoon, then passing
southward across Maui County tonight before reaching the Big
Island early Tuesday morning. Showers will increase as this
boundary moves across the island chain and favor windward and
mauka areas. However, the strong trades should be sufficient
enough to blow showers over to some leeward areas with ease.

Model guidance is indicating that the moisture field associated
with the frontal boundary will stall over or just south of the Big
Island. Cooler and slightly drier air will fill in behind the
boundary. If the front hangs up over portions of the Big Island,
the lingering instability along with this moisture field may allow
for a prolonged period of wet weather for some areas on the Big
Island. The instability will also keep the threat of
thunderstorms near or over parts of the the Big Island. Much will
depend on how far south the moisture field gets.

Trade winds are then expected to drop off slightly as we head
into the holiday weekend as low pressure moves closer to the state
from the west. Besides the lessening of the winds, the effects of
this low on our weather is yet to be determined due to differences
in the global models. Much will depend on how far east the low
moves. The closer the low gets, the higher the potential for
increased rainfall chances. The trend in the guidance keeps the
lower far enough west that significant rainfall in not expected at
this time.

&&

.AVIATION...
Breezy easterly trade winds will continue to generate tempo
moderate turbulence on the lee side of the mountains through
tonight. See AIRMET TANGO below. Otherwise scattered showers will
mainly be limited to windward areas, possibly generating a period
of MVFR ceilings and visibility. A band of enhanced moisture
dropping across the island chain late tonight, could increase the
need for additional AIRMET SIERRA for MTN OBSC.

AIRMET SIERRA remains in effect for northeast thru east facing
mountain slopes above 2500 feet for Big Island due to clouds and
showers.

AIRMET TANGO remains in effect for all island over and immediately
south through west of mountains for tempo moderate turbulence
below 9000 feet.

&&

.MARINE...
A nearly stationary, very strong area of high pressure centered
far N of the islands will support fresh to near-gale force trade
winds this week. A bit of a surge in wind speeds is expected
tonight and Tuesday as the high pushes the remnants of a front
over the islands from the NE. An ongoing Small Craft Advisory
(SCA) now includes all waters through Thursday due to a
combination of winds and seas. Moisture associated with the
frontal remnants may stall near the Big Island tomorrow through
Thursday, potentially triggering heavy showers and/or
thunderstorms as the island atmosphere will remain unstable.

While the fetch of trade winds to the E of the islands is
somewhat limited, a steady supply of short- and medium-period
wind waves/swells will be arriving from the N through NE for the
next several days, primarily due to the gradient between the high
and a slow-moving, gale-force low off the U.S. W coast. This will
result in increased seas, with rough and choppy surf along shores
exposed to the N and E. Surf along exposed E facing shores may
become large enough to warrant a High Surf Advisory (HSA) by
tonight or Tuesday, but peak surf heights along N facing shores
will remain below HSA heights. Very little swell energy from the
NW is expected this week, with increased potential for long-period
NW swell to arrive late next weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
waters-

&&

DISCUSSION...Burke
AVIATION...Powell/Rozanski
MARINE...Birchard