Heavy Rain in Hawaii, November 13 to 17, 2021

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
418 PM HST Sat Nov 13 2021

Corrected grammar in the 4th paragraph of the discussion.

.SYNOPSIS...
Expect increasing shower activity tonight as moisture moves in
from the south, and a cold front approaches from the northwest.
Locally heavy rainfall and isolated thunderstorms are possible
through Sunday night.

&&

.DISCUSSION...
As anticipated, have made some changes to the forecast from
midweek onward to reflect the trends in the models of east to
southeast flow remaining in place, in the wake of the frontal
system that will be impacting the islands over the next 36 to 48
hours.

Aside from high clouds moving over the islands today partially
blocking the sun, the weather has remained fairly quiet, as
expected. Showers on the Big Island have been a bit heavier than
in recent days, but by and large rainfall has been on the light
side over most areas. A couple of the windward Big Island gages
have reported a quarter to a third of an inch of rain this
afternoon. The overnight sounding at Hilo had reported 1.3 inches
of precipitable water (PW) and we were expecting it to increase
this afternoon. This afternoon's sounding reported 2.0 inches,
which is just a little faster than the models had expected.

The cold front that will be impacting our weather for the next bit
continues to advance towards the islands from the northwest.
Thunderstorm activity along the front, which is making is way
across the offshore waters northwest of Kauai, have decreased
since daybreak, but we anticipate another resurgence as we head
into tonight, with the possibility of thunderstorms near the
smaller islands after midnight, and near the Big Island earlier
with the enhanced moisture at that end of the state. We also
anticipate some showers to form out ahead of the approaching front
as the night wears on, mostly after midnight.

Expect our weather across the islands to become wetter over the
next 12 to 24 hours. There remains good model agreement with
higher PW values spreading northward over the central islands
tomorrow, and then staying at or slightly above normal for much of
the week. Near normal upper level temperatures at the southern
end of the state in the short term mean that any thunderstorms
that do develop in that area will primarily be due to the surge in
moisture. Any showers or thunderstorms that develop across the
region, could produce locally heavy rainfall. While not expecting
widespread flooding, localized or nuisance flooding is possible
with this event.

The front is expected to stall near Kauai and become a more
diffuse entity as it gets closer to the islands, however its
presence will continue to impact our local weather through much of
the week. The dissipating front is expected to transition to a
weak trough near or over the islands during the Sunday through
Tuesday time period. While the atmosphere will stabilize a bit as
this transition occurs, which will lessen the chance for
thunderstorms and heavy rainfall beyond tomorrow night, a wetter
pattern is expected to continue through the week.

As mentioned above, winds are expected to remain in an east to
southeast flow after the remnants of this front dissipate. With PW
values remaining at or slightly above normal, expect showers to
continue with a focus on the east to southeast sides of the
islands.

&&

.AVIATION...
Light southeasterly flow is carrying thick bands of clouds and
showers towards east facing slopes and coasts of the Big Island.
AIRMET Sierra for mountain obscuration will likely remain in
effect for this area into Sunday morning.

Elsewhere across the state this afternoon, the air mass is mostly
dry and mostly stable. Shower activity is limited and focused
over island interiors. Layered mid and high clouds, the first
signs of an an approaching cold front, will continue to stream in
from the southwest. Light icing is expected in these layers above
FL200.

A weakening cold front is moving in from the northwest and is
expected to stall near Kauai late tonight and Sunday. The front
will pull in tropical moisture from the south and the air mass
will become increasingly unstable. Bands of rain will move in from
the south and west beginning around midnight. Thunderstorms are a
possibility on the Big Island this evening and statewide after
midnight.

AIRMET Tango for moderate upper level turbulence has been
canceled as the associated jet streak has weakened. Conditions may
redevelop early Monday when a deep upper level trough is forecast
to approach the state from the west.

&&

.MARINE...
A cold front in the far northwest offshore waters will stall and
weaken near Kauai late tonight and Sunday. Light to moderate
southerly winds ahead of the front will shift to light to moderate
northeasterly behind the front. Winds elsewhere will become
light to moderate easterlies Sunday through Tuesday as the front
weakens to a trough near the central waters. Thunderstorms and
locally heavy rainfall are possible through early next week. A
more typical trade wind pattern is expected to return by mid week
and hold through the remainder of the week.

Surf along north and west facing shores will hold in the small to
moderate range as a reinforcing short period, moderate northwest
swell fills in late tonight into Sunday. Overlapping short
period, moderate north swells will arrive Sunday night into
Monday and again Tuesday night into Wednesday. Small background
south swells will keep surf from going flat along south facing
shores through early next week. Surf along east facing shores will
remain small through mid week due to weak trade wind regime.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...M Ballard
AVIATION...Bedal
MARINE...Almanza