Hail and Heavy Rain in Hawaii, February 3 to 5, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Wed Feb 3 2021

.SYNOPSIS...
A strong cold front approaching Kauai will bring significant
changes to the weather across the islands as it moves down the
island chain later today and tonight. Locally windy conditions
are expected today. In addition, cloud cover and showers will
increase with a threat of locally heavy rainfall. There is also a
chance of thunderstorms with gusty winds. Expect the northerly
flow behind the front to bring drier and cooler air over the
islands from tonight into the weekend.

&&

.DISCUSSION...
A strong cold front appears to a short distance northwest of
Kauai early this morning. This feature, which has been moving
toward the east-southeast at around 15 mph, is accompanied by
overcast layered clouds with embedded showers and isolated
thunderstorms. The background low-level flow ahead of this feature
is from the south to southwest. The winds have increased ahead of
the front, so a Wind Advisory has been issued for Kauai County
and Oahu. The windy conditions with localized strong gusts will
likely be most prevalent over mountain ridges and along the
northeast and east sides of the islands. Note that these windy
conditions may eventually spread to portions of Maui County later
today.

Clouds and showers continue to increase over the western islands.
As the front nears Oahu later this morning, a sharp upper-level
trough digging toward the state is expected to cause increasing
atmospheric instability. This scenario would likely enhance
precipitation ahead of and along the front. This will likely
cause locally heavy rainfall with the potential for water ponding
issues on some roadways on Oahu during the morning commute. The
leading edge of the front is then expected to move across Maui
County during the afternoon.

As the front moves through Maui, it is expected to gradually
weaken as it approaches the Big Island this evening.
Thunderstorms are still possible ahead of and along the front,
and the main issues will likely be related to any thunderstorm
development. Temperatures will be cold enough for snow over the
summits of Mauna Kea and Mauna Loa on the Big Island, and there
could be several inches of snow from some time this afternoon into
tonight. As a result, Winter Weather Headlines may be needed for
the Big Island Summits later today or tonight.

The forecast guidance shows surface high pressure building behind
the front. This will produce northerly flow across the entire
state. These northerly winds will likely transport significantly
cooler and drier air down across all islands. As the dew point
temperatures drop into the 50s due to the cool air advection,
expect minimum temperatures in the low 60s for many areas, with
select location dropping into the 50s tonight and Thursday
nights. Note that record minimum temperatures in some locations,
such as on Kauai, are in the 50s, so it is not out of the question
that a new record low temperature or two may be set somewhere in
the state later this week.

&&

.AVIATION...
A front near Kauai has increased south winds with terrain
enhancement accelerating wind speeds around and over terrain and
through aligned valleys. The terminals located in Maui County and
the Big Island are expected to see enhanced southerly winds
Wednesday due to this terrain acceleration. Low visibility is
expected as the front pushes through Kauai this early morning,
Oahu mid morning, and Maui County and the Big Island later today.
Expect widespread MVFR and pockets of IFR and LIFR associated
with the heaviest rains. Thunderstorms will accompany the front
with strong gusty winds expected. AIRMETs for mountain obscuration
and strong surface winds will continue through the day. Behind
the front, north winds and improving conditions are expected.

&&

.MARINE...
A strong cold front will slide southeastward down the island
chain today and tonight, shifting east of the Big Island by
daybreak Thursday. Strong south-southwest winds are expected in
advance of the front, with strong northwest winds filling in
following the frontal passage. Winds will shift northerly Thursday
through Friday night at moderate to gentle speeds. High pressure
building north of the islands will then bring a return of moderate
east-northeasterly trades over the weekend, strongest on Sunday.

A Small Craft Advisory (SCA) is now in effect for most waters
through 6 PM this evening, and extends through 6 AM Thursday for
the waters surrounding Kauai and Oahu, as well as the windward
Maui County waters the waters. The SCA will likely need to be
expanded to additional waters Thursday through Saturday due to a
large northwest swell moving through the islands.

The offshore NOAA buoys 51001 and 51101 show the incoming west-
northwest swell continue to build this morning, although some
wind wave contamination is making it difficult to determine if
it's running on time when compared to WW3 guidance. Given
significant wave heights are running pretty much spot on at the
moment, the swell is likely coming in on time as predicted. This
swell will continue to build from northwest to southeast down the
island chain today and tonight, peak on Thursday, then gradually
lower Thursday night through the weekend. Surf is expected to
build to warning levels along north and west facing shores of
Kauai tonight, with low-end warning level surf expected to reach
north and west facing shores of the Oahu, Molokai and north facing
shores of Maui on Thursday. Surf will remain elevated through at
least Friday night, before dropping below advisory levels on
Saturday and remaining below advisory thresholds through the
middle of next week.

The more west-northwest direction of this swell is expected to
bring warning level surf to west facing shores of the Big Island
from late Thursday afternoon through early Friday as well.

East shore surf will remain small through the middle of next week.
A slight boost in short period surf is expected along south
facing shores today ahead of the front, with the surf then
lowering tonight and remaining small through the middle of next
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu.

Small Craft Advisory until 6 PM HST this evening for Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters.

&&

DISCUSSION...Houston
AVIATION...Morrison
MARINE...Jelsema