Gale Warning 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
502 AM HST Tue Jul 18 2023

.SYNOPSIS...
Wind speeds will increase rapidly from today through Wednesday as
Tropical Storm Calvin produces strong and gusty winds as it
passes near the Big Island and just south of Maui County, Oahu,
and Kauai County. Damaging wind speeds are possible across most of
the state as Calvin approaches and passes by each island. Heavy
showers and flooding conditions are expected due to deep unstable
tropical moisture forced up the windward mountain slopes of
islands in Maui and Hawaii Counties. Calvin will likely weaken
into a Tropical Depression by Wednesday and maintain a westward
track moving away from the island chain by Wednesday night. Expect
improving weather and wind conditions statewide by Thursday as we
transition back to breezy trade wind pattern.

&&

.DISCUSSION...
A high pressure ridge north of the region will drive Tropical
Storm Calvin westward into Hawaiian waters later tonight. Weather
conditions will change rapidly today through Wednesday as
Calvin, currently around 450 miles east-southeast of Hilo Hawaii,
continues to quickly move towards the Big Island. The rapid east
to west motion of Calvin will tend to limit Tropical Storm impacts
to roughly a 12 to 18 hour time period over any one island.

Wind Impacts:
Calvin remains on a high confidence westerly track with the ridge
north of the islands providing the main steering flow. This storm
will continue to weaken as it approaches the Big Island due to
cooler water temperatures, the ingest of stable dry air, and some
vertical wind shear. However, Tropical Storm Force winds remain in
the forecast as the center of the system passes near the south
side of the Big Island tonight through early Wednesday morning. A
Tropical Storm Warning remains in effect for the Big Island and
surrounding waters due to the island's proximity to the Tropical
Storm Force wind speeds surrounding the storm. Wind speeds will
also increase statewide today through Wednesday as the storm
remains on a track passing south of the smaller Hawaiian Islands.
These winds will strengthen due to the increasing pressure
gradient between the ridge to the north and the low pressure
center associated with Calvin passing to the south. This means
that wind speeds over the islands in Maui County will at least
reach Wind Advisory thresholds and may exceed High Wind Warning
criteria in some areas, even as Calvin weakens and passes to the
south. Wind speeds will accelerate over and downwind of mountain
slopes with stronger gusts expected. A Wind Advisory remains in
effect today for all islands in Maui County, A High Wind Watch was
also issued for Maui County starting tonight for stronger winds
expected on Wednesday. Additional Wind Advisory and High Wind
Watch or Warning products may need to be extended to Oahu on
Wednesday as Calvin passes to the south. For the moment, Kauai
County will see less wind impacts from Calvin based on the latest
storm track and intensity.

Rainfall Impacts:
Deep unstable tropical moisture surrounding Calvin will be driven
up mountain slopes due to the strong wind forcing as the Tropical
Storm passes each island. This means the potential for flash
flooding remains elevated even for islands not directly in the
path of the Tropical Storm. How far north these heavy rainfall
impacts will occur is the forecast challenge with this event. The
highest potential for heavy rain and flooding are expected along
the windward slopes of the Big Island and windward slopes of the
eastern half of Maui. Storm total rainfall amounts in the 4 to 8
inch range should be expected for the eastern slopes of the Big
Island from North Kohala to the Hamakua Coast to Hilo to South
Point, with 1 to 2 inches of rain forecast over the Kona side of
the island. Maui may see between 2 to 6 inches of storm total
rainfall from the North Shore near Haiku to East Maui near Hana.
Other islands will see lower amounts between 0.25 to 2 inches,
with locally higher amounts possible favoring windward mountains
and slopes. The Flood Watch remains in effect with the potential
for flash flooding starting this evening over the Big Island and
Maui County. This Flood Watch was expanded in coverage to include
Oahu, Kauai and Niihau this morning, with enhanced flood threats
to Oahu forecast to start early Wednesday morning potentially
impacting the western islands.

By Thursday, Calvin will continue to weaken and pass quickly west
of the island chain. High pressure will build in quickly as the
storm exits the region, producing more stable breezy trade wind
weather with rapidly decreasing shower trends. This trade wind
weather will continue through the upcoming weekend with upward
trends in shower activity on Saturday and Sunday, mainly along
windward and mountain slopes.
&&

.AVIATION...
Breezy northeast to easterly trade winds will continue this
morning, with clouds and showers favoring windward and mountain
locations. Periods with MVFR conditions in and around clouds and
showers are expected. Winds will steadily rise from east to west
as Tropical Storm Calvin approaches and passes over or just south
of the Big Island later today through tonight, then south of the
smaller islands Wednesday. Conditions will quickly deteriorate
from east to west, especially on the Big Island as Calvin moves
through.

AIRMET TANGO remains in effect below 9000 ft over and immediately
south through west of mountains on all islands due to tempo
moderate turbulence.

AIRMET SIERRA for mountain obscuration will likely continue due
to bands of low clouds with embedded showers moving through.

&&

.MARINE...
Tropical Storm Calvin, located around 400 nautical miles east-
southeast of Hilo, Hawaii, is tracking west at 19 knots. Calvin is
forecast to enter the eastern offshore waters today, then track
near or over the Big Island and its surrounding waters later today
into Wednesday. A Tropical Storm Warning is in effect for the
offshore waters. Calvin will continue westward Wednesday as it
weakens, while passing south of the smaller islands. A return of a
more typical trade wind pattern is expected Thursday through the
weekend.

A Tropical Storm Warning is in effect for Alenuihaha Channel and
all waters surrounding the Big Island today into Wednesday. The
Gale Watch has been upgraded to a Gale Warning for the Pailolo
Channel and Maalaea Bay today through Wednesday.

Elsewhere, fresh to strong easterly winds will become strong
statewide today through Wednesday as Calvin passes over or near
the Big Island tonight into Wednesday. Small Craft Advisory is in
effect for rest of the coastal waters today through Wednesday.

Coastal impacts associated with Calvin will include rapidly
building surf for exposed windward coasts tonight through
Wednesday. The High Surf Warning for the Big Island has been
expanded to include exposed east facing shores in Maui County
starting tonight through Wednesday. Peak surf heights will be
short-lived and will likely occur late tonight into Wednesday
morning. Although confidence is low, some minor overwash and
erosion can't be ruled out for exposed low- lying coastal areas as
the surf peaks. For rest of the state, surf along east facing
shores will likely reach advisory thresholds Wednesday. Otherwise,
no significant swells are expected through the weekend. &&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from this evening through Wednesday afternoon for Maui
and Hawaii Counties.

Flood Watch from late tonight through Wednesday afternoon Oahu,
Kauai and Niihau.

Wind Advisory until 6 PM HST this evening for Maui County.

High Wind Watch from this evening through Wednesday afternoon
for Maui County.

Tropical Storm Warning for Big Island.

Tropical Storm Warning for Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

High Surf Warning from 6 PM this evening to 6 PM HST Wednesday
for Kahoolawe-Maui Windward West-Molokai Windward-Molokai
Southeast-Windward Haleakala-Kipahulu-South Haleakala-Big Island
Southeast-Big Island East-Big Island North.

Gale Warning until 6 PM HST Wednesday for Maalaea Bay-Pailolo
Channel.

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

&&

DISCUSSION...Bohlin
AVIATION...Gibbs
MARINE...Almanza