National Weather Service Raw Text Product

323 
FXHW60 PHFO 152003
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Sat Jul 15 2023

.SYNOPSIS...
Moderate to breezy trade winds will prevail this weekend, 
bringing brief showers to windward and mauka areas during nights 
and mornings. Beyond the weekend, tropical cyclone Calvin is 
expected to be gradually weakening as it passes very close to, and
possibly over, the islands from Tuesday night through Wednesday 
night, bringing the potential for locally strong winds, flooding 
rain, and high surf. Models remain in good agreement on Calvin's 
track, but some questions remain as the system is still closer to 
Baja California than the Big Island. Stay tuned for forecast 
updates as Calvin moves closer to Hawaii over the next couple of 
days. 

&&

.DISCUSSION...
Early morning soundings from Lihue and Hilo showed that inversion
heights have risen over the past 24 hours, ranging from roughly 
5,000 to 7,500 feet. However, ample dry air aloft remains in place
over the islands with the strong inversions. A 700mb trough is 
located just east of the state this morning and has effectively 
veered the trade wind flow to the northeast. As this trough moves 
west through the weekend, expect a modest increase in low clouds 
and trade wind showers from the previous couple of days, which is 
already noted on satellite and radar imagery this morning with 
more clouds and showers moving into windward and mauka locations 
than yesterday morning. However, given the strong subsidence 
inversions, any trade wind showers should be brief and favor 
windward areas mainly in the overnight through morning hours each
day. Only isolated showers are expected over leeward areas, with 
the exception of leeward Big Island, which will see typical 
afternoon cloud buildups and scattered showers each afternoon this
weekend. High pressure both northeast and northwest of the 
islands will maintain moderate to locally breezy winds through the
weekend.

The forecast beyond Sunday will revolve around Hurricane Calvin, 
which as of this morning's 5am HST advisory issued by the National
Hurricane Center (NHC) in Miami, Florida, is located about 1400 
miles west-southwest of the southern tip of Baja California or 
over 1700 miles from Hilo. The system, which reached its peak 
intensity yesterday, has slowly begun to weaken and is now a 
category 2 hurricane as it continues to move west-northwest at 
16mph. It is expected to continue to gradually weaken as it 
approaches closer to the Hawaiian Islands and encounters cooler 
sea surface temperatures and increasing vertical wind shear. As 
the previous discussion mentioned, NHC continues to issue 
advisories on Hurricane Calvin at this time, but the storm is 
expected to enter the Central Pacific Hurricane Center (CPHC) area
of responsibility when it reaches 140W on Sunday night as a 
tropical storm. 

The official forecast track has been consistent in bringing 
Calvin very close to, or over, the main Hawaiian Islands Tuesday 
night and Wednesday. It remains a little too soon to nail down 
specific details for impacts to land areas, though it does appear
that at least some impacts from Calvin are possible. Impacts for 
specific islands will be highly dependent on the track of the 
system. For example, since the strongest winds are expected to be
in the northern portion of Calvin, if the center passes north of 
the islands most of the strong winds would remain offshore, 
whereas if the center passes to the south the islands could be 
within the stronger wind field. One additional consideration 
regarding impacts to island weather is the forward speed of 
Calvin, which is expected to be moving relatively quickly. This 
should preclude widespread impacts for an extended period of time.
With that said, any of the typical tropical cyclone hazards of 
strong winds, heavy rainfall, and high surf will be possible as 
early as Tuesday but most likely Tuesday night through Wednesday.

A return to a typical trade wind weather pattern is expected from 
Thursday into next weekend as the remnants of Calvin move quickly 
W away from the islands, likely as a trough.

As the public and marine forecasts for Hawaii will be highly 
dependent on bulletins from NHC/CPHC issued at 11 am/pm and 5 am/pm 
HST, our regularly scheduled forecast issuances will likely be
modified at some point this weekend, with updates issued shortly 
after these bulletins.

&&

.AVIATION...
Moderate to locally breezy trade winds under stable conditions 
will continue today with isolated showers and scattered low clouds
favoring windward and mauka areas. Expect mainly VFR conditions
across the area with occasional MVFR under passing showers.

AIRMET Tango for tempo moderate mountain lee turbulence remains 
in effect through tonight. Otherwise, no additional AIRMETs are 
anticipated at this time. 

&&

.MARINE...
A surface ridge far north of the state will likely maintain 
moderate to locally strong trade winds into Monday. A Small Craft 
Advisory (SCA) remains in effect for the typically windier waters 
adjacent to the islands of Maui County and the Big Island this 
weekend. This SCA will likely need to be extended and expanded
into other areas early next week as a tropical cyclone approaches.

Based on the latest advisory issued by the National Hurricane 
Center in Miami, Florida at 5 AM HST Saturday morning, Hurricane 
Calvin is forecast to enter the Central Pacific Hurricane Center's
area of responsibility Sunday night as a weakening tropical 
storm. Tropical storm conditions associated with Calvin may reach 
the eastern Hawaiian Offshore Waters starting on Tuesday. After 
that, Calvin is expected move near the main Hawaiian Islands 
starting Tuesday night or Wednesday. If so, tropical storm 
conditions may develop over the waters adjacent to the Big Island.
and possibly Maui, from Tuesday night into Wednesday. Given 
uncertainties about the track, size, and intensity of Calvin as it
approaches the state, all marine interests in the Hawaiian 
Islands should monitor the latest forecast advisory product issued
by the National Hurricane Center in Miami, Florida under WMO 
header WTPZ23 KNHC.

A small southeast swell and background swell energy from the 
south-southwest should maintain small surf along exposed south 
facing shores this weekend. Modest, choppy surf will persist along
east facing shores this weekend due to the trade winds. An upward
trend in surf is expected along east facing shores early next 
week due to the arrival of a medium-period easterly swell, which 
is being generated by the captured fetch associated with Hurricane
Calvin. Depending on the track, size and intensity of Calvin, 
surf will likely reach the High Surf Advisory threshold along east
facing of most islands starting Tuesday night or Wednesday. There
is a possibility that some east facing shores, such as the Big 
Island or Maui, may have a brief period of surf reaching the High 
Surf Warning criteria in the Tuesday night / Wednesday morning 
time frame based on the latest wave model guidance.

&&

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

&&

$$

DISCUSSION...Vaughan
AVIATION...Ahue
MARINE...Kino