467
FXHW60 PHFO 040125
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
325 PM HST Fri May 3 2024

.SYNOPSIS...
Breezy trades will become slightly more unstable with pockets of
heavier showers possible through the weekend, mainly windward and
mauka. A drier moderate trade wind pattern then becomes
established by the middle of next week.

&&

.DISCUSSION...

Regional satellite shows a closed upper low positioned just SE of
170N/30N. Modest convection, including a few thunderstorms, are
noted within its core and beneath the cold pool aloft (-15C at
500mb) several hundred miles NW of the Hawaiian Islands. A shortwave
trough wrapping around the southeast periphery of this low
enhanced trade wind showers last night and is now lifting north of
the area. As a consequence, showers have decreased in coverage
during the last several hours. Otherwise, trade winds have
settled in and will take the edge off of afternoon heat compared
the last several days as showers focus over windward and mauka
zones in routine fashion.

Trades will then acquire a decreasingly stable character as the
aforementioned upper low opens up and shears east along the northern
periphery of the subtropical jet. The inversion is modeled to weaken
and lift by a few thousand feet which will increase the depth of
trade wind showers as they advance across the area. This will
facilitate slightly greater rain rates and increase leeward
penetration of showers. Waves of forcing rotating around the low
will be responsible for periodically increasing shower coverage
Saturday night through early next week. During this time, the cold
pool aloft will move directly overhead and is modeled to moderate to
around -11C at 500mb. Lapse rates will still be adequate for pockets
of deep convection to develop Sunday night through Monday night,
mainly during the overnight periods. Isolated thunderstorms cannot
be ruled out during this period, but confidence at this stage is too
low to warrant explicit inclusion in the forecast. Meanwhile, a
separate thunderstorm threat may manifest over the Big Island where
a plume of steep lapse rates on the order of 7C/km is modeled to
extend northeastward over the eastern end of the state. This will
contribute to an increased probability of pockets of heavy showers
and/or isolated thunderstorms Sunday and Monday afternoon,
particularly as the upper jet provides maximum venting aloft on
Sunday followed by the upper wave itself pivoting through on Monday.
Confidence at this stage once again falls short of inclusion in the
forecast but will be given consideration in future forecast cycles.

Pressure falls over a broad swath of the Central Pacific will then
ease the gradient leading to a drier moderate trade wind pattern by
the middle of next week.
&&

.AVIATION...

Breezy easterly trades will continue through the weekend as high
pressure builds in from the NE. Moisture riding in on the trades
will focus clouds and showers over windward and mauka areas, with
the greatest shower activity during the late night and early
morning hours. A few showers may spill over to leeward areas at
times. VFR conditions will generally prevail, but MVFR conditions
will be possible in any heavier showers that develop.

AIRMET Sierra is in effect for tempo mountain obscuration for
windward and mauka portions of the island chain. This AIRMET will
likely be needed through tomorrow morning.

AIRMET Tango is in effect for mod turb below 9000 feet downwind
of island terrain. Conditions are borderline, but should continue
through Saturday.

AMD NOT SKED in for Midway due to missing wind speed and
direction

&&

.MARINE...

High pressure far north-northeast of the state will produce fresh
to strong trade winds across Hawaiian waters for the next several
days. These stronger trades may ease slightly early next week as
the high begins to move farther to the northeast. An advanced
scatterometer (ASCAT) pass today continued to show Small Craft
Advisory (SCA) level winds across most coastal water zones, a
trend which will continue through the weekend. As a result, a SCA
remains in effect for all Hawaiian waters through early Monday
morning.

Surf along north and west facing shores remains tiny this
afternoon, but a small, long-period northwest swell will fill in
this evening and tonight, peak on Saturday, then gradually
decline into Sunday. Observations from Buoy One (51001) suggest
that forerunners from this swell should begin arriving at Kauai
late this afternoon, followed by Oahu this evening, and finally
Maui and the Big Island during the overnight through early morning
hours. This swell will increase surf heights noticeably for north
and west facing shores tonight through Saturday, but will remain
in the small category. Another small, long period northwest swell
will fill in late Sunday, maintaining small surf along north and
west facing shores into early next week.

Small background south swells will continue into Saturday,
providing tiny surf for south facing shores. Forerunners of a
small, long period south swell may arrive Saturday afternoon with
a slow onset through Sunday providing a boost in surf along south
facing shores into early next week.

Rough and choppy surf along east facing shores will increase
slightly on Saturday and hold into early next week as strong
trades persist.

&&

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


&&

$$


DISCUSSION...JVC
AVIATION...Farris
MARINE...Vaughan