Area Forecast Discussion
Issued by NWS Honolulu, HI

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993
FXHW60 PHFO 031859
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
859 AM 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 is
currently enhancing trade wind showers where local moisture is the
deepest, mainly over Windward Big Island and Maui. Otherwise, breezy
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 today as high pressure
builds in from the NE and pushes the surface trough NW of Kauai
further away from the state. 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
continue for a few more hours this morning and may be needed
again late tonight.

AIRMET Tango is in effect for mod turb below 080 downwind of
island terrain. Conditions are borderline, but should continue
through the day.

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

&&

.MARINE...

Latest ASCAT shows fresh to locally strong winds have arrived as
a high pressure builds far north of the state. These stronger
trades will continue through the weekend and drop a notch or two
early next week as the high moves far northeast of the state. The
Small Craft Advisory (SCA) for typical windy waters around Maui
and the Big Island has been expanded to all waters through early
Monday morning. Some area may drop out earlier as winds begin to
ease due to the high shifting northeast.

Tiny surf will persist along north and west facing shores
through much of today. Forerunners of a long period northwest
swell is expected to fill in this afternoon and peak Saturday,
bringing small surf along north and west facing shores. 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 bump up a notch Saturday and hold
hold into early next week as strong trades hold.

&&

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


&&

$$


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