Drought in Hawaii, April 1 to 30, 2024

Area Forecast Discussion
National Weather Service Honolulu HI
900 PM HST Sun Mar 31 2024

.SYNOPSIS...

Moderate trade winds will continue through tonight, before
gradually strengthening through the first half of the week. Trades
will likely become strong by the middle of the week, then remain
strong into next weekend. Shower coverage will increase Monday.
Clouds and showers will mainly favor windward areas, and
potentially persist through much of the week.

&&

.DISCUSSION...

Locally pressure gradients have weakened slightly in the past 24
hours as the now 1030 mb high lingers far northeast of the state
while a front drapes across the middle of the Pacific 500 miles
northwest of Kauai. Another stronger 1037 mb high is developing
behind the front and is producing near gale force winds based on
the latest ASCAT. A ridge extending from the high to northeast
continues to produce moderate trades with a band of clouds and
showers just upstream of the island chain this evening. This band
of clouds and showers will increase low level clouds and chances
of showers mainly for windward and mauka locations as it moves
through much of the day Monday. Following behind this band in a
more expansive stratocumulus deck that should keep scattered
clouds and showers in the forecast through Tuesday.

Local pressure is expected to increase over the next several days
as the strong high to the far northwest continues to strengthen
as it tracks far north of the state. Trade winds will respond be
increasing to breezy levels Monday night and windy levels Tuesday.
Meanwhile, the approaching front will stall and weaken to a trough
several hundred miles northwest of the islands keeping the gradient
and stronger winds northwest of the state.

In the upper levels an elongated trough deepening just northwest
of the state will cool temperatures and help destabilize the
tradewind inversion. Temperatures at the summit could drop in the
single digits Monday night into Tuesday, thus a slight chance of
freezing rain on the upper slopes of the Big Island summit zones
remains in the forecast through Monday night.

By late Tuesday into Wednesday, several impulses rotating through
the upper level trough will lead to a mid- level cutoff low, initiating
surface cyclogenesis along the stalled trough, about 550 miles
northwest of Kauai Tuesday night into Wednesday. If the upper
level low develops close enough to the state, locally heavy
showers could develop, especially for the western end of the state
Wednesday. Regardless, the nearby instability and moisture should
be enough for enhanced windward showers across much of the state.
At the same time, the surface high will move eastward well north
of the islands. The global models are predicting this high to be
around 1050 mb, which will further tighten the gradient over the
islands as the surface low features moves west resulting in strong
easterly winds through mid- week. These strong winds could
require a wind advisory and possibly even a high wind warning for
some of the windier locations. However, there are still enough
uncertainty between the model runs and it is still to early to
pinpoint high wind impacts. Thus we will continue to monitor as
these events unfold.

Winds will ease ever so slightly Thursday and Friday as the
anomalously strong high pressure to the north begins to weaken
and drifts farther east. The upper level disturbance will persist
very near or over the state that could help to keep active trade
wind showers into the weekend.

&&

.AVIATION...
A weakening cold front will approach and stall northwest of the
islands this week as a strong high pressure system slowly builds
in across the Central Pacific basin. Easterly trade winds will
strengthen over the next few days blowing up into the breezy to
locally windy range. A band of clouds, the remnants of an old
frontal band, will drift into the islands from the north through
Monday with increasing shower trends and brief MVFR conditions
over all islands favoring windward and mountain airfields.

No AIRMETs are currently in effect. AIRMET Sierra for mountain
obscurations will be needed later tonight from Molokai to the Big
Island as thisband of low clouds runs up over the mountains.
AIRMET Tango for moderate low level turbulence over and southwest
through northwest of mountains will also be needed on Monday.

&&

.MARINE...
High pressure far northwest of the coastal waters will track east
and strengthen to near ~1052 mb as it passes north of the islands
Tuesday and Wednesday. A surface trough is expected to develop
between the coastal waters and the strong high pressure. The high
will linger well north of the islands as it gradually weakens
Thursday and Friday. Easterly trade winds will gradually
strengthen over the next couple of days, approaching gale-force
speeds in the channels when speeds peak around midweek. A Small
Craft Advisory (SCA) will likely be issued for the typical windier
waters around Maui County and the Big Island as winds increase
Monday. The SCA will likely be expanded to all zones as winds and
seas increase Tuesday and Wednesday. Guidance trends will be
monitored to determine the need for a Gale Watch for the windier
zones.

The small, short to medium period northwest swell that arrived
today will linger through Tuesday. A developing fetch of north
winds to the north and northwest of the islands will generate a
moderate (~6 feet) short- period (10-12 seconds) north swell that
will build Tuesday and peak Wednesday before diminishing.

With the onset of strong trade winds, rough choppy wind waves are
expected to increase through the week with a peak from Wednesday
through Friday. It is likely east facing shores will exceed High
Surf Advisory (HSA) criteria during the second half of the week.
South facing shores will remain tiny through the week. An anomalously
strong low pressure in the distant southeast Pacific has an
associated fetch that may deliver a small, medium- period south-
southeast swell by the end of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Almanza
AVIATION...Bohlin
MARINE...Shigesato