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Hawaiian Islands Synoptic Discussion and Guidance

053
FXHW60 PHFO 250633
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Wed Apr 24 2019

.SYNOPSIS...
Trade winds will continue through Thursday with a high pressure
ridge lingering north of the Hawaiian Islands. An approaching cold
frontal system will weaken the trade winds on Friday, light
southeast winds will produce a modified light and variable sea
breeze regime with increasing shower trends into the weekend.
An upper level trough will likely enhance local area showers
across the state from Sunday through Monday. Trade winds return on
Tuesday as another high pressure center builds in from the west.

&&

.DISCUSSION...
A 1030 MB surface high far north of the islands will continue to
produce trade moderate trade winds across the state through
Thursday. An upper level ridge will keep fairly dry weather
conditions in the forecast for most islands with an increase to
windward showers for Maui and the Big Island on Thursday and
Friday.

The weather pattern changes from Friday into this weekend as a
cold front approaching the islands from the northwest and an upper
level trough weakens the high pressure ridge north of the state.
Trade winds transition to light southeasterlies allowing land and
sea breezes to develop over each island. Clouds will develop over
island interior sections in the day with clearing at night. The
upper level trough over the islands will create more unstable
weather conditions with increasing shower trends across the
region from Friday through Sunday.

A stronger upper level short wave trough will move in from the
northwest from Sunday night into Monday with the surface cold
front stalling out just north of the island chain. Some timing
and intensity differences remain in the model guidance with this
upper level system. However, we're still forecasting a trend of
increasing rainfall, locally heavy at times, with the possibility
for thunderstorms from Sunday night into Monday.

&&

.AVIATION...
Similar to the last several nights, light to moderate east
northeast trades will persist across the islands. Expect a slight
chance of showers along the windward coastal and mauka area,
especially affecting Maui and the Big Island. Otherwise, VFR and
dry conditions elsewhere.

&&

.MARINE...
Moderate to strong easterly trades associated with a ridge north
of the islands will continue through Thursday, then weaken Friday
into the weekend as a cold front approaches. Winds will steadily
veer out of the east-southeast direction and trend down into the
light to moderate range Saturday (strongest eastern end of the
state), then continue into early next week. Land and sea breeze
conditions with a light background east-southeast component will
result near the coasts through this time, especially for the
waters west of Maui. In addition to the winds, the shower coverage
will likely increase later in the weekend into early next week as
an upper trough and cold front begin to influence the local
weather. Although confidence remains low at this point, a
thunderstorm or two can't be ruled out Sunday through Monday as
the upper trough moves over the area.

Small but steady surf will continue along north facing shores
through the weekend due to a couple of recent systems passing north
of the islands. Swell directions associated with these small
sources will transition from the north-northwest to the north
Thursday, then to the north-northeast Friday through the weekend.

A slightly larger northwest swell associated with a gale that has
developed around the Date Line near 40N (around 1300 nm NW of the
islands) is forecast to arrive late Sunday, peak Sunday night into
Monday, then ease into midweek. Surf will respond along exposed
north and west facing shores, but remain well below advisory levels.

A similar trend will continue along south facing shores due to the
recent active pattern over the southern Pacific within Hawaii's
swell window. A series of long-period pulses, spaced a couple of
days apart, out of the south are expected through the first half
of next week. For the long range, the powerful, near hurricane-
force, system passing south of Australia today will continue
eastward Thursday through Saturday. As it passes south of Tasman
Sea and New Zealand, a decent fetch driving seas into the 20 to
25 ft range this weekend may translate to a small south-southwest
swell (Tasman source) for the islands next weekend (1st weekend
in May) if the fetch sets up as projected.

Minimal surf is anticipated along east facing shores due to the
local trades that are forecast to trend down locally and upstream of
the state over the weekend into next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Alenuihaha
Channel.

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Gibbs

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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