Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 251401

Area Forecast Discussion
National Weather Service Honolulu HI
401 AM HST Thu Apr 25 2019

Moderate trade winds will continue today 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 into Monday. Trade winds return on Tuesday
as another high pressure center builds in from the west.


GOES infra-red satellite imagery this morning shows patchy stable
cumulus clouds upstream from the Hawaiian Islands. Doppler radar
shows isolated to scattered showers across the Hawaii region with
the best shower activity just southeast of the Big Island.

A 1030 MB surface high far north of the islands will keep a
surface ridge over the state for one more day. Expect moderate
trade winds to continue under a stable upper level ridge axis.
Strong subsidence aloft will limit cloud heights and shower
activity today. This stability aloft shows up as a 5000 to 6000
foot subsidence inversion on the morning 12Z upper air soundings
from Lihue and Hilo. A slight weakening of this inversion over
Maui and the Big Island will allow scattered showers to develop
over windward slopes of both islands after sunrise.

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.


Light to moderate easterly trades will persist across the islands
today, generally in the 10-15 KT range. Expect a slight chance of
showers over the windward coast, slopes, and mauka areas.
Otherwise, dry conditions with VFR forecast elsewhere.


Moderate to strong easterly trades associated with a ridge north
of the islands will continue today, 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
today, 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 yesterday will continue
eastward 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.


Small Craft Advisory until 6 PM HST this evening for Alenuihaha



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