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Hawaiian Islands Synoptic Discussion and Guidance

966
FXHW60 PHFO 260138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Thu Apr 25 2019

.SYNOPSIS...
An approaching cold front will erode the ridge to the north of the
islands. Trade winds tonight will weaken with some southeasterly
winds. The lighter winds will lead to some local land and seas
breezes this weekend. An upper level trough digging into Sunday
night into Monday will enhance shower activity and bring the
possibility of thunderstorms and heavy showers.

&&

.DISCUSSION...
Most of the forecast changes this afternoon are from Saturday
afternoon through Monday, and focus on the convective nature.
In general beefed up PoPs, introduced the possibility of heavy
rain and a slight chance of thunder.

Trade winds are expected to weaken overnight and Friday as a front
moving from west to east erodes the ridge to the north of the
islands. While the front is expected to stay north of the islands,
it will turn the background flow to the southeast. Winds are
expected to weaken enough that some locations will experience some
land and sea breezes Friday, and moreso Saturday. We can expect an
increase in clouds and showers during the afternoon hours in this
regime. Will have to keep an eye on the strength of the southeast
flow to see if that overrides the lighter winds.

Heading into Sunday, our attention turns to an upper level trough
approaching from the northwest, and an upper level shortwave
along the trough that swings through the islands Sunday night and
Monday. The GFS and ECMWF continue to be in good agreement with
the instability that this will bring to the islands for the end of
the weekend and the start of the new week.

The forecast has been updated to indicate the possibility of
heavy rain and the slight chance of thunderstorms Sunday afternoon
through Sunday night for the western islands and adjacent waters.
Monday the forecast shifts the threat to the interior of the
islands. The coldest 500 mb temperatures swing through Sunday
night and early part of Monday, along with an increase in
precipitable water. In addition the models show an increase in
CAPE values Sunday night and into Monday, particularly over and
just east of the islands. Will be monitoring future runs to
determine if the area of thunderstorms needs to be expanded or if
the threat will linger into Monday night or Tuesday.

A high pressure system makes a return north of the islands
Tuesday, however left over moisture upstream of the islands could
be carried in on the trades. Heading into the second half of the
week, there are some model differences that will need to be
monitored, but for now, the forecast reflects a trade wind regime.

&&

.AVIATION...
Trade winds will be in the light to moderate range through tonight
as a high pressure ridge remains north of the state. However,
with a front approaching the area from the northwest, winds will
begin to veer to the southeast Friday.

VFR will prevail, with only isolated areas of showers and lower
ceilings in windward and mauka sections over the individual isles.

No AIRMETs are currently in effect.

&&

.MARINE...
Moderate to fresh easterly trades associated with a ridge north
of the islands will remain through tonight 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. The current north swell will begin to veer north-
northeast on Friday and will hold through the weekend. A slightly
larger northwest swell is expected to slowly build through the day
Sunday and peak Sunday night into Monday and gradually ease
through midweek. Surf will 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, more small long-period swells
are expected through next weekend due to active storms south of
New Zealand. Although some of these storms were large, none of
them had a direct aim towards Hawaii, thus limiting the surf
potential along south facing shores.

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...
None.

&&

$$


DISCUSSION...M Ballard
AVIATION...Kinel
MARINE...Kino

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



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