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Hawaiian Islands Synoptic Discussion and Guidance

194
FXHW60 PHFO 251935
AFDHFO

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

.SYNOPSIS...
A ridge to the north will maintain moderate trade winds today. An
approaching cold front will weaken the trades and allow the
background flow to become southeast on Friday. This 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.

&&

.DISCUSSION...
No changes to the forecast this morning, with no short term
changes expected.

A high pressure ridge to the north will maintain the moderate
trade wind flow today. A front approaching from the northwest will
start to erode the ridge tomorrow, which will result in a
weakening of the trade flow initially, and then a switch to a more
southeasterly flow. Winds could go light enough for some local
land and seas breezes this weekend, so we can expect a few more
clouds and showers over interior parts of the islands.

The bigger weather story is an upper level trough approaching
from the northwest Sunday night through Monday. The ECMWF depicts
a more defined trough approaching Kauai than the GFS right now.
Both models have 500 mb temperatures around -11 to -12 celsius but
the ECMWF has a little more moisture with the system. Temperatures
this cold at 500 mb are below normal for April, and the
precipitable water (PW) being forecast in the models between 1.30
and 1.50 inches is well above normal. The ECMWF is forecasting a
max PW near 1.60, while the GFS is a little drier. The GFS
forecast Lifted Index for Sunday night over Kauai is around -3.
All said, this will be an unstable airmass moving over at least
the western end of the island chain. The current progression of
the trough has it approaching from the northwest and then moving
to the east, which may keep the most of the focus over Kauai and
Oahu.

Will likely be making some changes to the forecast for Sunday
night through Monday to beef up the PoPs and possibly introduce
the wording for some heavier rain. Will likely hold off on the
introduction of thunderstorms while monitoring the next couple of
model runs.

High pressure is expected to build in north of the area Tuesday
into Wednesday which will at least briefly return trades to the
area. However some moisture leftover from the above mentioned
front may be riding in on the trades.

&&

.AVIATION...
Trade winds will be in the moderate range today as a high
pressure ridge remains north of the state. 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 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 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 this evening for Alenuihaha
Channel.


&&

$$


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

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



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