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Forecast Discussion for Honolulu, HI

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000
FXHW60 PHFO 240645
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST Sun Sep 23 2018

.SYNOPSIS...
An area of low pressure northwest of Kauai will lift northeastward
over the next couple days, dragging a weakening front toward the
islands. Deep tropical moisture and showers associated with the
front will overspread the western end of the state Monday night
and Tuesday, then linger in the vicinity of Kauai through the
middle of the week as the front stalls out and slowly dissipates.
Hot and muggy conditions can be expected across much of the
island chain as winds shift around to the south and southeast
ahead of the front. A more typical trade wind weather pattern is
expected to return Friday through next weekend as high pressure
builds to the north of the islands.

&&

.DISCUSSION...
Currently at the surface, a 1005 mb low is located around 750
miles northwest of Kauai, with a front extending southeastward to
near 24N 163W, then south-southwestward to near 14N 170W.
Meanwhile, a large 1032 mb high is centered around 1950 miles
northeast of Honolulu. Light to moderate trade winds continue to
prevail across the eastern end of the state this evening, while
light southeasterly boundary layer flow has taken hold over Kauai.
Infrared satellite imagery shows mostly cloudy skies in windward
areas from Oahu to the Big Island, with partly cloudy conditions
in most leeward locales. Some more extensive cloud cover is seen
over Kauai where some mid and high cloud is streaming in overhead.
Radar imagery shows scattered showers over windward areas from
Oahu to the Big Island, with a few isolated leeward showers
beginning to wind down with the loss of daytime heating. Meanwhile
over Kauai, scattered showers are streaming into southern and
eastern shores due to the southeasterly boundary layer flow. Main
short term concerns revolve around the timing and progression of
deep tropical moisture into the islands, as well as the potential
for some heavy rainfall over the western end of the state.

A closed upper level low several hundred miles northwest of Kauai
will shift slowly eastward tonight through Monday night, allowing
the weak front west of Kauai to inch closer to the western end of
the island chain. The upper level low will then open up and lift
northeastward Tuesday through mid week as another highly amplified
upper level trough digs well to the northwest of the state. This
will likely halt the eastward progression of the front somewhere
near or just west of Kauai Tuesday into Tuesday night, with the
front or its remnant moisture then lingering over this area
through Thursday and possibly into Thursday night. Both the GFS
and ECMWF then indicate that high pressure will build north of the
state Friday through the weekend, allowing a more typical trade
wind weather pattern to overspread the island chain. Of note, both
the GFS and ECMWF show a tropical cyclone south of the state next
weekend, so this will need to be monitored closely in the coming
days.

As for forecast details, the GFS and ECMWF have come into better
agreement with the timing and progression of deep layer moisture
associated with the front west of the state moving into the
western islands during the early to middle part of the week. The
deepest tropical moisture with precipitable water values around or
in excess of 2 inches appears to get held up over Kauai, with PW
values dropping off to near normal or slightly above normal
levels to the east. As a result, we will continue to advertise the
highest rain chances over Kauai through much of the work week,
with more of a convective shower regime in a sea breeze driven
environment from Oahu eastward to the Big Island. There is some
indication that heavy rainfall and even isolated thunderstorms
could affect Kauai, particularly Monday night through Tuesday
night. Will take a close look at the latest model guidance as it
continues to come in this evening and make a decision on the need
for a Flash Flood Watch for the island of Kauai with the morning
forecast package. By Wednesday, the best large scale forcing for
ascent will have lifted far enough northeast of the state that we
should be left in a convective mainly sea breeze driven shower
regime statewide, with these conditions continuing through
Thursday. More typical trade wind weather featuring windward and
mauka showers is then expected Friday through next weekend.

&&

.AVIATION...
Surface trough just west of the islands will continue to bring
synoptic southeast winds to much of the areas. As the wind flow
parallels the orientation of the island chain, winds will become
light enough over the islands to allow land breezes to develop.

Passing low clouds and showers will remain mostly confined to the
windward and mauka areas through early Monday morning, with
isolated MVFR ceiling possible. Some clearing is expected over
the interior areas as land breezes develop overnight. Otherwise,
VFR conditions will prevail.

No AIRMET is anticipated through early Monday morning.

&&

.MARINE...
An area of low pressure far northwest of Kauai will continue to
move slowly northeast and east through the middle of the week.
In response, surface winds will be light to moderate and out of
the southeast through Monday. A weakening front associated with
the low is expected to approach the coastal waters surrounding
Kauai Monday night into Tuesday. This will result in increasing
showers which may be locally heavy at times, and winds veering
more south to southwest Tuesday into Wednesday for the western
coastal waters. Elsewhere, southeast winds are expected to persist
with some scattered showers through the middle of the week. High
pressure is expected to build across the area Thursday into Friday
leading to the return of trade winds.

Surf will remain below advisory levels along all shores through
the week. A small to moderate north-northwest swell is expected
Thursday into Friday. Small, long-period southwest and south
swells will give surf along south facing shores a small boost
throughout the week, but remain well below advisory level.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Hui
MARINE...Kino

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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