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Hawaiian Islands Synoptic Discussion and Guidance

153
FXHW60 PHFO 230706
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
906 PM HST Sun Sep 22 2019

.SYNOPSIS...
A plume of deep tropical moisture moving from east to west across
the island chain will keep conditions a bit unsettled through
Monday night. Drier weather will overspread much of the state
Tuesday through Friday, with showers favoring windward and mauka
areas at night, and leeward and interior areas during the
afternoon and early evening hours. More typical trade wind weather
is expected to return next weekend.

&&

.DISCUSSION...
Have updated the forecast with some tweaks, but mainly to raise
PoPs for most windward areas after midnight as a band of showers
upstream approaches the islands. This band should drag across the
state late tonight into Monday. The local airmass is quite moist
with PWs exceeding 2 inches, so even with relatively marginal
upper level support, orographic enhancement and mesoscale
convergence zones could yield locally heavy rainfall to a few
windward areas late tonight and early Monday, and have added this
to the grids.

&&

.PREV DISCUSSION.../ISSUED 430 PM HST Sun Sep 22 2019/

Currently at the surface, a weak surface trough of low pressure is
located a few hundred miles southwest of the Big Island, while a
1027 mb high is centered around 1150 miles north-northeast of
Honolulu. The gradient remains rather light across the western
islands with sea breezes present in many areas, with moderate
trade winds in place over Maui County and the Big Island. Aloft,
an upper level low is evident in water vapor imagery around 250
miles west-northwest of Kauai. Visible satellite imagery shows
mostly cloudy skies across much of the state due to a combination
of scattered to broken high clouds streaming overhead and daytime
heating driven low cloud development. Radar imagery shows
scattered to numerous showers over the Big Island and Maui, with
little shower activity over the other islands at the moment. Main
short term concerns continues to revolve around the potential for
locally heavy rainfall and thunderstorms through Monday.

Today through Tuesday,
Model solutions are in good agreement showing showing the upper
level low west-northwest of Kauai retrograding further to the
west over the next couple days, dragging the weak surface trough
south of the islands, steadily westward. Meanwhile, deep layer
ridging is forecast to build well to the northeast of the state
through the period. These features are expected to drag a plume of
deep tropical moisture with precipitable water (PW) values of 2.0
to 2.3 inches from east to west across the island chain over the
next couple days. The deep tropical moisture is already in place
over Maui County and the Big Island, and is expected to
overspread Oahu by early this evening. The plume will then
overspread Kauai late tonight or Monday morning, with some drier
air (PW values below 2.0 inches) moving into the Big Island
Monday afternoon. This drier airmass will continue to spread
westward through Maui County and Oahu Monday night, and shift west
of Kauai on Tuesday.

The threat for heavy rainfall and flash flooding does not appear
particularly high given the more unstable air associated with the
upper level low not aligning with the deeper tropical moisture
through the period. That said, given the high PW airmass in place,
some locally heavy downpours can't be ruled out through early
this evening and again Monday afternoon and evening, particularly
over the Big Island. Will keep a mention of thunder in the
forecast for the Big Island through this evening and again Monday
afternoon. Elsewhere, thunderstorms are not anticipated.

Showers are expected to favor interior and mauka areas through
early this evening, then transition over to windward areas
later tonight through Tuesday as the trades return from east to
west across the island chain. Given the high PW airmass in place
however, a few showers will remain possible in leeward areas
through the period as well, with the highest chances for rain
during the afternoon and early evening hours on Monday.

Tuesday night through Saturday,
There remains some differences in the long range periods, but the
models have come into better agreement compared to 24 hours ago.
Both the GFS and ECMWF linger the band of deep tropical moisture
over or near the western end of the state Tuesday night through
Friday, with the GFS painting a bit wetter scenario for Oahu and
Kauai than the ECMWF which is a bit further west. Overall, winds
are expected to remain rather light through Friday, and this
should result in a more windward shower focus during the overnight
and early morning hours, and an interior/mauka shower focus
during the afternoon and early evening hours. More robust trade
winds are expected to return next weekend, and this should bring a
return of more typical trade wind weather to the island chain.

&&

.MARINE...
Moderate to fresh east-southeast winds are expected to make a
brief return Monday and into Tuesday. Winds could reach Small
Craft Advisory (SCA) levels in the typical windier areas near
Maui County and the Big Island. Will be monitoring the latest
model runs and observations tonight and tomorrow to determine if a
SCA is necessary.

Winds will once again be disrupted by midweek in part due to a
front developing to the northwest of the islands. This will
maintain a east-southeast flow with winds weakening to light to
moderate levels.

A series of small swells from the southwest and south are
expected through the week. A new moderate period south-southwest
swell is due Monday and will be followed by a larger long-period
swell later in the week. This latter swell may reach advisory
levels along south facing shores late Thursday into Friday. The
current northwest swell will decrease tonight and surf is expected
to steadily trend down on Monday along north facing shores. A few
other small to moderate northwest swells will be possible towards
the end of the week from the low pressure system developing
northwest of the state over the next few days.

&&

.AVIATION...
With the atmosphere becoming less unstable, the chances for
widespread heavy downpours and thunderstorms are diminishing.
However, isolated thunderstorms over the Big Island are still a
possibility, especially Monday afternoon. Trade winds, veering to
the east-southeast at times, are returning across the island
chain tonight. Winds will be in the light to moderate range over
most areas through the next day or two, with winds speeds
occasionally reaching breezy levels in some locations.

VFR will prevail over most locales, with brief to TEMPO MVFR in
passing showers and lower ceilings mainly affecting windward and
mauka sections.

&&

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

&&

$$

DISCUSSION...R Ballard
PREV DISCUSSION...Jelsema
MARINE...M Ballard
AVIATION...Kinel

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



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