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Hawaiian Islands Synoptic Discussion and Guidance

584
FXHW60 PHFO 180138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Sun Nov 17 2019

.SYNOPSIS...
An unsettled weather pattern will persist across the island chain
through Tuesday, with locally heavy rainfall and thunderstorms
possible statewide. The airmass will stabilize a bit Tuesday night
and Wednesday, but a showery wet trade wind pattern is expected
to hold into next weekend. Breezy to windy conditions could
develop late next week into next weekend.

&&

.DISCUSSION...
A 1026 mb high is centered around 800 miles north of Kauai,
driving light to moderate north to northeast trade winds across
local waters this afternoon. Water vapor loop shows a digging
upper trough a couple hundred miles north of Kauai. It is this
feature that will continue to trigger unsettled weather across the
state well into the coming work week. Thunderstorms mark the
eastern flank of the upper trough, with the closest thunderstorms
just outside our coastal waters zones to the north and northeast
of the islands. Earlier heavy rainfall across Kauai and Oahu have
collapsed, but our airmass remains unstable, moist and poised to
support potentially heavy rainfall on short notice.

High pressure will build eastward well to the north of the
islands and drive strengthening trade winds across local waters.
Moderate to locally breezy conditions should develop by this
evening and continue through tonight, with a slight easing
expected on Monday. A further easing of the trades into the light
to moderate range is expected Monday night through Tuesday night
as a new front approaches from the northwest. Models show the
trades should then strengthen again Wednesday and Wednesday night,
with breezy to windy conditions expected Thursday into next
weekend.

The moist unstable airmass will affect us through at least
Tuesday as the upper trough closes off into an upper low over the
island chain. Unsettled conditions, including the possibility for
heavy rainfall and thunderstorms, will continue through at least
Tuesday and a Flash Flood Watch remains in effect for the entire
state through Monday afternoon. This Flash Flood Watch may need to
be extended into Tuesday for all or portions of the island chain.
Models show the upper low will open into a trough Tuesday night,
then linger over the state through the remainder of the week. As a
result, we should see conditions stabilize a touch beginning
Tuesday night, but a showery wet trade wind pattern will likely
continue into next weekend.

&&

.AVIATION...
Abundant moisture along with an unstable airmass in place across
the area, will cause locally heavy showers and thunderstorms to
occur through Monday.

High pressure passing by north of the area will produce northeast
trade winds through Monday. Occasional MVFR conditions are
possible over north through east facing slopes as moisture
embedded in the trades interacts with island terrain. AIRMET
Sierra is currently posted for mountain obscuration over portions
of Kauai. This may be cancelled or expanded to other islands
overnight.

&&

.MARINE...
A High Surf Advisory (HSA) in effect for most north facing shores
of the smaller islands will be extended through tonight, though
the HSA for the west facing shores of Oahu, Molokai, and the Big
Island will be cancelled. The swell across the state has been
decreasing, but it remains somewhat elevated this afternoon.
Looking ahead to the middle of this week, a new long period
northwest swell arriving early Wednesday may cause surf to reach
the HSA criteria along most north and west facing shores of the
smaller islands from late Wednesday through Thursday.

The remnants of a dissipating front continue to linger over the
western part of the state, while a surface trough near the Big
Island is moving slowly west. The threat of thunderstorms will
continue to exist over the next day or two across the coastal and
offshore waters due to an upper level low developing over the
area.

With the current swell decreasing and winds becoming marginally
weaker, the Small Craft Advisory (SCA) has been cut back to the
typically windy zones around the Maui County and the Big Island.
However, winds are expected to drop below SCA thresholds by
sometime during the day Monday. They will ramp back up to SCA
levels later in the week as high pressure strengthens north of the
area.

Surf will remain small along east facing shores through the middle
of the week. The surf will increase along east facing shores
during the second half of the week as the trades become more
robust. Expect choppy surf to increase along east facing shores
with surf heights possibly nearing advisory levels this coming
weekend. Minimal background swells are expected along south facing
shores through most of the week with a few tiny southwest swells
possible near the middle of the week.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Monday afternoon for all Hawaii
islands.

High Surf Advisory until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Burke
MARINE...Kinel

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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