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Temp5.4 C
RH39 %
WindNE 14 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

915
FXHW60 PHFO 251339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Sun Oct 25 2020

.SYNOPSIS...
Southeast to south winds will maintain a few heavy showers near
Kauai this morning and send light showers over windward slopes of
the Big Island and Maui. Afternoon sea breezes will produce
clouds and spotty showers over interior sections of all islands.
Unsettled weather will develop under southeast winds late Monday,
bringing the potential for heavy showers and thunderstorms to
portions of the state through at least mid week.

&&

.DISCUSSION...
A slightly unstable southerly flow persists near Kauai, while
drier southeasterly winds prevail across the rest of the state.
Surface high pressure far northeast of the state is driving the
southeasterly winds, which veer southerly near Kauai due to
persistent troughing northwest of the islands. An upper level
trough digging toward the state from the northwest is creating
instability that is generating a few heavy showers and
thunderstorms around Kauai and Niihau this morning. Expect daytime
sea breezes to overpower the background winds, leading to
afternoon clouds and spotty showers over interior sections of all
islands, especially on Kauai.

As the upper trough advances toward the islands Sunday night and
early Monday, high clouds will thicken overhead, and the
atmosphere will slowly destabilize. At this time, the best chances
for enhanced showers, and possibly thunderstorms, appear to be
across the western end of the island chain Monday afternoon,
especially on Kauai.

Wet and unsettled conditions are looking increasingly likely late
Monday into mid week. The upper level trough will drive a front
near Kauai, but the main factor for heavy rainfall and potential
flooding will likely be a deep moist southerly flow developing
ahead of the front. The latest runs of the GFS and ECMWF show an
upper level jet stream setting up over over the the western half
of the island chain Monday night and persisting through at least
Tuesday. While this suggests a greater flood threat for Kauai and
Oahu, model differences still mean that there could be heavy rain
state-wide.

Forecast confidence decreases by Thursday and worsens later in
the week. The GFS and ECMWF show significant differences in the
evolution of the upper level trough. The GFS stalls the feature
nearby into Saturday, keeping a moist southeasterly flow in
place. The ECMWF develops a cut off upper low and sends it
westward, allowing a drier trade wind flow to build in. Given the
stark model differences, the current forecast broad brushes
scattered showers over the state during this time.

&&

.AVIATION...
The moisture axis draped over Kauai will support isolated to
scattered showers through early this morning. Likewise, windward
showers will continue over Maui and Big Island into early Sunday
morning. Dry air will support mostly clear skies from Molokai
through Oahu during this time, a trend which has been reinforced
by land breeze development. Prevailing VFR is forecast for all
locations. Sea breeze development will favor inland cloud build-up
and a few leeward showers late this morning and early this
afternoon.

No AIRMETs in effect.

&&

.MARINE...
Strong high pressure northeast of the state and a cold front
approaching from the northwest will keep moderate to fresh
southeast winds over the eastern islands, and light and variable
winds over the western end of the state during the next few days.
As the front moves closer, the potential for heavy showers and
thunderstorms will increase.

The current long-period northwest swell continues to run around 3
feet higher than predicted levels at the NOAA buoys northwest of
the state. Meanwhile, the Hanalei and Waimea buoys continue to
trend up early this morning, and are nearing advisory levels. The
swell will continue to fill in during the next few hours and peak
at low-end advisory levels along exposed north and west facing
shores of the smaller islands today into tonight. The swell will
gradually lower late tonight through the middle of the week. A new
long-period northwest swell will be possible next weekend.

Surf along south facing shores will remain small with a mix of
long-period south and short-period southeast swells moving through
the islands during the next 7 days. A combination of east swell
associated with a fetch of trades off to the east of the islands
and a small northeast swell, will keep small surf in place along
east facing shores through Tuesday. East shore surf is expected to
trend well below normal levels Wednesday through late in the week
as the upstream trades diminish and the northeast swell fades.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai and for north
facing shores of Maui.

&&

$$

DISCUSSION...Wroe
AVIATION...JVC
MARINE...Jelsema

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