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Hawaiian Islands Synoptic Discussion and Guidance

451
FXHW60 PHFO 281920
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
920 AM HST Tue Jan 28 2020

.SYNOPSIS...
Stable and rather dry conditions will persist through Thursday,
featuring light and variable winds across the western half of the
state and east to southeast winds with light windward showers on
the Big Island and Maui. A front will stall near Kauai on Friday,
bringing wet and unsettled conditions across the western end of
the state into Saturday as trade winds rebuild. Typical trade wind
weather is due on Sunday, with trades veering southeasterly again
early next week.

&&

.DISCUSSION...
A stable and rather dry conditions persist. A subtropical ridge
sitting about 175 miles north of Kauai is driving easterly trade
winds across the Big Island and a southeasterly background flow
from Maui to Kauai. This places the western half of the island
chain in the wind shadow of the Big Island, allowing land breezes
to dominate this morning. Scant rainfall has been recorded on
these islands during the past 24 hours, as precipitable water is
below January normal. Meanwhile, a pocket of shallow moisture has
been affecting windward Big Island and east Maui, but only
isolated showers have been observed. The atmosphere remains very
stable, as a mid level ridge overhead is maintaining a strong
inversion between 4,000 and 6,000 ft. Expect sea breezes to
produce afternoon clouds and only isolated light showers across
interior terrain.

Stable and rather dry conditions will dominate through Thursday.
The subtropical ridge will drift toward the state over the next
24 hours, further weakening and veering the background flow and
allowing land and sea breezes to dominate most areas from Kauai to
Maui while easterly trade hang on across the Big Island. By late
Wednesday, the subtropical ridge will be pushed over the islands
by an approaching front, disrupting the the trade wind flow state-
wide. The mid level ridge will remain in place overhead and
maintain stable conditions. As a result, expect light windward
showers to favor Big Island and east Maui, and the rest of the
state will be under a stable land and sea breeze pattern,
featuring afternoon clouds and a few light showers with clearing
at night.

Wet and unsettled weather will likely develop across the western
end of the state Thursday night and Friday. A front will drop
down from the northwest and stall near Kauai, while surface high
pressure passing north of the front ushers in building trade
winds. An upper level trough will pass over the islands on Friday
and could create enough instability to trigger heavy showers. The
latest runs of the GFS and ECMWF have been weaker with the upper
level trough, which would likely limit any chances of unsettled
weather to the deeper moisture along the front near Kauai and
perhaps Oahu. Elsewhere, building trade winds are expected to
focus modest showers across windward slopes.

Moderate trade winds are in store for the weekend, while wet
conditions could persist across the western end of the state. The
GFS and ECMWF show the front dissipating near Kauai and perhaps
Oahu, as the upper level trough moves eastward down the island
chain. While we cannot rule out heavy showers over the western end
of the state, the deepest moisture and greatest instability will
likely be out of phase. This favors mainly windward rainfall,
wetter on Kauai and Oahu. Both models suggest mid level ridging
will build overhead on Sunday as trade winds hold, favoring a
typical pattern of windward rainfall.

&&

.AVIATION...
Once again, a surface ridge of high pressure north of the state
will allow for light easterly flow across the region. VFR
conditions with partly cloudy skies will prevail, along with a
sea breeze development this afternoon. Isolated light showers may
form over the interiors and elevated terrain, but not expecting
anything significant or widespread.

&&

.MARINE...
A nearly stationary ridge just north of the area will keep winds
light and variable across Kauai and Oahu, and moderate to locally
fresh out of the east to southeast over Maui County and the Big
Island through most of the week. An approaching front from the NW
will shift the ridge over the island chain Thursday, weakening
winds around Maui County and Big Island and generating unsettled
weather.

A dynamic weather pattern across the Pacific will keep surf
elevated along north and west facing shores beginning tonight.
This next northwest swell is expected to peak at advisory levels
by Wednesday morning then lower slightly and remain near advisory
levels through Thursday. A slightly larger swell, from the
northwest to north-northwest, is expected to fill in Thursday
night and peak Friday at solid advisory levels, before slowly
declining through the weekend.

Surf along exposed east facing shores will remain rather small
through the period due to the lack of persistent trades locally
and upstream across the eastern Pacific. An increase in surf is
possible over the weekend as trades build behind the front.

Surf along south facing shores will remain small through the week.
However, background south swells should be enough to keep things
from going flat.

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...TS/Almanza

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



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