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Hawaiian Islands Synoptic Discussion and Guidance

753
FXHW60 PHFO 220631
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Sun Oct 21 2018

.SYNOPSIS...
A ridge of high pressure to the north of the state will keep a
moderate trade wind flow in place through the work week and into
the upcoming weekend. Clouds and showers will favor windward and
mauka areas through the period, with a stray shower reaching
leeward areas from time to time. A slightly wetter than normal
trade wind pattern may develop during the middle to latter part of
the week.

&&

.DISCUSSION...
Currently at the surface, a weak west to east oriented ridge of
high pressure is centered around 450 miles north of Kauai, and
this is driving moderate trade winds across the island chain this
evening. Meanwhile, a weak trough of low pressure is located
around 500 miles southeast of the Big Island. Infrared satellite
imagery shows clear to partly cloudy skies in most areas, with
radar imagery showing a few showers moving into windward areas
with some leeward spillover. Main short term concern over the next
couple days revolves around rain chances.

Model solutions are in good agreement with the large scale
synoptic pattern over the next week, showing a ridge of high
pressure holding in place between 25N and 30N through the middle
of the week. A weakening front is then forecast to approach the
islands from the northwest late in the week, then stall out just
to the north and west of Kauai over the upcoming weekend. Moderate
trade winds will prevail through much of the week, with perhaps a
bit of an uptick in the trade wind speeds Tuesday night through
Wednesday night as a weak trough of low pressures passes by to the
south of the state. We could see a bit of downtrend in the trades
late in the week and over the upcoming weekend as well when the
front approaches and stalls out to the north of the state.

As for sensible weather details, a fairly dry trade wind pattern
should hold in place tonight through Monday night, with mainly
windward and mauka showers. Trade showers should pick up a Tuesday
through Wednesday night, particularly across the eastern end of
the state, as the weak trough of low pressure slides by to the
south of the island chain. A slightly wetter than normal trade
wind shower pattern may develop statewide Thursday through the
upcoming weekend, as precipitable water values are forecast to
climb above normal into the 1.5 to 1.9 inch range.

&&

.AVIATION...
A high pressure ridge north of the islands will continue to
produce a moderate trade wind weather regime within a mostly
stable environment over the next 24 hours. A few passing showers
will move through the trade wind flow with scattered shower
activity favoring windward and mountain areas with only isolated
activity elsewhere.

No AIRMETs are in effect and none are expected through Monday.

&&

.MARINE...
Trade winds will persist through end of the work week due to a
surface ridge north of the islands. The pressure gradient will
likely increase over the area by late Tuesday. This will result
in an uptick in wind speeds, with Small Craft Advisory (SCA)
conditions developing over the typically windier waters adjacent
to the Big Island and Maui County from Tuesday night through
Wednesday night. Seas are expected to remain below the SCA
criteria this week.

The current south swell continues to affect the islands early
this evening. The buoys near the south shores continue to show
significant wave heights of 4 to 5 feet with wave periods of
near 15 seconds. Therefore, a High Surf Advisory (HSA) remains in
effect for south facing shores of all islands through Monday
afternoon. This south swell will gradually subside from Monday
night through Tuesday. A new long-period south-southwest swell
arriving Wednesday may cause surf to approach the HSA threshold
along south facing shores starting Wednesday night. A reinforcing
south swell arriving Friday will keep surf elevated along south
facing shores as we head into next weekend.

Surf is expected to remain below the HSA criteria along all other
shorelines across the state through the end of the work week.
Rough surf along east facing shores will increase slightly during
the next few days as the trades strengthen. A moderate north-
northwest swell arriving late Monday night and Tuesday, is
expected to peak Tuesday night or Wednesday. This swell will
slowly decline Wednesday night. A reinforcing north-northwest
swell arriving Thursday will maintain moderate surf along most
north and west facing shores through early Saturday. A new north-
northwest swell arriving late Saturday will likely cause surf to
build along most north and west facing shores from Saturday night
through Sunday.

See the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu (SRDHFO) for additional details on surf and swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai-
Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-Kona-South Big Island-Big
Island North and East-Kohala.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston

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