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Hawaiian Islands Synoptic Discussion and Guidance

059
FXHW60 PHFO 221353
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Fri Nov 22 2019

.SYNOPSIS...
Windy and showery trade wind weather is expected today through
Saturday. Showers will be most prevalent over windward and mauka
areas, with showers spilling over into leeward communities from
time to time. The trades are expected to ease a bit next week and
shift around to the east-southeast as a trough of low pressure
develops west of the state. Showers will continue to favor
windward and mauka areas next week, particularly Maui and the Big
Island, with some afternoon and evening shower development
possible over interior and leeward locales.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high is centered around 1500
miles northeast of Honolulu, while a trough of low pressure is
located around 450 miles south-southeast of South Point on the Big
Island. The resulting gradient is producing breezy to locally
windy conditions across the island chain early this morning.
Aloft, an upper level low is evident around 250 miles southwest
of Kauai, with broad upper level troughing extending eastward
across the Big Island. Infrared satellite imagery shows mostly
cloudy conditions across the state. Radar imagery shows scattered
to numerous showers moving into windward areas, with isolated to
scattered showers reaching leeward locales. In addition a few
isolated thunderstorms have been seen through the overnight hours
across the island chain. Main short term concerns revolve around
the potential for thunderstorms today and strong trade winds
through Saturday.

The upper level low and associated upper troughing over the state,
is keeping conditions unstable enough to produce thunderstorms
from time to time early this morning. The most unstable conditions
will remain over the island through the early morning hours, so
will keep a slight chance of thunderstorms in the forecast across
the majority of the state. The most unstable conditions should
have pushed off to the southwest of the islands later this morning,
but some moisture wrapping into leeward sections of the Big
Island will necessitate a slight chance of thunderstorms during
the afternoon hours.

Otherwise, upper troughing lingering over the state should keep a
showery trade wind pattern in place through at least Saturday
night, with showers most prevalent over windward and mauka areas.
The upper level troughing will begin to exit to the northeast as
upper ridging builds in from the west Sunday through early next
week, and this should bring a decrease in shower coverage to the
area. The boundary layer winds will also begin to shift around to
the east-southeast early next week, with showers favoring windward
areas, particularly Maui and the Big Island, along with some
afternoon/evening shower development over leeward and interior
sections of all islands.

Forecast details become more uncertain towards the middle of next
week. The 00Z GFS has trended further east with a trough of low
pressure and associated deep moisture, bringing some wet weather
to Kauai and possibly Oahu Tuesday through Thursday. The 00Z
ECWMF on the otherhand, remains consistent in keeping the trough
of low pressure and deeper moisture just to the west of the state.
Given the continued uncertainty in the forecast, we will wait
until details become more clear before making any significant
changes to the forecast. As a result, the forecast will reflect an
east-southeasterly trade wind shower pattern favoring windward
and mauka showers, with some interior and leeward shower
development each afternoon and evening.

As for winds, expect the trades to increase across the state
this morning, then peak this afternoon through Saturday as high
pressure strengthens northeast of the state and a trough of low
pressure slides by to the south. A Wind Advisory remains in effect
for most of the state through 6 PM this evening, and there is the
potential that the advisory may need to be extended through the
day on Saturday. For now will let the day shift monitor
observations to see if the advisory can be scaled back to some of
the more typically windy zones or if it will need to be extended
in time for the entire state. The trades should begin to edge
downward Saturday night and Sunday, but are expected to remain
breezy statewide. Breezy conditions will likely continue across
windward sections of Maui and the Big Island through early next
week, although we should see trades ease across the remainder of
the state as the boundary layer flow shifts around to the east-
southeast. Winds should gradually trend lower during the middle
and latter part of next week as the trough of low pressure sets up
to the west of the island chain.

&&

.AVIATION...
A strong high pressure system northeast of the state will produce
breezy to locally windy trades into the weekend. A weak upper
level low will continue to produce unsettled weather with
scattered to numerous showers favoring windward and mountain areas
of each island. Isolated thunderstorms will develop across the
state through the morning hours. Mostly VFR conditions are
expected for leeward areas with periods of MVFR conditions along
windward slopes of all islands. Some of these stronger showers
will drift over leeward areas especially in the overnight to early
morning hours.

AIRMET Sierra is in effect for mountain obscuration along the
north and east slopes of Kauai Oahu Molokai Maui and the Big
Island this morning. This AIRMET may decrease in coverage after
20Z.

AIRMET Tango remains in effect for tempo moderate turbulence over
and immediately south thru west of mountains on all islands below
8000 feet. Another area of moderate turbulence has developed
between FL250 and FL350 due to the subtropical jet stream. Some
aircraft have reported moderate turbulence in both areas over the
past 12 to 18 hours.

&&

.MARINE...
Trade winds will continue to strengthen today and Saturday with
speeds reaching gale force in the windier waters around the Big
Island and Maui County. A Small Craft Advisory remains in effect
for the rest of the waters. Enhanced winds and seas will remain
through Saturday with a slow decline in strength expected Saturday
night and Sunday. The short period east swell will bring advisory
level surf to east facing shores today and through the weekend.

A northwest swell will slowly decline through today. A reinforcing,
moderate northwest swell will arrive Saturday morning and peak
Saturday night, bringing near advisory level surf for exposed
north and west facing shores on Sunday. A smaller northwest swell
is expected by the middle of next week.

Lastly, a series of a small background southwest swells will
maintain small surf along south facing shores through the forecast
period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-South
Big Island-Big Island North and East-Kohala.

High Surf Advisory until 6 PM HST Saturday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Gale Warning until 6 PM HST Saturday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Morrison

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



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