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Hawaiian Islands Synoptic Discussion and Guidance

868
FXHW60 PHFO 220712
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
912 PM HST Thu Nov 21 2019

.SYNOPSIS...
Trade winds will strengthen Friday and will bring windy and
showery weather 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 veer 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 1029 mb high is centered around 1400
miles northeast of Honolulu, while a trough of low pressure is
located around 450 miles southeast of Hilo. The resulting gradient
is producing breezy conditions across the island chain this
evening. Aloft, an upper level low is evident around 200 miles
southwest of Kauai, with broad upper level troughing extending
eastward and over the central islands. Infrared satellite imagery
shows mostly cloudy conditions across most of the state. Radar
imagery shows scattered to numerous showers moving into windward
areas, with the brunt of the activity affecting Maui at the
moment. A few showers are trickling over into leeward areas.
Meanwhile some thunderstorms are seen over coastal waters
southwest of Kauai in closest proximity to the upper level low, as
well as over windward sections of Maui. Main short term concerns
revolve around the potential for thunderstorms tonight across the
smaller islands and strong trade winds over the next couple days.

The upper level low and associated upper troughing over the state
appears to be a bit stronger than previously advertised by the
numerical model guidance, with some convection seen just upstream
of Maui with cloud tops between 35 and 40 kft. This lines up well
with an area of enhanced moisture with precipitable water values
around 1.7 inches, and surface based CAPE values of around 1500
J/KG. This region of more unstable air is forecast to slide
westward across the smaller islands overnight, and could spark a
thunderstorm or two. As a result, we have added a slight chance of
thunderstorms to the forecast overnight across the smaller
islands.

This area of enhanced moisture and instability should have pushed
off to the southwest of the smaller islands by daybreak, but we
will monitor trends closely through the night in case thunder
needs to be added to the forecast during the morning hours Friday.
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 next 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
tonight, then peak in the Friday through Saturday time frame 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 on Friday. There is the potential
that the advisory may need to be extended through the day on
Saturday. 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
state.

&&

.AVIATION...
A strong high pressure system northeast of the state will produce
breezy to locally windy trades winds into the weekend. A weak
upper level low will continue to produce scattered to numerous
showers favoring windward and mountain areas of each island.
Isolated thunderstorms are possible across the state through the
overnight 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 hours.

AIRMET Sierra in effect for mountain obscuration along the north
and east slopes of Maui tonight. This AIRMET may expand to cover
other islands by Friday morning.

AIRMET Tango remains in effect for tempo moderate turbulence over
and immediately south thru west of mountains on all islands below
8000 feet. Some aircraft have reported moderate turbulence in
these areas earlier today.

&&

.MARINE...
Trade winds will continue to strengthen tonight with speeds reaching
near gale force in some of the channels Friday due to a strengthening
surface high far northeast of the area. A Small Craft Advisory (SCA)
is currently in effect for all Hawaiian waters, due to the large
northwest swell and locally strong trades and rough, choppy seas.
The SCA is in effect through 6 pm HST Saturday, but will likely
to be extended through the weekend. A Gale Watch is in effect for
the channels around Maui County and the Big Island for Friday.

A northwest swell peaked Thursday and will slowly decline Friday.
A High Surf Advisory (HSA) is in place for all affected areas
until 6 AM Friday. A reinforcing, moderate northwest swell will
fill in this weekend for another round of advisory level surf for
exposed shores. A smaller northwest swell is expected for the
middle of next week.

As a result of the strong trades, east facing shores of most
islands will see advisory level surf over the weekend. 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 from 6 AM to 6 PM HST Friday 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 AM HST Friday for Niihau-Kauai
Leeward-Waianae Coast-Oahu North Shore-Molokai Leeward-Maui
Central Valley-Kona.

High Surf Advisory until 6 PM HST Saturday for Kauai Windward-
Oahu Koolau-Molokai Windward-Maui Windward West-Windward
Haleakala.

High Surf Advisory from 6 AM Friday to 6 PM HST Saturday for
Olomana-South Big Island-Big Island North and East.

Gale Watch from 6 AM HST Friday through Friday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

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

Small Craft Advisory from 6 PM Friday to 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