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Hawaiian Islands Synoptic Discussion and Guidance

173
FXHW60 PHFO 300214
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
414 PM HST Mon Nov 29 2021

.SYNOPSIS...
Wet and unsettled weather will remain this week. Gusty winds
tonight will ease Tuesday and remain at moderate speeds through
midweek before increasing Friday. Windward and mountain areas will
have the best chances of rain over the next few days before
widespread rainfall and the potential for flash flooding pushes
over the islands Friday and into the weekend.

&&

.DISCUSSION...
A strong high pressure ridge north of the state is producing
gusty trade winds, while showers riding those trades are bringing
quick hitting downpours to windward and mountain areas. Some of
these showers are reaching leeward locations as well.

The center of an upper low and an associated surface trough are
sitting just east of the Big Island with the low providing
instability and the trough adding low level moisture. This
combination is allowing thunderstorms to form over, and in the
vicinity of the Big Island this afternoon. The upper low and
surface trough will move slowly east tonight and Tuesday, allowing
those isolated thunderstorms to spread as far west as Maui
County. By Wednesday, the upper low will weaken and the trough is
expected to dissipate, thus, the chance of thunderstorms will
decrease. However, local downpours will likely remain as the low
sits in place over the islands.

Moderate easterly trade winds will continue through Wednesday
before a cold front approaching from the northwest disrupts the
trades Thursday. On Friday, the front will sweep down the island
chain from the north, affecting Kauai initially and the central
islands through the day. Heavy rain is expected along and in the
vicinity of the front with gusty north winds behind the front.
Global models have been consistent with the front reaching the Big
Island Friday evening and then stalling over the weekend. Aloft,
a potent upper level low will accompany the front adding a healthy
amount of instability. If the low is as strong as the models are
advertising, this instability will fuel the chances of flash
flooding over the island chain through the weekend and possibly
into next week. With the surface front stalled in place, southerly
winds will develop across the state starting Saturday and
continuing through Monday. In summary, expect periods of inclement
weather, along with bouts of fair weather, starting Friday and
continuing into next week.

&&

.AVIATION...
An upper low near the Big Island will gradually weaken through
Tuesday night. The low will provide instability across the eastern
half of the state. Abundant low level moisture along an
associated surface trough will move over all islands tonight and
linger through the day Tuesday allowing for potential for heavy
showers and thunderstorms, especially for Maui County and the Big
Island. Expect MVFR ceilings and visibility within heavier
showers.

AIRMET Sierra for mountain obscurations has been cancelled this
afternoon, but expect the next area of clouds and showers to fill
in this evening with AIRMET Sierra likely going back up for Maui
County and Big Island.

AIRMET Tango is posted for tempo moderate turbulence below 7000
feet due to the breezy trade winds through 04Z this evening. Low
level winds will taper off this evening as the surface trough
arrives. Mid to high levels will see an increase in turbulence
between FL250 to FL350 as the upper trough moves in. AIRMET Tango
has been posted at these flight levels for tempo moderate
turbulence over Maui and the Big Island. This area of turbulence
should continue through the night and decrease Tuesday morning as
the upper trough begins to weaken.


&&

.MARINE...
A surface trough moving through the state has brought unstable
conditions over the eastern offshore waters and around the Big
Island. A few thunderstorms did develop around the Big Island this
afternoon with scattered thunderstorms over the eastern offshore
waters. Locally strong NE trade winds today will begin to veer
toward the east tonight and decrease slightly as the trough
weakens. The Small Craft Advisory has been cancelled for many
areas as the winds continue to decrease over the next several
days. For waters around the Big Island, a Small Craft Advisory
remains in effect through Tuesday due to winds up to 25 kt.

Some lingering instability from the upper level low could produce
some isolated thunderstorms through Tuesday night primarily over
the eastern half of the state.

Towards the end of the week, guidance is indicating a powerful
cold front moving through the state around Friday. Strong north to
northwest winds will follow behind the front and we could see some
unsettled weather ahead and along the front. In addition, a very
large north swell will begin to fill in Friday and peak over the
weekend. Very rough ocean conditions are expected Friday through
the weekend.

The current northwest swell will steadily decrease through
Tuesday. A series of small northwest swells are expected Tuesday
through Thursday. A much larger north-northwest swell is expected
over the weekend and will likely bring warning level surf along
north and west facing shores.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...Morrison
AVIATION...Foster
MARINE...Kino

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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