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Temp-2.9 C
RH28 %
WindENE 2 mph
RoadClosed
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Hawaiian Islands Synoptic Discussion and Guidance

508
FXHW60 PHFO 270558
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
758 PM HST Fri Feb 26 2021

.SYNOPSIS...
Strong and gusty trade winds will continue into early next week,
while a trough aloft keeps the island atmosphere unstable. Clouds
and showers will focus over windward areas, with some occasional
heavy downpours. Increasing moisture near Maui and the Big Island
will bring a more widespread threat of heavy windward showers, and a
slight chance of thunderstorms. Typical trade wind weather will
continue through much of next week, with showers favoring windward
and mauka areas and spreading into leeward communities from time
to time.

&&

.DISCUSSION...
Currently at the surface, a 1043 mb high is centered around 1600
miles northeast of Honolulu, while another 1033 mb high is
centered around 1550 miles northwest of Kauai. In between these
two highs, a nearly stationary south-southwest to north-northeast
oriented front is located around 950 miles northwest of the
Garden Isle. The gradient over the local area remains rather tight
this evening, with breezy trade winds prevailing across the entire
island chain. Infrared satellite imagery shows partly to mostly
cloudy skies in windward areas, and clear to partly cloudy
conditions in most leeward locales with the exception of leeward
Big Island where mostly cloudy conditions persist. Radar imagery
shows scattered to numerous showers moving into windward and mauka
areas, with a few showers spreading to leeward areas from time to
time. Main short term concerns revolve around the Flash Flood
and Winter Weather headlines, as well as the trade wind trends
through the weekend.

High pressure northeast of the state will settle southward and
weaken during the next couple days, while the high northwest of
the state builds eastward and strengthens to around 1044-46 mb by
late Sunday and Sunday night due north of the island chain. This
high will settle southward and weaken slightly early next week,
followed by a new 1044 mb high building northwest of the state
during the middle to latter part of next week. Overall, breezy to
windy trades are expected to continue through the weekend and
into early next week, with the trades peaking late Sunday through
early Tuesday. Wind Advisories will likely be required once again
for portions of the state during this time. The trades should ease
a bit for the middle and latter part of next week, but will likely
hold at moderate to breezy levels.

As for the remaining weather details, the island atmosphere will
remain unstable through the weekend and into next week, due to
the presence of a persistent trough aloft, and its associated cold
pool. As a result, inversion heights will be pretty much
eliminated, allowing for heavier showers to affect windward areas
and sending showers into leeward communities more regularly as
well. While these showers will be progressive and will not linger
over any one area for a prolonged time, high-resolution and global
model guidance continues to indicate the likelihood of repeated
development over the windward slopes of Maui and the Big Island
during the next couple days. This persistent rainfall is expected
to lead to increasing ground saturation and rising levels in
streams and rivers, with additional rainfall potentially resulting
in flash flooding as the weekend progresses. As a result, a Flash
Flood Watch (FFA) remains in effect for windward portions of Maui
and the Big Island. An expansion of the FFA to windward portions
of the other islands is not anticipated.

The cold temperatures aloft and deeper-than-normal moisture means
that the Big Island Summits will receive periods of wintry weather,
with the primary threats being freezing rain leading to icy
roadways, although snow showers will mix in as well. A Winter
Weather Advisory remains in effect through Monday morning, and
may need to be extended as the trough aloft lingers well into next
week.

&&

.AVIATION...
Locally strong easterly trade winds will continue this weekend
as an area of strong high pressure at the surface remains stalled
north-northeast of the the state. However, slight weakening
should allow winds to ease a bit compared to the past couple of
days. For now, AIRMET Tango remains in effect to account for low-
level mechanical turbulence over and immediately south through
west of the mountains of all islands. Additionally, AIRMET Tango
also continues to account for sustained wind speeds greater than
30kts in the Alenuihaha Channel and south of the Big Island, but
this is expected to gradually wane throughout the night.

As an upper disturbance moves over the island chain, expect
increasing chances for heavier showers and more widespread MVFR
cloud cover, particularly over eastern sections of the Big Island
and Maui. This could present some mountain obscuration issues
beginning during the early morning hours. Finally, thunderstorms
cannot be ruled out for portions of the Big Island and Maui during
the day Saturday, though confidence in timing and coverage
remains too low to include in the TAFs at this time.

&&

.MARINE...
With robust high pressure remaining far north of the area into
next week, trade winds will remain breezy to strong over coastal
waters.

Also, seas will remain elevated and rough with the persistent
strong trades over the next several days. Thus, a Small Craft
Advisory (SCA), for winds and seas, has been posted through
Sunday afternoon for all coastal waters, and may need to be
extended beyond that time frame.

With locally generated wind waves, and strong trade winds upstream
from the state, surf along east-facing shores will remain large
and turbulent. To account for this, a High Surf Advisory will
remain in effect through Sunday night for Kauai, Oahu, Molokai,
Maui, and the Big Island. Too, some of this trade wind energy will
wrap around into adjacent northern and southern exposures.

As the current northwest swell fades, a reinforcing moderate
northwest swell will fill in and peak late Saturday into early
Sunday. The following northwest swell will likely be larger than
this weekend's and raise surf to near advisory levels along
northern and western exposed shores, beginning late Wednesday into
early Thursday. Small swells from the southern hemisphere will
keep south shore surf from going flat through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through Sunday afternoon for Maui Windward
West-Windward Haleakala-Haleakala Summit-South Big Island-Big
Island North and East-Big Island Interior.

Winter Weather Advisory until noon HST Monday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters-

&&

$$

DISCUSSION...Jelsema
AVIATION...Vaughan/M Ballard
MARINE...Kinel

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