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Hawaiian Islands Synoptic Discussion and Guidance

599
FXHW60 PHFO 271320
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
320 AM HST Sat Feb 27 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 1041 mb high is centered around 1550
miles northeast of Honolulu, while another 1037 mb high is
centered around 1450 miles northwest of Kauai. In between these
two highs, a nearly stationary south-southwest to north-northeast
oriented front is located around 850 miles northwest of the
Garden Isle. The gradient over the local area remains rather tight
early this morning, with breezy trade winds prevailing across the
entire island chain. Infrared satellite imagery shows mostly
cloudy to overcast skies in most windward areas, while clear to
partly cloudy conditions prevail in most leeward locales. Radar
imagery shows scattered to numerous showers moving into windward
and mauka areas, with a few showers spreading into leeward
communities 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 or drizzle 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...
Early this morning, shower activity along with MVFR level clouds
and visibility have become more widespread along eastern sections
of Maui and the Big Island. This trend is expected to continue
throughout the day, with occasional heavy showers and even a few
thunderstorms possible in these areas. AIRMET Sierra remains in
effect for tempo mountain obscuration for Maui, but it is possible
that this will need to be expanded to include the Big Island
later this morning as showers and cloud cover continue to
gradually increase. For the rest of the state, VFR conditions
should prevail with showers favoring north through east sections
of the islands.

Additionally, strong high pressure remains in place northeast of
the main Hawaiian Islands. This will keep winds breezy to locally
strong today, though they will not be quite as strong as the
previous couple of days. AIRMET Tango remains in effect for low-
level mechanical turbulence over and immediately south through
west of the mountains of all islands. For now, AIRMET Tango also
accounts for sustained surface wind speeds of 30kts in the
Alenuihaha Channel and south of the Big Island, but this may be
allowed to expire this morning as wind speeds weaken slightly.

&&

.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 small to
moderate northwest swell will fill in and peak late today into
early Sunday. The next 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. Lastly, south-facing shores will
see surf boosted modestly by small swells from the southern
hemisphere, along with wrap from the east by the trade wind swell.

&&

.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...M Ballard/Vaughan
MARINE...Kinel

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



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