Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 101322

Area Forecast Discussion
National Weather Service Honolulu HI
322 AM HST Sat Apr 10 2021

Light winds will prevail statewide today, with moderate northeast
trade winds redeveloping over Kauai and Oahu on Sunday, then
gradually strengthening statewide early next week. A period of
unsettled weather is possible tonight through Monday, as a potent
low aloft passes over the area, bringing the potential for
thunderstorms, and a few locally heavy downpours. A more settled
weather pattern is expected thereafter.


A high to the distant N and a persistent low to the NE of the
islands are maintaining light to locally moderate NE trade winds
over western parts of the chain this morning, with lighter winds
over the E end of the chain. Satellite and radar indicate that
showers remain widely separated, but have increased in coverage and
intensity ever so slightly. Overnight soundings shows an increase in
PWAT to near 1.2", with a robust subsidence inversion still intact
near 9000`. Otherwise, scattered high clouds are moving from W to E
while dew points in the upper 50s to lower 60s F have allowed
overnight low temperatures to drop into the lower- to mid-60s F.

The low-level pressure gradient near and NE of the islands will
become quite weak today as troughing to the distant NE drifts W and
(partially) moves between the islands and the trade-wind-supporting
high. Light and variable winds will develop over Maui and the Big
Island with light NE winds over Kauai and Oahu, with a few windward
showers expected this morning. Winds will be sufficiently light
statewide to allow afternoon sea breezes to drive some cloud
formation over leeward and interior portions today. There is some
uncertainty as to how unstable the island atmosphere will become in
the short term as a developing shortwave trough approaches from the
N, but the forecast calls for scattered showers.

A potent shot of mid-level cold air is expected to accompany the
aforementioned shortwave trough aloft that will sharpen as it moves
over the islands tonight through Monday. This will significantly
destabilize the island atmosphere as mid-level temperatures plummet
from -8C to -16C, but PWAT will remain on the lower side (near
1.2"). Locally derived guidance depicts thunderstorm chances
steadily increasing tonight, peaking Sunday, then slowly diminishing
and shifting E on Monday. While instability will be high, allowing
thunderstorms and a few heavy rain cells to develop, the relatively
low PWAT will likely preclude the threat of widespread heavy
rainfall that would lead to significant flooding.

Periods of wintry weather will be possible over the Big Island
Summits and upper slopes with any deeper convection that develops,
and the freezing level may become low enough to support a brief
period of wintry weather over the summit of Haleakala - with the
most likely time being Sunday/Sunday night.

Models indicate surface low pressure will linger NE of the islands
into the middle of next week, leading to moderate NE winds over
Kauai and Oahu, and lighter NE winds over Maui County and the Big
Island, with mostly dry and stable conditions in place. A weak cold
front may move into the area from the N around Thursday.


A light to moderate trade wind flow will persist through tonight.
The weakness of this flow and daytime heating will lead to local
daytime sea breeze over the interior and lee areas of the smaller
islands, resulting in some afternoon clouds and possibly a few
showers. Leeward Big Island faces a similar scenario this afternoon
as well. This trade wind flow will take on a more northeast
component as the day progresses, and continue through Sunday.

Thus far, trade wind clouds and showers are affecting mainly the
windward and mountain areas, causing brief periods of MVFR ceiling
and vis. Tops of these showers are around 8k feet, with higher tops
to 12k feet in the stronger showers. Look for the frequency of these
showers to become less by mid morning. VFR will be primary flying
weather for today.

A strong upper level disturbance will be coming in tonight from the
north, leading to a slight chance for a thunderstorm, especially
over the eastern part of the island chain.

No AIRMETS, and none are expected through this afternoon.


Moderate northeast trade winds will trend down through the weekend
as the ridge weakens to the north. Winds should become light enough
for a land and sea breeze regime to setup for most areas. The
background flow will shift out of a more northerly direction
(stronger Kauai and Oahu waters) due to a persistent trough/low
lurking northeast of the state. In addition to light winds, a few
heavy showers and thunderstorms will become a possibility tonight
through Sunday as an upper disturbance drops south over the islands -
best chance windward waters and eastern half of the state. Northeast
trade winds will gradually return early next week, potentially
reaching the fresh to locally strong category by midweek.

Surf along south facing shores will gradually rise over the weekend
as a long-period, south-southwest swell arrives from recent activity
within Hawaii`s swell window near New Zealand. This source should be
fully filled in by Monday, then hold into midweek before lowering.
Heights should peak just below the advisory threshold (8 ft faces)
early next week.

Surf along north and west facing shores will remain small through
the weekend, then trend up late Sunday through early next week as a
small, west-northwest swell arrives from a gale that developed near
the Kurils earlier this week. Although confidence remains low, a
moderate, short-period north-northeast swell is depicted late next
week due a strong gradient setting up once again to the north
between strong high pressure near the Date Line and low pressure to
its east.

Surf along east facing shores will lower over the weekend as the
persistent north-northeast swell fades and the local winds diminish.
Surf will remain small early next week, then trend up by the second
half along exposed coasts as the previously discussed north-
northeast swell arrives.





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