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Temp-4.7 C
RH20 %
WindENE 12 mph
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Hawaiian Islands Synoptic Discussion and Guidance

282
FXHW60 PHFO 260635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Thu Feb 25 2021

.SYNOPSIS...
Strong and gusty trade winds will persist across the region into
early next week due to a series of strong surface highs to the
north and northeast of the islands. Rainfall will remain focused
over windward slopes, but an upper level disturbance will give an
added boost to these trade showers. The disturbance may even kick
off a few thunderstorms Friday and through the weekend over the
eastern end of the island chain.

&&

.DISCUSSION...
An exceptionally strong and broad 1047 mb surface high is centered
nearly 1600 miles north of the islands this evening. It is the
main source of of the strong and gusty easterly trade winds.

A Wind Advisory remains posted for portions of the Big Island and
Maui County through Friday. There is a small handful of sensors
registering sustained winds of around 30 mph and gusts to around
40 miles per hour, across Maui County and the Big Island. The
trade showers have been limited area-wide, but if any, they are
favoring the windward and mountain areas. Showers have been absent
across leeward Big Island this afternoon and evening, but there is
a small shower or two located off the Kona coast. Satellite
imagery shows a field of widely dispersed showers mostly upwind
of Maui and the windward coast of the Big Island from Upolu Point
to the Puna district. Do expect these trade showers to pick up a
little more as night progresses.

Looking ahead into the weekend, the trade winds are expected to
ease off slightly starting late Friday through Saturday. This
temporary lull comes about as a new high starts moving in from
the west to replace the surface high currently located 1600 miles
north- northeast of the islands, which will be moving southeastward
and weakening. The new high is not as strong as the previous but
it is still rather strong at 1043 mb at a point far north of the
islands Sunday night. The high then eases southward and weakens
to 1037 mb by Monday night. Under this scenario, trade winds are
likely to regain very wind status by Saturday night, leading to
another Wind Advisory for Maui County and northern Big Island.
More details are forthcoming.

The forecast becomes more challenging as an upper level low with
a pool of cold air of minus 13 to minus 14 degree C at 18k feet
sinks over the islands from the north in the next 24 hours, but
favoring the eastern part of the island chain. It is about minus
11 degree C currently at 18k feet area-wide. The air mass over
the islands is not all that very stable now, and the approaching
upper low and cold air will only destabilize air mass further more
in the next 24 hours. So we are anticipating an uptick in heavier
trade showers, area-wide. The strong and breezy trade winds will
help move along most of these showers, meaning the lee side of
the smaller islands will get their share of showers. We anticipated
the wind- sheltered leeward Big Island to be an active Friday
afternoon with an outside chance of a thunderstorms. The potential
for a thunderstorm spreads over to the rest of the Big Island and
windward Maui on Saturday, and persisting through Sunday. The
area with the highest risk of sustained heavy rainfall will likely
be windward Big Island, though a Flash Flood Watch does not
appear warranted at this time.

The upper low gives way to a trough by Monday, and this cold pool
of minus 14 warms slightly to minus 12 degree C. Both the GFS and
old ECMWF suggest things to improve across the smaller islands
Sunday night into Monday, but keeping the Big Island unstable.

It should be noted at this point of Monday, the GFS differs from
the old ECMWF run, where the GFS maintain a windy trade wind flow,
while the EC lighten the trades.


&&

.AVIATION...
Strong easterly trade winds will continue tonight and tomorrow as
an impressive surface high remains rooted roughly 1600 miles northeast
of the state. As a result, AIRMET Tango remains in effect to
account for low level mechanical turbulence for areas over and
immediately south through west of mountains. Additionally, since
strong winds are forecast to persist through tomorrow, AIRMET
Sierra remains in effect to account for surface winds greater than
30kts in the Alenuihaha Channel and south of the Big Island. Low
level wind shear will also continue to be a concern at some area
airports.

Models indicate MVFR clouds and showers will be possible overnight
and during the early morning hours for some locations along east-
facing sections of the islands. This appears most likely at Lihue
and Hilo, but will continue to monitor trends and update as needed
for other east-facing locations. Otherwise, VFR conditions should
prevail during the day tomorrow.

&&

.MARINE...
A very strong high northeast of the Hawaiian islands and a trough west
of the area are creating a tight pressure gradient that is producing
strong east winds. A Gale Warning is in effect for the Alenuihaha
Channel and the Big Island Leeward Waters through Friday evening.
A Small Craft Advisory (SCA) is in effect for the rest of the Hawaiian
Coastal Waters through the weekend. Winds will subside a bit Friday
night as the high weakens. Winds are likely to strengthen again over
the weekend as a new high build north of the area. Winds could reach
gale force again Sunday night or Monday.

A long fetch of strong northeast and east trade winds is producing
elevated seas. A High Surf Advisory in effect for exposed east facing
shores will remain up for the next several days. Surf elsewhere will
be below advisory levels. Several long-period northwest swells are
expected over the next week. These swells will peak tonight into
Friday, Saturday night into Sunday, and Wednesday into Thursday.
The mid-week swell is expected to be the largest of the three. Some
trade wind swell will wrap into N and S facing shores. A small swell
from the southern hemisphere is also expected the next couple of
days.

&&

.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.

Wind Advisory until 6 PM HST Friday for Molokai-Kahoolawe-Maui
Windward West-Maui Leeward West-Maui Central Valley-Windward
Haleakala-Leeward Haleakala-South Big Island-Big Island North
and East-Kohala-Big Island Interior.

Small Craft Advisory from 6 PM Friday to 6 PM HST Sunday for Big
Island Leeward Waters.

Gale Warning until 6 PM HST Friday for Alenuihaha Channel-Big
Island Leeward Waters.

Small Craft Advisory until 6 PM HST Sunday 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-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Southeast
Waters.


&&

$$

H Lau/Vaughan/Kinel/Donaldson

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