Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 210135

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST Sat Jan 20 2018

Moderate trade winds will deliver a steady stream of clouds and
showers the next couple of days as moisture associated with a
dissipated front combines with a weak disturbance aloft. Showers
will favor windward and mauka areas, but leeward areas could also
receive some rainfall. Winds may diminish Thursday and Friday.


A 1032 mb surface high centered about 1650 miles ENE of the islands
has an associated ridge that extends to about 350 miles N of Kauai.
The high is supporting moderate to fresh E to ESE winds on the large
scale, with the flow veered somewhat from normal due to a front NW
of the area that is weakening the ridge near and W of 160W. An E-W
oriented band of showery low clouds (associated with an old front)
extends for thousands of miles to the E of the Big Island (all the
way to Baja California), with this moisture NW-SE oriented over
windward waters. This cloud band has fueled an increase in dew
points as well as clouds and showers, with mostly cloudy skies
prevailing over most of the state this afternoon. Water vapor
imagery highlights a zonal flow aloft over the area, with a weak
trough currently passing over the islands, and a sharper trough
about 700 miles NW of Kauai prompting the development of layered
clouds to the W of the area. Afternoon soundings show a weakened and
elevated inversion, with PWAT near 1.2 inches.

Radar, satellite and objective analyses highlight an E-W gradient in
wind speeds across the area, with winds stronger to the E due to a
weakness in the pressure gradient near and NW of the islands. As
this convergent flow interacts with the terrain of the Big Island
and Maui, downstream eddies in the wind have been affecting sensible
weather elsewhere in the islands over the past day or so. As an
example, 2 to 4 inches of rain have been recorded in the last 24
hours in the wettest locations on Oahu and Kauai, with some
unexpected rainfall in leeward areas. Additionally, pilots have been
reporting areas of low-level turbulence and wind shear as they
encounter these eddies. These highly variable features increase
forecast uncertainty in the details of the short-term forecast.

Weak high pressure is expected to build NW of the islands tonight,
reinforced by a stronger high building even farther NW of the area
Sunday and Monday. Moderate to occasionally locally breezy trade
winds will prevail over the area as this occurs, with the flow
backing to a more typical ENE to E direction Sunday, continuing
through Tuesday. This should result in clouds and showers having a
more typical windward and mauka distribution. This new high will
then move SE into a position NE of the islands by mid-week, as
developing low pressure to the NW sends a cold front toward the
islands. The approaching front will turn winds to the ESE/SE again
Wednesday and Thursday, with light and variable winds possible by
the end of the week as the front draws closer.

Forecast models indicate that the aforementioned showery low cloud
band will fuel a somewhat wet trade wind weather pattern for the
next several days (potentially into Wednesday) as it streams in from
the E. The trough aloft to the NW will pass close to the islands
Sunday through Tuesday, and associated cool mid-level temperatures
are expected to make for a slightly unstable atmosphere. While
showers will focus mainly over windward areas, some showers may
develop and/or move over leeward areas. Another trough developing W
of the islands later in the week may keep island weather somewhat


A persistent band of clouds and showers remains anchored over the
windward slopes and waters this afternoon, and may linger over the
same areas tonight. AIRMET Sierra is in effect for IFR ceilings and
visibility within the showers for Kauai, Oahu, Molokai, Maui and Big
Island through the afternoon. AIRMET Sierra will likely continue
through at least the evening hours across windward areas. Leeward
sides of the smaller islands also remain under AIRMET Sierra for
tempo mountain obscurations until this evening. This AIRMET may need
to be extended into the evening as well.

AIRMET Tango is in effect for moderate to isolated severe low level
turbulence over and downwind of the terrain of all islands. This
AIRMET will likely continue through 04Z. Moderate turbulence may
continue tonight downstream of the mountains/islands.


Moderate to fresh trade winds will continue over the local waters
with a surface high located east-northeast of the state, moving off
to the east. Another high is forecast to build in behind this one,
continuing the breezy trade wind pattern. Trade wind swell generated
to the east of the state will keep seas above 10 feet through at
least Sunday, so the Small Craft Advisory remains in effect through
Sunday for most zones, but may need to be extended in a later
forecast package. The High Surf Advisory (HSA) for east facing
shores continues through Sunday as well.

Surf is expected to remain below advisory levels for all other
shores through this weekend. A small northwest swell (330 degrees)
is expected to arrive tonight and fill in Sunday, with a reinforcing
long-period, west-northwest (300 degrees) swell due in Sunday night.
The WNW swell will peak Monday night and gradually decline through
Wednesday. This second swell may warrant an HSA for exposed west and
north facing shores Monday and Tuesday.


High Surf Advisory until 6 PM HST Sunday for East facing shores
of Kauai Oahu Molokai Maui and the Big Island.

Small Craft Advisory until 6 PM HST Sunday for Kauai Windward
Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-
Big Island Southeast Waters.



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