Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 171402

Area Forecast Discussion
National Weather Service Honolulu HI
402 AM HST Tue Jan 17 2017

Trade winds will build slowly from east to west across the state
through Wednesday as a ridge of high pressure pushes north away
from the islands. The trades will then ease briefly Thursday and
Friday as a front begins to approach from the northwest. Limited
showers will favor the windward and mountain areas through the
week. A change in weather is then expected over the weekend as a
front pushes southward through the island chain, bringing an
increase in showers and strong trade winds to the Aloha state.


Currently at the surface, a 1023 mb high is centered around 1250
miles east-northeast of Hilo, with the ridge axis extending
westward to a location around 160 miles north of Kauai. This is
resulting in moderate trade winds over the Big Island and Maui
County, with light trades over Oahu and Kauai. Infrared satellite
imagery shows variably cloudy skies in place across the state,
with radar imagery and rain gauge observations showing shower
activity mainly limited to windward sections of the Big Island
early this morning. Main short term concern for today revolves
around summit winds on the Big Island.

Today through Friday,
The high pressure ridge axis will lift northward slightly through
Wednesday, allowing the trade winds to gradually strengthen across
the island chain. Light to moderate trades are expected across
Kauai and Oahu today, while moderate trades continue across Maui
County and the Big Island. The trades will peak at moderate
levels on Kauai and Oahu tonight and Wednesday, with locally
breezy trades expected across Maui County and the Big Island. A
front will then approach from the northwest Thursday and Friday,
with the trades easing back to light to moderate levels across the
state. A dry and stable weather pattern is expected through the
work week, with inversion heights remaining suppressed due to
mid-level ridging just north of the state and precipitable water
values remaining around or below 1 inch. The showers that do
develop are expected to remain limited primarily to windward and
mauka areas.

Winds on the Haleakala summit have dropped below advisory levels
early this morning, so the Wind Advisory has been cancelled here.
The winds may increase to advisory levels once again late tonight
and Wednesday. The Big Island summits have also seen a slight
decrease in winds overnight, so the High Wind Warning has been
replaced with a Wind Advisory which is in effect until 6 pm this
evening. This advisory may need to be extended in time later today
as winds will flirt with the advisory level threshold right
through Thursday.

Friday night through Monday,
The weather pattern will change over the weekend as a front
pushes southward through the island chain. There remains some
timing differences between the GFS and ECMWF, so will utilize a
blended forecast approach for forecast details. Overall, it
appears that the front will be closing in on Kauai Friday night,
then push southward through the island chain Saturday through
Sunday. The GFS shows the front hanging up near the Big Island
through early next week, while the ECMWF allows the front to clear
the island chain. With the approach of the front and the break
down in mid-level ridging Friday night, we should see an increase
in shower activity mainly across windward and mauka areas. Showers
coverage will increase in association with the front Saturday
through Sunday, with showers likely reaching leeward areas of the
smaller islands as well. Model solutions differ by early next
week, so will keep more shower activity in place across windward
sections of the Big Island, while showing a drying trend for the
remainder of the island chain. In addition to the increase in
showers associated with the front, the trade winds will become
strong over the weekend and possibly lingering through early next
week, with the trades potentially reaching wind advisory levels
across much of the island chain.


The low-level trade wind flow, which is slightly stronger near
Maui County and the Big Island compared with Kauai and Oahu, will
continue to transport low clouds and isolated light showers into
the windward facing slopes and coasts of the islands today.
As a result, brief periods of MVFR CIGS are possible in passing
low clouds and -shra over some windward sections, especially
windward Maui and the Big Island. Otherwise, VFR conditions are
expected to prevail.

There are currently no AIRMETS in effect, and none are
anticipated this morning. As the trade winds continue to
strengthen, there is a possibility that AIRMET TANGO may be
required for TEMPO moderate low-level turbulence leeward of the
mountains on some islands later today or tonight.


Doppler radar derived winds from the Kauai NEXRAD shows solid 15
kt trades blowing there this hour. So, the firmer trades have
finally reached the western end of the island chain. However,
the ASCAT pass overnight determined the winds in the Alenuihaha
Channel and waters south of the Big Island to be blowing just
below Small Craft Advisory (SCA) criteria. This condition will be
short lived as we expect strengthening trades today. We expect
the trades to reach SCA criteria of 25 kt this afternoon in the
Alenuihaha Channel and waters south of the Big Island, and spread
over to adjacent Maui County waters tonight and the windward
waters of the Big Island. The strong trades will continue through
Wednesday before easing off a bit Thursday and Friday.

If not the wind that will warrant an SCA, the seas will, brought
on by an elevated WNW swell slated to arrive very late tonight
and spread down the island chain Wednesday. Therefore, the SCA
will cover many of the marine zones. On Thursday, the trades will
back off slightly while the WNW swell starts subsiding to where we
may begin scaling down the SCA.

As for the surf, as noted above, warning level surf is expected
with the WNW swell for most N and W facing shores of the smaller
islands starting as early as tonight and running through Wednesday
night. The swell will peak late Wednesday, then drop to advisory
level by Thursday night. Due to its west component, the Kona
coast of the Big Island will likely have warning level surf
Wednesday night into early Thursday.

As noted above, the trades are slated to ease a bit Thursday
through Friday as a front approaches from the northwest. The
front will move through the islands Saturday and Saturday night,
bringing with it blustery NE winds, and large, rough, short-period
seas for at least the rest of the weekend. This will result in SCA
conditions to near gale force winds over all Hawaiian waters, and
a High Surf Advisory for E facing shores.


Wind Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until 6 PM HST Wednesday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST
Wednesday for Maui County Windward Waters-Maalaea Bay-Pailolo
Channel-Big Island Windward Waters.



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