Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 171955

Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Sat Nov 17 2018

A showery cloud band will move over Maui county today, spreading to
the Big Island by tonight, with the band eventually moving south of
all islands Sunday night. Locally strong trade winds and fewer
showers are expected after the cloud band passes, with winds easing
slightly on Sunday. Stronger trade winds and brief windward showers
are expected Monday and Tuesday, with lighter winds and increased
showers possible toward the middle to end of next week.


Currently, a 1026 mb surface high is centered far N of Kauai, while
an ENE-WSW oriented band of broken to overcast showery low clouds
extends over portions of the islands. This band is the tail end of a
frontal boundary that is tied into a low centered far NE of the
islands. Little in the way of air mass change exists across the
boundary at our latitude, but stronger NE winds and slightly lower
dew points lie to its N.

Latest GOES-17 visible imagery shows the most showery portion of the
decaying frontal boundary currently over Maui county and portions of
Oahu, with stable (and much less showery) low clouds beginning to
spread over Kauai from the NE. Water vapor imagery shows an upper-
level trough axis over Kauai, with broken to overcast high clouds
streaming over Maui county and the Big Island in the associated
southwest flow aloft. Based on model analyses, a weak mid-level
cutoff low is centered SW of the islands.

The forecast anticipates the surface high moving slowly SE through
the weekend, effectively pushing the weak frontal boundary southward
through the islands while supporting increasing NE trade winds in
the short term. The boundary is currently clearing Kauai, and is
expected to move S of Oahu this afternoon while Maui county receives
some soaking showers that will primarily affect windward areas
today. Decreasing showers are expected on Maui by early Sunday as
they increase on windward Big Island, with the boundary likely
moving S of the Big Island by Sunday night.

Additionally, the upper trough will slide slowly E and flatten
through Monday, leading to a gradual clearing (from W to E) of the
high clouds from skies over Maui and Hawaii counties later today
into tonight. Meanwhile, the weak mid-level low will linger SW of
the islands through the weekend before dissipating early next week.
The effects of this feature are beginning to look less impactful,
and the chance for winter weather on the Big Island summits appears
to be diminished as the shower band associated with the weakening
frontal boundary contains mostly shallow moisture.

After the boundary passes, NE winds are expected to briefly surge
(becoming locally strong) while cloud and shower coverage will
diminish. As the high dissipates on Monday, a new and stronger high
will build far N of the islands, and trade winds will remain breezy
Sunday into Monday. As this new high builds SE toward the islands
Tuesday and Wednesday, strong trade winds are expected, with GFS and
ECMWF both indicating that moisture associated with another weak
frontal boundary will be pushed over the islands in the
Wednesday/Thursday time frame. This new high is then forecast to
weaken while continuing to move E. Toward the end of the week,
latest model runs indicate a weakening and veering of the low level
winds, as a sharp trough digs toward the area from the NW. Lots of
uncertainty with the forecast at that point due to limited model
consistency thus far.


High pressure will build north of the state resulting in moderate to
fresh trade winds at the low levels over the next several days. Low
clouds and showers will move in with the trades along a weakening,
eastward-tracking frontal boundary through this weekend, resulting
in periods of MVFR conditions and mountain obscurations for windward
sides of the islands. The boundary has progressed eastward enough
that AIRMET Sierra has been dropped for Kauai and Oahu, but remains
in effect for tempo mountain obscurations along windward sides of
Maui County and the Big Island.

High level clouds cover the central and eastern portions of Hawaii
this morning along the subtropical jet, and will gradually shift
eastward as an upper level trough moves over the islands through the
weekend. Strong directional and speed shear persists over the island
chain due to the jet, so AIRMET Tango remains posted for tempo
moderate turbulence in FL250-400 layer for the entire area. The
upper trough will cause some instability in the atmosphere resulting
in some enhancement in the showers.


Trades are forecast to hold in the fresh to strong category
through the weekend as high pressure builds north of the state in
the wake of a dissipating front pushing south across the islands.
Visible satellite data showed this frontal boundary extending
west southwest across the Oahu and Maui County waters with wind
speeds up to 25 knots across coastal waters. Trades may reach the
strong category across most waters Tuesday through Wednesday with
near gales possible across the typically windier waters between
Maui County and the Big Island. Seas will remain around the
advisory level (10 ft) over most waters into tonight before
lowering Sunday through Monday as the current large north swell
lowers. As a result, a Small Craft Advisory will remain up through
tonight across most waters due to a combo of winds and seas, then
across the windier zones through the day Sunday.

Surf along north and west facing shores will hold around advisory
levels at least through the first half of the day before
gradually lowering through the rest of the weekend as the swell
eases. Observations at the PacIOOS nearshore buoys are remaining
steady in the 9-11 ft range this morning, which is well above
predicted levels. The High Surf Advisory has been extended
continues through 6 pm HST today. Expect this source to shift out
of a more northerly direction through the weekend as it fades.

A reinforcement out of the northwest is expected to fill in late
Monday, peak Monday night into Tuesday, then fade through midweek.
This source will be from a storm-force low that was depicted in
the Thursday evening analysis over the far northwest Pacific near
the western Aleutians. ASCAT data during this time showed a
decent sized area of strong to gale-force westerly winds
associated with it focused at the islands within the 300-320
directional band. Guidance remains in decent agreement and shows
this feature tracking eastward to the Date Line today. Surf
associated with this source should remain below the advisory
levels along north and west facing shores as it fills in and peaks
early next week.

Surf along east facing shores will gradually rise over the weekend
and early next week as the trades increase into the fresh to strong
category, especially next Tuesday as trades reach the strong
category across most waters. Limited fetch upstream of the state,
however, should keep the surf below advisory levels for east
facing shores through this time.

Small surf will continue along south facing shores with mainly
background southern Pacific pulses moving through.


High Surf Advisory until 6 PM HST this evening for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai, and for north
facing shores of Maui.

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

Small Craft Advisory until 6 AM 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.



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