Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 051945

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Thu Dec 5 2019

A fast-moving disturbance aloft will lead to thickening layered
clouds, which are expected to bring a period of widespread light to
moderate rain tonight and Friday before diminishing. Additionally,
moderate trade winds will deliver low clouds and a few showers to
windward areas. Light winds and less clouds are expected over the
weekend, with mostly clear nights and mornings giving way to partly
cloudy afternoons, with just a few interior showers. Trade winds
will briefly return early next week, but may diminish again by the
middle of next week.


In summary, a somewhat unsettled (albeit short-lived) weather
pattern is anticipated over the next 24 hours or so, as a potent
shortwave trough aloft passes quickly from NW to SE over the area.
After the passage of the shortwave late Friday, a more benign
weather pattern is expected for the weekend as winds turn light and
variable, and a mid-level ridge builds over the islands. A weakening
front will approach from the N early Monday, then dissipate near the
islands, as it will have little support aloft as the ridge weakens
only slightly. This feature will likely add increased low-level
moisture to the light to moderate trade wind flow provided by a weak
high pressure cell passing N of the area early next week, resulting
in a modest increase in windward clouds and showers. Winds may turn
light and variable again by the middle of next week as another front
approaches. Latest guidance depicts this as a more vigorous cold
front that is followed by strong and cool NE winds in about a week
from now.

In the meantime, the passage of the shortwave (now near 26N 170W, or
about 700 miles NW of Kauai) is expected to bring an increasingly
thick swath of mid- and high-level clouds over the state. These
layered clouds will likely bring a period of widespread light to
moderate stratiform rain to most areas. These clouds are expected to
bring a period of snowfall to the Big Island summits, and a Winter
Storm Watch remains is in effect for impacts expected late tonight
and Friday. Additionally, winds over the Big Island summits have
increased ahead of the shortwave, and a Wind Advisory is posted
through today, but this may need to be extended.

Confidence in the details of the short-term forecast are lower than
normal, but we do generally expect moderate trade winds to persist
through Friday, supported by a high pressure cell to the N. However,
forecast models are depicting a surface low passing several hundred
S of the islands Friday into Saturday, apparently in response to the
large scale lift provided by the trough aloft. If this low is
stronger than expected, and/or passes closer to the islands than
expected, winds could be stronger than forecast, and locally heavy
rainfall (and even thunderstorms) could potentially develop. Will
continue to monitor satellite trends and latest guidance to see how
to best shape the forecast in this dynamic regime.

By Friday night, the shortwave will be exiting to the E, and a ridge
aloft will build over the area from the W as a surface front
approaches from the N. A surface ridge extending toward the islands
from the ENE will be suppressed over the islands, leading to light
and variable winds over land. Land breezes should keep skies
relatively clear during nights and mornings, while afternoon sea
breezes drive cloud formation over interior and mauka areas. The
ridge aloft will limit associated shower coverage and intensity.

The front will likely dissipate as it stalls near Kauai and Oahu
late Sunday into Monday, while a weak high passes quickly E along
28N. The high will support a brief period of trade winds late Sunday
into Tuesday, with some remnant frontal moisture potentially fueling
a few windward showers. Another front approaching from the NW will
lead to lighter winds by Wednesday, with this front potentially
moving down the island chain next Thursday/Friday, followed by
strong and cool NE winds.



A deck of mid to high clouds is continuing to fill in across the
island chain this morning and some light icing is possible from
FL130 to FL220. Below that deck, things are not looking much better,
particularly over windward areas of the Big Island, where AIRMET
Sierra is in effect for tempo mountain obscuration above 2000 feet.
Isolated ceilings of around 1500 feet are also possible this morning
for the AIRMET area. A few PIREPs from the area confirm that
conditions are indeed poor.

Elsewhere across the state, shower activity is increasing as an
upper level disturbance closes in on the area from the west.
Periods of MVFR conditions will become more frequent with these
clouds and showers later this afternoon and AIRMET Sierra may need
to be extended to other islands.


Overnight ASCAT pass showed solid areas of 20 knots over the coastal
waters, consistent with surface observations. High pressure to the
north will maintain those winds today, but expect a slow downward
trend starting tonight.

A trough moving overhead will increase showers tonight and Friday,
while an associated upper level trough provides some additional
instability near the islands. A slight chance of thunderstorms is in
the forecast for areas northeast of the islands, with portions of
the northern coastal waters included in the threat.

A long-period north swell is expected to build today into Friday,
before turning to the north-northeast and increasing a bit further
over the weekend. The swell is expected to peak Friday night through
Saturday night, with advisory level surf along north facing shores,
and warning levels for east facing shores. Advisory level surf is
possible with additional northwest swells Sunday night and into the
first half of the new week.


Wind Advisory until 6 PM HST this evening for the Big Island

Winter Storm Watch from this evening through Friday afternoon
for the Big Island Summits.



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