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Hawaiian Islands Synoptic Discussion and Guidance

405
FXHW60 PHFO 060149 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
349 PM HST Thu Dec 5 2019

.SYNOPSIS...
A fast-moving disturbance aloft will bring thick layered clouds that
will deliver a period of widespread light to moderate rain tonight
into Friday before diminishing. Additionally, moderate trade winds
will deliver low clouds and a few showers to windward areas. Light
winds, less clouds and drier conditions 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.

&&

.DISCUSSION...
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 shortwave moves E of all islands by Friday night, 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 strengthening 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 trough (now near 24N
169W, or about 650 miles NW of Kauai) is expected to bring an
increasingly thick swath of mid- and high-level clouds over the
state tonight and Friday. These layered clouds will likely bring a
period of widespread light to moderate stratiform rain to most areas
before clearing the area from NW to SE through the day. These clouds
are expected to bring a period of snowfall to the Big Island
summits, and a Winter Storm Warning has been issued, for impacts
expected late tonight and Friday. Additionally, winds over the Big
Island summits remain strong enough to warrant a Wind Advisory,
which has been extended through Friday.

Confidence in some of the details of the short-term forecast are
lower than normal, but we 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 miles S of the islands Friday into Saturday (with the
ECMWF keeping a surface low S of the Big Island through Sunday),
apparently in response to the large scale lift provided by the
trough aloft. If this low is stronger 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. A slight chance of thunderstorms remains in the forecast
for some coastal zones, and was added to the forecast for portions
of Maui and the Big Island on Friday afternoon.

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 will extend over the islands from the NE,
leading to a diminished wind flow that will lead 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 approaching 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.

&&

.AVIATION...
Windward Big Island remains the winner in terms of lowered ceilings
as we head into the mid afternoon hours. At the other end of the
island chain, cloud cover appears to be increasing from west to
east. This will continue to be the case as we head into the
overnight hours as an upper level disturbance nears the state from
the west. Rain will become more widespread along with lowered
ceilings overnight, which is reflected in the 00z TAFs. AIRMET
Sierra for tempo mountain obscuration above 1500 feet remains in
effect for north through east sections of the Big Island. At this
time, it does not seem that an extension of this AIRMET is warranted
for Kauai and Oahu, but tempo MVFR conditions are possible with any
of the heavier showers.

Icing appears to be becoming more of a concern with these thickening
clouds from Molokai westward, so anticipating issuance of AIRMET
Zulu for tempo moderate icing between FL140/FL260. Some wave
features can also be seen on visible satellite north of Kauai and
Oahu, so believe that some tempo moderate turbulence is possible in
this region. However, Ellrod values are not very impressive through
the overnight hours, so will wait on issuing any kind of mod-severe
turb AIRMET/SIGMET at this point.

&&

.MARINE...
High pressure to the north will maintain moderate easterly winds
tonight, but expect a slow downward trend starting Friday.

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 northeast of the islands, with portions of the northern
coastal waters included in the threat.

Overlapping long-period swells from the north tonight into Saturday,
then north-northeast and a bit larger this weekend will result in
large surf for north and east facing shores of most islands. The
north swell will build tonight and reach the 8-foot High Surf
Advisory (HSA) level for east facing shores Friday, but should
remain below the north shore advisory level until later Friday
evening. As the swell shifts around to the north-northeast this
weekend, swell and surf heights will increase further and peak
Friday night through Saturday night, with advisory level surf along
north facing shores, and possibly warning levels for east facing
shores. Headed into next week, advisory level surf is possible with
a couple of northwest swells beginning Sunday night into Monday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM Friday to 6 PM HST Sunday for East
facing shores of Kauai Oahu Molokai Maui and the Big Island.

Wind Advisory until 6 PM HST Friday for Big Island Summits.

Winter Storm Warning from 9 PM this evening to 6 PM HST Friday for
Big Island Summits.

&&

$$

DISCUSSION...Birchard
AVIATION...Dye
MARINE...TS

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

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