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Hawaiian Islands Synoptic Discussion and Guidance

238
FXHW60 PHFO 231357
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 AM HST Sat Feb 23 2019

.SYNOPSIS...
A surface trough near the Big Island will keep increased clouds and
showers over the eastern end of the island chain into Sunday, with a
slight chance of thunderstorms through tonight. Meanwhile, weak high
pressure to the northwest will support mostly dry weather over Kauai
and Oahu today, then delivering just a few windward showers Sunday
and Monday. Increased windward showers are possible statewide
Tuesday and Wednesday. Light north to northeast winds will gradually
strengthen on Sunday, with stronger and cooler northerly winds
expected for most of next week.

&&

.DISCUSSION...
A light wind pattern remains in place statewide this morning. A
persistent NE-SW oriented surface trough over the Big Island
continues to fuel cloud and shower development near its axis, with a
few thunderstorm clusters observed near the Big Island overnight.
Broken to overcast low and mid-level clouds continue to blanket
skies from Maui county eastward, streaming over the area from the
SW. Meanwhile, skies over Kauai and Oahu are mostly clear as the
light winds have allowed land breezes to push most clouds offshore.
The morning soundings highlight a moisture and stability gradient
across the state, with PWAT near 0.8" and a subsidence inversion
observed in Lihue, and PWAT near 1.6" and an unstable atmospheric
profile noted in Hilo.

Little overall change to this pattern is expected into Sunday,
except that the trough will begin to drift E. Light to moderate
winds will prevail, with N to NE winds and mostly dry weather W of
the trough axis, and E to SE winds (and enhanced cloudiness) near
and E of the trough axis - primarily affecting the the Big Island. A
shortwave trough now passing overhead will move E and weaken later
today, with another impulse aloft expected to pass overhead tonight
into Sunday. While these shortwaves may act to enhance moisture
along the trough axis, they are expected to have little impact on
sensible weather in the relatively dry air mass lying over most of
the island chain. With this in mind, a slight chance of
thunderstorms has been included in the forecast for the Big Island
and adjacent waters through tonight.

The passage of this second shortwave will act to finally shunt the
surface trough E and away from the Big Island on Sunday. While winds
over the Big Island summits have recently eased, the passage of
these shortwaves may increase winds to near advisory levels later
this weekend.

A pair of weak surface highs passing N of the islands will maintain
a light to moderate N to NE wind flow over most of the state through
the weekend. Moisture associated with a decayed front will become
embedded in this flow tomorrow into early next week. This moisture
is expected to fuel a modest increase in clouds and showers along N
and E facing slopes as it moves slowly S down the island chain.
Forecast models continue to portend a stronger shortwave trough
passing over the area from Tuesday through Wednesday, with very cold
500 mb temperatures near -19C moving over the islands. This impulse
may enhance the frontal moisture over the islands, and produce a few
thunderstorms, but confidence is not sufficiently high to include
them in the longer range forecast as of yet. This trough could
potentially bring another round of winter weather to the summits and
upper slopes of the Big Island, and potentially Maui.

After this shortwave passes, models suggest that the longwave
pattern near and N of the islands will once again take the form it
has for most of February. In this pattern, high pressure NW of the
islands combining with low pressure to the NE will support a cool
and breezy NW to N wind flow. Shallow bands of clouds and showers
would likely periodically move over the islands from the N if this
large scale pattern were to redevelop as advertised.

&&

.AVIATION...
Patches of mid-level clouds will continue to stream across the
eastern end of the island chain today, with light, background low-
level flow as a weak surface trough lingers near the Big Island.
Land breezes have cleared out most of the low clouds, except over
the Big Island and isolated spots on windward Maui. The light winds
will allow for cloud build-ups over the island interiors again this
afternoon, as well as some showers for the eastern half of the state
and a slight chance of thunderstorms around the Big Island. AIRMET
Sierra remains in effect for tempo mountain obscuration on the Big
Island, and is now expected to continue through this afternoon. VFR
conditions will prevail across the rest of the islands.

&&

.MARINE...
A surface trough near the Big Island and a weak surface high
pressure system north of the area will maintain light northeast
winds in the vicinity of the smaller islands most of this weekend.
East of the trough, light to moderate southeast to south winds will
persist across the windward Big Island waters this morning. Periods
of heavier showers and a slight chance of thunderstorms are also
expected over portions of the Big Island waters into tonight. The
surface trough will begin to weaken and shift eastward later today
and tonight as a new surface high pressure system builds northwest
of the area, and the tail end of a weak front passes by to the
north. This will cause light north to northeast winds to fill in
across the state, including the Big Island, by Sunday. A new surface
front is forecast to approach the area from the northwest early
Tuesday. The arrival of this feature in the islands is expected to
cause increasing north winds and rough seas. This will result in
Small Craft Advisory (SCA) conditions developing across portions of
the Hawaiian coastal waters starting late Tuesday.

A declining east swell will cause surf to slowly ease along east
facing shores this weekend. Surf along north and west facing shores
of the state will remain well below typical wintertime heights
through early Monday. A moderate north-northwest swell arriving
Monday is forecast to peak Monday night, and then gradually decline
early Tuesday. The forecast models continue to indicate a surface
low pressure system located east of Japan will deepen this weekend,
with the associated winds increasing to hurricane force. As this
strong system moves rapidly east and then northeast, the captured
fetch will send a large, long-period northwest swell towards the
islands. Based on the latest model guidance, the swell will likely
arrive on Tuesday and build through Wednesday, before peaking
Wednesday night. The surf produced by this northwest swell is
forecast to approach the High Surf Warning criteria along most north
and west facing shores of the smaller islands late Wednesday and
early Thursday. In addition, a large north swell will spread down
across the area starting late Wednesday night, which will keep rough
surf elevated along most north facing shores from Thursday into
Friday. The large swells will also contribute to elevated seas
Wednesday and Thursday, which will likely maintain SCA conditions
across most Hawaiian Waters through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Birchard
AVIATION...TS
MARINE...Houston

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



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