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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 220212
Area Forecast Discussion
National Weather Service Honolulu HI
412 PM HST Wed Mar 21 2018
Cloudy conditions with breezy trades will continue through Friday.
Showers will focus over windward areas with highest chances across
the western portion of the state over the next couple of days. An
upper trough will approach from the west and bring unsettled
weather across the state Friday night and Saturday. Improving
conditions will be possible Sunday into early next week.
Cloudy and breezy trade winds continue across the state late this
afternoon due to a strong high pressure system to the north.
Water vapor imagery shows an upper trough positioned near the
dateline with plenty of middle to upper level clouds drawn up over
the state from the southwest along along the subtropical jet.
Shortwave energy tracking along the eastern flank of the upper
trough is producing an area of thunderstorms along a moisture axis about
250 miles northwest of Kauai at this time. A few lightning
strikes have been detected close to Kauai throughout the day, but
still remaining offshore. That said, as this shortwave energy
tracks near the northwest end of the state over the next couple of
days, a slight chance for thunderstorms is possible. This has
been added to the forecast for Kauai and surrounding waters
through Thursday afternoon, and possibly longer.
At the low levels, radar and visible satellite show broken to
overcast clouds and isolated showers continue to carry in with the
trade winds, focusing over the typical windward and mountain
locations. A few showers will pass over to leeward sides of the
smaller islands at times due to the breezy trades. On the Kona
slopes of the Big Island, some moderate showers have formed due to
the daytime heating and sea breezes, but should taper off after
sunset. Tonight and Thursday, Kauai looks to have the best chance
of rain given the higher precipitable water values directly to
the south and west. Otherwise, expect the current weather
conditions to continue into Thursday for the rest of the state.
Model consensus supports breezy trades holding through Friday.
The aforementioned upper trough to the west is forecast to slowly
move east toward the state through the weekend, passing over the
islands on Sunday. The upper pattern will continue to support mid
and high clouds through Saturday, then decreasing Sunday as the
upper trough goes by.
Conditions will begin to evolve Friday into the upcoming weekend
as the upper trough draws closer, bringing deeper layered
moisture and more instability in the atmosphere. Despite the
model spread between various solutions that has been shown in each
cycle over the past few days, most indicate weak surface low
pressure developing along an approaching trough in the vicinity of
the islands Saturday with an abundance of deep moisture being
drawn northward (2 inches precipitable water values). A
combination of falling upper heights, instability, lift, and
moisture will lead to a wet weekend with a few thunderstorms,
especially late Friday through Saturday. Trade winds will also
weaken and shift out of the south to southeast Friday night as
the low develops and lifts away north of the state.
Sunday through early next week, improving conditions are expected
from west to east as drier air fills in and the upper trough and
surface low lift northeastward and away from the area. Guidance
depicts light west winds, potentially giving way to overnight land
breezes and afternoon sea breezes. Forecast confidence, however,
remains low through this extended period provided the spread
between solutions shown.
A surface ridge NE of the area will slowly move southeast over the
next couple of days, maintaining breezy, ENE trades across the state.
AIRMET TANGO is in effect for low-level turbulence over and S through
W of the mountains and is expected to continue through tonight.
Clouds and light showers will persist over N and E sections of
all islands, with tempo MVFR ceilings and vis continuing through
tonight and likely tomorrow as well. AIRMET SIERRA for mountain
obscuration for N and E sections of all islands expected to remain
in place through tonight.
Low pressure to the northwest of Kauai will slowly drift ESE
toward the islands supporting a slight chance for thunderstorms
from Kauai westward through tomorrow afternoon.
Windward coastal buoys indicate that the large NNE swell appears
to have peaked in the 11 to 12 ft range late this morning.
Although the peak has passed, the swell will decrease slowly and
will remain large through tonight. The High Surf Warning for
north-facing shores may be dropped to an advisory later tonight
or early Thursday due to the higher threshold of 25 ft. Surf is
not expected to fall below the warning threshold of 15 feet for
east-facing shores until Thursday afternoon or Thursday night.
This surf event is especially of concern to east-facing shores
which do not typically receive waves at these sizes. Strong surges
are also forecast for harbors exposed the swell.
No other significant swells are expected through the rest of the
forecast period. A small to medium sized north-northwest swell is
expected late Sunday or Monday with surf well below advisory
level. South-facing shores can expect to see a slight increase in
surf this weekend as a small southwesterly swell arrives.
Breezy trades will continue for another day. These winds combined
with the large NE swell will maintain Small Craft Advisory
conditions across all of the coastal waters through Thursday.
High Surf Warning until 6 PM HST Thursday for north and east
facing shores of Niihau, Kauai, Oahu, Molokai, Maui and the Big
Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office