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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 180654
Area Forecast Discussion
National Weather Service Honolulu HI
854 PM HST Fri Nov 17 2017
Low pressure far to the north of the islands will drop south over
the next couple of days, dragging a weak cold front southward
through the islands Saturday through Sunday. The front will be
accompanied by a band of clouds and showers, with showers favoring
northern slopes and coasts. After the front comes through, cool
north and northwesterly winds are expected Saturday night through
Monday. The low to our north will move away from the islands
Tuesday and Wednesday, and trades will gradually return. A large,
strong high pressure area to our north is expected to bring breezy
to windy trades for Thanksgiving Day and next Friday.
Currently a stacked area of low pressure, surface and aloft, is
located around 950 miles to the north of Honolulu. Meanwhile, a
weak cold front is located around 250 miles north of Kauai.
Infrared satellite imagery shows partly to mostly cloudy skies
across the smaller islands, while mostly clear conditions prevail
over the Big Island. Radar imagery shows some scattered showers
over windward areas of the smaller islands, with mainly rain free
conditions in leeward locales. Main short term concern revolves
around rain chances over the next few days.
Tonight through Tuesday,
The stacked low to the north of the State will slow its
progression as it drops southward tonight and Saturday, then
stall around 600 miles to the north-northeast of Honolulu Saturday
night. The stacked low will then edge eastward Sunday through
Monday, before lifting northeastward and away from the island
chain Monday night and Tuesday. A weak cold front will also shift
southward through Kauai around noon on Saturday, through Oahu
late Saturday afternoon, Maui County Saturday night and through
the Big Island on Sunday. Light northerly winds will develop over
Kauai and Oahu tonight, while light and variable winds and land
breezes dominate across Maui County and the Big Island. The
northerly flow will increase over the western islands on Saturday,
with lighter northerly winds allowing for sea breezes across Maui
County and the Big Island. The north-northwesterly winds will
then overspread Maui County Saturday night and the Big Island on
Sunday. Light to moderate north-northwesterly flow will continue
Sunday night, shift more northerly on Monday, then shift around to
the northeast on Tuesday.
As for sensible weather details, showers will favor northern
slopes and coasts through the period, with an increase in showers
expected in association with the front as it moves down the island
chain. Some showers will spill into leeward locations from time to
time as well, particularly as the front moves through. In
addition, some afternoon/evening shower development will be
possible in leeward areas of Maui County and the Big Island on
Saturday due to sea breeze enhancement.
Following the frontal passage, cooler weather will prevail, with
surface dewpoints dropping into the 50s in many areas, making it
feel very comfortable across the island chain.
Summit level winds continue to bounce between advisory and warning
levels this evening, but they should increase solidly back to warning
levels later tonight through Sunday as the upper low moves
closer. Winds at the summits should then begin to trend down
Sunday night and Monday as the upper low begins to open up and
shift eastward and away from the island chain.
Tuesday night through next Friday,
A strong area of high pressure will build eastward well to the
north of the State, increasing the gradient across the islands and
shifting the winds back into a more typical trade wind direction.
The trades are expected to increase to moderate levels on
Wednesday, with breezy to windy conditions then expected for
Thanksgiving Day and Friday. With the returning trades, a more
typical windward and mauka shower pattern is expected to resume
across the island chain.
A cloud band associated with a weak cold front will move into
Kauai and Oahu from the north on Saturday. Expect scattered clouds
and enhanced showers in the afternoon especially over mountains
and along northwest through northeast slopes of all islands.
No AIRMETs are in effect, and none are expected overnight.
Winds have trended down across the waters and will gradually shift
out of the north overnight into Saturday as a frontal boundary and
low pressure dive southward across the region. Guidance supports
this more northerly flow holding through the weekend and into next
week as low pressure associated with this frontal boundary stalls
north of the state. Winds should remain below advisory levels
through the weekend, but the rather unusual wind directions will
result in different areas of terrain acceleration, such as through
the Kaulakahi Channel on Saturday and Saturday night and the
Hamakua Coast on Sunday. Moderate trades return Tuesday and
Wednesday next week.
Surf along east facing beaches will gradually trend down over the
weekend as the trades relax and the flow shifts out of the north as
the aforementioned front drops south into the area and seas continue
Surf along north facing shores will remain up through much of the
period due to overlapping northerly swells moving through the local
waters. The anomalous pattern across the northern Pacific featuring
storms diving southward across the eastern Pacific from the
Aleutians/Gulf of Alaska to near the islands due to a blocking
ridge that remains established over the central Pacific is
projected to continue through much of next week.
The latest analysis showed a strong pressure gradient between 1040
mb high pressure centered near the Aleutians around 45N and 1010 mb
low centered diving southward across the eastern Pacific several
hundred nautical miles north of the islands. Persistent strong- to
gale-force northerly flow locked in between these two synoptic
features focused toward the local area over the past couple of
days will translate to the next large northerly swell through the
second half of the weekend and into the upcoming week. Provided
the proximity of this generation source to the local area, no
significant timing or swell magnitude differences are depicted
between the ECMWF-Wave and WAVEWATCH III solutions as this source
moves through the islands.
The north (350 deg) swell will begin to fill in Saturday night, peak
Sunday into Monday before temporarily trending down Tuesday into
Wednesday. The ECMWF solution is a bit more bullish with the low
dropping farther south and depicts a secondary peak late Monday
through Monday night. The overnight package will support a blend
of these two solutions to express uncertainty. Surf along north
facing shores will near warning levels around the peak Sunday,
then likely hold through Monday night above advisory-levels (15
ft faces) before easing Tuesday into Wednesday.
As the swell is peaking along north facing shores Sunday into
Monday, the progressive pattern across the northeast Pacific is
projected to continue as another gale- to storm-force low emerges
over the Gulf of Alaska near Kodiak Island and tracks south-
southeastward across the northeast Pacific Monday through Tuesday. A
reinforcing northerly swell associated with this source is expected
to fill in by Wednesday night, likely translating to surf nearing
the warning mark along north facing shores once again by Thursday.
This source will hold through the second half of the week before
easing into the weekend.
Spectral densities at the offshore buoys south of the islands are
showing some energy out of the south-southwest within the 15 to
17 sec bands. This is from recent activity across the southern
Pacific near New Zealand and the Tasman Sea. Surf along south
facing shores will reflect this and likely hold through the
weekend at small levels. Similar sources are expected into next
week that will keep the surf from going flat along southern
See the latest collaborative nearshore and swell forecast for Oahu
for more details on the surf at: www.prh.noaa.gov/hnl/pages/SRF.php
High Wind Warning until 6 PM HST Sunday for Big Island Summits.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office
This page is maintained voluntarily by the MKWC and the UHMET faculty, staff, and students.
It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman