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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220202

Area Forecast Discussion
National Weather Service Honolulu HI
402 PM HST Thu Jan 21 2021

Trade winds will strengthen as high pressure builds northeast of
the state. Showers will favor windward slopes, with a few heavy
showers possible this weekend and early next week as an upper
trough lingers across the islands. Meanwhile, trade winds will
shift out of the east to east-southeast and become breezy and


Water vapor imagery shows an upper trough over the state late this
afternoon. Associated high clouds are streaming up from the
southwest over the Big Island. 00Z soundings show stable mid-layers
with relatively cool 500 mb temperatures around -11 C which
contributed to some of the locally heavy showers passing through

At the surface, a 1034 mb high is centered far northeast will
drive fresh trade winds across the state tonight. The trades will
carry in low clouds and showers that will focus over windward and
mauka areas this evening. Showers will trend down for the smaller
islands later tonight as upstream moisture appears limited and
guidance shows drier air building in. Showers will linger through
the night into Friday morning for windward Big Island. Overall
shower coverage should decrease by Friday.

High pressure will continue to build into the weekend resulting in
winds becoming fresh to strong. Another upper trough will induce a
high amplitude surface trough about 500 miles west of the main
Hawaiian Islands which will result in our winds veering east
southeasterly this weekend. Temperatures aloft will remain cold,
but will likely have limited influence Friday and Saturday due to
the midlevel inversion and less moisture to work with.

Deeper moisture from the southeast will begin to move up from the
south on Sunday into the middle of next week with the strong east
southeast winds continuing. This will likely result in an
increase in clouds and showers statewide, particularly across
windward and southeast slopes. In addition to the shower increase,
layered mid and high clouds will keep cloudy skies in the
forecast for most of the state much of next week.


A strong surface high will remain far northeast of the area over
the next couple of days. This will produce locally strong east
winds over the main Hawaiian Islands over the next 24 hours. These low
level winds will carry some clouds and showers over eastern
sections of the islands, especially during the night and early
morning hours. These clouds and showers may produce localized
MVFR ceilings and visibilities at times.

Airmet Sierra remains in effect for northeast Big Island this
afternoon for mountain obscuration from a widespread MVFR level
cloud deck. However, these clouds are expected to begin breaking
up over the next few hours.

Airmet Tango remains in effect for moderate turbulence over and
southwest through west of the mountains of all islands. Strong
surface winds are expected to continue over the next several days.


A 1034 mb surface high is centered far to the north northeast of
the area. This is resulting in moderate to locally strong trade
winds. A Small Craft Advisory (SCA) is currently posted for the
typically windy waters around Oahu, Maui County and the Big
Island. The high will get a reenforcement over the weekend and on
into early next week strengthening to around 1044 mb. The result
will be for trade winds to increase across the entire area. This
combined with higher seas from both a large northwest swell and
the trade winds generated seas, will result in SCA criteria being
met across the entire area.

A small northwest swell will linger through the day Friday. Long
period forerunners from a larger northwest swell will fill in late
Friday and Friday night. This swell is expected to peak on
Saturday with high end advisory or low end warning level surf
expected. The direction of this swell is from 310 degrees and the
primary period during its peak will be around 18 seconds. This
swell will then lower gradually Sunday and Monday.

Model guidance indicates surf will become rather small for this
time of year along most north and west facing shores Tuesday and

Meanwhile, high pressure will be strengthening north of the area
resulting in rather robust trade winds by this weekend and on into
early next week. Short period choppy surf will gradually rise
and a High Surf Advisory will likely posted and remain in effect
for several days.

Small, mainly background southerly swells are expected over the
next several days. A slightly larger south southwest swell from
the Tasman sea is expected to gradually fill in on Monday, peak
Monday night and Tuesday, then lower gradually Wednesday through


Small Craft Advisory until 6 PM HST Sunday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman