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Temp-2.2 C
RH6 %
WindE 12 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 270630

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Wed Feb 26 2020

A stable and breezy trade wind flow will continue to focus modest
rainfall along windward slopes through Thursday. High pressure
will strengthen northeast of the state on Friday, possibly
boosting trades further through the weekend. An upper low will
drift by during this time, leading to an increase in showers,
some possibly heavy with a slight chance of thunderstorms. Wet
conditions may persist into early next week.


Gusty trade flow continues as a surface ridge persists far north
of the main Hawaiian islands. Latest soundings show a dry and
stable airmass, with PW values less than 0.8 inches and strong
inversions near 6500 feet. Satellite loop shows patchy broken low
clouds across windward slopes of the islands, while radar shows
scattered moderate showers within these clouds. Expect little
change through tomorrow. The ridge far north of the state will
break down as another, strengthening high moves in from the
western Pacific. Trade winds should be more or less unchanged
during this transition. The ridge aloft will erode as an upper
low far east of the state deepens, but stable conditions will
hold with modest rainfall remaining focused over windward slopes.

Trades will increase Friday through the weekend as the surface
high strengthens to around 1040 mb as it becomes parked far
north-northeast of the state. However, the upper low expected to
drop southeastward toward the state during this time may weaken
the local surface pressure gradient, keeping the strongest trade
winds to the northeast of the islands. At this time, it still
appears that we will see an increase in trades, possibly requiring
a Wind Advisory.

As the upper level low moves overhead, PW values will not change
substantially, but very cold temperatures aloft will produce
unstable conditions that will likely generate spotty showers, some
briefly heavy, and perhaps a few thunderstorms. Rainfall will
continue to be focused along windward slopes, but the gusty trades
will easily push fast-moving, briefly heavy, showers across
leeward areas. In addition, winter weather will be possible on the
high summits, and freezing levels could drop low enough to affect

Models show the upper low dissipating and lifting northward on
Monday, while an associated surface trough is pulled over the
islands from the east. Trades should decrease and shift out of
the southeast if the trough is strong enough. Convergence on the
east side of the trough could lead to another period of wet
weather for some windward areas.


A strong surface high far northeast of the main Hawaiian Islands
will maintain locally strong and gusty northeast to east trade
winds through Thursday. As a result, AIRMET TANGO remains in
effect for moderate low-level turbulence below 8 thousand feet
south through west of the higher terrain on all the islands.

The robust low-level trade wind flow will continue to transport
patches of low clouds with embedded showers into the northeast
facing sections of the islands through Thursday afternoon. This
will produce brief periods of MVFR conditions due to lower
ceilings and reduced visibilities in trade showers. Otherwise,
expect VFR conditions to prevail.


Strong to near gale force trade winds are expected to prevail
into Sunday, with wind speeds gradually increasing as a surface
high builds far N and NE of the islands. Winds are expected to be
strongest from Friday night into Sunday. Forecast uncertainty
increases thereafter as a potent mid-level low develops over the
area, and induces a surface trough near the islands. When and
where the surface trough forms, and where it moves, will have
profound affects on wind speed and direction from late Sunday into
next week. The forecast currently anticipates a weak trough
developing just E of the islands Sunday, then moving W and
sharpening somewhat Monday, with winds responding by veering to
the SE and weakening. The destabilizing low aloft will bring the
potential for thunderstorms from Friday into next week.

With trade winds increasing near and upstream of the islands,
combined seas will build, and a Small Craft Advisory (SCA) is in
effect for all zones through the weekend. Surf along E facing
shores will also increase over the next several days, with surf
rising to advisory-levels by Saturday. Latest guidance places a
belt of enhanced trade winds immediately upstream of the islands
late in the weekend as the surface trough develops, leading to a
surge in mid-period ENE swell/wind waves Sunday/Monday, with
warning level surf along E facing shores a distinct possibility.

A High Surf Advisory (HSA) for N and W facing shores has been
dropped, but another long-period WNW swell arriving Thursday into
Friday will push surf heights back up close to advisory levels
along exposed shores. Pending the swell's arrival at upstream
buoys, a new HSA will likely be issued Thursday. This swell will
gradually diminish through the weekend, with no other significant
NW swells in the forecast through the middle of next week.


Small Craft Advisory until 6 PM HST Sunday for all Hawaiian




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