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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 221411

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Mon Jul 22 2019

A trough of low pressure will pass a few hundred miles to the south
of the islands through Tuesday, spreading clouds and showers from
southeast to northwest across the island chain. Drier weather and
decreasing cloud cover is expected by Wednesday. Moderate to locally
breezy trades and typical windward and mauka showers with the
occasional leeward spillover for the smaller islands will then
prevail into next weekend.


Moisture from a trough of low pressure south of the islands has
spread over the Big Island overnight, with clouds and showers
affecting the extreme southern portion of that island early this
morning. This trough of low pressure will continue to track
northwest, almost parallel to the main Hawaiian islands through
Tuesday. Even bulk of the moisture will stay south of the islands,
do expect some moisture from this system to reach the rest of the
islands late today, especially Oahu and Kauai. Early morning Hilo
sounding has moistened up rather dramatically comparing to Sunday
afternoon, though the airmass is still only slightly unstable.
Abridged early morning Lihue sounding showed the same trend. High
pressure to the far north, coupled with this passing trough of low
pressure, are bringing moderate to locally breezy trade winds to the
islands through the next couple of days. There is also an upper
level low located northwest of the area. As such, this upper level
feature will keep airmass in the island vicinity somewhat unstable.
Expect much of the unsettled weather will remain south and west of
the islands today. The only exception is the Big Island, where
atmospheric conditions will be unstable and moist enough to see a
slight chance of thunderstorms during the afternoon and evening
hours. Rest of the islands will see passing showers carried by the
brisk trade winds, though the lee areas of Oahu and Kauai may see an
increase in clouds and showers as the moisture edges north late.

The upper level low will be far west enough by Tuesday for more
stable atmospheric conditions to spread from the east, though Kauai
weather may still be under the influence of the upper level feature
as well as the moisture from the surface trough. Unsettled weather
thus may still affect western and southern coastal waters of Oahu
and Kauai Tuesday, with improving conditions for Oahu waters later
on Tuesday. Rest of the islands will see a return of the typical
trade wind weather, with passing low clouds and showers affecting
mainly windward and mountain areas. Lee areas of the Big Island will
see mainly afternoon and evening clouds and showers.

Latest GFS indicated a surface trough may deepen northwest of the
islands by the middle of the week, with high pressure current far
north of the area moving to the northeast. Forecast solutions also
show high pressure aloft moving to near the islands by the later
part of the week. Looks like the islands will see seasonal trade
wind weather for the most part by then, with locally brisk trade
winds continuing.


A high will remain nearly stationary far north of the main Hawaiian
islands as a trough passes southwest of the area. The pressure
gradient between these systems will maintain locally strong east
winds over the area. AIRMET TANGO is in effect for moderate
turbulence below 8,000ft and this AIRMET will likely continue
through tonight.

The low level winds will carry scattered to broken low clouds over
east sections of the smaller islands. Isolated MVFR ceilings are
possible over these areas, but we don't expect the ceilings to be
extensive enough to require AIRMET SIERRA for MTN OBSC.

Moisture from the trough passing southwest of the area may fuel
heavy showers and isolated thunderstorms over the Big Island this
afternoon. AIRMET SIERRA may be needed for MTN OBSC.


Fresh to locally strong easterly trades will continue through the
week as high pressure remains north of the area and weak low
pressure passes to the south. Stronger winds will remain across the
typically windier locations from Maui County to the Big Island due
to terrain accelerations. The current Small Craft Advisory has been
extended through Wednesday.

Heavy showers and a few thunderstorms will remain a possibility
across the leeward waters of all islands through Tuesday night as
the aforementioned area of low pressure to the south tracks
northwestward, then passes to the west by midweek.

Rough surf will continue along east facing shores due to strong
onshore trades. A slight downward trend is expected for a brief
period through Tuesday as the upstream trades relax. A slight uptick
is expected once again Wednesday into Thursday as upstream trades

Surf along south facing shores will remain small today, then begin
to trend up tonight through midweek out of the south- southwest due
to a recent system that passed near the Tasman Sea. Heights should
remain below advisory levels as this swell moves through and peaks
Tuesday. A south-southeast long-period swell associated with a
recent compact gale southeast of the Tuamotus will be possible
Thursday night through Friday. Surf will rise along exposed shores
but should remain below advisory levels.


Small Craft Advisory until 6 PM HST Wednesday for 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
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