Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 am HST Tuesday Sep 2 2014
light trade winds will strengthen slightly today...and remain light
to moderate for most of the upcoming week. The trade winds will
deliver a few showers to windward areas...especially during nights
latest observations show that a light easterly flow prevails in the
low levels...with winds light enough to allow localized land breezes.
The winds are being supported by a distant high that is relatively
weak...with weak troughs between the high and the islands
contributing to a loose pressure gradient...as is the nearly
stationary remnant circulation of Marie...about 1300 miles NE of
the islands. Although skies immediately upstream of the islands from
Kauai to Molokai contain only isolated showers...an area of showery
low clouds lies over windward waters out to 200 miles east of Maui and
The Big Island...moving west at 10 to 15 miles per hour.
Today...abundant sunshine is expected over leeward areas...while
windward areas will be partly cloudy with a few brief showers.
Showers/clouds are expected to be a little more active through
tonight over windward Maui and Big Island as the aforementioned area
of moisture moves west. After this modest moisture source passes...
limited moisture is expected through Thursday. Increased moisture
loosely associated with the remnant of Marie passing far NE of the
islands may increase windward showers from Thursday night through
Saturday. A middle-level ridge near the islands will ensure that the
atmosphere remains strongly capped at about 8 to 10 kft.
Trade winds are expected to be slightly stronger today as compared
to the last 2 days...as the troughs northwest of the islands weaken and the
surface ridge to the north strengthens somewhat. The overall synoptic
pattern does not support a strong trade flow this week however...but
rather a light to moderate flow. Therefore...one of the biggest
forecast questions over the next couple of days involves the
strength of the trade winds...and whether or not afternoon sea
breezes will become sufficiently strong/widespread to drive cloud
and shower development over leeward and interior areas. Although a
slight increase in leeward sky cover is in the forecast grids for
the smaller islands today...no significant probability of precipitation are indicated. Some
uncertainty as to how this will verify. The leeward Big Island is
the exception...where leeward clouds and showers are expected every
afternoon and evening through the forecast period.
windward and mountain areas may experience brief MVFR conditions as
weak trade wind flow pushes a few showers ashore...especially over
the eastern end of the island chain. No airmets in effect.
elevated surf will continue along S facing shores this week.
Although surf is currently below advisory levels...forerunners of a
very long period S swell /20-25 seconds/ are expected to build today
and tonight...although at low heights. Latest observations from buoy
51004 and the cdip buoy at Barbers Point show initial energy from
this swell is beginning to arrive. As this swell fills in...it will
produce surf near advisory levels along S facing shores...likely
from Wednesday into Thursday. Will hold off on the issuance of an
advisory to see if buoy data shows greater amplitude swell
heights...but an advisory may be necessary later today for surf
nearing advisory levels on Wednesday.
Forecast models are indicating that a fairly large gale low will
develop northwest of the islands this week...generating a northwest swell that
arrives around Monday. Since the low has yet to develop...too
early for specifics...but resultant surf looks to remain below
advisory level heights along north and west facing shores.
Winds and seas will remain below Small Craft Advisory thresholds
at least for the next several days.