Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
330 PM HST Friday Mar 27 2015
moderate to locally fresh trade winds will continue over the weekend
and early next week...before diminishing Tuesday. Mostly dry weather
will continue for most of the weekend...with a slight increase in
windward showers expected late Sunday and Monday...especially on
Kauai and Oahu. A disturbance passing north of the islands on
Tuesday will weaken the trade winds...and may briefly lead to
increased windward showers. The disturbance will move east of the
area on Wednesday and Thursday...with typical trade wind weather
expected through the end of next week.
moderate to locally fresh trade winds are being supplied by a 1030
mb surface high far NE of the islands...and an associated ridge
about 500 miles north of Maui. A ridge aloft is supporting a stable
atmosphere...and a mostly dry trade wind weather pattern is in place
over the islands. Afternoon soundings show a strong subsidence
inversion based near 6 kft...and precipitable water values near 0.8 inches...which
is about one Standard deviation below normal for March/April. While
stable low clouds have been banked up along windward slopes through
the day...and along leeward slopes of The Big Island this
afternoon...very little rainfall has been observed. Expect a slight
moistening of the trade flow overnight to allow for a few windward
showers...but they are expected to be brief and light.
A mostly dry trade wind weather pattern is expected through most of
the weekend...as a middle level ridge remains overhead...and trade
winds gradually diminish as the surface ridge north of the islands
retreats eastward. A new high building northwest of the area on Sunday will
push remnant frontal moisture toward the islands from the north on
Sunday and Monday. Although this moisture is not expected to make it
as far S as the islands...weakly convergent low level flow will lead
to increased trade wind showers...mainly over Kauai and Oahu...from
late Sunday into Monday. The atmosphere will continue to be
relatively stable so these showers are not expected to be heavy.
The trade wind flow could be disrupted around Tuesday as a
progressive middle-level trough passes north of the islands...and induces
the formation of a surface trough near the islands. This is seen in
deterministic GFS guidance...but most GFS ensemble members are
weaker than the deterministic run with the depth of the middle-level
trough...and the strength of the surface trough. Latest European model (ecmwf)
guidance shows the trough aloft passing north of the islands...with
little in the way of a surface reflection. Given these solutions...
the forecast leans away from the lighter winds depicted by the
deterministic GFS and leans toward the gefs/ECMWF...which keeps
light to moderate trade winds for most of next week. The passing
disturbance may briefly destabilize the atmosphere on Tuesday and
Wednesday...leading to increased windward showers. Trades could be
light enough on Tuesday for sea breezes to drive afternoon leeward
predominantly VFR conditions are expected at all terminals through
29/00z. No airmets are in effect and none are anticipated tonight.
a late morning scatterometer pass that sampled waters east of Oahu
showed winds at or above Small Craft Advisory criteria in waters north
and S of The Big Island...but below criteria in other marine zones.
Latest guidance from global and mesoscale models indicate that winds
near 25 knots will continue over waters north and S of The Big Island
overnight...where a Small Craft Advisory is in effect through 6 am Saturday. Trade
winds are still expected to gradually ease through Saturday...and
while a slight increase in wind speeds is expected Sunday/
Monday...the expectation is that winds will remain below Small Craft Advisory speeds
before winds diminish again somewhat Tuesday.
Not much surf to speak of currently...as lingering south-southwest and north-northwest
swells gradually fade away through Saturday...and a small
long-period south-southeast swell builds. A long-period northwest swell will build and
peak on Sunday...with peak surf heights nearing advisory levels
along exposed north and west shores. This swell will gradually diminish in
height and period through Tuesday. At its peak...this swell will
bring combined seas close to the 10 feet threshold for Small Craft Advisory issuance.
Swells arriving from the S will be relatively low in magnitude...but
sufficiently large to keep some surf along S facing shores through
the period...with the strongest source being a south-southeast swell that is
currently on the rise. The recently updated Oahu surf discussion
/srdhfo/ contains details on the sources of the expected swells.
Small Craft Advisory until 6 am HST Saturday for Alenuihaha
Channel-Big Island leeward waters-Big Island southeast waters.