659 
FXHW60 PHFO 190141
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
341 PM HST Mon Oct 18 2021

.SYNOPSIS...
A drier, more stable airmass is expected over the islands through
mid week along with weakening trade winds. Trades are expected to
strengthen again late in the week, along with an uptick in shower
activity.

&&

.DISCUSSION...
Little change in the forecast philosophy this afternoon, and the
forecast remains on track. The morning ASCAT pass showed 23 knots
still in the Alenuihaha Channel. The models and our forecast
expected winds to be on the decline a bit today, so feel this was
to be expected. Additionally, the high clouds have largely left
the islands as expected. 

The high pressure ridge north of the islands continues to edge
closer as the front further north sinks southward. This will 
continue the gradual decline in trade winds through Wednesday. A 
new high building further north of the islands during the second 
half of the week will help trades to gradually increase towards 
the end of the week and into the weekend.

The upper level low to the west that was responsible for ushering
in the high clouds has, as expected, moved to the west. Expecting
this motion to continue, and that upper level feature to no longer
be an impact to our weather.

Another upper low/trough developing north of the islands and
sinking southward over the islands during the latter part of the
week will likely help to boost shower activity, but how much
remains to be seen. Satellite precipitable water immediately
upstream of the islands supports the short term drying trend we
expect. The GFS and ECMWF both bring an increase in precipitable
water (PW) with this upper level feature, but the GFS is wetter 
than the ECMWF. The forecast does reflect an uptick in showers 
during the second half of the week, but will have to watch future 
runs to determine how much of an impact this will have on our 
local weather.

&&

.AVIATION...
As a surface ridge located north of the state moves southward and
weakens through Wednesday, trade winds will gradually weaken as
well. For now, AIRMET Tango remains in effect for moderate low-
level turbulence over and immediately south through west of the 
mountains of all islands. However, this AIRMET may be cancelled 
later this afternoon or evening given the waning surface winds. By
tomorrow through mid-week, localized land/sea breezes are 
possible with as background flow becomes rather weak.

Mid-afternoon satellite and radar imagery show relatively few 
areas of low clouds and showers across the state, with isolated
showers and low clouds favoring windward areas and their adjacent
waters. Cloud coverage has also increased across the Kona slopes
of the Big Island, though ceilings remain in the VFR category.  
Although isolated MVFR conditions are possible throughout the
period in any heavier showers that do develop, VFR conditions are
expected to prevail state-wide.

&&

.MARINE...
Trade winds are already beginning to diminish, with only a few
20 to 25-knot wind barbs left in the Alenuihaha channel with the 
morning ASCAT pass. Given the decreasing trend indicated by the 
hi-res models, the Small Craft Advisory (SCA) has been dropped for
all remaining waters. The moderate trades will gradually trend 
down over the next few days, due to a series of fronts from the 
far northeast Pacific weakening the ridge north of the islands. 
These fronts will reach the northern offshore waters Wednesday and
Thursday, though both are expected to fall apart before reaching 
the Hawaiian Islands. High pressure reestablishes north of the 
islands Thursday through Friday, bringing a return of locally 
strong trades, favoring the typically windy waters around Maui 
County and waters south of the Big Island.

As the trades ease, the surf along the east facing shores will 
lower a little each day as well. The boost in the trades on 
Friday will lead to increasing surf along the east facing shores 
over the upcoming weekend.

A series of north-northwest to north swells are expected to move
through the coastal waters this week. A small pulse that arrived
earlier today will be reinforced by a slightly larger north- 
northwest swell Tuesday night through Wednesday. An even larger 
north swell follows Thursday night and Friday, with surf likely to
reach advisory level for north facing shores, before slowly lowering
over the weekend. In addition, Kahului Harbor and other north 
facing harbors will be susceptible to harbor surges from this 
north swell. 

Small surf along the south facing shores will get a bit of a boost 
beginning early Tuesday as a small, but long (14 to 16 second period)
south swell due in tonight. This swell will linger through Wednesday
before tapering off through the end of the week. A similar swell 
could arrive over the upcoming weekend. 

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Alenuihaha 
Channel-Big Island Leeward Waters-Big Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Vaughan
MARINE...TS