Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 AM EDT Sat Jun 21 2014

Valid 00Z Sun Jun 22 2014 - 00Z Sun Jun 29 2014

The eastern half of the subtropical Pacific is dominated by 2 very
large high pressure cells (21N 122W and 32N 170W). Embedded
between these features--lies an upper-level trough (along 24N
151W) and closed circulation at 500 millibars.

The 21/00Z GFS/ECMWF and Canadian deterministic solutions offer a
wide spectrum of possible outcomes for the trough along 151W and
generally thought to stay away from each and any of them after
24/12z (day 3). The main reason---how they handle the departing
upper-level trough northeast of the Big Island. What has been a
very stable forecast---and really should be---is slightly altered
now by solutions that are grappling with the expansive highs and
the outcome of the Subtropical Ridge over the Pacific.

The trough and embedded circulation in place continues to be
lifted northeastward through day 2...as the high/ridge north and
northwest of Kauai migrates eastward. The 21/00Z ECMWF lets the
migratory mid-latitude trough---scheduled to do so on
Monday---pass over the system northeast of the Big Island. In
effect...allowing the cutoff circulation to remain
intact...separated and migrating westward along 30N through the
heart of the broad ridge complex.

The 21/00Z Canadian and 21/00Z-06Z GFS hold to a northeastward
progression of the trough through Tuesday...then these solutions
'part ways'. Even the 21/06Z GFS is significantly different than
its 21/00z run.

From yesterday...
The main feature of the short-term upper-level weather pattern is
a slow-moving trough northeast of the Big Island. This feature
lifts northeastward and away from the forecast area on Monday---as
an upper-level ridge migrates eastward from the central Pacific.
Local enhancement of the shower activity will continue into Monday
with a subtropical ridge along 31N latitude emerging across the
central and eastern Pacific. The deep-layered ridge settles and
solidifies over a vast portion of the Subtropical Pacific---and
the pressure gradient/tradewinds increase in intensity by midweek
(days 4-5).

The cutoff solutions in the deterministic solutions previously
mentioned should not override the predominant idea of the 21/00z
GEFS/ECENS---a ridge in the Subtropics broadens and expands across
a large portion of the Pacific. Recommend a blend of the two to
handle the synoptic-scale flow from the surface to 500mb millbars.
What swirl(s) can develop and linger...AND propagate within the
frame work of this ridge will certainly be a challenge...and may
be at a scale that involves convectively-induced feedback.

Vojtesak