Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
825 AM EDT Tue Apr 25 2017

Valid 00Z Wed Apr 26 2017 - 00Z Wed May 03 2017

An upper ridge of high pressure and associated surface high will
be in control through 12z Friday, as seasonable trades are
anticipated. However, the upcoming weekend into early next week
appears quite unsettling, a strong digging mid-level upper trough
with additional upstream upper dynamics will begin approaching the
state from the north and west. This feature is currently rounding
the southwestern base of a large upper trough/vortex, which is
based near the southern chain of the Aleutian islands. Along with
the short wave will be a strong surface cold front, which reaches
the northwestern portion of the islands by 00z/29. The full
latitude upper trough will allow a plume of anomalous moisture
content to pool in vicinity of the state and potentially heavy
rain. The model guidance and ensembles all generally show this
scenario into next weekend, but with decent spread on a potential
closed mid to upper low. Most solutions including the gfs and
ecmwf show upper dynamics closing off, a cut-off low forming and
this circulation continuing to drop south toward the state. In
fact, the ecmwf retrogrades this feature to north and then west of
the islands by next Tuesday. The 00z gfs had a similar solution
but now the 06z gfs has trended in shifting the upper low through
the state and downstream by early next week. The two means, gefs
and ecmwf, position the low right over the state for the weekend
and until the spread reduces this may be the proper way to
approach the forecast. If you believe the operational versions of
the gfs and ecmwf, this means a significant chance for locally
heavy rain for much of the islands, as 1.5 inch pws consumes the
state. A quite beginning to the forecast but highly unsettled for
this weekend into next week.

Musher