Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
823 AM EDT Fri Jun 21 2019

Valid 00Z Sat Jun 22 2019 - 00Z Sat Jun 29 2019

A deep upper-level low (-5 standard deviations at 500 hPa) is
situated several hundred miles northwest of Hawaii early this
morning, and is expected to remain northwest of the state through
the weekend. This along with surface high pressure will northeast
of the state will keep relatively light trades (10-20 kt) in place
into early next week. Models/ensembles show general consensus that
this feature should very slowly migrate eastward toward Hawaii by
next Tue-Wed, but solutions differ on the extent to which heights
may fall across the state. The ECMWF and CMC were the most
aggressive deterministic solutions in that regard, while the GFS
was a bit less amplified with the feature by that time. Ensemble
means showed smaller differences by the middle of next week
relative to the deterministic guidance, and thus prefer an
ensemble approach to the forecast by that time rather than any one
deterministic solution. Models do agree that moisture should
increase, with PWs among deterministic solutions by the middle of
next week across Hawaii ranging from 1.8 to well over 2 inches.
This should support areas of heavy rainfall, although confidence
is low at this point as to exactly when/where this would occur due
to the range of solutions. ECENS probabilities show the greatest
chance for heavy rainfall across the western Hawaiian islands from
late Tue into Wed, while GEFS probabilities show the most
significant heavy rains remaining north of the state.
Additionally, the approach of this system will likely disrupt the
trades, with the potential for a period of southeasterly or
southerly flow at the lower levels, especially if the trough is on
the more amplified side as shown by the ECMWF.

Ryan