Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
826 AM EDT Mon Apr 29 2019

Valid 00Z Tue Apr 30 2019 - 00Z Tue May 07 2019

Models show good agreement that an upper-level shortwave will
cross Hawaii through Tue before moving east of the state by Wed.
Scattered (mostly windward) showers with locally heavy rain will
be possible as this wave passes. In the wake of the shortwave, a
brief period of weak upper-level ridging is expected to build
across the state through Thu, bringing a somewhat drier air mass
and a reduction in shower activity, and a continuation of light to
moderate trades. By late week into the weekend, models/ensembles
show broad agreement that a shortwave should begin to amplify near
40N/170W which is then expected to quickly deepen/close off as it
dives southeast toward Hawaii. Deterministic solutions differ on
the exact evolution and track of this feature, but they all show a
similar overall scenario. The ECMWF ensemble even shows 500 hPa
heights reaching -5 to -6 standard deviations just north of Hawaii
by Sun. At the surface, such an upper-level feature would be
associated with a an occluded low pressure system with a cold
front sweeping across Hawaii over the weekend. An increase in
moisture along/ahead of the front could result in a period of more
widespread showers across the state by Sat-Sun. The exact position
of the upper low by Sun will play a significant role in the
sensible weather impacts for Hawaii, with a more western position
(as shown by the CMC) resulting in a longer period of enhanced
rainfall potential, and a more easterly track (as shown by the
GFS) sweeping any frontal boundary across Hawaii much more
quickly. At this point, confidence in the overall scenario is
above average, but confidence is below average in the specifics of
the upper low evolution, and thus the sensible weather impacts for
Hawaii.

Ryan