Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
821 AM EDT Sun Jul 16 2017

Valid 00Z Mon Jul 17 2017 - 00Z Mon Jul 24 2017

Initially, an elongated positively-tilted upper trough sits over
the Hawai`ian island chain accompanied by widely scattered light
shower activity drifting west-southwestward. Over the course of
the next couple of days, models carry this trough axis slowly
toward the west nearing the International Dateline. There is
somewhat of a lull in the upper pattern in between the
aforementioned trough and approach of what is currently Fernanda,
a major hurricane which is currently well east of Hawai`i. For the
most part, the track forecast provided by the National Hurricane
Center mimics output from the 06Z GFS/00Z ECMWF. It is currently
being advertised as a weakening tropical cyclone by late Thursday
with additional decrease in strength likely based on some of the
recent model output. Sufficient shearing of the mid-level
circulation is noted in all guidance although residual tropical
moisture will likely push toward the islands late in the period.

At the surface, a persistent anticyclone will be a mainstay
feature throughout the forecast with the position of this high
pressure drifting toward the west-northwest in time. This general
synoptic pattern favors the persistence of trade winds across the
region, generally with a strength in the 15 to 20 knot range.
Depending on the strength of Fernanda, some additional enhancement
in the local wind fields is possible as models show an uptick by
late Thursday, especially over the Big Island and Maui.
Additionally, some further wind shifts are likely into next
weekend depending on the timing and strength of the tropical
circulation. And looking at the rainfall forecast, windward
showers should be the primary theme with the usual local
enhancements within regions of upslope as well as any mesoscale
boundaries. The better chances for more substantial rain will
occur later in the period as precipitable water values jump over
1.50 inches as tropical moisture ahead of Fernanda moves through
the islands. The 06Z GFS/GEFS mean as well as the 00Z CMC are
notably wetter given their faster surface low track. Of course the
slower 00Z ECMWF/ECMWF ensemble mean are a lot drier given the
system is still east of the region next weekend.


Rubin-Oster