Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Sat Jul 22 2017

Valid 00Z Sun Jul 23 2017 - 00Z Sun Jul 30 2017

Tropical Storm Fernanda, currently about 600 miles east of Hawai'i
per the latest CPHC advisory, is forecast to continue weakening
and become post-tropical by late today as it continues on a
west-northwestward path. The remnant low is forecast to pass north
of the Islands Sunday into Monday. Models have come into better
agreement showing a weaker low center with a track just north of
Hawai'i. A brief disruption to the easterly trades is possible as
the system passes by, but the trades should quickly resume in its
wake. An increase in precipitable water values ahead of the system
may also increase shower activity by Sunday and Monday, although
the core of deeper moisture associated with the system is forecast
by both the GFS and the ECMWF to remain north of the Islands.
Please refer to the CPHC advisories for the latest information on
Fernanda.

Otherwise, an expansive surface high is forecast by all guidance
to remain anchored well northeast of Hawai'i through the next
week, which should keep 10-20 kt surface easterlies prevalent
across the Islands with the typical associated shower activity.
Tropical Storm Greg is forecast by NHC to continue on a generally
westward to west-northwestward track through the next several
days. Greg is forecast to become Post-Tropical by late Wednesday,
still well east of Hawai'i. Models show some spread with respect
to whether the remnant low from Greg will make it to Hawai'i
before dissipating. The ECMWF and GEM global are quite a bit
faster than the GFS and bring Greg's remnant low to just east of
the Big Island by Saturday afternoon. Ensembles show a rather
large range of spread, but at this time there seems to be a
somewhat greater clustering of EC/GEFS/CMC ensemble members around
the faster deterministic ECMWF/GEM solutions.


Ryan