Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
829 AM EDT Fri Jul 21 2017

Valid 00Z Sat Jul 22 2017 - 00Z Sat Jul 29 2017

A couple features of note will affect Hawai'i over the next week.
Tropical Storm Fernanda, currently about 800 miles east of Hawai'i
per the latest CPHC advisory, is forecast to continue weakening
and become post-tropical as it moves west-northwestward over the
next couple days. The remnant low is forecast to pass north of the
Islands Sunday into Monday. Models show some spread with respect
to how far north of the Islands the system will pass, with the GFS
and UKMET a couple hundred miles farther north as it passes the
Islands relative to the ECMWF and GEM global. The current shallow
nature of any convection associated with the system would favor
the farther south track of the ECMWF/GEM, as shown by the latest
forecast track from the CPHC. Depending on the exact track, 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. All models keep system east of Hawai'i through next Friday.


Ryan