Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
806 AM EDT Sun Jul 23 2017

Valid 00Z Mon Jul 24 2017 - 00Z Mon Jul 31 2017

The remnant low from Tropical Storm Fernanda, currently about 300
miles east of Hawai'i per latest satellite imagery, is forecast to
continue weakening into a surface trough as it moves generally
westward. The system is forecast to pass just north of Hawai'i
today into early Monday. A brief disruption to the easterly trades
is expected 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, 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.

Otherwise, an expansive ridge of high pressure is forecast by all
guidance to remain anchored well northeast of Hawai'i through
Thursday, which should keep 10-20 kt surface easterlies prevalent
across the Islands with the typical associated shower activity. By
late in the week, the ridge break down somewhat as an upper trough
amplifies across the north central Pacific. This will drive a cold
front southward across the central Pacific, although the front is
forecast by all guidance to remain well north of Hawai'i through
next weekend. 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. There has been a general trend
since this time yesterday on weakening the remnant low quicker.
The ECMWF/GEM/GFS all now weaken the low to a trough by Sunday and
turn it southwestward, away from Hawai'i. Ensembles have also
trended toward weakening the remnant low more quickly, with only a
handful of EC members bringing the remnant low to Hawai'i next
weekend.


Ryan