Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
823 AM EDT Sat Aug 04 2018

Valid 00Z Sun Aug 05 2018 - 00Z Sun Aug 12 2018

As a weakness in the mid/upper-level ridge shifts west of the
Hawai`ian island chain, all eyes will move to Hurricane Hector
which is expected to pass to the south of the Big Island by
Wednesday. Compared to the official forecast track from the
National Hurricane Center, the 00Z UKMET is a quick outlier.
Beyond this period, the latitudinal spread increases with the
06Z/00Z GFS and 00Z CMC to the north of the previous two ECMWF
runs. A compromise between these two solutions appears reasonable
as either seems plausible at this time. In its wake, the pattern
flattens out considerably although ensemble spaghetti plots depict
a lot of noise given general uncertainties.

The surface pattern will initially be dominated by moderate
east-northeasterly trade winds given abundant high pressure over
the mid-latitudes. As Hurricane Hector approaches the Big Island
from the southeast, low-level wind fields should pick up
considerably. The 06Z GFS shows a 50 knot wind barb possibly
affecting this region by midday Wednesday although this is the
more northern solution. Additionally, this will be highly
dependent on the tropical cyclone size and intensity. After the
system exits to the southwest, prevailing trades will resume,
albeit in a slightly more weakened state relative to the start of
the period.

Regarding impacts, beyond the usual trade driven shower activity,
Hurricane Hector could easily bring heavy rainfall to the southern
most islands depending on its track. While the guidance keeps this
axis tucked to the south, it would not take much for this to
change. Of course rough seas and strong swells can be anticipated
as well. As the system moves west of Hawai`i, there may be an
additional threat for locally heavy rainfall, particularly in the
00Z ECMWF given a spike in precipitable water values.


Rubin-Oster