Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
759 AM EDT Sat Oct 31 2015

Valid 00Z Sun Nov 01 2015 - 00Z Sun Nov 08 2015


The 31/00Z deterministic GFS/ECMWF were two good pieces of
forecast guidance --- heading into the 3/12z time frame (early
Tuesday) --- in their depiction of the subtle southerly
flow/moisture surge setup and disappearance of the seasonal
trades/tradewind inversion for Hawai'i.

The surface reflection's northward migration into the mid-latitude
flow pattern between 155W-160W migration --- is to an extent ---
tied to a 'yet-to-form' mid-latitude upper-level
circulation---approaching Hawai'i from the northwest.

The 31/00Z GFS/ECMWF are in very good agreement with the depth and
energetic character of this upper-level feature. Its influences
are likely to emerge in the next 24 hours ---from an area of the
central Pacific very near 40N 170W --- and both model runs slowly
drift it southeastward towards Kauai through the next 48-60 hours.

Beyond the day 4 period (generally beginning on/after 5/00Z), the
GFS looks a bit too fast with a deep wave sweeping across the
eastern Aleutians --- versus the ECMWF. This deep eastern Aleutian
wave does appear to have some influence on the Subtropics for the
latter half of the Hawai'i forecast (northwest swell setup?)---but
not sure there was much need to use a deterministic approach
beyond day 5.

I would recommend using a blend of the two deterministic solutions
(up to 60%) until 4/12z, then edge away from the deterministic
GFS/ECMWF solutions. If I had a preference --- looking at what
will transpire in the Alaska forecast today ---would keep some of
the 31/00z deterministic ECMWF between 4/12z and 5/12z to dovetail
into a 70/30 blend of the ECENS/GEFS means by 6/12Z. This should
transition the sequencing of upper-level wave energy migration
from a deep and complex upper feature moving from the southeast
Bering Sea to the Gulf of Alaska by the end of this forecast
period.

Vojtesak