Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
829 AM EDT Fri Jul 24 2015

Valid 00Z Sat Jul 25 2015 - 00Z Sat Aug 01 2015

24/00Z ECENS/GEFS ensemble guidance maintains a 'progressiveness'
to the northern Pacific mid-latitude flow pattern from Asia to
North America---but their deterministic solutions are 'tainted' by
the introduction of tropical energy and moisture---generating
large differences by days 4-5 in the Gulf of Alaska and invof 20N
120W.

The spurious nature of the deterministic runs force the usage of
the an ECENS/GEFS mean blend by 72 hours (27/12Z). A brief
explanation of why? follows below.

Satellite imagery indicates a rather extensive and broadening
region of upper-level westerly flow emerging across the western
half of the Subtropical Pacific. And the shorter-term Island
forecasts may need to contend with a little more northeastward
spread of ITCZ-derived cloudiness than originally anticipated by
the global model guidance. Some of the alteration is tied to a
weak, compact BUT persistent mid-level circulation invof 27N 174W.
This features continues to pull tropical moisture nnewd along its
southeast quadrant but there seemed to be enough shear and the
feature is shallow enough---to limit its impacts on the
lower-level northeasterly trades.

Moving into the medium range period---the 24/00Z model cycle
generated larger than normal spread. And not
surprisingly---remnants of 'Halola' and what interaction it
may/may not have with the Monsoon Trough located invof the
Marianas are looming medium range concerns.

A third, and not least important medium range forecast
concern---is found off the Mexico's west coast. In short, there
are three possible waves generated invof 130W, and the
deterministic ECMWF is the most aggressive with all three.
Question is---can the deterministic ECMWF be 'trusted' with its
24/00Z scenario across all of the Pacific Rim (the northern
hemispheric portion of it).

Would recommend a 70/30 ratio of the ECENS/GEFS means beyond
27/12Z and continue to expect variability in the deterministic
guidance for a few more cycles---until the GFS/ECMWF can come into
better agreement with the energy and moisture exchanges from
Subtropics to mid-latitude.

Vojtesak