Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
804 AM EDT Mon Sep 15 2014

Valid 00Z Tue Sep 16 2014 - 00Z Tue Sep 23 2014

The 15/00Z GEFS/ECENS ensemble means were very representative of
the flow pattern across the Pacific Ocean through day 5
(20/12Z)---before differences 'crop up' along the western tip of
the Aleutians and downstream in the Gulf of Alaska.

The forecast for the central Pacific remains essentially
unchanged---with the western portion of the Subtropical Ridge
rebuilding and migrating eastward to a position near 35N 160W by
Wednesday (17/12Z). 

The ridge then flattens---migrating east-southeastward to
approximately 33N 142W heading into Friday (19/12Z). The 'flatter'
Subtropical Ridge axis solution in the central Pacific along
30N-35N latitude results in a brief return of the more typical
'Northeast Trades' from Wednesday 'til the Friday/Saturday time
frame. The low-level flow pattern then begins to gain
amplitude---the result of mid-latitude energy from the southern
Bering Sea transferring downstream to the Gulf of Alaska and the
presence of another 'energized' Aleutian Low in the Gulf of Alaska.

The biggest differences---in the 15/00Z means and deterministic
runs---were tied to a re-positioning (a latitude and longitude
issue) of the broad cyclonic gyre in the northeast Pacific. The
consequence of this will be to the detriment of the Subtropical
Ridge. Recommend a 50/50 blend of the 15/00Z ECENS-GEFS after day
4 (Friday) to handle the changes over the mid-latitude Pacific and
character of the Subtropical Ridge axis.

For Hawai'i, trades should be on the increase around Wednesday
with the positioning of the Subtropical Ridge axis northeast of
the Islands to give the ITCZ and cloud debris embedded within it a
chance to spread northward and northwestward over the
Islands---primarily between day 2 and day 4.

Vojtesak