Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
827 AM EDT Wed Aug 19 2015

Valid 00Z Thu Aug 20 2015 - 00Z Thu Aug 27 2015

The 19/00Z GEFS/ECENS and their deterministic runs looked good
through 24/12z and continue to carry the quasi-stationary TUTT
cell another day invof 30N 160W---then drift the feature northwest
of 165W.

And like yesterday---not sure if the deep-layered ridge centered
near 46N 146W will be the catalyst for the TUTT cell's
'drift'---but there still appears to be a burst of moisture and
energy originating in the Tropics (between 140W-150W along 10N)
emerging around the 20/00Z and 20/06Z that bears watching.

By 23/00Z, the 19/00Z ECMWF and 19/06Z deterministic GFS both
carry a surface wave invof 156W between 16N and 17N latitude that
has origins in the Tropics. The wave's mid-level moisture and
precipitation shield will be migrating northward towards the Big
Island and Maui/Lanai through 24/00Z.

Then the forecast becomes a little less obvious--- being that the
TUTT cell has moved northwest--- the low-level
east-northeasterlies are more prominent north of the Islands and
'counter-balancing' the low-level southerly sub-synoptic scale
winds embedded withing disturbance to the south of the Islands.
The blend of the deterministic GFS/ECMWF QPF signals a very wet
period for the Big Island on Saturday into Sunday (23/00Z-24/00Z).
The interesting feature of the pattern---the deeper convection
blossoms downwind of the Big Island---but over coastal waters, and
appears to develop and persist invof the low-level convergent wind
fields (trade wind and sub-synoptic scale).

In either case, the potential for locally heavy rains continues
this upcoming weekend. And after the weekend, the flow within the
sfc-700mb layer remains rather light and variable---with weak
convergence (possibly a surface trough) remaining invof Hawai'i
into Tuesday. 

Vojtesak