Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
711 AM EST Mon Nov 20 2017

Valid 00Z Tue Nov 21 2017 - 00Z Tue Nov 28 2017

Models and ensembles maintain a mean trough aloft during the
period, generally oriented northeast-southwest over the state.  In
this pattern the precipitable water values will likely remain
below normal at least through the weekend.  As a result
windward-focused but occasional leeward shower activity should
stay on the lighter side.  Bundles of sheared shortwave energy
embedded within the overall mean trough may provide a little extra
organization to rainfall at times.  Toward the beginning of next
week some guidance is suggesting a potential for a northward drift
of moisture toward the Big Island in response to evolution of
energy within the upper trough.  The small scale of the shortwave
energy and typical model verification seven days out in time
combine to yield low confidence in a specific scenario.  Thus at
this time would recommend a compromise for resolving guidance
differences by next Monday.  

Expect low level winds initially from the north to become
northeasterly over the next day or so as low pressure to the
northeast of the state continues lifting away and a central
Pacific surface ridge extends southeastward.  A more typical
east-northeasterly trade pattern will develop late week onward as
high pressure continues to push southeastward.  Guidance is
consistent in showing a fairly tight pressure gradient with
brisk/windy trades late this week and perhaps to some extent into
the weekend.  By next weekend and early next week the 00z GEFS
mean differs from other guidance in holding high pressure farther
north and then northwest of consensus but this appears to have
only modest impact on the surface pattern over the islands.

Rausch