Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
827 AM EDT Wed Sep 24 2014

Valid 00Z Thu Sep 25 2014 - 00Z Thu Oct 02 2014

An amplified mid-latitude pattern will be featured north of the
Hawaiian Islands with a 594-dm ridge initially centered over
30N/165W. A west-to-east oriented ridge should remain sprawled
over the latitude encompassing the islands throughout the forecast
period with any meaningful height falls displaced well to the
north. By 30/1200Z...the 06Z GFS/GEFS mean are a pair of the more
aggressive solutions digging a longwave trof east of 170W.
Compared to other deterministic/ensemble guidance, this appears to
be more of an outlying solution. Would recommend staying away from
the 06Z GFS/GEFS mean during the latter half of the period.

At the surface, a dome of high pressure is forecast to slide
eastward along roughly 30N while gradually strengthening in time.
In response to the increasing pressure gradient, the
east-northeasterly trade winds should pick up in intensity by late
Thursday. Some models depict 20 to 25 knots of low-level flow at
times from late Thursday until Saturday. Gradually the trades are
expected to slacken from late in the weekend into the early/middle
part of next week. The more outlying 06Z GFS suite of solutions
would favor a warm advection regime as winds shift to
southeasterly. However, again not recommending this solution at
this time. Overall, precipitation should be a little more active
during the Thursday through Saturday time frame given the increase
in the trades as an enhanced plume of moisture funnels over
Hawaii. The 00Z ECMWF depicted 1.75 to 2 inch precipitable water
values over the islands with an increase in precipitation during
this period. Once the trades begin to weaken, the tropospheric
moisture content begins to decrease with a return to the more
usual windward showers and enhanced convection across the favored
upslope terrain.


Rubin-Oster