Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
820 AM EDT Tue Apr 24 2018

Valid 00Z Wed Apr 25 2018 - 00Z Wed May 02 2018

Guidance agrees in principle regarding the pattern evolution that
will promote increased potential for significant rainfall over the
central/eastern islands by the weekend and early next week, but
differ in some of the specific details.

Through Wednesday and to some extent into Thursday expect high
pressure northeast of the state to support brisk trades with
shower activity tending to favor windward areas.  Then a rapidly
amplifying central Pacific trough aloft will likely close off an
embedded low to the north of the area, around 30-35N latitude,
while developing surface low pressure anchors a front that moves
into the islands.  Among current guidance the 00z/06z GFS runs
show a farther south/southwest upper low position than most other
solutions in the Friday-Sunday time frame.  A more intermediate
position in the 00z GEFS mean suggests hedging away from the GFS
scenario.  Meanwhile there are differences in southeastward extent
of the front that will serve as the focus for potentially enhanced
rainfall in association with moisture being pulled up from the
south (precipitable water values of at least 1.75-2.00 inches). 
Perhaps due in part to differences aloft, the GFS generally holds
the front a little farther northwest than other guidance,
generally over the eastern half of the state.  On the other hand
by early next week the 00z GEFS mean tracks the surface low and
trailing front/moisture axis a little southeast of consensus. 
Overall a solution close to or just a fraction slower than the 00z
ECMWF seems to provide the best intermediate forecast at the
surface and aloft.  Even with the differences in frontal position
and corresponding rainfall duration/intensity over eastern
locations, there is decent confidence in a drier pattern with
north/northeasterly winds over the western islands by the weekend
and early next week.

Rausch