Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
727 AM EST Mon Dec 12 2016

Valid 00Z Tue Dec 13 2016 - 00Z Tue Dec 20 2016

...Heavy rainfall threat for the weekend into Monday...


Showers over the region ahead of a weak cold front to the north
and a surface trough to the southwest will continue into Tuesday.
After the boundary pushes through, winds will turn northerly and
weak/brief ridging should mean a relatively dry day on Wednesday. 
Trades will continue to veer to northeasterly and then easterly.
By Thursday, split flow in the central north Pacific will promote
ridging into the Gulf of Alaska and a closed low well to its
southwest near 30N/165W. Ensembles continue to trend a bit closer
together toward the deeper/slower side as most consistently
advertised by the GEFS ensembles. The 06Z GFS and 00Z ECMWF (and
Canadian) form a good cluster between the farther southwest 00Z
GFS and parallel GFS and farther northeast 00Z ECMWF ensemble
mean. This is also near the 06Z GEFS mean. By the weekend into
Monday, some of the GEFS and ECMWF ensembles take the upper low
eastward but the increasing majority of ensembles stall the upper
low just to the west of Hawai'i. A heavy rainfall pattern is more
likely for the region but a shift to the east with the upper low
position may guide the moisture axis east of the region. Ensemble
range in 7-day precipitation totals remains quite large -- from
about a half inch to over 20 inches over the central and
especially eastern islands. The 00Z GFS was an extreme example
among the ensembles. Ensemble means' QPFs have been increasing
over the past few days and the GEFS mean (relative to its model
climatology) were nearing maximum values for this lead time (5-7
days) this time of year per the Ensemble Situational Awareness
Table.


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