Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
726 AM EST Sat Feb 10 2018

Valid 00Z Sun Feb 11 2018 - 00Z Sun Feb 18 2018

A broad upper low spinning to the east of the Hawai`ian island
chain will continue its very slow migration toward the east before
shearing by early next week. The accompanying surface reflection
will feature weakening pressure gradients thus resulting in a
lighter wind field, generally out of the north-northeast by
Sunday. There may be a brief uptick in the coverage of rain
showers as a plume of higher precipitable water pivots back toward
the islands.

Into early next week, the focus shifts upstream as a broad upper
trough swings east of the International Dateline. Along the
southern extent of these height falls, the guidance continues to
advertise a band of shortwave energy peeling off during the
Monday/Tuesday timeframe. Relative to yesterday, the biggest shift
in the operational/ensemble guidance is a more northward placement
of this upper low. Instead of approaching from the west of
Hawai`i, a vast majority of the ensemble members have moved toward
a solution which perhaps grazes Kauai and Ni`ihau. By Wednesday,
there is quite a bit of lateral spread with the 06Z GFS/GEFS mean
on the quicker side while the 00Z CMC/UKMET/CMC ensemble mean
places well to the west with the 00Z ECMWF/ECMWF ensemble mean in
the middle. Except for the 06Z GFS/GEFS mean which continue to
accelerate this feature eastward, other solutions favor a gradual
eastward migration with potential absorption by another upper
trough to the north. Suffice to say, there is quite a bit of model
spread relative to the previous day. Given there is a slow moving
closed low in the picture, would likely steer away from the
quicker 06Z GFS/GEFS mean here. Considering the surface pattern,
winds shift toward a more southeasterly direction as a warm
advection regime ensues. Depending on the strength of the upstream
surface wave, there may be an increase in wind speeds but
solutions are variable. Outside of the 00Z CMC solutions, a
frontal passage should occur sometime during the middle/latter
part of the work week with the quicker GFS depicting this
occurrence on the earlier side. The upcoming pattern should
bolster chances for precipitation but it has been reduced relative
to yesterday's forecast as the guidance has changed. The driest
solution would be the 06Z GFS/GEFS mean given the more progressive
nature of the pattern. The 00Z ECMWF/CMC/UKMET all support some
period of more organized rainfall but uncertainty plagues any
additional details.


Rubin-Oster