Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
722 AM EST Mon Dec 26 2016

Valid 00Z Tue Dec 27 2016 - 00Z Tue Jan 03 2017

As has been in the forecast for days, a mean upper ridge over much
of the Central Pacific will advance eastward in response to
lowering heights in the mid-latitudes. A split in the jet stream
is forecast to allow a shortwave to detach from the prevailing
westerlies and eventually close off south of 40N/150W by sometime
Thursday depending on the model. To conclude the work week, this
circulation should continue to dip southward toward the vicinity
of 30N while potentially retrograding a bit. Ensemble spaghetti
plots have indicated a dichotomy among the solutions with the
ECMWF members favoring a more eastward position which has little
support from other 06Z/00Z guidance. There are no indications this
closed low will approach the Hawai`ian island chain although
residual troughing should extend toward the region from roughly
Thursday onward.

The surface pattern will initially remain conducive to moderate to
occasionally stronger easterly trades given the presence of an
anticyclone northeast of the islands. An even stronger dome of
high pressure, potentially reaching 1045-mb, is expected to move
east of the International Dateline leading to winds over Hawai`i
shifting to a more northeasterly direction. Brisk winds
accompanying this regime will spread across the islands with
speeds picking up into the 20 to 30 knot range by mid-week.
Regardless of model choice, all solutions favor the trailing end
of the baroclinic zone to cross through the region although
low-level convergence seems to be rather weak. As the pressure
gradient weakens into this weekend, there should be a decrease in
the intensity of the northeast to east-northeasterly flow.

Showers should be primarily driven by the period of enhanced
trades although infusion of lower precipitable water air should
keep the activity somewhat scattered in nature. There may be an
uptick in precipitation along the southwestern flank of the
frontal zone as indicated by the 00Z CMC/UKMET solutions. However,
the weak surface convergence may play a role in tempering rainfall
amounts which suggests those aforementioned solutions may be a tad
heavy.


Rubin-Oster