Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
729 AM EST Wed Dec 20 2017

Valid 00Z Thu Dec 21 2017 - 00Z Thu Dec 28 2017

...Heavy rain threat through Thursday...

A deep upper trough over the central Pacific will keep conditions
wet over the Hawai`ian island chain through Thursday. Abundant
moisture pooling along the parent surface boundary continues to be
a conduit to active convection, particularly offshore of Maui
where thunderstorms were observed just east of the island. The
frontal zone will be in the process of sweeping southward as
attendant mid-level height falls project eastward. Consequently, a
cold advection regime is anticipated into the early part of the
weekend with northerly low-level prevailing. At its peak on
Thursday/Friday, wind speeds maximize around 20 to possibly 25
knots before slackening in response to a weakening pressure
gradient. Over the weekend, the mid/upper-level pattern is
forecast to shift to a more west-northwesterly direction as the
stronger jet energy resides up over the mid-latitudes. It should
be a quiet period in terms of precipitation given the
semi-persistent low-level northerly to northeasterly flow.

Moving into next week, in advance of upstream amplification in the
vicinity of the International Dateline, low-level winds turn
easterly leading to an increase in windward showers across
Hawai`i. At this point, models have been in reasonable agreement
until they diverge with the approach of the next potent trough.
Ensemble spaghetti plots agree on an even more amplified system
than the one currently in place. While the 06Z GFS/00Z ECMWF/00Z
CMC all show this scenario unfolding, the 00Z ECMWF as well as its
previous run favor an upper low cutting off just west of the state
by December 26-27. This latter evolution would favor an impressive
period of heavy rainfall over the northern most islands. However,
other solutions are more progressive and would keep this axis of
precipitation to the north. Regardless, of solution, warm
advection should pick up considerably as brisk southeasterly flow
ensues. An eventual frontal passage is likely but the timing is
highly model dependent. While the 00Z ECMWF is plausible, it does
not have a whole lot of ensemble support except from its own
members. It definitely bares watching given the noted heavy
rainfall signature.


Rubin-Oster