Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
710 AM EST Mon Dec 11 2017

Valid 00Z Tue Dec 12 2017 - 00Z Tue Dec 19 2017

Cold front still appears on track to move across Hawai'i Tue/Wed
as an upper trough amplifies north of the islands.
Models/ensembles still show good agreement that a portion of the
trough will cut off into an upper low just east of Hawai'i Wed
night/Thu and then slowly drift westward into the weekend. By
Sun-Mon the upper low should begin to become absorbed into another
full latitude trough axis extending from just northwest of Hawai'i
to the Gulf of Alaska. Sufficient moisture will be associated with
the frontal passage Tue/Wed to support scattered showers and
perhaps even a few localized areas of heavy rainfall. The more
significant threat for rainfall will occur late in the week as the
upper low cuts off and drifts west, allowing deeper moisture to be
pulled northward across the state. Thus, expect numerous showers
(and even some threat for thunder given the upper low passing
overhead) by Fri-Sat, especially focused along east facing terrain
given expected low-level easterly flow of 20-25 kt. ECENS QPF
probabilities show about a 20 percent chance for exceeding 2
inches of rainfall across the Big Island and Maui for the 48-hour
period ending at 00Z Sun, with a 35-45 percent chance for
exceeding 1 inch - continuing a gradual climb in these
probabilities noted over the past several days. Thus, confidence
continues to increase in the potential for at least locally heavy
rainfall across portions of the Big Island and Maui by late in the
week. As the next upper trough amplifies northwest of Hawai'i by
early next week, expect a broad plume of tropical moisture to
continue streaming across portions of Hawai'i until the trough
axis and any surface frontal boundary can clear the state.

Ryan