Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
730 AM EST Tue Dec 19 2017

Valid 00Z Wed Dec 20 2017 - 00Z Wed Dec 27 2017

...Heavy rain threat Wednesday into Thursday...

The synoptic-scale pattern will feature a broad upper low
positioned to the north of the Hawai`ian island chain. This will
allow slow moving height falls to traverse through the region with
the parent surface front briefly stalling in the vicinity of the
Big Island on Wednesday. A gradual eastward jog of this upper
trough should eventually allow the cold front to sweep south of
Hawai`i by late Wednesday night. Before this occurs, a
particularly wet period is in store for the region as a 1.75 to 2
inch precipitable water air mass streams through. Based on the 00Z
ECMWF, some 6-hour rainfall totals could reach the 1 to 2 inch
range. This would particularly be the case over the higher terrain
where local orographic forcing would come into play.

By Thursday/Friday, in the wake of the longwave trough passage, a
relaxing of the flow is likely as mid-level winds shift to a more
west-northwesterly direction. This will focus a period of cold
advection across the islands which favors a more northerly
component in the surface wind fields. There may be an
intensification of these winds into the early part of the weekend
as a surface ridge builds to the north. Additionally, a general
shift to a more northeasterly direction is expected although the
00Z CMC shows warm advection rather although its solution appears
to be an outlier. Looking toward next week, a return to amplified
flow is likely although details remain uncertain with the
evolution. There is plenty of support via ensemble spaghetti plots
showing a longwave trough position just east of 170W by next
Tuesday morning. This should ultimately lead to an intense period
of warm advection although models vary with the upstream frontal
position. Eventually, another heavy rainfall threat would be
expected by next Tuesday across Kauai based on the 06Z GFS/00Z
ECMWF/00Z CMC solutions.


Rubin-Oster