Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Tue Dec 24 2019

Valid 00Z Wed Dec 25 2019 - 00Z Wed Jan 01 2020

...Heavy rain possible across the western Hawaiian islands Tuesday
night through Wednesday...

An upper-level low initially west of Hawaii is forecast to move
northeastward through the middle of the week as it gets influenced
and eventually picked up by stronger westerlies farther north. A
cold front is forecast by all guidance to sweep eastward
toward/across Hawaii by Wed south of the upper-level/surface lows.
A plume of deep tropical moisture along/ahead of the front along
with enhanced instability associated with the upper-level
shortwave are expected to result in the potential for heavy
rainfall, especially across the western Hawaiian islands, starting
Tue night and continuing into Wed and possibly Thu. The GFS and
ECMWF continue to show some differences that impact the degree of
heavy rain threat, with the ECMWF continuing to be the more
bullish of the two solutions, with PWs (near 2.25 inches) and
resultant rainfall amounts. The GFS continues to weaken the
moisture plume more quickly as it moves eastward, and keeps the
heaviest rainfall amounts north of Hawaii, likely driven by closer
proximity to the best mid/upper-level forcing. Confidence is still
moderate at best given the continued spread among the guidance on
the specifics of the event, however the latest runs of the UKMET
and CMC continued to be more along the lines of the ECMWF in terms
of a more significant heavy rain threat, leaving the GFS as the
one deterministic outlier with less significant rains for Kauai
and Oahu. ECENS probabilistic precipitation guidance shows the
greatest chance for heavy rains across Kauai and perhaps as far
east as Oahu between 00Z Wed and 12Z Thu, while similar GEFS
probabilistic guidance resembles the GFS in keeping the highest
chance for heavy rains north of Hawaii. Given the synoptic setup
and anomalous PWs (+3 to +4 standard deviations), there seems to
be sufficient synoptic support for heavy rainfall - and the
potential for at least a 2-3 inches of rain seems there across the
western islands (with the potential for locally higher amounts,
especially across Kauai). Strong/gusty winds may also accompany
the front through the western islands on Wed.

The cold front is forecast to sweep eastward across the Hawaiian
islands through late in the week before stalling and washing out.
Guidance is in general agreement that the threat for heavy
rainfall should diminish eastward with time as the upper-level
support quickly pulls away from the state. Thus, expect scattered
(locally heavy) showers to persist across Hawaii into next
weekend. As the front passes it will disrupt the trades, with a
period of light and variable winds possible by Thu night-Fri. More
typical trades should resume by next weekend as the front
dissipates and high pressure once again builds north of the state.

Ryan