Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 AM EST Sun Dec 22 2019

Valid 00Z Mon Dec 23 2019 - 00Z Mon Dec 30 2019

...Heavy rain possible across the western Hawaiian islands by
midweek...

Windy conditions may persist into Sunday across Hawaii but winds
should gradually lessen through the day and into Sunday night as
surface high pressure north of the state weakens and moves
eastward. By Tue-Wed models show good consensus that an upper low
initially well west of Hawaii should begin to move northeastward
as is influenced by strong westerlies farther north. A surface
front trailing the upper low is expected to focus a band of deep
tropical moisture, with a plume of very high PWs moving gradually
eastward along/ahead of the front. Models agree on this general
scenario, which could pose a heavy rain threat to the western
Hawaiian islands by Tue night into Wed. The GFS and ECMWF still
differ on precisely how deep the moisture in the plume will be,
with the ECMWF/ECENS much more bullish than the GFS. The ECMWF
shows PWs by Wed across Kauai rising into the 2.50-2.75 inch
range, while the GFS is substantially lower, near 2 inches. On the
other hand, both the ECENS and the GEFS show PW standardized
anomalies of +3 to +4 standard deviations across the western
Hawaiian islands by Tue night/early Wed. Additionally, ECENS
probabilities for exceeding 2 inches of rainfall in 24 hours have
increased substantially over the past couple days, now peaking
across Kauai from 12Z Wed to 12Z Thu around 50 percent. Similar
GEFS probabilities are quite a bit lower, on the order of 20
percent. Given the synoptic signals and growing ensemble support,
however, confidence has gradually increased in at least the
possibility for a significant heavy rainfall event across Kauai by
midweek. Additionally, a period of strong/gusty winds may also
accompany the system. The remaining differences among guidance
with respect to the intensity of the moisture plume associated
with this system will hopefully be resolved over the next 1-2 days.

The cold front is forecast to sweep eastward across the remaining
Hawaiian islands through late in the week before stalling and
washing out, however the widespread heavy rainfall threat is not
expected to accompany the front eastward the the upper-level
support quickly races eastward, cutting off the mechanism for
strong moisture inflow into the region. 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