Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Sun Dec 05 2021

Valid 00Z Mon Dec 06 2021 - 00Z Mon Dec 13 2021

...Significantly heavy rainfall is forecast today through Tuesday,
which could cause catastrophic flooding...

The forecast for significant heavy rainfall and potential for
flooding/flash flooding for the early part of the week is still on
track for Hawaii. A closed mid-upper low is expected to form
Sunday to the west of Hawaii around 25N/165W, while an upper high
resides well east of the state, and a surface low and high
generally reflect the positions of the upper-level features. This
pattern will lead to mean southerly flow across Hawaii through
much of the period, directing deep tropical moisture northward
from the ITCZ. Model guidance agrees that precipitable water
values will be over the 95th percentile beginning today and
lasting through at least Tuesday, with the axis of highest PWs
drifting slowly east to west across the state as a couple of
surface troughs could focus the moisture. Heavy rainfall should
follow this pattern and several inches of rain can be expected,
along with flooding/flash flooding potential. The HREF for Hawaii
shows southern parts of the Big Island in particular could get hit
hard given upslope enhancement, with 15 to 20 inches of rain
possible over the 48 hour period. Meanwhile, Blizzard Warnings
currently in effect for the highest elevations of the Big Island
will expire early Sunday. Gusty winds remain a threat through
Sunday night, especially for higher elevations. By the second half
of the week, the upper low is forecast to drift westward while an
upper high encroaches toward Hawaii, eventually lessening heavy
rain chances as a more typical trade wind pattern sets up.

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