Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Mon May 15 2023

Valid 00Z Tue May 16 2023 - 00Z Tue May 23 2023

Another day of moderate to locally breezy northeasterly trade
winds looks on tap for Monday behind the cold frontal passage a
day or so ago and with a surface high to the northwest of the
state. Trades are forecast to trend more easterly by around
Tuesday-Wednesday and weaken a bit as the high shifts north. Not
too many showers are expected given a relatively dry airmass in
place, but those that do form should focus on windward/mountain
areas.

By around midweek, model guidance continues to indicate an upper
trough axis setting up to the west of the state, which would then
track east eventually over Hawaii Thursday and Friday. The upper
trough should draw in higher moisture levels and reduce the
typical trade wind inversion, which will provide for increasing
shower coverage with heavy rain becoming likely. Ample tropical
moisture will be in place, with even the ensemble means from the
GEFS and the ECMWF showing climatological precipitable water
anomalies over the 90th percentile, and the individual
deterministic GFS and ECMWF indicating PW anomalies over the 95th
percentile and 3-4 standard deviations above average. This pattern
would also turn winds more southerly, helping tropical moisture
stream in. Confidence in this wetter pattern continues to
increase, though with remaining differences in the details, such
as the persistence of the upper trough through late week and
beyond. GFS/GEFS runs have showed the trough cutting off a weak
upper low and moving more slowly east/slower to erode than most
other guidance. Amplified moisture levels do look to persist into
the weekend, but depending on how the upper and lower level
dynamical features evolve, there may be less support for heavy
rain into the weekend and beyond. Thus how long this wetter
pattern will last remains uncertain and will continue to be
monitored.


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