Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Wed Feb 15 2023

Valid 00Z Thu Feb 16 2023 - 00Z Thu Feb 23 2023

...Heavy rainfall/flooding threat especially during
Thursday-Saturday within a longer term wet pattern...

In principle, guidance is agreeable and consistent with a pattern
evolution that will favor an extended period of enhanced rainfall
potential from late this week into next week, with the best
consensus for heaviest rain and flooding threat during
Thursday-Saturday.  From the weekend into next week there are
still some differences regarding exact position of the
southeast-northwest band of supporting moisture.  These
differences are very important for resolving details of the
precipitation forecast at specific locations but are too small to
have meaningful predictability several days out in time.

Expect the upper low initially tracking just north of the state to
curl around to the west by Thursday, while a surface trough (with
a low to the north) passes through the state from east to west and
switches winds from northerly to southeasterly.  Then guidance has
the upper low drifting south-southeast after Thursday, with the
deep-layer southeasterly flow bringing up abundant moisture into
the islands.  Within the Thursday-Saturday period that has the
best signal for heaviest rainfall, GFS/ECMWF runs in particular
highlight Friday as the day when extremely heavy rain may fall
over southeastward-facing terrain of the Big Island.  Through the
weekend the GFS continues to show more tendency than other
guidance for spurious surface waves to track northwestward over or
near the islands, while by Sunday the GFS/GEFS mean are weaker
than the ECMWF/CMC and their means for the upper ridge that could
briefly build in from the northeast.

During the first half of next week the guidance is maintaining the
recent theme of a shortwave trough approaching from the west and
possibly pulling off another upper low, which would keep the
persistent moisture axis within southeasterly flow over or at
least very close to the main islands into the middle of next week.
 The 00Z GEFS/CMC means and 12Z ECMWF mean are all very similar
aloft through next Wednesday.  At least in a broad sense the 00Z
and more so 18Z GFS are more similar to the means aloft late in
the forecast, versus the 00Z ECMWF/CMC that use some of the trough
energy to close off a compact upper low well to the north and have
a weaker upper ridge to the northeast of the state.  In this time
frame, most guidance suggests that the moisture axis may shift a
little southwest of its Thursday-Saturday position but generally
within a range of staying over the islands or being a little
southwest (generally seen in ECMWF/ECMWF mean runs) such that the
western islands would still be in/near the highest precipitable
water values.  It will take additional time to resolve differences
this small in scale.  Either way, some degree of heavy rain
potential will continue into the middle of next week for some
areas.

Rausch