Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sun Feb 12 2023

Valid 00Z Mon Feb 13 2023 - 00Z Mon Feb 20 2023

Models and ensembles continue to advertise a trend toward a much
wetter pattern over the next week.  The most agreeable and
consistent theme in the forecast is that an upper low initially
just east of the state should wobble westward to a position over
the islands by Tuesday-Wednesday, and then jump to the
west-southwest later in the week as an upper ridge passes by to
the north.  The upper low should still be to the southwest or
south of the main islands through next weekend.  Meanwhile the
operational models (with less-defined support from the means)
agree that a surface trough with possible embedded wave should
cross the islands from east to west around Wednesday, and possibly
into Thursday per the slower 00Z GFS/CMC.  Not surprisingly the
ensemble means tilt toward the timing of their parent model but
the GEFS mean does hint at slightly faster timing than the
GFS/CMC.  Thus would favor a compromise timing for now.  Details
of any embedded surface low have low predictability due to the
small scale and dependence on shortwave impulses that are
difficult to resolve multiple days out in time.  This surface
feature would alter the rainfall pattern relative to the
windward/mountain-focused regime supported by brisk to strong
trades between now and Monday (possibly into Tuesday).

Once the upper low repositions itself after midweek, most guidance
maintains relative agreement into Friday with a pronounced surge
of moisture arriving from the southeast and a corresponding
increase of heavy rainfall potential late in the week.  At this
point the GFS already starts diverging from other guidance and
continues to do so the rest of the period as it brings multiple
compact surface lows northward and diverts the axis of greatest
moisture northward as well.  This appears to be at least partially
due to the model being deeper than the others with the upper low. 
The GEFS mean tilts in the GFS direction regarding the moisture
axis but at least depicts low level flow closer to the
east-southeast trades seen in other guidance after the midweek
surface trough passes through.  On the other hand, the 00Z ECMWF
has made a somewhat westward adjustment with the moisture axis
during next weekend with the 12Z ECMWF mean partially leaning that
way as well.  The CMC takes a bit longer for this westward shift
to occur.  Overall prefer an average of non-GFS guidance from late
week through the weekend.  This would lead to some heavy rainfall
potential possibly extending into at least the first part of the
weekend, with strong high pressure to the north-northeast
promoting east-southeasterly trades and primarily windward
emphasis for rainfall, and then the axis of greatest moisture
possibly drifting a bit west by Sunday.

Rausch