Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
729 AM EST Sat Feb 16 2019

Valid 00Z Sun Feb 17 2019 - 00Z Sun Feb 24 2019

Guidance continues to show medium-small scale complexities to the
expected evolution, which may take until a short range time frame
to get resolved.  However there are similar and persistent themes
that point toward a significant heavy rainfall threat focused over
the eastern islands in the Tuesday-Wednesday time frame, with
activity gradually trending lighter and less widespread by late
week and next weekend.

Consensus shows a low at the surface and aloft initially northeast
of the main islands performing a cyclonic loop to a position west
of the islands by Monday.  Some shower activity is possible but
should be tempered by an axis of relatively drier air pushing from
east to west across the state.  In this time frame there continues
to be the complicating factor of mid-latitude shortwave energy
aloft that may close off a separate upper low farther north or
northeast, and in this respect the 00-06Z GFS are less supported
by most other models (though the CMC has some modest similarity)
and majority of ensemble members.  At least for the pattern
closest to the state the 00Z GEFS/ECMWF means cluster well with
the 06Z FV3 GFS and 00Z ECMWF/UKMET/CMC so would recommend this
group of solutions into the early part of the week.

Although the upper low begins to eject northeastward with the
approach of upstream shortwave energy by Tuesday-Wednesday, latest
GEFS/ECMWF means maintain good continuity and agreement with the
idea that during that time frame moist southeasterly low level
flow will bring precipitable water values reaching 2-3.5 standard
deviations above climatology into the eastern half of the main
islands.  It remains to be seen if PWATs reach as high as
indicated in the 00Z ECMWF but the good continuity/similarity of
the ensemble means do provide some added confidence in the heavy
rainfall threat over and near the Big Island in this time frame.

The aforementioned upstream energy will continue along in the form
of a trough whose core should pass north of the islands.  As flow
aloft becomes west-northwesterly, expect moisture to decrease with
time after midweek.  In addition the surface trough lingering over
the area should push eastward, leading to northeasterly winds that
will tend to focus remaining shower activity over favored windward
terrain.  For this latter part of the forecast both the GEFS/ECMWF
means continue to favor a non-GFS scenario--with drier air being a
little slower to push into the islands versus the 00Z GFS in
particular.

Rausch