Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
728 AM EST Thu Feb 07 2019

Valid 00Z Fri Feb 08 2019 - 00Z Fri Feb 15 2019

Recent clustering and reasonable stability of guidance are
providing increased confidence in the forecast of unusually deep
and southward low pressure that may bring a period of
strong/damaging winds and high waves to the state during the
weekend into the start of next week.

Today and tomorrow an initial upper shortwave crossing the region
will support an area of low pressure to the north-northeast of the
main islands.  A trailing cold front will pass through and usher
in brisk northerly winds and drier air, with an axis of lowest
precipitable water values (1-2 standard deviations below
climatology) passing through from northwest to southeast on
Saturday.

Models/ensembles continue to show a dramatic amplification of
central Pacific energy leading to a closed low forming just north
of 30N latitude by early Saturday and then drifting southward to
around 25N by early Monday.  At the low's most extreme combination
of depth and southward extent, latest GFS/ECMWF runs and their
ensemble means suggest that 500mb/850mb heights could reach as low
as 4-6 standard deviations below normal.  In response to this
upper low there is good consensus among the guidance that the
surface low north of the state will deepen and loop back to the
south, possibly getting as close to the islands as 24N latitude
around Sunday/early Monday.  By this time frame the most prominent
guidance difference involves the 00Z ECMWF whose low track strays
somewhat west of other models and ensemble means (including the
ECMWF mean).  Note that its solution is not extreme enough to be
an outlier though.  The old 12Z/06 ECMWF is more representative of
majority clustering.  Guidance continues to show the threat for
strong northerly/northwesterly winds and high waves on the western
side of this system, with one or more areas of rainfall also
embedded within the fast flow.

After early Monday guidance generally shows the upper low opening
up while the upper trough persists but becomes gradually weaker
with time.  As has been the case in recent days for the 5-7 day
portion of the forecast, the 00Z GEFS/ECMWF means provide a good
intermediate starting point--keeping an upper low in place to the
north longer than the GFS/CMC runs but not quite as far west as
the aforementioned questionable 00Z ECMWF.  With minor differences
the 00Z CMC mean generally supports the other means.  There is
still a lot of uncertainty among models/members for specifics of
energy flowing around the mid-latitude upper high, with this
energy possibly having some influence on the upper low of interest
after Monday.  Based on the ensemble mean scenario, expect some
degree of elongated surface low pressure to persist just
north/northeast of the state through midweek and possibly sag
southward over the area into Thursday, maintaining some degree of
unsettled weather.  Again confidence in any specific scenario
surface/aloft (along with associated sensible weather effects)
becomes fairly low especially by Wednesday-Thursday.

Rausch