Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
729 AM EST Thu Feb 14 2019

Valid 00Z Fri Feb 15 2019 - 00Z Fri Feb 22 2019

The state should remain under a very unsettled pattern in the
coming days as a central/east-central Pacific Rex block pattern
keeps upper troughing and embedded low near the state into the
first half of next week.  Expected evolution will maintain the
threat for periods of heavy rainfall over portions of the state.

Latest models and ensemble means are fairly agreeable into
Saturday/early Sunday.  An upper low center will track southeast
then east over the main islands, likely reaching a position
northeast of the Big Island during the day Friday.  Then the
feature should return back to the west over the course of the
weekend.  As this occurs a surface trough initially east of the
islands should develop embedded low pressure as it likewise
returns westward.  This surface trough will separate northeasterly
and southeasterly low level winds.  Guidance shows there could be
a narrow axis of relatively lower precipitable water values near
this feature, which could briefly temper rainfall potential a bit.

From late weekend onward model/ensemble details diverge regarding
what happens with energy in the southern part of a mid-latitude
trough and its effect on the upper low that likely reaches to the
northwest of the main islands by next Monday.  Broadly speaking
there are two clusters, the ECMWF/CMC/UKMET and the ECMWF/CMC
means which are quick to pull the trough energy southward into the
overall low circulation, and 00-06Z GFS runs that delay the
pulling off of trough energy leading to a separate upper low in
the area of 30-40N latitude.  Some individual ensemble members
offer the GFS scenario as a possibility but few enough do it that
even the GEFS mean's surface and moisture parameters appear closer
to the ECMWF/ECMWF mean cluster.  Preferring this majority
scenario, expect surface troughing to lie somewhat westward of the
GFS runs during Monday-Thursday.  At the same time the position of
the upper low should enhance the southwesterly flow of lower
latitude moisture across the region.  In fact by 12Z Wednesday the
GEFS/ECMWF means are both remarkably similar in showing PWATs
reaching 2-3 standard deviations above normal over the southeast
half of the main islands.  The literal 00Z ECMWF shows even more
extreme moisture anomalies and significant heavy rainfall
potential.  Thus the midweek period in particular will have to be
monitored very closely for this concern.

Rausch