Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
726 AM EST Sun Feb 17 2019

Valid 00Z Mon Feb 18 2019 - 00Z Mon Feb 25 2019

The dominant focus for adverse weather continues to be with an
axis of deep moisture expected to move toward the eastern islands
from the east over the next couple days and bring the threat for
heavy rainfall/flash flooding from late Monday into midweek. 
There are still differences in details for this event and the
subsequent drying trend but there is broad agreement in principle
for the heavy rainfall threat followed by somewhat lighter and
more scattered rainfall.

Guidance agrees that the upper low now north-northeast of the
state will curl around to the west of the main islands by early
Tuesday.  There is somewhat more spread for the exact track of
associated surface low pressure, with upper low position seemingly
favoring a compromise among the latest GFS/ECMWF runs.  A broader
surface trough separating northeasterly from southeasterly winds
will progress westward as well.  Then upstream shortwave energy
will eject the upper low northeastward.  Most guidance is
pinpointing Tuesday as the time when highest precipitable water
values will reach the eastern islands and the Big Island in
particular, with the ECMWF/ECMWF mean showing higher PWATs than
the GFS/GEFS mean.  Either way moisture will be anomalously high
with even the means reaching 3-4 standard deviations above normal
over the Big Island at 00Z Wednesday.  Based on comparisons with
CMC/UKMET rainfall forecasts and possibility for a period of
training/repeat activity--plus a hint of northward-moving surface
waviness nearby--there does appear to be the potential for heavier
totals than seen in the 00-06Z GFS runs even if not quite as high
as ECMWF runs.

By late week and next weekend the ECMWF/GFS have reversed compared
to yesterday in terms of their relative drying trends.  Now the
ECMWF is much more aggressive in lowering PWATs than the GFS. 
This is due in part to the 00Z ECMWF being stronger than other
solutions with the shortwave crossing the area Thursday-Friday
thus pushing surface troughing farther east than consensus.  In
this time frame the ECMWF is not an outlier relative to the full
ensemble envelope for the shortwave aloft but fairly extreme
though.  There is also the possibility that a weaker impulse just
upstream could briefly slow the drying trend as well.  The ECMWF
mean holds onto more moisture than the operational run while the
GEFS mean is a tad lower than operational GFS runs--leaving an
average of these means as a reasonable starting point.  There is
decent agreement that high pressure to the northwest will
gradually extend a ridge to the north of the islands late week
into the weekend, favoring a trend toward moderate northeasterly
winds and shower focus over windward terrain.

Rausch