Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
728 AM EST Fri Feb 15 2019

Valid 00Z Sat Feb 16 2019 - 00Z Sat Feb 23 2019

Today's guidance continues to show the potential for an extended
period of unsettled weather with areas of heavy rainfall possible
at times.  Rain may begin to decrease in coverage and intensity
toward the end of next week.  Over the past 24 hours the
models/ensemble means have adjusted some of their details for
expected evolution with the end result being slightly better
clustering than was the case previously.  However the full
ensemble spread remains considerable so there is only a modest
increase of confidence. 

Current solutions agree well into Monday.  An upper low center
initially just northeast of the Big Island will likely perform a
cyclonic loop and reach a position just to the northwest of the
main islands by early Monday.  As this occurs a north-south trough
(separating northeasterly and southeasterly low level flow) will
progress westward across the area while embedded low pressure
tracks just north of the main islands.  Proximity of the upper low
will maintain the threat for periods of enhanced rainfall and
isolated thunderstorms, though guidance does show a band of
relatively lower precipitable water values crossing the islands
from east to west during the weekend and early next week--albeit
with some differences in timing.

By Tuesday there is increasing spread for the latitude of the
upper low.  At that time the 00Z ECMWF mean is a bit farther north
than most other solutions including the 00Z operational ECMWF. 
Meanwhile a mid-latitude shortwave still poses a complicating
factor.  Current majority cluster has adjusted to some variation
of earlier GFS runs that formed a separate upper low along 30-40N
latitude, but the surface trough over/near the state during the
first half of next week now appears to be a compromise between
yesterday's majority that was somewhat farther west and the
eastward GFS.

Even with the spread for the upper low and pattern to the north,
there is reasonable consensus that by Tuesday-Wednesday the
combination of upper low position (even as it begins to lift
northeastward) and southeasterly low level flow will bring
considerable deep moisture into the area.  GEFS/ECMWF means are
consistent in showing PWATs reaching at least 2-3 standard
deviations above climatology for this time of year over at least
the eastern half of the main islands with somewhat higher max
values indicated in operational GFS/ECMWF runs.  It will be in
this time frame that the heavy rainfall threat should be highest. 
Note that the operational 00Z ECMWF brings highest PWATs somewhat
farther west than other solutions especially around Thursday.

Models/means still show continued ejection of the upper low during
the latter half of the week in response to approach of upstream
shortwave energy.  This trailing energy may serve to maintain an
overall mean trough aloft just west of the main islands into
Friday, keeping conditions unsettled, but the overall trough looks
to be somewhat flatter.  This should support at least a gradual
decline of deep moisture.

Rausch