Prognostic Meteorological Discussion
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FXHW01 KWNH 211219

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Wed Mar 21 2018

Valid 00Z Thu Mar 22 2018 - 00Z Thu Mar 29 2018

Typical detail uncertainties persist but guidance continues to
agree in principle on a pattern that should be favorable for a
heavy rainfall threat this weekend.  An upper trough approaching
from the west late in the week will likely pull abundant deep
moisture into the area from the southwest with precipitable water
values possibly approaching or reaching two inches at some
locations Saturday and possibly extending into Sunday depending on
speed of the system.  In addition there continue to be indications
of an accompanying surface trough/embedded waviness that would
provide an added focus for rainfall.  Overall the
Saturday-Saturday night period has higher confidence for heavy
rainfall with timing differences becoming evident by Sunday
(00z/06z GFS slower than the 00z ECMWF and UKMET/CMC leaning
faster than the GFS/ECMWF average).  The upper trough should pass
overhead by Monday with flow aloft gradually trending
west-southwesterly as a deep trough develops over the central
Pacific by the middle of next week.  Expect any shower activity
during the first half of next week to be on the lighter half of
the spectrum with some land/sea breeze influence given persistence
of an ill-defined surface pattern.

In the shorter term, expect brisk winds from the east or southeast
around the periphery of east-central Pacific high pressure along
with a tendency for windward focus of showers.  Most guidance
continues to suggest that the ECMWF may be a bit overdone with the
eastward extent of heavier rainfall near the western islands from
now into Friday.


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