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Hawaii Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1215Z Mar 17, 2018)
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Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
815 AM EDT Sat Mar 17 2018

Valid 00Z Sun Mar 18 2018 - 00Z Sun Mar 25 2018

...A heavy rain/flash flood threat in the western islands next
Thursday and Friday...

The models forecast an inverted trough to move east to west across
the islands with scattered showers possible in the western
islands.  The trough passage is followed by high pressure building
to the north of the islands Sunday.  The anticyclonic flow
provides a temporary drying period and less shower coverage,
predominantly in windward terrain.

A transition occurs early in the week as the high pressure ridge
drifts east of the islands.  Low level flow becomes southeast to
south and low level moisture returns to the region. There is a
lack of large scale low level convergence so showers will continue
to be driven by terrain interactions.

A new system moves slowly in from the west/Papahanaumokuakea
Marine National Monument which has an associated cold front that
drifts slowly towards the western islands late next week and
weekend. Precipitable water values are forecast to rise into the
1.75-2" ballpark.  Low level convergence increases as the front
approaches, so showers should increase alter next week in the
western islands, with the gfs and ecmwf showing higher coverage
and amounts Thu and Fri as a result.  The high available moisture
which would lead to the possibility of high rain rates.
Considering wet conditions from recent rainfall, flash flooding is
a possibility.

The models and ensemble means are in better than average agreement
on the movement of the boundary through the end of next week.