Prognostic Meteorological Discussion
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FXHW01 KWNH 201218
PMDHI

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Fri Apr 20 2018

Valid 00Z Sat Apr 21 2018 - 00Z Sat Apr 28 2018

Showers across Hawai`i will begin to decrease in coverage and
intensity by tonight as the deepest moisture retreats south of the
islands and an upper trough which has enhanced shower activity
moves east. From Sat and into next week, heights will quickly rise
across Hawai`i as an upper ridge expands west of the state. This
will allow much drier air (PWs near or below 1 inch) to spread
across the state, significantly reducing the coverage of any
windward showers. Low-level easterlies should also increase into
the 20-30 kt range by mid-week as the surface pressure gradient
tightens a bit. Models/ensemble show general agreement that this
drier pattern should persist into late next week, before a
northern stream Pacific trough amplifies northwest of the state.
Models show the amplifying trough pushing a cold front and
pre-frontal surface trough toward Hawai`i by Friday. Solutions
vary on timing but at this time an ensemble mean approach is
preferred. The system should begin to disrupt the low-level
easterlies by Friday, with moisture/PWs increasing as well. With
the best height falls remaining north of Hawai`i, however, models
are not overly aggressive with increasing shower activity in
association with this system.

Ryan


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