Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Wed May 15 2024

Valid 00Z Thu May 16 2024 - 00Z Thu May 23 2024

...Heavy rain threat mid-late week and into the weekend...

In principle the latest guidance has not changed much for the
overall pattern evolution during the period, likely supporting a
multi-day heavy rain event extending from midweek into the weekend
followed by a return to more typical trade conditions next week.

Guidance consensus still shows an unseasonably deep/south upper
low settling just west of 30N 160W by Wednesday-Thursday, followed
by a gradual northwestward wobble through the weekend.  By next
week solutions diverge some for exactly how upstream energy may
feed into the overall upper trough.  The surface low associated
with the upper feature should push a front into the central
islands Wednesday-Thursday, with the surface boundary retreating
gradually westward thereafter as upper heights over the state
slowly rise.  The developing deep-layer southerly flow should
initially bring precipitable water values (PWATs) to at least
1.50-1.75 inches, with the best focus for heavy rain and
thunderstorms over the central islands Wednesday-Thursday. The
heavy rain axis may begin drifting back into the western islands
by Friday with that part of the state remaining the most likely
area to see heavy rainfall during the weekend as PWATs potentially
rise to at least 2.00 inches for a time.  Flash flooding will be a
threat during this multi-day heavy rainfall event.  Guidance
generally agrees that the axis of enhanced rainfall should retreat
beyond the western islands after Sunday.  Moderate east-southeast
trades next week should support shower focus over windward areas
as well as potentially other locations when background flow is
light enough. There is some spread for how much drier air may move
in from the east with the 00Z ECMWF/12Z ECMWF mean being more
pronounced in that regard than the 00Z GFS/GEFS. An intermediate
solution looks reasonable for both the period of heavy rain threat
and PWATs within the trade pattern next week.  Continue to monitor
forecasts over the coming days as guidance refines the specific
details.

Rausch