Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Tue May 14 2024

Valid 00Z Wed May 15 2024 - 00Z Wed May 22 2024

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

The forecast remains on track for an unseasonably deep/south upper
low to settle just west of 30N 160W by Wednesday-Thursday,
followed by only a slight north or northwest drift thereafter. 
Consensus of latest guidance continues to show the associated
deepening surface low near the upper center pushing a front as far
east as the central islands mid-late week, then retreating
gradually westward as upper heights over the state slowly rise. 
Expect the developing deep-layer southerly flow to bring an
initial increase of 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 mid-late week.  The
dominant clustering of dynamical and machine learning guidance
still depicts a shift back to the western islands by the weekend
with additional moisture streaming up from the south to bring
PWATs up to at least 2.00 inches for a time.  Flash flooding will
be a threat during this multi-day heavy rainfall event.  By
Monday-Tuesday most solutions suggest that the surface boundary
should drift a little farther west while the moisture/rainfall
axis becomes less pronounced, leading to a drier trend over the
main islands.  The one model diagnostic comparison of note is that
from Sunday or Monday onward the 00Z GFS flattens the southeast
side of its upper trough/builds in more ridging from the east
versus most other dynamical models/means and machine learning
models.  As a result this GFS run may be a little quick with its
eventual westward shift of the heavy rain axis.  The prior 18Z GFS
compared better into early Monday but then also started to stray
from the majority somewhat.  Continue to monitor forecasts over
the coming days for any potential adjustments in specific details.

Rausch