Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Fri May 07 2021

Valid 00Z Sat May 08 2021 - 00Z Sat May 15 2021

Guidance agrees that an upper low will be centered near the
western part of the main islands at the start of the period,
supporting the potential for areas of heavy rainfall and possibly
thunderstorms.  The upper low will open up and shear out to the
northeast during the weekend so expect a lighter trend for
rainfall that should extend into next week as lower precipitable
water values move in.

By mid-late week models and ensembles are showing the development
of another upper trough with a possible embedded low, within an
axis from well northeast of the state to east of or over the main
islands.  Model ensemble spread is great enough to yield fairly
low confidence in resolving specific details that will be
important in determining the levels of moisture and stability over
the state.  ECMWF runs have been fairly consistent in showing the
upper trough axis over the state for a period of time and the new
00Z run even closes off a compact upper low over the islands. 
This leads to the 00Z ECMWF maintaining more moisture over the
region than other guidance.  The 00Z CMC mean and 12Z ECMWF mean
both offer some support for the general ECMWF axis aloft while the
operational CMC and GFS/GEFS mean are farther east to varying
degrees and so they are drier. The low predictability of details
in this type of evolution would recommend an intermediate
solution, tilted a bit in the direction of the ECMWF/CMC ensemble
means.

Expect at most moderate trades from the weekend into next week
with the state in the far southwest periphery of eastern Pacific
high pressure and remaining fairly far away from a central Pacific
high that tracks northeastward.  In contrast to the spread for
details aloft later next week, guidance provides a more agreeable
depiction of strengthening high pressure north of the islands by
Thursday-Friday and in turn a stronger trend for trade flow. 

Rausch