Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Wed May 12 2021

Valid 00Z Thu May 13 2021 - 00Z Thu May 20 2021

Guidance is consistent in maintaining the trade pattern through
the next week along with below-climatology precipitable water
values that should keep windward/mountain-focused showers
(possibly straying at times to some leeward locales) on the light
side.  There is still some late-period spread and day-to-day
variability for the upper low expected to settle northeast of the
state from Thursday through at least the weekend.

As for the specifics, the models and ensemble means generally show
the trades strengthening between now and Friday with fairly brisk
trades persisting the rest of the period as a western-central
Pacific surface high ultimately replaces the high initially
northeast of the state.  Meanwhile guidance is also similar in
showing a gradual decrease of precipitable water values during the
latter half of this week and remaining at below normal values into
the first half of next week.  For the upper low settling to the
northeast of the state Thursday onward, consensus is good into
Saturday.  Then guidance diverges from late weekend into next
week.  GFS runs continue to eject it eastward faster than other
guidance and the GEFS mean is not far behind.  Interestingly the
00Z CMC mean and 12Z ECMWF mean retrograde the upper low while the
00Z ECMWF/CMC/UKMET all show a gradual drift to the east. 
Compared to other guidance the CMC mean looks particularly
questionable with its strength/longitude of the upper ridge to the
north, while better compatibility of the 12Z ECMWF mean with other
guidance at mid-latitudes raises the question of whether its upper
low retrogression is overdone.  Thus would ultimately favor the
00Z ECMWF/CMC/UKMET for a specific forecast.  Sunday-Monday still
looks to be the time frame when there could be some rainfall
enhancement as an upper trough elongates southwest from the upper
low, but the prevailing below normal moisture should keep totals
fairly modest.      

Rausch