Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
814 AM EDT Sat Sep 14 2019

Valid 00Z Sun Sep 15 2019 - 00Z Sun Sep 22 2019

A retrograding upper low to the north of the state this weekend
into the start of next week will weaken the gradient to the south
of mid-latitude surface high pressure, leading to light/locally
moderate trades early in the period.  At the same time an area of
enhanced moisture will progress northwestward across the region. 
Thus expect some locally enhanced shower activity with focus along
windward areas in some cases and sea breeze boundaries where
background flow is weak.

Over the course of this coming week the best clustering of
guidance shows a drier trend for precipitable water and rainfall,
with an upper ridge moving into the region from the east.  The
combination of the initial northern upper low's departure and
possible development of low pressure well west of the main islands
may help to strengthen the trades a bit.  Around midweek the 00Z
GEFS mean has rectified its suspicious surface and upper level
forecasts from 24 hours ago but still brings more low latitude
tropical moisture across the state than other guidance, so would
recommend the GFS/ECMWF/ECMWF mean in principle.

By late week and early next weekend the most agreeable aspect of
the forecast is persistence of moderate trades, though differences
develop with specifics of mid-latitude high pressure.  However
guidance diverges considerably with energy aloft that may approach
from the northeast, with corresponding differences for moisture
trends and the source of moisture.  The 00Z ECMWF/ECMWF mean are
fast and west-southwestward enough to eventually pull up moisture
from the south by next weekend, while the GFS holds the energy far
enough east to result in a band of moisture reaching the area from
the north.  Such medium to smaller scale flow details have low
predictability 6-7 days out in time so confidence in any specific
solution is minimal.

Rausch