Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
815 AM EDT Fri Aug 25 2017

Valid 00Z Sat Aug 26 2017 - 00Z Sat Sep 02 2017

Surface high pressure north of the state is forecast by all
guidance to gradually weaken over the next few days as a broad
area of negative height anomalies develops across the north
central Pacific. Next week, models show general agreement that
ridging will reestablish northeast of the Hawaiian Islands,
resulting in a broad mid/upper-level blocking pattern across the
north and central Pacific. The weakening surface high north of the
Islands should result in a slight weakening of the trades through
early next week. A relatively weak tropical wave is expected to
pass south of Hawaii Sunday into Monday, which should result in an
increase in precipitable water values across the state. The ECMWF
keeps the deepest moisture (PWs > 2 in.) a bit farther south than
the GFS, primarily affecting the Big Island. The GFS spreads 2+
in. PWs farther north, affecting the majority of the Hawaiian
Islands. Thus, expect the potential for an increase in shower
activity, especially across the Big Island by Sunday/Monday.

In the wake of this tropical wave, easterlies should increase a
bit (to near 20 kt), as the ridge reestablishes itself northeast
of Hawaii through midweek. Dry air advecting from the subtropical
ridge Northeast of Hawaii will once again lower PWs and likely
result in drier than average conditions. By late next week, the
digging trough across the north central Pacific will push a cold
front southeastward, remaining well northwest of Hawaii through
Friday. The ECMWF develops a relatively significant wave along the
front several hundred miles northwest of Hawaii, while the GFS
depicts a broad/less wavy frontal zone. The existence of this
frontal wave may have an effect on whether easterlies remain
dominant through the end of next week, or whether low-level flow
takes a more east-southeasterly component, which would likely
increase moisture and precip chances if that occurred.

Ryan