Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
715 AM EST Tue Nov 21 2017

Valid 00Z Wed Nov 22 2017 - 00Z Wed Nov 29 2017

Models show the large scale pattern across the central/northern
Pacific changing over the next week. This will occur as a Rex
Block which has been persistent for weeks well west of Hawai'i
breaks down. Deterministic model solutions as well as ensembles
show good agreement on this scenario. An elongated area of
negative 500 hPa height anomalies has been persistent from Hawai'i
northeastward toward the Gulf of Alaska over the past week
downstream of the Rex Block. This elongated trough is expected to
gradually weaken over the next week in response to the weakening
block.

At the surface, high pressure is forecast by all guidance to
remain anchored north of the Islands through the weekend before
drifting eastward early next week as a weakness in the ridge
develops over the central Pacific, in the vicinity of Hawai'i.
Northeasterly surface flow of 10-15 kt will gradually veer to a
more east-northeasterly direction and strengthen to 15-25 kt
through late this week and into the weekend as the surface high
gradually shifts east. As long as the surface ridge remains north
of the Islands it will remain a source of relatively dry air, and
precipitable water values should remain near or just below 1 inch
through Saturday. This should keep shower activity relatively
sparse and confined to windward terrain. From Sunday onward, Pwat
values should increase as low-level flow changes in response to
the developing surface weakness, pulling higher values (>2 inches)
northward across the eastern Islands. Models differ a bit with
respect to this moisture return, and the ECMWF is more aggressive
with increasing Pwats early next week (and thus shows greater
potential for heavy rainfall by that time. EC ensemble
probabilities show a significant degree of uncertainty here as
well, but the potential exists for heavy rainfall by early-mid
next week across the Big Island if sufficient moisture return is
able to occur.

Ryan