Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
704 AM EST Wed Nov 22 2017

Valid 00Z Thu Nov 23 2017 - 00Z Thu Nov 30 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 10-20 kt will gradually veer to a
more easterly direction and strengthen to 15-25 kt over the next
few days 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 (near 2 inches) northward across
the eastern Islands. Models differ a bit with respect to this
moisture return, and in contrast to yesterday, the GFS is now more
aggressive than the ECMWF with increasing Pwats early next week,
and thus shows greater potential for heavy rainfall by that time.
Despite the wavering shown by the deterministic ECMWF with respect
to moisture return early next week, EC ensemble probabilities
suggest somewhat increased confidence for heavy rainfall, with
48-hour probabilities (ending 00Z Wed) of exceeding 2 inches of
rain having increased to near 30 percent across the windward
terrain of the Big Island. The GFS and ECMWF are in agreement that
another push of dry air will make its way across Hawai'i by
Wednesday, bringing an end to any threat for heavy rainfall.

Ryan