Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
725 AM EST Wed Feb 28 2018

Valid 00Z Thu Mar 01 2018 - 00Z Thu Mar 08 2018

To commence the forecast period, a broad upper trough centered
just east of the International Dateline will allow a series of
weak perturbations to lift northeastward toward the Hawai`ian
island chain. This particular setup will focus a steady period of
strong low-level east-southeasterly flow with sustained winds in
the 25 to 30 knot range at times. A broad area of showers should
accompany this regime with precipitable water values likely
exceeding 1.50 inches. However, the focus of this activity should
drift westward in time as the north/south oriented inverted trough
is expected to retrograde in time. Consequently, the surface winds
will turn more easterly while weakening in time given a reduction
in the synoptic-scale pressure gradients. Some lingering chances
for showers will continue although the more concentrated activity
is forecast to stay west of the islands.

Toward the conclusion of the work week, this elongated trough axis
should evolve into more a closed off feature while lifting
north-northwestward into the mid-latitudes. By early next week,
the pattern shifts toward an omega block with an upper ridge
centered over the middle of the Pacific. The corresponding
negative height anomaly should dominate generally east of 150W
longitude. This will eventually place the western extent of the
mean positively-tilted longwave trough over Hawai`i. While model
spread began on the lower end, there are a series of detail
differences within this band of height falls which suggests a more
ensemble-based approach for much of next week. At this point,
there is the greatest overlap between the 06Z GEFS/00Z ECMWF
ensemble means. With this shift in the pattern, the prevailing
surface winds will turn more east-northeasterly by late Sunday as
low pressure forms northeast of the island chain. This is forecast
to continue through much of the week with winds picking up at
times given the 1040-mb anticyclone positioned over the central
Pacific. Ultimately it will be a drier period given the influx of
lower precipitable water air into the region.


Rubin-Oster