Area Forecast Discussion for Hawaii

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Issued: Mar 31, 2015 8:30 PM HST

Synopsis
A low level trough over the W half of the island chain will gradually dissipate tonight. The trades will return on Wednesday and become locally strong, with showers favoring windward and mountain areas through Friday. Enhanced moisture southeast of the state could bring increasing shower activity over the Big Island during the upcoming weekend.

Discussion
The cloudy skies and lingering showers across Oahu will continue to decrease through the rest of the evening hours as an upper level shortwave trough pushes further to the NE of the aloha state, and its associated dissipating low lvl trough moves down to the Kauai channel or maybe Oahu tonight. This trough is hardly recognizable on satellite and radar data, and should not bring any more significant impacts weatherwise across the area through the rest of this evening. Models show subsidence aloft behind the departing shortwave trough, which will help in bringing a more stable airmass across the area.

At 8 pm HST, radar and IR satellite data showed cloud coverage over Kauai gradually clearing out with only localized patches of low clouds lingering over interior areas. On Maui county, showers linger over leeward areas, but with accumulations remaining light. Some lingering showers can also be observed over the SE slopes of Mauna Loa in the Big Island, with the afternoon cloud build-ups now slowly dissipating.

Cloud motion on earlier visible and current infrared satellite imagery suggest that winds across the state are starting to shift to a more E flow, which will gradually strengthen from later tonight through Wed morning. By Wed afternoon, a rather robust trade wind pattern should establish with no sea breezes expected and shower activity shifting to windward and mountain areas. Only exception will be the leeward Big Island where onshore flow should bring afternoon cloud cover and a few showers. The returning trades will be supported by a broad sfc high pressure system far NNE of the island chain regaining strength in the wake of the upper shortwave trough.

The forecast philosophy for the rest of the work week remains unchanged with a trade wind weather regime expected to prevail through Fri. This solution is still supported by ECMWF and canadian models. Gfs, on the other hand, remains as the outlier by bringing another collapse of the trades starting late Thu and into Fri, while bringing a very moist airmass at the mid levels from the SE and S of the islands. But it seems that GFS is not handling well the evolution and eventual solution of a mid/upper trough approaching Kauai from the NW in that same time frame, bringing a much more aggressive rain event for the upcoming weekend. Thus, it seems reasonable to keep following the ECMWF solution, with perhaps a less aggressive weakening trend of the trades on Fri, and wait for upcoming model guidance before making any significant adjustments to the forecast philosophy for the weekend. But regardless of the final outcome, it is likely that enhanced moisture SE of the state will bring increasing showers across the Big Island Sat and Sun.

Aviation
The trend through Wednesday is for gradually rebuilding trade winds and an increasingly stable island atmosphere. Vfr conditions will prevail through the period, with overnight land breezes acting to clear most leeward areas. The returning trade winds will focus limited moisture along windward slopes and coasts, where isol MVFR vis/cig in SHRA can be expected.

Marine
Winds will remain below advisory criteria through Wed morning, then speeds will increase and likely reaching advisory levels by Wednesday afternoon over the typically windier waters around the Big Island and Maui county.

The current small northwest swell will dissipate by Wednesday. A small south-southeast swell will also be on a gradual decline over the next couple of days, followed by a small south-southwest swell for the second half of the work week.

Watches/Warnings/Advisories
None.


DISCUSSION...REYNES

AVIATION...BIRCHARD