Issued: May 22, 2013 8:30 PM HST
A pair of surface troughs north of the islands are keeping a light wind regime across the islands through Thursday. Sea breezes will likely bring more afternoon showers on Thursday with best chances of rain over interior and mountain areas. Light trades will begin establishing late Thursday, then a more typical trade wind weather regime will return for the upcoming weekend.
Latest wv satellite data shows an upper lvl trough low located some 800 miles N of the main Hawaiian islands, while sfc analyses show two decaying troughs within 300 miles N and NW of the island chain. Meanwhile, a sfc high remains centered far to the NE of the islands. This synoptic scenario is keeping generally light winds across the area with remnant convection from earlier sea breeze activity still lingering over central Oahu, leeward Haleakala in Maui, and the SE slopes of Mauna Loa over the Big Island. Most of this convective activity should clear out after midnight as land breezes intensify. However, with latest soundings showing pwats around one and a half inches, enough moisture will remain in place tonight for some isolated showers to linger into the overnight hours.
The aforementioned sfc troughs will continue to decay on Thu, but their influence will still keep weak gradients over the islands. Thus, the expected return to a trade wind pattern will be slow with weak ne/e flow starting to establish Thu afternoon, and gradually intensifying towards the end of the week. Therefore, another round of afternoon sea breezes is likely again Thu afternoon and evening with showers favoring interior and mountain areas. As the troughs dissipate and high pressure strengthens NE of the islands, a more typical trade wind weather regime will return for the upcoming weekend. Gfs and ECMWF show fair agreement in this overall scenario with upstream moisture feeding windward and mauka showers as the trades establish.
Partial clearing should occur over the islands while the coastal waters may see an increase in low clouds overnight. Hence, prevailing VFR conditions are expected at all taf sites, though may see some local MVFR vis conditions in the vicinity of PHTO due to volcanic emissions, and brief MVFR ceilings are possible over coastal waters.
The south swell will keep decreasing and remaining below advisory criteria during the next several days. A northwest swell is near its peak and will last into Thursday before gradually diminishing.
Trade winds will strengthen through the week, and are expected to reach small craft advisory levels over the windier locations near Maui county and the Big Island by the weekend.