Issued: Jul 04, 2009 4:00 AM HST
Synopsis
Trade winds will gradually increase across the state through the weekend as high pressure strengthens to our north. Clouds and showers will favor windward and mauka areas, with only light total rainfall amounts expected.
Discussion
Water vapor imagery shows an upper low northeast of the islands, with a trough extending southwestward across the state. This trough has helped to weaken inversion heights, but 12Z soundings from Hilo and Lihue show the inversions strengthening at around 6-8kft. The Hilo sounding has warmed up to 5C between 800mb and 500mb, a sign that the upper trough is already beginning to move westward. The poes amsu total precipitable water sensor shows drier areas upstream to the east northeast of the state. It also shows a larger area of high pw' s moving toward the Big Island from the southeast, which appears to be associated with high-level moisture' clouds on the east side of the upper trough.
The GFS and ECMWF are pretty similar showing the upper trough continuing to move westward across the state, as an upper level high builds in from the east. As heights builds, the surface high far north of the state will strengthen and become more of a dominant feature across the central Pacific. By tuesday' wednesday, it takes on a position northeast of the state that is more typical of the summertime season.
Surface pressures across the state have shown steady 1-1.5mb pressure rises through the night. Trade winds will continue to increase through early next week, and remain at moderate to fresh levels through the end of the week. The GFS shows an increasing amount of low-level moisture getting picked up in the trade flow. However, inversion heights will strengthen and lower under the building mid' upper level ridge, keeping rainfall amounts limited.
Fire weather
Limited rainfall from the drier trade wind pattern will continue over the holiday weekend. Minimum relative humidity values today will be similar to or slightly higher than Friday, thanks to an area of clouds' moisture moving across the state. Rh values will be around 50 across leeward sections, above the red flag threshold of 45 percent. Wind speeds and the kbdi also remain below critical values.
Marine
Wave heights at the kilo nalu buoy have continued to slowly lower through the night. 06Z wavewatch iii shows this diminishing trend continuing through the weekend. Surf along south facing shores will remain below advisory levels on Saturday, and diminish further Sunday and Monday.
As the surface high builds across the eastern Pacific through early next week, the broad fetch of stronger winds will generate a moderate short period easterly trade swell. This swell will be greatest mid' late week. With the stronger pressure gradient, winds may also reach small craft levels by mid-week, initially across the windier channels.
Aviation
A more typical trade wind cloud' shower pattern is expected today. Vfr conditions will again be the rule, with the typical exception of mvfr at phny due to the higher elevation of the airport. Showers may drop ceilings down to mvfr across windward sections, but they will be brief and passing.