FORECAST DISCUSSION

Synopsis

A strong ridge north of the area will keep trade winds locally breezy through Friday. Slight weakening of the trades is possible Friday night and Saturday as a weak front passes far north of the state, then a new high will build in to our north Sunday and Monday with trades strengthening once again. The trades will focus showers primarily over windward slopes. Some showers will spread leeward over the smaller islands, mainly nights and mornings. Afternoon showers are possible over the lee slopes of the Big Island.

Discussion

Tranquil trade wind weather prevails across Hawaii on this Thursday evening. Radar shows isolated light showers moving east to west within the trade flow, with rain gauge data showing almost all land stations remaining dry during the past several hours. Surface analysis depicts a 1030 mb high centered about 1400 miles NE of PHNL, with a ridge extending westward to the north of the state, and a cold front far to the NW near Midway atoll. Aloft, a mid/upper level ridge is centered a few hundred miles NE of the state, with a very dry/stable airmass over Hawaii. Mimic-tpw satellite imagery shows a generally dry airmass over and around the state, with estimated pwats around 1 inch. The 00Z soundings also show a dry and stable airmass in place, with pwats ranging from 1 to 1.2 inches and strong inversions based around 7500 to 8000 feet. Overnight, expect showers to increase, mainly across windward areas, as a band of clouds moves in from the east. While many windward areas will experience these passing showers, rainfall amounts should remain mostly light. The existing forecast appears to be in good shape, and no updates are planned for this evening. Little change to the ongoing trade wind weather pattern is expected through the next several days. Breezy trades will persist on Friday as the ridge holds firm to our north, then should weaken slightly Friday night into the weekend due to the weak cold front currently near Midway passing by well to our north. A new high is then forecast to build north of the state Sunday through Monday, which will cause trade winds to increase once again. Showers and clouds will continue to favor mainly windward sections, especially during the night and early morning hours. There will also be the chance for scattered showers each afternoon and early evening on the Kona slopes. Persistent ridging aloft with only shallow moisture available will keep most of the shower activity on the light side. As the surface high shifts slowly eastward to our north next week, models show moisture from the dissipated frontal remnants getting caught up in the trade wind flow and eventually moving SW across the islands. The latest GFS has come into better agreement with the earlier ECMWF run on the timing of this, showing the stripe of enhanced moisture reaching the state Tuesday afternoon and moving across the area through Wednesday. Will await additional model runs to confirm this faster timing before making any adjustments to the existing forecast.

Aviation

Moderate to fresh trade winds will focus clouds and light showers over the windward and mauka areas. Airmet tango remains in effect for tempo moderate low level turb over and immediately S through W of the mountains due to the trades. Conditions are expected to persist through tonight and possibly Friday as well. Tempo MVFR cigs may develop late tonight along windward slopes and coasts. An airmet for mtn obscuration may become necessary for east facing slopes late night or early Friday.

Marine

The late morning ascat pass along with more recent mesonet obs and high-resolution model output all suggest that small craft advisory /sca/ conditions are ongoing across the typically windy waters around Maui county and the Big Island. The SCA remains posted through 6 am Friday. Hi-res model output also suggests that the advisory may need to be extended for these windy areas through Friday and possibly into the weekend. Will see if the overnight ascat pass provides any additional information before making a decision on this. The Hilo, Pauwela and mokapu point buoys all show short period seas of 7-9 feet due to trade wind swell. This swell should decrease slightly over the next couple days as the high to the northeast weakens, then pick back up early next week as the next high builds north of the area. Small swells from the northwest and south are also in store through the rest of this week and into this weekend. Surf along south facing shores may increase Sunday and Monday as a swell from east of New Zealand reaches the islands.

HFO Watches/Warnings/Advisories

Small craft advisory until 6 am HST Friday for Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.

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DISCUSSION/MARINE...JACOBSON AVIATION...BEDAL

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