FORECAST DISCUSSION

Synopsis

Trade winds in the light to moderate range will allow leeward sea breeze development this afternoon, resulting in an increase in leeward shower activity later today. A mid-level trough will shift west of the state tonight, giving high pressure to our north a tighter grip on trade flow and generating breezy and locally windy conditions Tuesday through Thursday. Drier and more stable conditions are expected Monday through Thursday, but a more showery trade wind pattern will return late in the work week as an upper level low approaches the islands from the east.

Discussion

A 1033 mb high far north of the main Hawaiian Islands is driving light to moderate trade winds across local waters this morning. Satellite loop shows a plume of broken to overcast low clouds extending west from the Big Island across waters south of the smaller islands, while patchy broken low clouds embedded within trade flow lie just east of the islands. Radar shows very little shower activity over the islands, with scattered showers noted across waters south of the islands within the Big Island plume. Overnight soundings show a neutral to slightly unstable airmass, with less than 1.2 inches of PW. Trade winds are light enough to allow development of afternoon sea breezes across leeward areas today. We expect trades will continue to push showers across mainly windward areas through the forecast period, but this will be augmented by sea breeze- generated leeward showers this afternoon. A mid-level trough will exit to the west of the state over the next few days, strengthening the hold the high to our north has on our trade flow. We expect trades to increase beginning tonight and Monday, then reach breezy to locally windy levels Tuesday through Thursday. In the short run, there may be more windward shower activity tonight due to the convergence associated with the returning trades. However, by Monday and continuing through Thursday, models show a drier and more stable airmass will move into the area. Models show an upper low will track eastward toward the islands Thursday night and Friday, then lift northward and away from the islands for the first half of next weekend. High pressure will persist north of the islands through the period, with breezy trade winds expected to continue through at least Thursday night. Trades are then expected to weaken Friday through Saturday as the upper low approaches from the east. We expect deep layer moisture will increase beginning Thursday night and remain over the islands through next Saturday. As a result, we should see wetter trades by the end of the work week, with showers continuing to favor windward and mauka areas.

Aviation

Light to moderate east to east-southeast background flow around the state has allowed land breezes to clear out most of the land areas early this morning. The light flow will also allow for a hybrid trade wind/sea breeze pattern to set up today. Leeward and windward areas should see cloud build up this afternoon. Some showers this afternoon may introduce brief periods of MVFR CIGS/VIS. There are no AIRMETS in effect.

Marine

Gentle to moderate trade winds will gradually increase later tonight and Monday as the surface ridge strengthens well north of the state. Fresh to locally strong trade winds will then prevail through much of the upcoming week, with a Small Craft Advisory (SCA) expected for the typically windy areas around the Big Island and Maui County. A small increase in short-period trade wind swell is due this week, though a more noticeable increase will come from a NE swell due to build later today through Monday. This swell was produced by a fetch of gales off the Oregon and California coast and will have periods of around 12 to 13 seconds. This creates some difficulty in sorting out the arrival of the swell at the NOAA buoys NE of the islands, since the the ongoing SSE swell is characterized by similar wave periods. We will monitor the NOAA and PacIOOS buoys through the next couple of days, as there is a low probability that this swell could potentially produce east surf near the advisory level on Monday and Tuesday. Some of this swell energy will also wrap into exposed north facing shores. Pulses of swell from the SSE and at times from the SSW will produce south shore surf near to just below summer average during the upcoming week. Extreme tides that have been observed over the past several days will trend back toward normal into the upcoming week. As a result, flooding impacts along the coast will diminish.

HFO Watches/Warnings/Advisories

None.

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DISCUSSION...Powell AVIATION...Eaton MARINE...Wroe

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