Marine Weather and Tide Forecast for Keokea, HI

DISCUSSION

The weak upper low that has lingered around the area during the last week is presently positioned northeast of the Big Island at 250mb and roughly over/immediately east of the island at 500mb where the center is analyzed at 583dm. The islands reside along the southern periphery of the attendant cold pool and observed 500mb temperatures are approximately -9C and -10C at Lihue and Hilo respectively as of 12 UTC this morning. This low will sag southeast and tend to deepen as the mid-latitude pattern eases southward during the next couple of days. At the same time, energy associated with a progressive mid- latitude trough will pinch off from the westerlies as it crosses 160W and close off in the vicinity of Kauai during Wednesday. The islands, residing between these two rather small scale lows, will be positioned below a narrow band of shortwave ridging aloft Wednesday into Thursday. Attendant height rises in the mid-levels may nonetheless be hard to come by as the overall geopotential height field remains depressed and even further deepens given that the weak low presently over/east of the Big Island is modeled to deepen to 579dm as it drifts eastward. As a consequence, little change in the thermodynamic profile is anticipated today into Thursday with the exception of the aforementioned upper-level ridging helping to quash afternoon thunder potential over the Big Island. Moderate to locally breezy trades will remain weakly stable during this time with pockets of heavier showers developing where low-level convergence is maximized, mainly Leeward Big Island along the sea breeze each afternoon. Here, nuisance flooding is not out of the question.

By Thursday, lapse rates steepen to 6.5 to 7.0C/km over the western end of the state, the GFS being notably colder and unstable aloft compared to the EC which settles the low further west. Regardless of the model differences, ridge amplification east of the Dateline will cause trades to strengthen during this time limiting the potential for areas of enhanced convergence to emerge anywhere except Leeward Big Island. As a result, expect some variety of moderately unstable trades to develop no later than Thursday and continue through at least Friday night. From a sensible weather perspective, this suggests quick-moving/transient trade wind showers some of which may be heavy, especially at night and during the morning. The potential for thunderstorms over leeward/interior portions of the Big Island on Thurs/Fri will be evaluated going forward as this area stands the greatest chance to experience organized heavy rainfall as diurnal convection develops each afternoon.

AVIATION

Trade winds will return to a more easterly direction tonight and strengthen into the moderate to locally breezy range from Tuesday onward as the ridge builds in north of the state. Expect passing showers mainly over windward and mountain areas and along leeward convergence cloud bands through Tuesday, with brief MVFR conditions possible within any shower bands. No AIRMETs are currently in effect.

MARINE

Trades will persist through the weekend, with the strongest winds expected late Wednesday and Thursday. Currently, a 1032 mb centered nearly 1400 nm northeast of the state is driving moderate to fresh trade winds. Surface low pressure will form far north of the islands tonight and push the surface high eastward on Tuesday. Meanwhile, another area of high pressure from the west Pacific will move into the central Pacific, then stall far north of Hawaii on Wednesday into Friday. As the new high moves in, expect trade winds to increase to fresh to strong levels on Wednesday or Thursday, when a Small Craft Advisory (SCA) will be needed for the typically windy waters around the Big Island and Maui. The high is forecast to retreat westward during the weekend, favoring a decrease in winds.

Surf along south facing shores will remain just below seasonal average into Saturday as pulses of south-southwest and southwest swell move through. Surf will decline Sunday and remain small into early next week.

East shore surf will remain near or just below seasonal average during the next couple of days, followed by a slight increase Thursday and Friday as trade winds build east of the islands. A gradual decline in east shore surf is expected during the weekend and early next week. Tiny surf will prevail along north facing shores this week.

HFO WATCHES/WARNINGS/ADVISORIES
None.