SynopsisWet trade wind conditions will persist across most of the state today. A cold front approaching the region from the northwest will cause our winds to weaken and veer from the southeast starting later today, and continuing through Friday. This will allow sea breeze convergence to trigger cloud build ups and locally heavy afternoon showers across island interiors Thursday and Friday. The front is expected to weaken to a surface trough as it reaches Kauai Friday. Locally breezy northeasterly trade winds will fill in behind this trough as it moves across the island chain this weekend.
DiscussionA 1031 mb surface high located near 36°N 142°W has a ridge extending west-southwest to a point about 450 miles north of Lihue. The tight pressure gradient south of these features is maintaining locally breezy trade winds across the state this morning. Mid- level ridging in the vicinity of the islands is providing relatively stable atmospheric conditions. However, the morning soundings from Lihue and Hilo show slightly elevated trade wind inversions of 8 to 9 thousand feet, with precipitable water values of 1.3 to 1.5 inches. The current satellite imagery and radar reflectivity data show scattered to broken low clouds and scattered showers associated with an old front in the vicinity of the islands. Most of these clouds and showers are affecting the windward sides of the islands.
This area of low clouds and showers is expected to sag slowly south and west today. The breezy trades will keep the showers moving across the windward and mauka areas today. A few of the showers will move over to leeward sides of the smaller islands, especially this morning.
The trade winds are expected to begin to weaken from late this afternoon through tonight as a cold front, which is currently about 1050 miles west northwest of Kauai, approaches from the northwest. This will cause the surface high and its associated ridge to weaken, and make the pressure gradient relax over the state. As a result, the surface winds will weaken and veer out of the southeast from later today through Friday. This weaker southeasterly wind regime will allow local afternoon sea breeze and nighttime land breeze circulations to develop over leeward and interior sections of most islands. Somewhat enhanced moisture values and slight atmospheric instability are also expected along and ahead of the front. The convergence of local sea breeze circulations over the islands will produce convective afternoon cloud build ups and scattered showers. Some of these showers may produce locally heavy rainfall across island interiors, mainly during Thursday and Friday afternoons.
The forecast models appear to agree that the front will weaken to a surface trough as it reaches Kauai Friday. As this trough moves across the island chain, northeasterly trades will spread from west to east across the state this weekend. These northeasterly winds may be locally breezy, especially over the western end of the state on Saturday. The winds will eventually weaken to moderate breezes from Sunday into next week. The remnant moisture associated with the former front/trough will also likely produce an extended period of wet trade wind conditions from Sunday into the middle of next week. Both the long range GFS and ECMWF models indicate the trades will focus showers mainly over the windward sides of the islands from Oahu to the Big Island.
AviationA surface ridge about 745 nm north of the state will maintain an easterly trade wind flow today. Mid level ridging is maintaining an inversion in the 7000 to 9000 ft range, which will keep the atmosphere somewhat stable, but a band of moisture moving along the trade wind flow will affect windward areas through much of the day. The band will shift west, likely affecting Kauai and Oahu tonight.
Airmet sierra for mtn obsc was expanded from Kauai to cover Oahu Molokai and Maui. This airmet will likely be extended into the afternoon and may need to be extended into tonight for Kauai and Oahu.
Airmet tango for low level turb remains in place to the lee of all islands. Radar VAD winds are showing primarily 20 kt or less below the inversion, but recent aircraft soundings out of hnl and ogg showed some 25 kt winds just below the inversion. Given these border line conditions, we will let airmet tango continue for now, but in absence of observations and an expectation of weakening winds, it may come down later today.
MarineA small craft advisory /sca/ remains posted for the typically windy waters adjacent to the islands of Maui county and the Big Island today. The winds are expected to decrease and veer out of the southeast by tonight. These weaker winds will persist through Friday. The forecast models continue to show increasing northeasterly trade winds spreading from west to east over the area this weekend. These winds may briefly reach the SCA criteria near Kauai on Saturday.
A small long period west-northwest swell has arrived at buoy 51101 northwest of Kauai, while forerunners from this swell are showing up at the Waimea bay and Barbers Point buoys. This swell will continue to fill in across the Hawaiian waters today, and will persist through Friday. A reinforcing north-northwest swell will arrive Saturday and continue into early next week. Small south swells will maintain background surf along south facing shores through Thursday. A long period south swell will arrive Friday and peak late Saturday above the seasonal south swell average, and then slowly subside into early next week. While surf will be elevated, surf heights are expected to remain just below the high surf advisory criteria along south facing shores this weekend. Along east facing shores, the breezy trade winds will continue to produce choppy surf today, then taper off through the end of the week as the winds weaken and veer out of the southeast.
HFO Watches/Warnings/AdvisoriesSmall craft advisory until 6 pm HST this evening for Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.