Issued: Dec 17, 2014 4:00 AM HST
A surface high northeast of the main Hawaiian islands will continue moving slowly eastward, which will allow trade wind speeds to weaken slightly later today and tonight. The trades will strengthen again starting Thursday as a new surface high passes north of the state. Moisture riding in on the trades will bring showers mostly to windward and mauka sections. Trade winds are expected to weaken and shift to a southeasterly direction this weekend ahead of an approaching front.
The relatively tight pressure gradient south and southwest of a 1026 mb surface high centered near 29°N 152°W, or about 650 miles northeast of Honolulu, is maintaining moderate trades across the main Hawaiian islands early this morning. Aloft, mid- to upper-level ridging is maintaining stable atmospheric conditions across most of the region. As a result, the low-level trade wind flow is transporting scattered to broken low clouds with brief light showers mainly into windward and mauka sections of the state based on loops of satellite imagery and radar reflectivity data. The trades are also carrying a few light showers over to leeward sections of some of the smaller islands.
The surface high is moving slowly eastward, so expect the gradient to gradually relax later today. This will cause a slight drop in trade wind speeds. At the same time, a surface front, which is about 700 miles northwest of Kauai, will shift eastward to a position far north of the islands. As this front weakens tonight, a new surface high will push eastward to a position about 700 miles north of Honolulu Thursday morning. This will reestablish the pressure gradient across the region, so expect a slight increase in trade wind speeds from Thursday into Friday.
A rather typical coverage of low clouds and showers carried by the low-level trade wind flow is expected to continue through tonight. This should result in a rather typical weather pattern of mainly night and morning showers falling over windward and mauka sections of the aloha state. With the stable atmospheric conditions continuing due to mid- and upper-level ridging over the area, expect overall rainfall totals and rates to remain relatively low. The forecast models indicate a drier air mass will move across the state late Thursday, which will likely continue into the weekend.
By this weekend, the high north of the state is expected to shift eastward, which will weaken the pressure gradient over the area. As the high continues moving eastward, the trade winds will not only weaken, but veer out of the east southeast or southeast. Note that the southeasterly flow may allow some volcanic haze from sources on the Big Island to be carried up over portions of the smaller islands this weekend. The light winds and relatively dry air mass will likely produce an inactive diurnal convective pattern this weekend. This weather pattern will feature some afternoon clouds mainly over leeward and interior sections of the smaller islands, but minimal shower coverage is forecast this weekend.
In the longer range, the forecast models are showing some big changes in the local weather pattern. Currently, the guidance shows a strong cold front will push in from the west northwest from late Sunday into Monday over the western end of the main Hawaiian island chain. Beyond that time period there were some significant disagreements in model output. The most recent run of the ECMWF model no longer stalls the front over the islands. It is now much closer to the GFS solution, which continues to show the cold front will push down the entire island chain early next week. We will continue to monitor the model output over the next couple of days to see if this convergence toward a consensus has taken place yet or not. In the meantime, there remains some uncertainly in the long-range forecast as we head into the holidays next week.
High pressure to the northeast of the islands will maintain moderate trades. Clouds and showers will continue to ride in on the trades, mainly affecting windward and mauka areas, and will at times bring MVFR conditions to those areas. Otherwise VFR conditions are expected to prevail.
A small craft advisory /sca/ remains in effect for the typically windy waters adjacent to the islands of Maui and Hawaii counties today. Trade wind speeds are forecast to briefly drop below the SCA criteria this evening. A SCA will likely be needed again for the normally windy marine zones from Thursday into Friday.
The circulations associated with a series of lows moving across the north Pacific ocean will produce northwest swells aimed at the main Hawaiian islands into early next week. A surface low developing near the international dateline around latitude 40°N is expected to send a long-period northwest swell toward the islands on Thursday. This swell is forecast to produce surf near the high surf advisory criteria along most north and west facing shores of the smaller islands from late Thursday into early Friday. A more powerful low east of Japan will send an even larger long-period swell toward the islands. Surf produced by this swell will probably reach the high surf warning thresholds along most north and west facing shores of the islands on Sunday. Note that the southern extent of the fetch area may push large breakers into the leeward coast of the Big Island. As a result, there may be more impacts on the west side of the Big Island compared with recent swells. This swell will also likely increase seas to near the SCA criteria across most of the coastal waters starting Sunday.