Area Forecast Discussion for Hawaii

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Issued: Sep 22, 2014 4:00 PM HST

Moderate trade winds will focus clouds and modest showers over windward areas during the next couple of days. Aside from the Kona slopes, leeward areas will be rather dry and hot, though humidity will be down compared to the past several days. A wet trade wind pattern will develop late Wednesday and Thursday and will persist through the weekend.

Moderate trade winds are back and will persist into the weekend. Island surface pressure readings are up an average of around two millibars from this time yesterday as the pressure gradient tightens across the region. The resurgent trades are being driven by surface high pressure sitting about 1000 miles to the northwest of Kauai. The high will remain in place for the next day then move slowly eastward through the rest of the week. As a result, trades may increase slightly on Tuesday and again over the northern islands on Thursday, followed by a decrease heading into the weekend.

The returning trade winds have brought some relief from the recent humid conditions. A drier air mass ushered in on the trades contains dew points a few degrees down from what we experienced recently, leading to a slight decrease afternoon relative humidity compared to yesterday. Even though leeward areas will remain seasonably hot, around 90, during the next couple of days, the slight drop in humidity along with the moderate trades will continue to provide some relief.

Through Wednesday, modest shower activity will be focused over windward areas, as well as the Kona slopes each afternoon. Although a well-defined inversion is absent in the afternoon soundings, mid level ridging is producing stable conditions, and with no significant features observed in the trade wind flow immediately upwind of the islands, windward rainfall will be modest while leeward portions of the smaller islands remain dry.

An increase in shower activity in the trade wind flow is due during the second half of the week. A band of moisture associated with remnants of a front will drift southward and stall over the islands Wednesday night and Thursday as the surface high passes by to the north. Moisture from the band will linger over portions of the state through Sunday, providing enhanced windward rainfall and likely spreading some showers to leeward areas. A mid to upper level trough is expected to move near the state from southeast Friday and Saturday, and this could trigger some heavy showers on the band. For now, the forecast remains broad brushed with higher pops and no mention of heavy rainfall. The details will become clearer over the next couple of days.

Trade winds will continue to strengthen through Tuesday. Clouds and showers will be focused primarily across windward and mauka areas, with brief MVFR conditions occurring in showers. Gfs forecast soundings show low-level winds approaching 25 knots briefly overnight. The inversion is expected to strengthen further tonight, and conditions will be increasingly conducive for low-level turbulence in the lee of the islands. If these conditions develop, we may need to issue an airmet for moderate low-level turbulence for late tonight/tuesday.

Trades will be at border line small craft advisory levels across typically windy waters around the Big Island and Maui through at least Friday. A recent ascat pass confirmed these conditions, and little change is expected in the short term. Winds may briefly increase a notch over the northern half of the state as a band of moisture stalls over the central portion of the island chain on Thursday.

Surf will remain around background levels through much of the week. A slightly higher swell, possibly up to 4 feet, is expected from the NNE Thursday through Saturday. A moderate SSW is due this weekend.