| Current Conditions | ||
| Temp |  | 9.4 C |
| RH | | 12 % |
| Wind | | NNE 19 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 499 FXHW60 PHFO 191943 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 943 AM HST Mon Aug 19 2019 .SYNOPSIS... A building ridge north of the state will maintain a trade wind pattern through the upcoming weekend. Showers will be focused over windward and mountains areas in this flow. Light to moderate trade winds will persist with periods of moderate to locally breezy trades in the middle of the week and again for the weekend. && .DISCUSSION... The overnight analysis depicts a 1017 MB high pressure system about 575 miles northwest of Kauai, with a building east to west oriented ridge. There is good model agreement with the weather pattern near the islands into the weekend. The surface ridge to the north strengthens, allowing the trade winds to rebound to moderate to locally breezy levels Tuesday night and Wednesday. Some weak troughing over the state could weaken winds a bit Thursday and Friday, but expecting winds to rebound again this weekend. The background trade wind flow will focus shower activity over the windward sides of the islands. Models show various batches of moisture riding in on the trades through the week, so there will be at times periods of increased shower activity. However, despite the periods of increased activity, it should primarily be more typical trade wind showers versus the drier than normal activity we have seen as of late. Towards the end of the week, moisture from an old front to the northeast of the islands will move towards the islands. The models have thus far been pretty consistent with the bulk of that moisture staying north of the islands, with some of it skirting Kauai and perhaps Oahu. Currently the forecast reflects a more typical trade wind pattern, but as we get closer will make adjustments based on the higher resolution models. Longer term, we are seeing model differences and run to run differences primarily with the GFS. The current forecast maintains the trade wind flow for the start of the new week. The forecast remains on track so no changes to the forecast at this time. && .AVIATION... Once again, an elongated surface high to the north of the state will act to maintain weak to moderate east northeast trades through this afternoon. Conditions will be rather dry except for isolated showers that develop either on the windward side of the islands or over the mountain ridges. After sun down, showers will increase in coverage along the windward slopes, possibly leading to MVFR conditions at times. No AIRMETs are currently in effect. && .MARINE... A nearly east to west oriented surface ridge located about 300 nm north of Lihue is forecast to weaken and shift southward later today. This will result in light to moderate northeast trade winds across the Hawaiian coastal waters. As a new surface high pressure system builds north of state from tonight through mid- week, east-northeast trade winds will gradually strengthen. Based on the latest forecast, trade winds are expected to reach the Small Craft Advisory (SCA) threshold starting Tuesday night across the typically windier waters adjacent to the islands of Maui County and the Big Island. These SCA conditions may persist over these waters through Wednesday night. A weak surface front approaching the area from the north later this week is expected to cause a slight disruption in the pressure gradient over the region. Therefore, the trade winds are forecast to weaken slightly from Thursday through Friday. Surf along south facing shores will slowly build through the day today as a new large, long-period south-southwest swell begins to spread across the Hawaiian waters. Long-period forerunners are starting to register on the offshore buoys at NOAA buoy 51002 and 51004, which should be filling in this afternoon. Surf is expected to rise above the seasonal average by late this evening and is expected to reach advisory levels by tonight. As this swell passed through the PacIOOS buoy at Pago Pago several days ago, it registered swell heights and energy levels slightly higher than our recent mid-July 2019 warning-level event along south facing shores. The possible limiting factor for this swell event in Hawaii was that the storm was slightly more compact compared with other high surf events. We will be monitoring observations in case a High Surf Warning might be required during the peak of this event on Tuesday or Tuesday night. The surf may not drop below the HSA criteria along south facing shores until Thursday based on the latest guidance. The small west swell associated with recent tropical cyclones in the Northwest Pacific will steadily trend down today. Surf along north and west facing shores of the smaller islands may increase from late Wednesday through Friday as small northwest swells spread down the island chain. Small, choppy surf may trend up slightly along east facing shores later this week as the trades strengthen. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...M Ballard AVIATION...Thomas MARINE...Kino Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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