| Current Conditions | ||
| Temp |  | 6.0 C |
| RH | | 52 % |
| Wind | | NE 27 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 197 FXHW60 PHFO 190126 CCA AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 325 PM HST Thu Jul 18 2019 .SYNOPSIS... High pressure far to the north will provide trade winds into next week, with wind speeds easing somewhat over the weekend. Clouds and showers riding in with the trades will favor windward and mountain areas, with showers most frequent during nights and mornings. An area of especially showery clouds will arrive overnight, bringing showers to many windward communities before departing Friday. A few light showers may briefly spread to otherwise dry leeward areas, with the exception being leeward Big Island, where clouds and a few showers are expected each afternoon and evening. && .DISCUSSION... High pressure will remain entrenched far N and NE of the islands over the next week, supporting a seasonable trade wind weather pattern. The high will support locally strong ENE trade winds initially, with wind speeds easing slightly by Saturday as it weakens a little. With trade winds blowing, clouds and showers will focus over windward areas, but a few brief showers will spread leeward on the smaller islands. On the Big Island, this is the wet season for the leeward slopes, where afternoon and evening clouds can be expected (nearly daily) to drop a few showers. Having said that, today has been a bit of an exception, with only patchy cloudiness noted. The leading edge of an area of open-celled showery cumulus clouds is moving in to windward Big Island and Maui from the E, and will bring a statewide increase in windward cloud and shower coverage overnight, especially as compared to the current mostly dry conditions. A few showers will spread leeward with the gusty trade winds, but accumulations in general will be limited as models depict this moisture moving W of the islands on Friday, and because the low- levels will be capped at about 7000 feet. Latest GOES-17 satellite imagery reinforces this notion, as the incoming showery low cloud field transitions to a more stable cloud field in an area about 300 to 800 miles E and NE of the islands. About 800-1000 miles to the E and NE, the cloud field becomes more showery, with GFS indicating these clouds leading to an increase in low-level moisture that arrives on Saturday. Hard to bank on the details of that verifying, with the general expectation being that the trade winds will deliver periods of clouds and showers to windward areas, especially during nights and mornings. Generally speaking, the pattern aloft will feature low pressure to the NW and a ridge to the SE, with the low acting to occasionally spread high clouds of varying thicknesses over the islands. Currently, water vapor imagery shows a weak low aloft centered about 400 miles N of the islands, with an associated E-W oriented trough. Afternoon soundings indicate that this feature is promoting instability above a strong low-level subsidence inversion based near 7000 feet, with the unstable layer starting at 12000 feet on Kauai. With the unstable layer based near 15000 feet on the Big Island, surface-based air parcels are not expected to be able to tap into the instability, and upslope convection is not expected, even as the trough aloft becomes N-S oriented over the area over the weekend. With the low-level inversion remaining in place, rainfall amounts will be limited, even as showery patches move through with the trade winds. && .AVIATION... A high will remain nearly stationary far northeast of the main Hawaiian islands and maintain locally strong east winds over the islands through Friday. AIRMET TANGO is in effect for moderate turbulence below 7,000ft and this AIRMET will likely continue into Friday. The low level winds will carry some clouds and showers over east sections of the islands. An area of broken low clouds moving over the islands from the east has produced an increase in cloud cover over east sections of the Big Island of Hawaii and east Maui but little in the way of low ceilings or visibilities. These clouds may become more active this evening and produce localized MVFR conditions over east sections of the islands. AIRMET SIERRA for MTN OBSC could be required. The area of showery low clouds should clear the islands Friday afternoon. && .MARINE... High pressure north of the state will maintain fresh to strong trade winds through Friday. The high is expected to weaken some over the weekend, which will help the winds ease slightly. Trade winds may drop to moderate to fresh speeds early next week for a brief day or two. The Small Craft Advisory (SCA) continues through Friday afternoon for all waters in response to the trade winds. The forecast calls for the winds to start to weaken Friday night, which will drop the SCA back to the typical windier areas around Maui and the Big Island for the weekend. Rough and choppy surf is expected to hold steady along east facing shores at heights just below the advisory threshold through Friday. As the winds ease over the weekend, surf is expected to drop slightly. Some small background south and southwest swells are expected through the weekend. An increase in surf along south facing shores is possible early next week as a couple small southwest swells arrive in the islands. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. Small Craft Advisory until 6 PM HST Friday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Big Island Windward Waters. && $$ DISCUSSION...Birchard AVIATION...Donaldson MARINE...Kino Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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