| Current Conditions | ||
| Temp |  | 1.0 C |
| RH | | 46 % |
| Wind | | NE 0 mph |
| Road | | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 056 FXHW60 PHFO 290159 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 359 PM HST WED SEP 28 2016 .SYNOPSIS... Locally breezy trade winds will focus showers across windward and mauka areas through Friday. Showers are expected to increase during the weekend, and possibly into early next week, as deeper low level moisture moves over the islands and a mid to upper level trough strengthens near the state. && .DISCUSSION... Water vapor imagery shows a broad upper level low east of the state, with a ridge just to the west. At the surface, high pressure is located far north of the state, with a trough to our northwest. 00z soundings from Lihue and Hilo show inversions around 6kft and precipitable waters of 1.1-1.2 inches. Early afternoon MIMIC total precipitable water imagery shows similar or higher moisture values within 200 miles upstream to the northeast. The surface high north of the state has kept most clouds and showers focused across windward sections, though rainfall has remained limited through the day. Most rain gages only measured up to a few hundredths of an inch over the past 12 hours (with one recent shower bringing 2 tenths of an inch to the Niu Valley gage on Oahu). Satellite and radar show showery clouds associated with the upstream moisture starting to move ashore over Maui/Big Island. These showers will spread to the rest of the state early this evening. Forecast soundings/cross sections show inversions/ moisture depth increasing from this evening through Thursday morning, then lowering during the afternoon. More typical trade wind showers are expected through Friday. Rainfall chances will increase heading into the weekend. The GFS/ECMWF show a nearly east to west oriented upper level trough setting up just north of the state on Friday and gradually strengthening through the weekend. This trough will erode the mid level ridge over the islands and weaken the inversion. Meanwhile, the models are showing deeper low level moisture moving in from the east. This should result in an increase in rainfall, especially across the eastern end of the island chain. The GFS and ECMWF had started to converge toward a similar solution early next week, but the 18z GFS seems to develop a problem with gridscale convective feedback. It now spins up a compact low over/north of the state Monday/Tuesday, while the ECMWF develops at most an inverted surface trough. During this time, moisture from the remnants of tropical cyclone Ulika will be carried near or over the islands by the trade wind flow. This could create the potential for heavy rainfall given the combination of tropical moisture and increasing instability that would accompany the upper low. While we will continue to keep the forecast wet through the extended, have held off on mentioning any heavy rain due to uncertainty with the pattern and timing. && .AVIATION... AIRMET Tango remains posted for low level turbulence south through west of mountain ranges as high pressure continues to provide for moderate to fresh trade winds across the area. VFR conditions prevail at most of the TAF sites with some brief MVFR conditions expected along windward slopes as moisture embedded in the trades interacts with terrain. && .MARINE... High pressure far north of the state will maintain locally strong trade winds across the area, and a small craft advisory is in effect for the typically windier areas near Maui and the Big Island. The morning ASCAT pass showed only 20 knots of wind through the Alenuihaha Channel, in an area that had 25 knots from the pass last night. High-resolution models picked up on this brief downward trend, and also show wind speeds increasing again this evening and continuing near small craft levels through much of the week. This advisory is in effect through Thursday, though it would need to extended if conditions remain steady as currently expected. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ Discussion/Marine...Bravender Aviation...Burke Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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