| Current Conditions | ||
| Temp |  | 4.1 C |
| RH | | 71 % |
| Wind | | NW 0 mph |
| Road | | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 656 FXHW60 PHFO 220153 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 353 PM HST Sat Oct 21 2017 .SYNOPSIS... Trade winds will diminish tonight with clouds and showers remaining focused over windward and mauka areas. Sunday, light and variable winds with afternoon clouds and sea breezes are expected. Some heavier showers over the eastern half of the Big Island are possible Sunday afternoon and night. Monday through Wednesday, southerly winds ahead of a front will bring deep tropical moisture with the potential for flooding for mainly the eastern half of the state. Cool and dry air will fill in behind the front. && .DISCUSSION... Trade will diminish tonight with showers relegated to mainly windward areas. By Sunday, a front approaching from the northwest will push a surface ridge over the islands allowing for light and variable winds over the smaller islands with widespread afternoon sea breezes and interior clouds and showers. Near and over the Big Island, southeasterly winds will bring increasing humidity and moisture from the tropics allowing for heavier showers starting Sunday and continuing Sunday night, mainly over the Kau, Puna, and Hilo districts. As the front approaches Kauai Sunday night and Monday, both the GFS and ECMWF are depicting a moisture convergence band forming over Oahu, Maui County, and the Big Island. The near surface convergence, tropical moisture, and island terrain will likely produce heavy rain, isolated thunderstorms, and the potential of flash flooding for areas of the eastern islands through Tuesday. Sufficient deep layer shear, as well as turning of the winds with height in the lower levels, suggest that some of those isolated thunderstorms could be strong or severe along and ahead of the front. Kona winds (southerly) will increase Monday with some potential for gusty winds accelerating down the terrain into windward areas of Kauai and Oahu. The front will push over Kauai Monday night bringing moderate rain with the heaviest falling over the north and west facing slopes of the Garden Isle. The front will push the aforementioned convergence band slowly eastward, likely pushing east of Oahu Tuesday, and then east of the Big Island sometime Wednesday. North winds behind the front will bring cool and dry conditions to Kauai Tuesday, and possibly Oahu Wednesday; while the convergence band keeps things moist and humid over Maui County and the Big Island. Model guidance is a bit cloudy for the second half of the week with the GFS maintaining a weak trough east of the state, while the ECMWF places a stronger surface low southeast of the Big Island. The forecast maintains light to moderate northeast trade winds with windward showers in the extended forecast. && .AVIATION... Moderate to breezy trade winds will trend down this evening as a ridge of high pressure approaches the islands from the north. AIRMET Tango for low level turbulence downwind of the mountains on all islands will likely be canceled later this evening. Ragged clouds and scattered showers will persist along windward slopes and coasts overnight. Partial clearing is expected across leeward slopes and coasts after sunset once weak sea breezes abate. Isolated MVFR CIG/VIS are to be expected in passing showers this afternoon may become more frequent across windward Big Island late tonight. Otherwise, VFR conditions will prevail all areas. && .MARINE... Trade winds continue to gradually diminish but are still blowing in the moderate to fresh breeze range across the coastal waters this afternoon. Will be discontinuing the Small Craft Advisory as we should see winds will drop off overnight and start turning SE as a subtropical ridge near 30N pushes rapidly S over the islands on Sunday. Kona winds from southerly directions will then take over and increase by Monday as a developing gale low gets cranking 600 nm or so to our N. Some of the guidance suggests that prefrontal konas might approach SCA levels on Monday or Monday night, something we will need to keep an eye on. There will also be fresh northerlies immediately behind the cold front as it pushes down the coastal waters from NW to SE Monday night and Tue, and these could also approach SCA criteria in some spots before the low starts to move away in the midweek time frame. Deep tropical moisture drawn northward ahead of the front is expected to lead to some thunderstorms over the coastal waters starting early Monday, mainly from around Oahu eastward. Sufficient deep layer shear, as well as significant turning of the winds with height in the lower levels, suggest that a couple of the storms could be strong or severe along and ahead of the front. Although short-period wind waves will continue to ease, building NNW and S swells Sunday and Monday are expected to keep combined seas elevated, but below the SCA criteria of 10 feet. A larger, shorter-period NNW swell building on Tuesday will likely lead to SCA in all zones as seas exceed 10 feet once again. The first NNW swell will produce surf near the advisory threshold along exposed N facing shores, and the second swell is expected to produce high surf along N facing shores. Advisory-level surf along S facing shores is possible Sunday and Monday, with the swell generated by a distant low near New Zealand last weekend. A small long-period W swell is possible next week, generated by large and slow-moving Super Typhoon Lan in the W Pacific. An even larger, possibly warning level NW swell is possible late in the week as Lan becomes extratropical in the NW Pacific. The GFS and ECMWF are in good agreement on the evolution of this feature and the fetch generation area. SE swell is also possible over the next several days. The recently updated Oahu Surf Discussion (SRDHFO) has details on the swell sources affecting the islands. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ Morrison...Discussion RBallard...Marine Bedal...Aviation Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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