Current Conditions | ||
Temp | ![]() | -0.9 C |
RH | ![]() | 96 % |
Wind | ![]() | SW 14 mph |
Road | ![]() | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 982 FXHW60 PHFO 231954 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 954 AM HST Sat Feb 23 2019 .SYNOPSIS... A surface trough will linger in the vicinity of the Big Island through the weekend. Coupled with impulses passing high overhead, locally heavy showers and isolated thunderstorms are likely for the Big Island and scattered showers for Maui over the rest of the weekend. High pressure northwest of the islands will maintain a mostly dry weather pattern for Oahu and Kauai through today. An uptick in windward showers is possible tonight through Monday as some of the low level moisture spreads westward from Maui and the Big Island. The high pressure will strengthen early in the new work week, with strengthening north to northeast winds, along with cooler temperatures. && .DISCUSSION... It is already an active morning with locally heavy showers occurring along the Hamakua coast of the Big Island. Therefore, the forecast was updated to include locally heavy showers for the lower elevation of the Big Island for the rest of today. Also, conditions are ripe for scattered evening showers, locally heavy at times, for the lee side of the Big Island through midnight tonight. We are watching out for Maui, and Molokai, where showers are already present. We won't rule out another forecast update to include locally heavy showers for this afternoon, should it trend toward that direction in the coming hours. The 2 am HST Hilo sounding show a moist and somewhat unstable air mass. There is an inversion at 25k feet, that will deter thunderstorms, but an approaching short wave high aloft, will likely destroy this high inversion, leading to a more favorable air mass for thunderstorms this afternoon and evening. The short wave trough high above, or upper level disturbance, is a series of two over the weekend. The one approaching the islands now is forecast to dampen out over the Big Island this evening. A stronger one is forecast to drop in early Sunday morning, then gradually move east, exiting the Big Island early Sunday evening. This second short wave will be strong enough to push the surface as per both the ECMWF and GFS solutions. Timing is critical, and what the GFS is saying, is that this second shortwave will clear things out before the maximum daytime heating Sunday afternoon, leading to a nicer day (Sunday) than today for Maui and the Big Island, in particular. The surface trough also gets nudged east of the Big Island by Sunday evening. The trough finally stalls some 100 miles off the Big Island coast on Monday, and stays there through Tuesday. The remnants or tail end of a front is forecast to reach Kauai Monday morning, then stall over the central islands Monday evening. It is a shallow front with cloud tops no higher than 10k feet. This means the windward and mountain areas will be socked in, while much clearer conditions prevail on the lee side. Things gets messy thereafter, Tuesday through Wednesday as another surface boundary drops in with aided by a vigorous looking upper level trough, leading to a slight chance of thunderstorms. Also, this upper level impulse will carry a pocket of very cold temps of minus 19 degree C at 500 mb over the islands. So, we could have another round of winter weather for the summits and upper slopes of the Big Island, and potentially Maui. As for the surface winds, the prevailing wind flow will be a light northeast breeze across the smaller islands and light and variable over and in immediate vicinity of the Big Island today through tonight. The northeast winds strengthen slightly as well overnight to moderate speed. As the surface trough over the Big Island gets pushed east of the island Sunday evening, a light to moderate northeast flow fills and lingers into next week, including through the next round of potentially unsettled weather. After this shortwave passes, models suggest that the longwave pattern near and north of the islands will once again take the form it has for most of February, with high pressure northwest of the islands and low pressure to the northeast. This setup favors a cool and breezy northwest to north wind flow. Shallow bands of clouds and showers would likely periodically move over the islands from the north if this large scale pattern were to hold && .AVIATION... A surface trough near the Big Island will keep winds light across the region today with land breezes developing in a weak northerly background flow. Expect SHRA with occasional +SHRA, associated with the feature to affect Maui and the Big Island through the remainder of this afternoon. Lowered vis and mountain obscuration will cause MVFR conditions at times. The rest of the islands may have isol low clouds, especially inland and over the mountain ridges and slopes, but generally VFR conditions. AIRMET SIERRA remains in effect for north to southeast sections of Maui and the Big Island due to mountain obscuration from low clouds and SHRA above 2500 ft. && .MARINE... Gentle northeast winds will prevail over most of the island chain into Monday. A weak surface high passing north of the state is driving the winds, while a surface trough stretching across the Big Island is leading to variable winds to the south of the Big Island. The trough will drift east slowly, but otherwise, little change is expected through the weekend, with thunderstorms possible around the Big Island into Sunday morning. Winds will increase through the first half of the work week. Winds will begin to strengthen late Monday or Monday night as a dissipating front sags southward over the islands and another surface high passes to the north. Winds will shift out of the north and build to fresh to possibly strong levels late Tuesday and Wednesday as a diffuse front moves down the island chain. Small Craft Advisory level winds will be possible during this time, followed by a decrease in the north winds on Thursday. Small surf is expected on all shores this weekend as an east swell fades and tiny northwest swell holds well below winter time average. A pulse of moderate northwest swell is expected on Monday, followed by a large, long period northwest swell that will build on Tuesday and peak on Wednesday and early Thursday. This large swell will produce surf around high surf warning levels for north and west facing shores. In addition, the combination of fresh to potentially strong north winds and seas from the winds, the northwest swell, and a short period north swell will lead to rough conditions that will likely require a Small Craft Advisory for most waters. Though winds will decline on Thursday, seas will remain elevated. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ H Lau/Thomas/Wroe Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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