High pressure to the northeast will move to the east over the coming days, and moderate trades will weaken. Increased shower activity is expected Monday through Wednesday as a front approaches from the northwest, and an upper level trough develops near the islands.
There have been some changes to the forecast with the afternoon package to refine various aspects. Through tomorrow night, the PoPs were adjusted towards the latest run of the National Blend of Models (NBM), with the exception of the Big Island were a blend of the GFS and ECMWF was used as the NBM seemed to be a little wet. For Monday and Tuesday, the NBM was heavily used for PoPs, with some nudging near the Big Island towards the GFS solution. Thunderstorm areas were reduced, following the previous trend of using the coldest 500 mb temperatures, however confidence is low for any thunderstorm development. Snow chances for the Big Island summits remains for Monday afternoon through much of Tuesday night, with summit temperatures reduced to be better aligned with that possibility. Monday night through Wednesday afternoon, the Big Island winds were adjusted upwards utilizing the GFS winds. More details on these changes below.
High pressure to the northeast will maintain a trade wind flow through the remainder of the weekend. A front far to the northwest will continue to move to the east, pushing the high to the east. This will in general weaken the trade winds for the start of the week, with the winds turning a bit south of east. The ECMWF keeps the front to the northwest of the islands, while the GFS would usher it closer to the islands Tuesday. Both models develop an upper level low over the western islands Monday, which merges with the upper level trough associated with the approaching front to the northwest Tuesday night. On the east side of the upper level low, winds will draw moisture towards the islands, with most of that moisture focused over the Big Island. The colder temperatures aloft will then bring the possibility for some wintry weather for the Big Island summits. This isn't an organized frontal system moving over the Big Island, so not expecting a significant winter event, but certainly can't rule out some snow showers.
The upper level low will bring colder temperatures to the islands, with the ECMWF and GFS both showing pockets of -10C, or slightly colder, Monday afternoon through Tuesday night. While this is on the colder side of normal for this time of year, its generally within 1 standard deviation. Additionally, the precipitable water values remain near normal in the areas with the coldest 500 mb temperatures. This would suggest not all parameters are lining up for the development of thunderstorms. While winds are weakening Monday, believe there will be enough of a trade wind component to inhibit thunderstorm development, but that could change as we head into Tuesday with winds weakening a bit more. That being said, at that time the question will remain as to whether or not the cold temperatures and precipitable water will line up.
That all being said, there remains a possibility, so have left slight chance of thunderstorms in the forecast. Anticipate that as the forecast period starts to be covered by the high resolution guidance (Sunday afternoon's run), we should get a better idea for how things will pan out. At this time, suspect the rain focus will be on the Big Island.
The second half of the week will have whatever remains of the front moving to the the east, and a new upper level low that develops along the frontal upper level trough moving to the west. A new high building in behind the front will bring returning trades to the region, which will carry some of the residual showers from the front into the islands.
Moderate trades are on the decline for the next few days. Low cigs and SHRA should be focused mainly over windward and mauka locations. MVFR conds possible in any SHRA, otherwise VFR prevails.
No AIRMETs in effect.
Moderate to locally strong E trades persist today then gradually weaken as high pressure to the NE drifts SE and weakens. An upper level trough forms SE of the coastal waters Monday causing moderate trades to then veer to ESE and slowly weaken through Tuesday. Additionally, a front will advance into the offshore waters by Tuesday afternoon bringing locally strong ENE winds behind it before weakening over the coastal waters midweek. There is the slight chance for thunderstorms Monday through Wednesday.
Surf along N facing shores remains small early this afternoon, but a small to moderate, medium to long period NW swell will fill in this evening into tonight in maintenance of elevated surf through the weekend. Small long period forerunners are evident on the NDBC buoys 51101 and 51001 early this afternoon. A moderate long period NW swell is then expected to arrive late Tuesday and peak Wednesday near High Surf Advisory thresholds before declining by the end of next week.
Surf along E facing shores will decline through the weekend due to the weakening of the local and upstream trade winds. Large choppy short period surf for N and exposed E shores is possible by the middle of next week depending on the evolution of the approaching cold front. For S shores, tiny background southerly swell remains through the forecast period.
None.
