000 
fxhw60 phfo 121957 
afdhfo 


Area forecast discussion 
National Weather Service Honolulu HI 
957 am HST Mon Nov 12 2018 


Synopsis... 
a ridge north of the islands will maintain moderate to locally 
breezy trade winds with a few brief showers over mainly windward 
and coastal areas through Tuesday. Heading into midweek, winds 
will weaken as the ridge is disrupted, which will allow for 
afternoon clouds and showers over the interior of the islands. 
Trades are expected to return for the second half of the week, 
with increasing trade wind showers. Periods of high clouds are 
expected beginning Tuesday, which could become thicker later in 
the week. 


&& 


Discussion... 
no changes to the forecast this morning. A high pressure ridge 
remains north of the islands, helping the winds transition to the 
more typical northeast trade wind direction. Expect moderate 
trades to persist, with some locally breezy locations. Overnight 
soundings continued to Show Low precipitable water (pw), and that 
is reflected upstream of the islands on the satellite derived precipitable water. 
Radar shows some showers riding in on the trades, but rainfall 
totals will remain low in this regime, with the focus mostly over 
windward areas. 


High clouds far northwest of the islands are expected to filter 
into the islands as early as Tuesday, and could become thicker 
Wednesday and Thursday. The ridge to the north gets disrupted 
Tuesday night and Wednesday, which will allow lighter winds over 
the islands. This will likely mean land and sea breezes Wednesday, 
which could mean an increase in clouds and showers over the 
interior of the islands. The amount of high clouds moving the 
islands at this time could however limit the amount of afternoon 
cloud development. 


The second half of the week should see trade winds build back in 
as a high with a high far northeast of the islands. Heading 
towards the weekend however, a front approaching from the 
northwest will help to erode the ridge north of the islands and 
increase low level moisture. A high pressure behind the front will 
move the front along, and the models show little organization with 
the front, particularly as it reaches the islands. At this time it 
looks like trades will linger into the start of the weekend, with 
enhanced trade wind showers as the boundary helps to bring an 
increase low level moisture in on the trades. 


An upper level trough forming northwest of the islands Friday 
afternoon/evening will bring colder temperatures aloft from 
the northwest. Models suggest an upper level low could develop 
along this trough, as it moves over the islands this weekend. As 
the colder temperatures move in from the northwest, they will 
encounter the increased moisture coming in from the east. The GFS 
and European model (ecmwf) both show precipitable water values reaching near 1.7 inches near the 
islands which is above normal for November, with 500 mb 
temperatures dropped to around -12 c. Will be monitoring future 
model runs to see what the trend is in the forecast, and to 
evaluate the instability this situation could provide. 


&& 


Aviation... 
typical trade pattern expected today with flow generally veering 
east northeasterly by this afternoon. Low stratocumulus clouds 
will push up against the windward side of the mountain slopes, 
producing MVFR conditions at times and periodic mountain 
obscuration. Moderate east northeast winds may generate light to 
moderate turbulence over and leeward of the Island Mountain 
ranges. Otherwise, VFR conditions persist. 


Airmet Tango in effect below 8000 feet for over and southwest of 
local mountain ranges for moderate turbulence, lasting at least 
through this afternoon. 


&& 


Marine... 
high pressure centered northwest of the islands will be moving 
eastward and will cause the northeast trades to veer more 
easterly through Tuesday. Wind speeds are currently holding at 
the fresh to locally strong range and is expected to continue 
through tonight and possibly into Tuesday. The current Small 
Craft Advisory (sca) is in effect through tonight for the Kaiwi 
Channel and the typical windy areas around Maui County and The 
Big Island. 


A front passing north of the islands will bring a decrease of 
trades Tuesday night through Wednesday night. Behind it, high 
pressure will be building in from the northwest Thursday, and 
fresh to locally strong northeast trades will be returning Friday 
into the weekend. 


The current north swell will continue to gradually subside 
through Tuesday. A larger north-northwest swell with a peak around 
7 foot swell height and a 14 second period, is expected to fill in 
throughout the day Tuesday and peak Tuesday night into Wednesday. 
Surf will likely reach advisory levels for exposed north and west 
facing shores, potentially as early as Tuesday. Then, a 
reinforcing northwest swell will arrive Thursday night into 
Friday with heights near the advisory levels across north and west 
facing shores. The south facing shores will continue to feel a 
small background southerly swell during the forecast period. 


&& 


Hfo watches/warnings/advisories... 
Small Craft Advisory until 6 am HST Tuesday for Kaiwi Channel- 
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island 
leeward waters-Big Island southeast waters. 




&& 


$$ 






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