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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 231958
Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Fri Jun 23 2017
A weak upper level disturbance passing over the islands this
weekend will weaken the trades and increase showers for some
areas. The upper level disturbance will weaken and move away early
next week as a a ridge of high pressure approaches the islands.
This will return more settled weather, and it will turn locally
windy starting about Tuesday.
Gentle to locally breezy trade winds today are being driven by a
faraway high pressure area about 1500 miles to our NNW. The high
will pass N of the islands near latitude 44N over the weekend. In
the meantime, models indicate a 700 mb trough about 300 mi E of
the Big Island will drift W toward the islands. This shallow
feature had been more active in recent days, but is now decoupled
from a deeper mid-level low. However, a digging mid- and upper-
level trough to the NW of us this weekend will help to reinvigorate
the 700 mb trough, and in fact lead to weak troughing at the
surface near or over the islands as well.
This combination of events will have two effects on our weather.
The first will be a weakening of the trades this weekend into
Monday, with light winds leading to nighttime land breezes and
daytime sea breezes for more locales. The second effect will
likely be an increase in shower intensity (and possibly coverage),
as aforementioned mid- and upper-level features weaken the trade
wind inversion. There could be some brief afternoon or evening
downpours over interior and leeward areas during the afternoons,
mainly for the northern main islands which will be closer to the
synoptic enhancement. If the low level flow veers enough (which
doesn't appear likely, but is a possibility), there could also be
some persistent showers over windward or southern sections of the
islands as well. Overall, it doesn't look like the nicest weekend
coming up, although not expecting a complete washout by any means.
By Tuesday, the 700 mb trough will be well NW of the islands and
lose it's hold on our weather. At the same time, a surface ridge
will be approaching the islands from the N and NW, and the models
agree that a drier airmass to the E will overspread the islands.
This should lead to more settled trade wind weather for the middle
to latter part of the week, with the tightening pressure gradient
leading to breezy to locally windy conditions.
A ridge of high pressure north of the islands will keep moderate
to occasionally breezy trade winds in place through the weekend,
but winds and seas are expected to remain below Small Craft Advisory
(SCA) levels. The trades are expected to strengthen early next week
as high pressure builds northeast of the islands, and an SCA for winds
will likely be needed for the typically windy waters at that time.
A new reinforcing SSW swell will peak near or just below advisory
levels today. Surf along south facing shores will remain above
average into early next week, but remain below advisory levels.
In addition to the new southerly swell, tides have been running
about a half a foot above predicted levels. These elevated tide
levels will increase the risk for minor coastal flooding when
combined with the above average south swell. For the next few
afternoons, the predicted high tides are around 2.5 feet above
MLLW (not including the extra 1/2 foot mentioned above). See the
latest Special Weather Statement (SPSHFO) for additional details
on the elevated tide levels and potential impacts.
Small trade wind and SE swells will continue into the weekend. In
the longer range, models indicate a northeast swell developing for
the beginning of next week. A High Surf Advisory may eventually
be needed for east facing shores if the swell comes in bigger than
anticipated. For more details on the surf, please refer to the
Oahu Collaborative Surf Forecast (SRDHFO).
High pressure north of the area will maintain moderate trade winds
over the islands today. Clouds and showers will tend to favor the
windward and mountain areas. An area of enhanced showers will
move through the central islands this morning and the Big Island
through much of the day. Windward areas should expect periods of
MVFR CIGS/VIS, mainly along the mountain slopes. Otherwise, expect
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office
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