Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 240148
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
348 PM HST Fri Jun 23 2017

.SYNOPSIS...
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.

&&

.DISCUSSION...
Gentle to locally breezy trades are blowing across the islands,
thanks mainly to a high pressure area about 1500 mi NNW of the
islands. The afternoon soundings show a capping inversion around
8-10kft, a little more elevated than previously. An area of
scattered to broken showery low clouds has been affecting the
islands today, but a lot of these have faded during the daytime.
Would expect to see showers fill in again tonight somewhat in this
environment.

Some relatively minor changes will occur this weekend. A weak 700
mb trough about 300 mi ESE of the Big Island will be drifting W
toward the islands at the same time a weak trough in the mid-to-
upper levels of the atmosphere will be digging toward the islands
from the NW. Neither of these two features is especially
impressive, but will be enough to weaken the trade wind inversion
further and could induce a weak surface trough over the islands
this weekend that will push just W of the islands by Mon. There
is some model disagreement as to how strong the surface trough
might be, will affect how much the low level flow weakens and
veers to E or ESE and thus where showers will be most likely.
Hybrid patterns with weak trades can make for challenging
forecasts when you dig into the details. A middle of the road
approach suggests that there will be some leeward and interior
afternoon and evening showers due to sea breeze convergence, along
with night and morning showers favoring windward sections and
possibly south shores as well. With MLCAPE forecast to increase to
500-1000 J/kg by Sunday over the northern main islands, any of
these could be briefly heavy, but moisture will be somewhat
limited and excessive rainfall appears unlikely.

By Tue, 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 us from the N and NW, and models agree that a
drier and more stable airmass will overspread the state. 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.

&&

.MARINE...
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 again 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 reinforcing SSW swell will peak this afternoon and evening.
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 still indicate a northeast swell
developing for the beginning of next week, but have scaled back on
the peak swell height by a foot or two. A High Surf Advisory is
looking less likely for this event, but may still 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).

&&

.AVIATION...
Moderate trade winds will continue tonight with a slight weakening
of the winds tomorrow with localized sea breezes will become more
probable. Clouds and showers will tend to favor the windward and
mountain areas. Windward areas can expect passing showers to carry
periods of MVFR CIGS/VIS tonight.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
R Ballard/Stall/Eaton



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