Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 272004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Tue Jun 27 2017

.SYNOPSIS...
Breezy tradewind weather, with only limited windward and mauka
showers, can be expected through Thursday thanks to stable air and
a ridge of high pressure to our north. The trades will become
weaker and wetter by this weekend, as an upper level disturbance
with less stable air passes over the islands.

&&

.DISCUSSION...
Our current breezy trades are being driven by a subtropical ridge
about 700 mi to our N. Our local airmass is relatively dry and
quite stable, with precipitable water (PW) of about 1.1 inch below
a stout inversion sloping from near 5000 ft over Lihue to 6500 ft
over Hilo. This is resulting in modest trade wind rainfall. Most
rain gauges around the state were dry over the last 24 hours, and
only a couple had rainfall over 1/10 of an inch. Guidance suggests
persistent conditions will result in a continuation of drier than
normal trade-wind weather through Thu.

The trade wind inversion weakens and lifts Thursday night into
Friday in response to a developing mid- and upper-level trough to
the E of the islands that will drift slowly W. The models suggest
this feature may cut off into a weak but closed 500 mb low over
the islands this weekend, with a weak and very broad surface
trough passing over the islands from E to W on Sat. Interestingly,
at the same time, a subtropical high N of the islands will
strengthen, helping to keep us from losing the trades altogether.
Trade wind speeds will probably reach a relative minimum this
weekend, in the gentle to locally breezy range. The models show a
marked increase in boundary layer moisture starting Thu night with
PW increasing to 1.5-1.6 inches, as well as some mid-level
cooling. By Friday, we should see a marked increase in trade wind
showers, especially when compared to the current rather dry
weather. Depending on the strength of the surface trough, we could
see increasing showers in interior, leeward, or S shore locales
too, particularly if the low level flow to the E of the trough
veers more than currently expected.

Strong high pressure to our N will return breezy to locally windy
trades to the state after the weekend. The mid level features
enhancing our showers should weaken and lift N away from us,
though lingering low level moisture may keep trade wind showers
somewhat active.

&&

.MARINE...
A Small Craft Advisory (SCA) remains in effect due to strong
trade winds over the typically windy waters adjacent to the Big
Island and the islands of Maui County. The SCA, which is
currently in effect through Thursday, will likely need to be
expanded to include the Kaiwi Channel and the Maui County
windward and leeward waters by mid-week. The latest forecast
keeps SCA conditions over the typically windy areas through
Friday, followed by slightly weaker trade winds this weekend.

The locally strong trades combined with a small northeast swell
generated by a fetch of gales west of Oregon and California, will
produce slightly elevated choppy surf along east facing shores
most of this week. Surf is expected to remain below the High Surf
Advisory criteria along east facing shores, however, some of the
northeast swell energy should wrap into exposed north facing
shores across the state. A small short-period northwest swell is
also forecast from Friday into this weekend.

Small pulses of swell energy from the southeast through southwest
will continue to produce near normal summertime surf along south
facing shores into mid-week. Reinforcing long-period south-
southwest and southwest swells will generate a slight bump in
surf heights along south facing shores from Thursday into this
weekend.

&&

.AVIATION...
Fresh to locally strong trade winds will continue through the
period. Morning soundings from PHLI and PHTO show a strong low
level inversion between 4-6 kft. The elevated winds combined with
a stable atmosphere and strong inversion will contribute to low
level turbulence over and immediately south through west of the
mountains. AIRMET TANGO is posted across the state for low level
turbulence. This will likely continue for most of the work week.

Predominately VFR conditions should be expected across the state.
Clouds and isolated showers will favor the windward and mountain
areas. Any showers should remain light and brief. A broad swath of
stable stratocumuli is pushing towards the islands. Windward
areas could see isol MVFR CIGS through the day, especially PHTO.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$
R Ballard/Foster/Eaton



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