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Hawaiian Islands Synoptic Discussion and Guidance

061
FXHW60 PHFO 221330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Thu Jun 22 2017

.SYNOPSIS...
High pressure far north of the islands will maintain gentle to
locally breezy trade winds through Friday. The trade winds will
deliver passing clouds and showers, mainly to windward areas. A weak
mid-level trough developing near the islands this weekend and early
next week may weaken the trades and bring an increase in showers.

&&

.DISCUSSION...
High pressure cells are centered far NNW and NNE of the islands this
morning, with a low and associated stalled front separating the two.
The low-level gradient S of the highs is supporting trade wind flow
over the islands, with speeds ranging from light in sheltered
leeward locations to breezy in the more exposed areas. A low aloft
near 25N141W has been triggering high-based thunderstorms (in an
area otherwise dominated by low-topped stratus) about 900-1500 miles
to the ENE of the islands over the past 24-36 hours, with the low
currently tracking NW, and associated thunderstorm coverage
diminishing. Overnight soundings continue to highlight a stable
island atmosphere, with a strong capping inversion based near 8 kft,
and a slight overnight increase in PWAT to near 1.25". Based on
radar and satellite observations, the inversion is sufficiently high
to allow clouds embedded in the trade flow to produce showers,
especially as they interact with island terrain.

Little significant change to the overall weather pattern is expected
through Friday, with passing clouds fueling a few showers, mainly
over windward areas. A mid-level ridge over the islands will slowly
and slightly weaken as the low aloft slowly moves N and gradually
dissipates NE of the islands. A trend toward slightly weaker trade
winds is anticipated Friday and Saturday as a weakness in the low-
level gradient, associated with the dissipating low aloft, moves
over the islands from the E. The GFS also indicates increasing low-
level moisture associated with this feature moving in on the trades
by Saturday.

On Sunday and Monday, a weak and subtle trough in the surface
pressure and wind fields is expected to move E to W across the area,
potentially leading to another slight decrease in trade wind speeds.
The GFS is still depicting a slightly sharper trough than the ECMWF,
and depicts a weak surface trough that passes over the islands
Sunday, resulting in a veered low-level flow that could turn surface
winds to the ESE into Monday. Although the low aloft is expected to
dissipate NE of the area, it leaves behind a weakness as a new
trough aloft moving in from the N helps to cool the mid-levels over
the islands. The combination of the incoming mid-level system and a
possibility for veered low level flow may bring increased shower
chances this weekend, and previous PoP forecasts have been left
unchanged. The best chance for additional showers will be mostly
windward and mauka, though possibly some sea breeze or upslope
showers as well if the winds weaken enough.

On Tuesday and Wednesday, the high currently to the NNW will move E,
and models agree that breezy ENE trades will prevail, delivering a
few showers to windward areas, with a ridge aloft building over the
area from the NW.

&&

.MARINE...
The current long period SSW swell will gradually decrease. However,
this swell will be reinforced by another above average south swell,
arriving this afternoon, and peaking late tonight or Friday.

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 high surf. For the next few afternoons, the
predicted high tides are around 2.5 feet, which does not include the
extra 1/2 foot mentioned above.

Other wave trains are affecting Hawaii as well, with a small out-of-
season northwest swell diminishing today, a small trade wind-
generated swell and a short period southeast swell. The southeast
swell will continue into the weekend and will add a small component
to the wave heights along south facing shores.

In the longer range, models indicate a northeast swell developing
for the beginning of next week. This swell will need to be monitored
as it interacts with the trade wind swell, and a High Surf Advisory
may eventually be needed for east facing shores if the swell comes
in bigger than anticipated.

Moderate to occasionally fresh trade winds may diminish slightly
over the weekend, with a Small Craft Advisory now in effect through
this afternoon for the typically windier zones around Maui and the
Big Island.

For more details on the surf, please refer to the Oahu Collaborative
Surf Forecast (SRDHFO) prepared by Pat Caldwell under WMO header
FXHW52 PHFO.

&&

.AVIATION...
High pressure far NE of the state will maintain light to moderate
trade winds over the islands through the period. Clouds and showers
will tend to favor the windward and mountain areas, with some
showers introducing brief periods of MVFR. Otherwise, expect VFR
conditions. No AIRMETs are expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for South facing
shores of all islands.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Discussion...Birchard
Marine...Kinel
Aviation...Bedal

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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