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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250627

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Sat Jun 24 2017

A mid level trough passing over the islands this weekend,
will weaken the trade winds and result in an increase in leeward
shower activity. The mid-level trough will shift west of the
State by early next week, with high pressure then building back
northeast of the islands. This will result in a drier and more
stable trade wind shower regime, with the trades increasing into
the breezy to locally windy range through much of next week. A
more showery trade wind pattern will return late in the work week
and next weekend, as an upper level low approaches the islands
from the east.


Currently at the surface, a 1034 mb high centered around 1600
miles north of Honolulu, is driving light to moderate trade winds
across the island chain this evening. Meanwhile aloft, a mid-level
700 mb trough is enhancing some of shower activity over the
western islands this evening. Infrared satellite imagery shows
partly to mostly cloudy skies in place across the State, with
radar imagery showing scattered windward showers and some
lingering shower activity over leeward areas as well. Main short
term concern revolves around rain chances.

Tonight and Sunday,
High pressure will track slowly eastward well to the north of the
islands, maintaining a light to moderate trade wind flow across
the State. Localized sea breezes will likely develop again Sunday
afternoon in the more sheltered leeward areas due to the weakened
trade wind flow. Meanwhile aloft, the weak mid-level trough will
shift westward, reaching Kauai Sunday afternoon. As for sensible
weather details, showers will continue to favor windward and
mauka areas through the period as the trades continue to blow
across the island chain. In leeward areas, showers should end
over the next couple hours as daytime heating is lost, with
showers redeveloping again Sunday afternoon in the weakened trade
wind regime. There could be a few enhanced showers develop Sunday
afternoon, particularly over Kauai in association with the mid-
level trough moving overhead.

Sunday night through Thursday,
The mid-level trough will exit to the west of the state, while
surface high pressure builds to the northeast of the island chain.
This will increase the pressure gradient across the islands,
resulting in strengthening trade winds Sunday night and Monday,
with breezy to locally windy conditions expected Tuesday through
the end of the upcoming work week. In addition to the
strengthening trades, with the exit of the mid-level trough to the
west of the islands, a drier more stable airmass will move into
the area. Precipitable water values will drop into the 1.0 to 1.2
inch range, below normal for this time of year, and this will
result in a drier than normal trade wind shower pattern. Showers
will continue to favor windward and mauka areas, with a stray
shower reaching leeward areas from time to time due to the
strengthening trades. Rainfall amounts are expected to remain
light through the period.

Thursday night through next Saturday,
Both the GFS and ECMWF are in good agreement showing an upper
level low tracking eastward toward the islands Thursday night and
Friday, then lifting northward and away from the islands for the
first half of next weekend. High pressure will continue to hold in
place north of the islands through the period, with breezy trade
winds expected to continue through at least Thursday night. The
trades are then expected to weaken Friday through Saturday as the
gradient slackens due to the approaching upper level trough. As
for sensible weather details, both models show deep layer moisture
increasing beginning Thursday night, with this enhanced moisture
remaining over the islands through next Saturday. As a result, we
should see a more showery trade wind pattern through the period,
with showers continuing to favor windward and mauka locales.


Light to moderate northeast to east trade winds will continue
through Sunday. Expect onshore sea breezes to develop mainly along
western slopes of all islands with cloud ceilings building over
mountain areas after 20Z. TEMPO showers and isolated MVFR
conditions are forecast.

No AIRMET's in effect or anticipated over the next 24 hours.


Light to moderate trade winds associated with 1034 mb high
pressure far north of the state will continue through the rest of
the weekend. Fresh to strong trades are expected to return through
the day Monday and continue through the week as the gradient
tightens over the region. Small craft conditions will be likely
through this period over the typically windier channel waters,
Maalaea Bay and south of the Big Island.

Surf along east facing shores will build and become rough late
Sunday through next week due to a combination of increasing trades
and a moderate northeast swell associated with a recent batch of
gales off the west coast. Surf will approach advisory levels along
east facing shores through this time, especially Monday through
Tuesday as the northeast swell peaks.

Although surf along south facing shores will trend down toward
average going into the upcoming week, small long-period reinforcing
pulses from the southern Pacific will keep things from going flat. A
blocking pattern has become established east of New Zealand, which
is leading to fetch regions associated with passing storms
setting up farther east than normal or more toward French
Polynesia. Another pulse from this region out of the south-
southeast is forecast to fill in Wednesday across the local waters
and bring the surf back up a notch going into the second half of
the week. For the long range, nothing significant is anticipated
at this time as model guidance continues to support this blocking
pattern holding down south.

Up north, mainly wrap into the typically exposed locations from the
northeast swell Monday through Tuesday and the increasing trades
will be expected for surf. Otherwise, nothing significant is
anticipated through the week.

Extreme tides that have been observed over the past several days
will trend back toward normal into the upcoming week. As a result,
flooding impacts along the coast will diminish.






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