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Temp3.4 C
RH15 %
WindENE 7 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

660
FXHW60 PHFO 270634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Mon Jun 26 2017

.SYNOPSIS...
A trade wind weather pattern can be expected over the next
several days as high pressure remains far to the north northeast
of the area. Clouds and passing light showers will favor windward
and mauka areas with most leeward locations remaining dry. An
upper low is forecast to pass westward just north of the area late
in the work week and over the upcoming weekend, bringing a more
showery trade wind pattern to the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high centered 1800 miles
north-northeast of Honolulu, is driving moderate to locally breezy
trade winds across the island chain this evening. Infrared
satellite imagery shows partly to mostly cloudy skies in place
across the State, with cloud cover most prevalent in windward
areas. Additionally, there is some jet stream cirrus enhancing
upper level cloud cover over the central islands as well. Radar
imagery shows some light showers moving into windward areas, with
rain free conditions elsewhere. Main short term concern revolves
around rain chances.

Tonight through Friday,
A ridge of high pressure will remain to the north and northeast of
the State through the period, keeping a breezy trade wind flow in
place across the island chain. The airmass will remain quite dry
through Thursday, with precipitable water (PW) values remaining
in the 1.0 to 1.2 inch range. PW values will then climb slight
above normal levels in the 1.3 to 1.5 inch range Thursday night
and Friday, as an upper level low approaches from the east. As a
result, we expect a dry trade wind shower pattern through
Thursday, with showers limited primarily to windward and mauka
areas. The trade showers should increase in coverage and intensity
Thursday night and Friday as the airmass moistens up and becomes
less stable.

Friday night through next Monday,
The GFS and ECMWF both show the trade winds weakening over the
weekend as weak surface troughing develops over the islands in
response to an upper level low north of the State. The trades are
then expected to strengthen early next week as the surface
troughing over the islands breaks down.

Aloft, the model solutions start in fairly good agreement Friday
night, but then diverge with respect to the handling of the upper
level low over the weekend. The GFS pivots the upper low north of
the islands Friday night through Saturday night, with the feature
then stalling out through early next week with a trailing upper
trough lingering over the State. The ECMWF on the other hand,
lifts the upper level low north of the islands Friday night and
Saturday, with the feature then shifting northwest and away from
the islands Saturday night through early next week as upper level
ridging builds in from the east.

Given the model differences, will continue to favor a blended
forecast approach until the details become more clear. As a
result, we expect a fairly moist and somewhat unstable pattern
will keep a showery trade wind regime in place through at least
Saturday night. We will then show a more typical trade wind
weather pattern Sunday through early next week, but this could be
wetter if the GFS is correct or drier if the ECMWF ends up panning
out. Showers will continue to favor windward and mauka areas
through the period, with some afternoon/evening shower development
likely over interior and leeward areas Saturday and Sunday
afternoon, in the weakened trade wind regime.

&&

.AVIATION...
The high pressure ridge north of the Hawaiian Islands will
produce moderate to breezy northeast to east trade winds. AIRMET
Tango is in effect for tempo low level moderate turbulence south
through west of mountains.

Tempo cloud ceilings and showers will move over mountain areas
and across north through east sections of all islands with
isolated showers forecast elsewhere. Brief mountain obscurations
are possible along windward mountain slopes and coasts.

&&

.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, may need to be expanded to
include the Maui County windward waters and the Kaiwi Channel 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
during most of this week. However, surf is expected to remain
below the High Surf Advisory criteria along east facing shores.
Some of the northeast swell energy will also likely wrap into
exposed north facing shores across the state into mid-week. A
small short-period northwest swell (2 ft, 10 sec) is forecast for
Friday and Saturday.

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 result in a slight bump in
surf heights along south facing shores from Thursday into this
weekend.

&&

.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.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston

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