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Hawaiian Islands Synoptic Discussion and Guidance

708
FXHW60 PHFO 261935
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
935 AM HST Sun Mar 26 2017

.SYNOPSIS...
A persistent ridge of high pressure northeast of the islands will
keep trade winds blowing through next weekend, with locally breezy
levels expected today and tonight. A dry and stable airmass will
keep showers limited and confined primarily to windward slopes
through the first half of the work week. An increase in trade wind
showers is then expected Wednesday through next weekend as upper
level troughing sets up over the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1028 mb high centered around 1050
miles northeast of Honolulu, is driving moderate to breezy trade
winds across the state this morning. MIMIC total precipitable
water (PW) imagery shows a dry airmass remaining in place across
the area, with PW values right around 1 inch. This correlates
well with the 12z soundings from PHTO and PHLI which came in with
PW values of 0.93 and 1.06 inches respectively. Additionally both
of the early morning soundings showed a stable airmass firmly
encompassing the island chain, with a sharp low level inversion
between 4 and 5 kft at each site. Satellite imagery shows some low
clouds moving into windward areas with the trades, while leeward
areas remain mostly clear. Meanwhile, radar imagery shows some
light showers moving into windward section of the Big Island and
just to the north of Molokai, with mainly rain free conditions
elsewhere. Main short term concern continues to focus around the
ongoing high surf headlines.

Today through Tuesday night,
High pressure northeast of the islands will keep a fairly tight
pressure gradient in place across the islands through tonight,
with moderate to breezy trades expected to continue. The trade
wind speeds are expected to drop off a notch Monday through
mid week, as high pressure lifts further northeast and away from
island chain. A dry and stable airmass is expected to limit shower
activity through the period, with PW values remaining around 1
inch. Showers will favor windward and mauka areas, primarily
during the overnight and early morning hours.

Wednesday through Saturday,
Model solutions are in good agreement showing high pressure
remaining anchored well to the northeast of the state through the
period, with a weakening front approaching from the northwest late
in the week. Light to moderate trades are expected through the
end of the work week, with localized sea breezes possible,
particularly across the western islands where the pressure
gradient won't be as tight. The trades are then expected to
increase back to moderate and locally breezy levels over the
weekend as a re-enforcing area of high pressure north of the
island chain, tightens the pressure gradient once again. Aloft, a
weakness in the upper level ridge is expected to move overhead
during the middle and latter part of the week, and this should de-
stabilize the airmass a bit. This in combination with increasing
boundary layer moisture resulting from the persistent easterly
trades, should lead to an increase in trade wind showers across
the islands. Showers will continue to favor windward and mauka
areas, with activity most prevalent during the overnight and early
morning hours.

&&

.AVIATION...
Locally breezy trade winds will continue today. The morning
soundings show the inversion height to be around 4.5 kft. AIRMET
TANGO for low level turbulence over and immediately south through
west of the mountains remains posted.

Scattered clouds and isolated showers will favor the windward and
mountain areas. Expect predominately VFR conditions, however some
brief passing showers may introduce short periods of MVFR
CIGS/VIS.

&&

.MARINE...
Hazardous boating and beach conditions will continue into Monday
before trending down into mid week due to a combination of fresh
to strong trade winds and a large west-northwest (WNW; 300-310
deg) swell that is currently moving down the island chain. These
conditions have led to a Small Craft Advisory for all Hawaiian
waters due to winds (25 to 30 kts, strongest in the channels)
and/or seas (10 ft or greater) through Monday. The large WNW
swell will generate life-threatening surf along exposed north and
west facing shores into Monday before slowly trending down into
mid week.

For the building WNW swell, the latest PacIOOS nearshore buoy
observations are remaining in good agreement with the latest wave
model guidance. However, earlier this morning a few observations
at the Waimea buoy spiked into the 9 to 12 ft range a couple of
times through the overnight hours. Although these random spikes in
wave heights appear to have been or could have been spurious (a
few feet higher than the fully exposed Hanalei buoy), the north
facing shores of Oahu remain in the warning. The west side of
Oahu was left in the advisory, due to the anticipated shadowing
associated with this westerly swell direction. Elsewhere, wave
heights are remaining around 9 ft at Hanalei and around 7 ft at
the Pauwela buoy. The offshore buoys northwest of Kauai have
peaked with heights ranging from 11 to 13 ft, which was the
predicted peak of this swell event.

Surf along east facing shores will remain small and choppy
through the upcoming week due to moderate to strong onshore winds
persisting.

Surf along south facing shores could see a slight increase late
today due to the previously discussed WNW swell wrapping around
the islands. A slight increase in surf along south facing shores
is possible Thursday through Saturday due to a long-period swell
(190 deg) associated with recent activity across the southern
Pacific.

For the outlook, another large long-period WNW (310 deg) swell
will be possible across the islands Thursday night through the
weekend. The latest deterministic and ensemble guidance remain in
decent agreement over the next few days and depict a storm-force
low developing off the coast of Japan tonight into Monday, then
tracking ENE to the Date Line Tuesday night into Wednesday while
intensifying to around a 970 mb low. A large batch of gale- to
storm- force westerly winds focused toward the region through
this time is forecast with seas peaking within the 30 to 40 ft
range. Swell across the islands associated with this storm will
likely lead to another period of warning-level surf and advisory-
level seas. More details will follow this week as this system
evolves and confidence increases with regard to specifics on
arrival times and impacts.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Kona-South
Big Island-Kohala.

High Surf Advisory until 6 AM HST Monday for Waianae Coast-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters-

&&

$$

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...Evans

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