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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 230711 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
911 PM HST Wed Feb 22 2017

High pressure north of the state will keep a moderate to breezy
trade wind flow in place through Friday, with the trades then
easing over the weekend into early next week. Clouds and showers
will favor windward and mauka areas through the weekend, with a
stray shower spilling over into leeward areas from time to time.
Deep tropical moisture is expected to lift northward into the
islands late in the weekend through the first half of next week,
bringing potentially unsettled weather to much of the state.


Currently at the surface, a 1030 mb high centered around 925
miles north-northwest of Honolulu, is driving moderate to locally
breezy trade winds across the island chain this evening. Infrared
satellite imagery shows mostly cloudy skies in place in most
windward areas, with partly cloudy conditions in leeward locales.
Meanwhile, radar imagery shows scattered light showers moving into
windward areas, with a few showers beginning to spill over into
leeward areas as well. Main short term concern for the next few
days revolves around rain chances.

Tonight through Sunday,
High pressure north of the state will keep moderate to breezy
trade winds in place through the remainder of the work week, with
the speeds diminishing slightly each day. The trades are expected
to diminish to light and locally moderate levels over the weekend,
as the area of high pressure weakens and slides eastward around
400 miles north of the island chain. Clouds and showers are
expected to favor windward and mauka areas through the period,
with the occasional shower reaching leeward areas of the smaller
islands. There might be a bit of an increase in the trade showers
Friday through Saturday, as a dissipated front brings a slight
increase in boundary layer moisture southward over the islands,
and an upper level trough moves overhead. Overall however, the
trade showers as well as rainfall amounts should remain light
through Saturday, as precipitable water values remain at or below
1 inch.

Both the GFS and ECMWF are now in good agreement showing deep
tropical moisture spreading northward into eastern sections of the
state Saturday night and Sunday, with precipitable water values
climbing to around 1.5 inches. This will likely result in an
increase in shower coverage and intensity particularly across the
Big Island and perhaps as far northward as eastern sections of
Maui. Elsewhere, a fairly typical trade wind shower pattern
should continue.

Sunday night through next Wednesday,
The GFS and ECMWF appear to be coming into better agreement with
respect to the evolution of the large scale synoptic pattern for
early next week. A large ridge of high pressure is forecast to
develop well northeast of the islands with a digging upper level
trough approaching the state from the west. This will assist in
shifting the low level boundary layer flow around to the east-
southeast, transporting deep tropical moisture with precipitable
water values of 1.5+ inches, northward and over the island chain.
There does remain some significant differences in the surface
pressure field between the GFS and ECMWF Monday night through the
middle of next week however. The GFS spins up a surface low which
tracks northward to the east of the islands, while the ECMWF spins
up a weaker low and tracks it slowly northward into the western
islands. Due to the differences between the reliable guidance,
confidence in the forecast remains below normal.

As for sensible weather details, with the increase in deep layer
moisture over the island chain, it appears likely that eastern
sections of the state will be fairly wet Sunday night through
early next week, and possibly right into next Wednesday. In fact
heavy rainfall and even thunderstorms are looking to be an
increasing probability for the early and middle portion of next
week across the Big Island. Elsewhere the forecast is more
uncertain, but given the differences between the models we will
keep rain chances in the forecast for all areas, with the highest
POPS over windward locales.


Trade winds supported by a surface high far NNW of the islands
will persist through Thursday as ridging aloft builds. The result
will be breezy trade winds delivering stable and showery low
clouds to windward slopes and coasts, with isolated MVFR VIS/CIG
in SHRA expected. VFR will prevail at the TAF sites.

AIRMET Tango remains in effect for low level turbulence to the S
and W of the terrain, and will likely remain in place through
Thursday. Otherwise no additional AIRMETs are anticipated.


Strong trade winds associated with a 1031 mb high pressure centered
north-northwest of the state will continue to generate rough boating
conditions across the island waters through Thursday. The highest
winds and seas will remain across the channels and south of the Big
Island through this time. High pressure will weaken Friday through
the weekend as a gale-force low develops to its north and drops
south to around 600 nautical miles north-northwest of the state.
This will allow the strong trades to relax or trend down into the
moderate to fresh range locally.

A new long-period west-northwest swell (310 deg) associated with a
recent hurricane-force low that developed over the northwest Pacific
this past weekend arrived earlier this morning (around 7-8 am HST)
at the offshore buoys northwest of Kauai. This swell will fill in
and build down the island chain through the overnight hours into
Thursday before trending down Friday. This swell appears to have
peaked between six and seven feet within the 13-15 second energy
band at the offshore buoys, which should correspond to peak surf
heights along exposed north and west facing shores remaining below
the high surf advisory threshold later tonight through Thursday.

Another, slightly larger, west-northwest (310 deg) swell associated
with a large area of gales south of a recent hurricane force low
near the western end of the Aleutian Islands should begin to fill in
through the day Friday, peak Friday night into Saturday, then
gradually ease through the remainder of the weekend. Advisory-level
surf will be likely at and around the peak of this swell along
exposed north and west facing shores. Arrival time of this source
may end up coming later than anticipated, considering recent trends
and current model differences between the ECMWF-WAVE and GFS-
WAVEWATCH III solutions shown (ECMWF is depicting a six to nine hr
later arrival time than the GFS-WAVEWATCH III at a point forecast
near Midway, where the swell is scheduled to arrive within the next
12 hrs). Later forecast packages will reflect this slightly later
arrival time.

A small northerly swell associated with a compact gale that was
analyzed around 1200 nautical miles north of the islands earlier
will fill in Thursday night and hold into Saturday before shifting
out of the northeast (020-030 deg) and rising Sunday night into
Monday. An earlier ASCAT pass clipped the western periphery of this
system and showed an area of 35 to 40 kt northerly winds favorable
for the local area.

Surf along east facing shores will remain near the advisory level of
8 ft (faces) tonight before steadily trending down Thursday through
the weekend as the trades weaken. Surf may begin to build once again
Sunday night into Monday as the aforementioned small northerly swell
shifts out of the northeast.

A small long-period (18-20 seconds) south-southwest (220 deg) will
fill in Friday night and hold into Saturday, which may support a
slight increase in surf along south facing shores through this


Small Craft Advisory until 6 PM HST Thursday for all Hawaiian




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