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Hawaiian Islands Synoptic Discussion and Guidance

810
FXHW60 PHFO 301651 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
651 AM HST Sun Apr 30 2017

.SYNOPSIS...
Unsettled weather will impact the island chain through early next
week, as a front remains in the vicinity of the central islands
and a potent upper level low pivots just to the south of the
western end of the state. Heavy showers and a slight chance of
thunderstorms are expected in association with these features,
with the most unsettled weather expected across the central and
eastern islands. A return to a more typical trade wind weather
pattern is then expected by the middle of next week.

&&

.DISCUSSION...
Currently at the surface, a Kona Low is located around 80 miles
north of Kauai and is shifting southward at around 20 mph.
Meanwhile, a nearly stationary front remains just to the west of
the Big Island. Infrared satellite imagery shows mostly cloudy to
overcast conditions across the state. Radar imagery shows a
steady stream of moderate to locally heavy rainfall moving
northward through Oahu and Maui County, with numerous showers
impacting the Big Island and scattered showers across Kauai.
Main short term concern continues to revolve around the potential
for heavy rainfall.

Today through Monday night,
Model solutions are in good agreement showing the Kona low
tracking south of Kauai today and tonight then stalling out and
weakening between 200 and 300 miles south of Kauai Monday and
Monday night. The heaviest rainfall threat is expected to continue
to focus over Maui County and the Big Island today, although some
moderate to locally heavy rainfall is also expected across Oahu.
On Kauai, the drier low level airmass will likely keep shower
coverage scattered despite the cold upper level low moving nearly
directly overhead. By tonight the nearly stationary front or
trough running through Maui County is expected to have dissipated,
with the deepest moisture and best instability shifting off to
the south of the island chain. As a result, we should see a
decrease in the potential for widespread heavy rainfall. That
said, with the upper level low remaining in close proximity to the
islands along with lingering deep layer moisture, scattered to
numerous showers will need to remain in the forecast tonight through
Monday night, with the best chances for rain expected across the
central and eastern end of the state.

The Flash Flood Watch remains in effect from Oahu eastward to the
Big Island through late this afternoon, with the highest
potential for flooding rains expected across Maui County and the
Big Island today.

The Winter Storm Warning remains in effect for the Big Island
summits through late this afternoon, while the High Wind Warning
remains in effect through late tonight.

The Wind Advisory for the lower elevations has been cancelled as
winds have remained below advisory levels through the night and
are expected to decrease further through the day today.

Tuesday through next Saturday,
There remains some differences between the GFS and ECMWF with
respect to how quickly ridging aloft builds in from the west and
shifts the lingering instability and deeper moisture off to the
south and west of the island chain. Overall however, the trend
will be toward improving conditions, with a more typical trade
wind pattern expected to overspread the state Wednesday through
next weekend.

&&

.AVIATION...
Surface and upper air instability will affect the island chain
through today and beyond. The features are slow-moving and will
take considerable time to clear the area. Periods of rain,
moderate to heavy showers, and isolated thunderstorms will
persist, with the most unstable conditions and MVFR (and isolated
IFR) weather occurring over the isles from Oahu to the Big Island
through the day.

AIRMETs for mountain obscuration are currently in effect for Maui,
Molokai, and Lanai; and for middle- to high-level turbulence over
Maui County, the Big Island, and their adjacent waters. Also, an
area of light icing is located from Molokai to the Big Island in
the layer from 120 to FL200.

&&

.MARINE...
The active weather pattern will persist into the upcoming week as a
Kona low continues to impact the area waters. The latest surface
analysis showed a frontal boundary extending south and southwest
near Maui County from a 1010 mb low centered around 200 nm northwest
of Kauai. Locally heavy showers and isolated thunderstorms will
remain in the forecast as this low drifts toward the southwest,
passing west of Kauai today through tonight. As this low begins to
track away from the state tonight into Monday, the previously
mentioned frontal boundary near Maui County will continue to weaken
and shift westward as a trough. Although a blend of the latest
guidance depicts strong southeast winds setting up over portions of
the Maui County and Big Island waters tonight into Monday, forecast
confidence remains too low at this point to include a small craft
advisory. Another front is forecast to pass north of the waters
Tuesday through Wednesday. Strong high pressure will build north of
the waters in its wake Wednesday through Thursday, likely
introducing a return of a more typical trade wind pattern.

Out at the beaches, higher than normal high tides combined with
advisory-level surf along south facing shores over the past couple
of evenings have led to water reaching areas that is typically
not observed. The latest preliminary water levels observed in
Honolulu continue to run around one foot above predicted levels.
The PacIOOS guidance that combines wave-runup (associated with the
surf) and water levels depicts this trend continuing into the
upcoming week during and around the times of high tides. As a
result, beachgoers can expect a large portion of some beaches to
become inundated during these high tide cycles. For the upcoming
few days (through Tuesday), the best chances for these conditions
will be through the evening hours, especially as a reinforcing
south swell fills in Sunday night through Tuesday. These higher
than normal high tides are forecast to trend back down toward
normal levels by the end of the week or below impact levels. This
reinforcing south swell should hold into midweek before trending
down and may near advisory levels once again Monday through
Tuesday. Mainly background southerly swells are expected through
the second half of the week.

Surf along north facing shores, especially for Kauai and Oahu, have
been rough over the past 24 hrs due to a combination of fresh
northerly winds and a short-period northerly swell that was
generated from a compact low centered a couple of hundred nautical
miles northwest of Kauai (reached gale force level earlier in the
weekend). Winds and surf are expected to trend down into the
upcoming week as the low passes to the west and weakens. Two small,
long-period, northwest swells associated with activity over the
northwest Pacific are forecast to fill in this week and generate
small to moderate surf along north facing shores. The first one
is forecast to fill in late Monday and hold through Tuesday night.
The second source is expected to arrive Wednesday and hold into
the second half of the week.

Small surf will remain likely along east facing shores into the
first half of the week. Surf will slowly begin to trend back up
later in the week as strong high pressure builds north of the region
and trade winds return.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Oahu-Molokai-Lanai-
Kahoolawe-Maui-Big Island.

High Wind Warning until 6 AM HST Monday for Big Island Summits.

Winter Storm Warning until 6 PM HST this evening for Big Island
Summits.

&&

$$

DISCUSSION...Jelsema
AVIATION...Kinel
MARINE...Gibbs

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



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