Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 300724
Area Forecast Discussion
National Weather Service Honolulu HI
924 PM HST Sat Apr 29 2017
Unsettled weather will impact the island chain through early next
week, as a front stalls out in the vicinity of Maui County and a
potent low aloft pivots southward across the western end of the
island chain. Heavy showers and 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.
Currently at the surface, a stacked area of low pressure (Kona
Low) is located around 275 miles north of Kauai and is shifting
southward at around 25 mph. Meanwhile, a nearly stationary front
remains just east of Maui and to the west of the Big Island.
Infrared satellite imagery shows mostly cloudy to overcast
conditions across the vast majority of the state, with some breaks
in the cloud cover over Kauai. Radar imagery shows a steady
stream of moderate to locally heavy rainfall with occasional
embedded thunderstorms moving northward across Maui County, with
showers also picking up over southern sections of the Big Island.
Across Oahu and Kauai in the drier airmass on the western side of
the front, showers remain isolated to widely scattered. Main short
term concern continues to revolve around the heavy rainfall
Model guidance is in fairly good agreement showing the Kona low
dropping steadily southward through the night. The stacked low
will begin to separate however, with the surface low tracking
southwestward to a location around 225 miles west-northwest of
Kauai by 18Z Sunday, while the closed upper low drops southward
over or just to the west of Kauai by 18Z Sunday. Both the ECMWF
and GFS show a steady stream of moderate to heavy rainfall and
thunderstorms continuing to move through Maui County overnight,
and this is where the most unsettled weather and highest potential
for flooding rains is anticipated. The ECMWF appears to shift the
moisture back westward toward Oahu by daybreak Sunday, but given
radar trends, support from the GFS, and the dry airmass firmly
entrenched over Oahu at the current time, only scattered showers
are expected overnight, with the same conditions expected across
Kauai. The Big Island is a bit more uncertain, but both the GFS
and ECMWF do indicate an increase in deep layer moisture streaming
over the island after midnight, so we do expect shower coverage
to increase through the night.
As for severe weather potential, there is plenty of deep layer
shear to support organized convection, but given the less than
ideal thermodynamic environment with surface based CAPE values
below 500 J/KG, along with marginally supportive mid-level lapse
rates, the probability of any organized severe weather is low.
Sunday through Monday night,
Model solutions are in good agreement showing the upper low
tracking southward Sunday and Sunday night 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 on Sunday, but the
ECMWF and to a lesser extent the GFS, do show some of the deeper
moisture getting drawn back westward into Oahu during the day.
Across Kauai, the drier low level airmass will likely keep shower
coverage scattered despite the cold upper level low moving nearly
directly overhead. By Sunday night 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 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 Sunday, with the highest potential for
flooding rains overnight expected across Maui County.
The Winter Storm Warning remains in effect for the Big Island
summits through Sunday, while the High Wind Warning remains in
effect through late Sunday night.
The Wind Advisory for the lower elevations of the Big Island
remains in effect as a result of the strong southerly winds
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 into next
Surface and upper air instability will affect the island chain
through Sunday 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/IFR
weather occurring over the eastern half of the state at least
AIRMETs for mountain obscuration are currently in effect for Maui,
Molokai, and Oahu; and for low- and/or middle-level turbulence
over all the islands.
The very 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 front extending south and southwest
near Maui County, with a 1009 mb low also centered around 275
miles north of Kauai. Locally heavy rain and scattered
thunderstorms will remain in the forecast as this Kona low drifts
toward the southwest, passing just west of Kauai Sunday through
Sunday night. Strong southerly winds are expected to continue
through Sunday east of the front, which includes the waters
surrounding the Big Island. As a result, the small craft advisory
for the Big Island waters will remain in place through early
Sunday evening. Forecast confidence in the strength of these
southerly winds around the Big Island remains low at this time and
may be further adjusted as more satellite observations come in
overnight. Winds are forecast to become light to moderate out of
the southeast Monday night through midweek as the low moves away
from the area. A return of a more typical trade wind pattern will
become a possibility through the second half of the upcoming week.
Out at the beaches, higher than normal high tides combined with
advisory-level surf along south facing shores have led to water
reaching areas over the past couple of evenings 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 will continue into
the upcoming week during and around the times of high tides. As a
result, beachgoers can expect the entire width 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 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 the compact gale-force low centered a few hundred
miles north-northwest of Kauai. Winds and surf are expected to trend
down into the upcoming week as the low passes to the west. A small
northwest (310-330 deg) long-period swell is expected to fill in
Wednesday and hold into the second half of the week. Otherwise, no
significant surf is expected at this time along north facing
beaches through the upcoming week.
Flash Flood Watch through Sunday afternoon for Oahu-Molokai-
High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Molokai-Lanai Makai-
Kahoolawe-Maui Leeward West-Maui Central Valley-Leeward
Haleakala-Kona-South Big Island.
Wind Advisory until 6 PM HST Sunday for Kona-South Big Island-
Big Island North and East-Kohala-Big Island Interior.
High Wind Warning until 6 AM HST Monday for Big Island Summits.
Winter Storm Warning from midnight tonight to 6 PM HST Sunday
for Big Island Summits.
Small Craft Advisory until 6 PM HST Sunday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.