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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 282019
Area Forecast Discussion
National Weather Service Honolulu HI
1019 AM HST SAT MAY 28 2016
A surface trough northwest of the state will keep winds light
through the weekend. A weak front will reach Kauai Sunday night
and Oahu on Memorial Day, then stall and dissipate over the
eastern islands by mid week. High pressure will build north of the
state behind the front, allowing the trade winds to return for
much of next week.
Water vapor imagery shows an upper level trough a couple hundred
miles northwest of the state, with a westerly jet on the south
side of the trough extending across the Big Island. At the
surface, a trough is located northwest of Kauai, extending to a
front farther to the north. 12z soundings show an inversion
around 9kft at Hilo and a weak one around 14kft at Lihue.
Precipitable waters at both locations were around 1.5 inches.
Early morning MIMIC total precipitable imagery shows drier air
over windward waters just northeast of the area from Oahu to the
Big Island, with the band of higher moisture from Kauai northward,
as well as across the leeward coastal waters.
There were still quite a few showers that lingered overnight,
despite the widespread land breezes that developed. A number of
rain gages on Oahu picked up a quarter to half an inch of rain
just over the past six hours. Regional radars have died down a bit
this morning, but are expected to light up again with afternoon
heating. A similar pattern is expected for today that we have seen
the past couple days, with onshore sea breezes and interior
clouds/showers developing by afternoon. High-resolution guidance
from the WRF NMM and ARW runs again show the strongest activity
located across Kauai and Oahu, closer to the moisture and
Some drying is expected over the next day or two, with
precipitable waters decreasing a bit, but overall little change is
expected in the sensible weather pattern. The surface trough
northwest of Kauai will pass over the islands, keeping winds light
and leading to a continuation of the locally-driven land and sea
breeze pattern. Convergence and heating/instability during the day
will lead to afternoon/evening showers. However, we may not see as
much clearing at night as would typically be expected due to
influence from the weak surface trough. This may account for the
showers that developed last night, and we may see a similar
situation for the next couple nights. Therefore have kept a slight
chance of showers for many areas during the overnight hours as
The 12z GFS and ECMWF are in better agreement with the weak front
expected to cross the area early next week, and within the high-
resolution model time frame there is decent agreement with the
approach through Monday. There are still some timing/placement
differences, which is not surprising given the weak steering and
lack of organized upper level dynamics. The main impacts for us
will be an increase in rain chances as the cloud band moves
through, and an increase in northeasterly trades behind it.
Current timing brings the band to Kauai on Sunday night and Oahu
on Memorial Day, leading to greater clouds and rainfall chances.
From Monday night into Wednesday, the front is expected to stall
and dissipate over the eastern islands. With a new high building
north of the state, this will lead to more of a wet trade wind
pattern, with moisture from the old front enhancing showers
across windward sections.
A light ESE wind flow on the large scale will be overridden by
island-scale sea breezes by mid-morning. As these winds converge
over the islands, low clouds and showers will increase, and then
remain in place into the evening hours. Greatest instability and
moisture are over Kauai and Oahu, where ISOL IFR conditions are
possible in +SHRA. These conditions will likely warrant the
issuance of AIRMET SIERRA for mountain obscuration.
Otherwise, light E winds are pushing low clouds and showers into
windward Big Island, and to a lesser degree, the other islands.
VFR will be the prevailing condition at most TAF sites, but MVFR
conditions are expected at PHNY for most of the day, mainly due to
ceilings. Brief MVFR conditions are possible at many of the TAF
sites in the afternoon as clouds and showers may drift overhead.
Winds will remain light through at least Sunday, but increase out
of the northeast as the new high builds north of the state. Winds
may approach small craft advisory levels where they are funneled
through the typically windier channels, but the current forecast
is maxed out at 20 knots. If the front dissipates faster or high
builds in stronger, we may need to increase the wind speed
forecast for a couple days mid week.
The current south swell continues to gradually diminish in size
and period, and will do so through the weekend. A small northwest
swell arriving Sunday will peak Monday well below the advisory
threshold. Another small south swell is expected late Tuesday
into Wednesday, before fading through the week.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office