NWS Bulletins for NPAC - MKWC
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Hawaiian Islands Synoptic Discussion and Guidance
784
FXHW60 PHFO 151400
AFDHFO
Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed May 15 2024
.SYNOPSIS...
A kona low developing about 600 miles north of Kauai today will
intensify and meander around well northwest of the islands into
next week. The low will turn our local winds southerly, pulling
copious moisture from the deep tropics over the islands. A slow
moving band of downpours and a few thunderstorms is expected to
develop near Oahu and possibly Maui county Wednesday night into
Thursday, then slowly shift westward toward Kauai by Friday.
Excessive rainfall and flash flooding will be a significant
concern. Other areas away from the main band could see spotty
downpours and localized flooding problems as well. The weather
over the Aloha State this weekend and beyond will depend highly on
the later evolution and movement of the kona low far to our
northwest.
&&
.DISCUSSION...
We continue to watch the strongly digging shortwave trough on
water vapor imagery early this morning to the NNW of the islands.
The late evening scatterometer pass showed that cyclogenesis may
be beginning as expected about 600 miles or so N of Kauai. Both
the GOES-derived and CIMSS-MIMIC total precipitable water fields
are showing a large wedge of relatively moist air pushing
northward across the area surrounding Hawaii, with even deeper
ITCZ moisture well to our S beginning to creep N under the
influence of the sharply digging trough aloft. A gradual increase
in the number of showers has been noted over the local area over
the last several hours. The showers over the offshore waters
growing stronger with gradually cooling cloud tops. The GLM and
ground- based lightning detection networks are showing a few
thundershowers over the northern offshore waters.
The convection allowing models suggest that today we should
continue to see a gradual increase in the number and intensity of
showers moving up from the south, mainly around Kauai and Oahu.
Starting tonight, as upper difluence moves over the low level
moisture convergence, the high resolution and global models
suggest a more organized band of heavy showers and embedded
thundershowers will develop over or near Oahu, which could linger
into Thu. There's quite a bit of uncertainty among the various
models about how intense this band could be, but utilized model
consensus and NBM to set up the location and timing of heaviest
QPF which continues to seem reasonable.
The Big Island may remain too far E to see much heavy rain from
this system, aside from some heavy afternoon showers or
thundershowers, especially over the slopes. However, not feeling
confident enough yet to consider dropping the Flood Watch for
that island and would prefer to wait and see how things evolve
over the next 12 to 24 hours, since active convection can
sometimes wreak havoc on what might appear to be reasonable CAM
solutions.
The global models and CAMs show that the band may break up or
weaken a bit by Thu night as the best difluence and stronger
shortwave energy shifts NE of the islands. Heavy showers will
still be possible but they may become more spotty and less
organized for a time before reintensification starting Friday. One
of the big questions that remains will be how far W will the band
be when it reintensifies under stronger upper level difluence Fri
into the weekend. The NBM thunderstorm probabilities show values
increasing again near Kauai on Friday before shifting westward to
the coastal and offshore waters W of Kauai Fri night. Global model
rain rates increase quite a bit within the band this weekend, but
by then hopefully the heaviest rain is falling out over the
water. Still, the models can change on these details and we'll
need to keep an eye on this.
One other thing to note is the possibility of some gusty
downslope winds on the normal-windward side of the islands. The
models don't seem to be super-stoked about the risk for damaging
downslope winds, but some gusts over 30 knots appear possible on
Wed into Wed night. Adjusted the wind gust grids to raise gusts in
the favorable downslope areas, using the University of Hawaii
WRF-NMM 950 mb winds.
The global models show that the middle and eastern islands this
weekend will have an odd combination of very moist air, with
precipitable water near 2 inches...but also a low level subsidence
inversion that may try to keep somewhat of a lid the moisture.
Nevertheless, with ESE or SE flow continuing, there should be some
fairly prolific afternoon showers in any areas shadowed by the
background flow. Kauai remains a big question mark during this
period as it will continue to be on the periphery of the deeper
moisture and strong dynamics just to the west.
There's considerable uncertainty after the weekend, with lingering
moisture gradually diminishing and the kona low far to the north
likely weakening, leaving a surface trough behind W of the islands.
There should be a continuing trend toward more settled weather
statewide, with weak mid-level ridging eventually trying to build
in. In the meantime, there will be a daily chance of spotty heavy
showers, mainly in the afternoons and evenings.
&&
.AVIATION...
An active weather pattern will continue through the rest of the
week, especially as a kona low develops about 600 nm to the
northwest of Kauai. Showers and a few thunderstorms will bring the
potential for periodic MVFR or lower conditions on and off
throughout the period. The main focus for showers and the
occasional thunderstorm will likely be around Oahu and Kauai
today and tonight as the kona low taps into some tropical moisture
to the south and pulls it up through the western half of the
state. Moderate southerly winds are expected today with higher
gusts possible within/near heavy showers or thunderstorms, then
winds will become light once again overnight.
AIRMET Sierra is in effect for all of the smaller islands and
windward Big Island for tempo mountain obscuration. Sierra may be
needed for IFR conditions and Tango for turbulence as the kona
low gets closer and conditions deteriorate.
&&
.MARINE...
Low pressure developing north of Kauai will usher a cold front
toward the state from the northwest today. The front will advance
eastward until stalling over Oahu late Thursday and weakening.
The remnant boundary will then drift west during the remainder of
the week. Heavy showers and isolated thunderstorms are expected
along and ahead of the front.
A series of overlapping, long period S swells will maintain near
to below average surf along S facing shores through the near
term. Renewed energy out of the southerly quadrant will then fill
in late tomorrow night bringing above average surf by Thursday.
Surf remains elevated in through the weekend.
The existing small medium period NW swell will peak today. By
tonight, a short period fresh NW swell associated with the
developing low will spread into the area. Bumped the official
forecast up slightly higher than guidance given the propensity for
the guidance to undersimulate the magnitude of fresh swell. The
forecast peak is near to just below the High Surf Advisory
threshold Thursday into Friday.
West facing shores see a bump in surf through the week due to
overlapping NW swell and S swells wrapping into exposed areas.
Surf along E facing shores remains well below normal through the
weekend.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday evening for all Hawaii islands-
&&
$$
DISCUSSION...R Ballard
AVIATION...Farris
MARINE...JVC
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office
Narrow the Menu List
Select Time Limit: 12 hours | 24 hours | 48 hours | 72 hours | No time limit
Select Product Type: All | Routine Bulletins/FCSTS | Warnings/Watches/Advisories | HAWN Weather | Tropical | Marine | Aviation | Daily Obs | Special
Hawaiian Islands Synoptic Discussion and Guidance
784
FXHW60 PHFO 151400
AFDHFO
Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed May 15 2024
.SYNOPSIS...
A kona low developing about 600 miles north of Kauai today will
intensify and meander around well northwest of the islands into
next week. The low will turn our local winds southerly, pulling
copious moisture from the deep tropics over the islands. A slow
moving band of downpours and a few thunderstorms is expected to
develop near Oahu and possibly Maui county Wednesday night into
Thursday, then slowly shift westward toward Kauai by Friday.
Excessive rainfall and flash flooding will be a significant
concern. Other areas away from the main band could see spotty
downpours and localized flooding problems as well. The weather
over the Aloha State this weekend and beyond will depend highly on
the later evolution and movement of the kona low far to our
northwest.
&&
.DISCUSSION...
We continue to watch the strongly digging shortwave trough on
water vapor imagery early this morning to the NNW of the islands.
The late evening scatterometer pass showed that cyclogenesis may
be beginning as expected about 600 miles or so N of Kauai. Both
the GOES-derived and CIMSS-MIMIC total precipitable water fields
are showing a large wedge of relatively moist air pushing
northward across the area surrounding Hawaii, with even deeper
ITCZ moisture well to our S beginning to creep N under the
influence of the sharply digging trough aloft. A gradual increase
in the number of showers has been noted over the local area over
the last several hours. The showers over the offshore waters
growing stronger with gradually cooling cloud tops. The GLM and
ground- based lightning detection networks are showing a few
thundershowers over the northern offshore waters.
The convection allowing models suggest that today we should
continue to see a gradual increase in the number and intensity of
showers moving up from the south, mainly around Kauai and Oahu.
Starting tonight, as upper difluence moves over the low level
moisture convergence, the high resolution and global models
suggest a more organized band of heavy showers and embedded
thundershowers will develop over or near Oahu, which could linger
into Thu. There's quite a bit of uncertainty among the various
models about how intense this band could be, but utilized model
consensus and NBM to set up the location and timing of heaviest
QPF which continues to seem reasonable.
The Big Island may remain too far E to see much heavy rain from
this system, aside from some heavy afternoon showers or
thundershowers, especially over the slopes. However, not feeling
confident enough yet to consider dropping the Flood Watch for
that island and would prefer to wait and see how things evolve
over the next 12 to 24 hours, since active convection can
sometimes wreak havoc on what might appear to be reasonable CAM
solutions.
The global models and CAMs show that the band may break up or
weaken a bit by Thu night as the best difluence and stronger
shortwave energy shifts NE of the islands. Heavy showers will
still be possible but they may become more spotty and less
organized for a time before reintensification starting Friday. One
of the big questions that remains will be how far W will the band
be when it reintensifies under stronger upper level difluence Fri
into the weekend. The NBM thunderstorm probabilities show values
increasing again near Kauai on Friday before shifting westward to
the coastal and offshore waters W of Kauai Fri night. Global model
rain rates increase quite a bit within the band this weekend, but
by then hopefully the heaviest rain is falling out over the
water. Still, the models can change on these details and we'll
need to keep an eye on this.
One other thing to note is the possibility of some gusty
downslope winds on the normal-windward side of the islands. The
models don't seem to be super-stoked about the risk for damaging
downslope winds, but some gusts over 30 knots appear possible on
Wed into Wed night. Adjusted the wind gust grids to raise gusts in
the favorable downslope areas, using the University of Hawaii
WRF-NMM 950 mb winds.
The global models show that the middle and eastern islands this
weekend will have an odd combination of very moist air, with
precipitable water near 2 inches...but also a low level subsidence
inversion that may try to keep somewhat of a lid the moisture.
Nevertheless, with ESE or SE flow continuing, there should be some
fairly prolific afternoon showers in any areas shadowed by the
background flow. Kauai remains a big question mark during this
period as it will continue to be on the periphery of the deeper
moisture and strong dynamics just to the west.
There's considerable uncertainty after the weekend, with lingering
moisture gradually diminishing and the kona low far to the north
likely weakening, leaving a surface trough behind W of the islands.
There should be a continuing trend toward more settled weather
statewide, with weak mid-level ridging eventually trying to build
in. In the meantime, there will be a daily chance of spotty heavy
showers, mainly in the afternoons and evenings.
&&
.AVIATION...
An active weather pattern will continue through the rest of the
week, especially as a kona low develops about 600 nm to the
northwest of Kauai. Showers and a few thunderstorms will bring the
potential for periodic MVFR or lower conditions on and off
throughout the period. The main focus for showers and the
occasional thunderstorm will likely be around Oahu and Kauai
today and tonight as the kona low taps into some tropical moisture
to the south and pulls it up through the western half of the
state. Moderate southerly winds are expected today with higher
gusts possible within/near heavy showers or thunderstorms, then
winds will become light once again overnight.
AIRMET Sierra is in effect for all of the smaller islands and
windward Big Island for tempo mountain obscuration. Sierra may be
needed for IFR conditions and Tango for turbulence as the kona
low gets closer and conditions deteriorate.
&&
.MARINE...
Low pressure developing north of Kauai will usher a cold front
toward the state from the northwest today. The front will advance
eastward until stalling over Oahu late Thursday and weakening.
The remnant boundary will then drift west during the remainder of
the week. Heavy showers and isolated thunderstorms are expected
along and ahead of the front.
A series of overlapping, long period S swells will maintain near
to below average surf along S facing shores through the near
term. Renewed energy out of the southerly quadrant will then fill
in late tomorrow night bringing above average surf by Thursday.
Surf remains elevated in through the weekend.
The existing small medium period NW swell will peak today. By
tonight, a short period fresh NW swell associated with the
developing low will spread into the area. Bumped the official
forecast up slightly higher than guidance given the propensity for
the guidance to undersimulate the magnitude of fresh swell. The
forecast peak is near to just below the High Surf Advisory
threshold Thursday into Friday.
West facing shores see a bump in surf through the week due to
overlapping NW swell and S swells wrapping into exposed areas.
Surf along E facing shores remains well below normal through the
weekend.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday evening for all Hawaii islands-
&&
$$
DISCUSSION...R Ballard
AVIATION...Farris
MARINE...JVC
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office