NWS Bulletins for NPAC - MKWC
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Hawaiian Islands Synoptic Discussion and Guidance
890
FXHW60 PHFO 191302
AFDHFO
Area Forecast Discussion
National Weather Service Honolulu HI
302 AM HST Mon May 19 2025
.SYNOPSIS...
A high pressure ridge north of the Hawaiian Islands will remain
slightly weakened by a stalled frontal boundary far northwest of
the state. Expect light to moderate easterly trade winds today
becoming moderate to locally breezy from Tuesday onward. An upper
level trough and embedded lows will modulate shower activity over
the region this week. Brief periods of showers remain in the
forecast favoring the typical windward and mountain areas.
Increasing shower trends and thunderstorms are possible from
Thursday into Friday with much more stable trends returning by
this weekend.
&&
.DISCUSSION...
Starting with the water vapor satellite channel this morning we
are looking into the upper levels of the atmosphere. The water
vapor imagery shows a few weak upper lows drifting along a narrow
west to east oriented trough. The upper low that produced periods
of showers over the past few days continues to weaken and drift
northward farther away from the island chain. More stable conditions
are anticipated as a result, with lower shower coverage statewide
lasting into Wednesday. Several bands of clouds linger in the
easterly trade wind flow moving towards each island, however the
steering winds on these clouds is slightly from a more southeast
direction, driving these clouds more parallel to most island
mountains. The southeast and east slopes of the Big Island are the
exception to this rule, with some enhanced showers favored over
the Kau and Puna Districts of the island during the daytime and
evening hours.
Local radar imagery shows scattered pockets of showers moving up
into windward mountain slopes on the trade winds this morning.
These passing shower trends will show decreasing trends a few
hours after sunrise, then show increasing trends during the
overnight to early morning hours the next day. Weather balloon
soundings at 2 AM HST (12Z) show temperature inversion heights as
measured at Lihue and Hilo are roughly between 5,000 to 7,000
feet. These heights indicate a fairly typical trade wind weather
pattern with brief passing showers.
The overall weather pattern remains fairly consistent with the
subtropical ridge locked in place north of the state. A few
wrinkles in the forecast include a slight increase in trade wind
speeds from Tuesday onward as the stalled front far northwest of
the state dissipates, allowing the ridge to strengthen slightly
across the region. Another wrinkle to the forecast will occur with
the shower activity. Less shower coverage remains in the forecast
through Wednesday as the upper low drifts farther away; increasing
regional stability. By Thursday, another stronger low pressure
system moves in from the north and anchors over the western
islands by Wednesday night. Instability aloft associated with the
cold air and wind divergence surrounding this low will briefly
increasing showers on Thursday and Friday, some of these showers
may be heavy at times. Isolated thunderstorm activity is possible
although too early to add to the weather forecast grids at this
time due to uncertainties in the track and strength of this next
upper low. More stable trends return by this weekend with
temperature inversion heights in the 5,000 to 6,000 foot range,
capping shower development statewide.
Forecast confidence in this pattern can be challenging as much
depends upon our forecast guidance and our ability to correctly
predict the precise movement and intensity of these fairly weak
upper lows. However that said, forecast confidence in the
easterly trade winds is high, with moderate confidence in these
shower trends, and lower confidence in possible thunderstorm
impacts for Thursday and Friday.
&&
.AVIATION...
A hybrid moderate easterly trade wind and sea breeze pattern will
prevail today. Daytime sea breezes will develop along terrain
sheltered western slopes of each island. Cloud bands will form
over each island during the late morning to the afternoon as sea
breezes and trade winds converge. Trade winds will strengthen into
the moderate to locally breezy range from Tuesday onward as the
ridge builds in north of the state.
Brief passing showers are possible mainly over windward and
mountain areas and along converging cloud bands through Tuesday.
MVFR conditions may develop within any shower bands.
No AIRMETs are currently in effect.
&&
.MARINE...
Surface high pressure will remain anchored far northeast of the
state through mid-week, followed by another surface high building
north of the region by late-week. Moderate to locally fresh
easterly trades should keep conditions just below Small Craft
Advisory (SCA) criteria today. However, a slight uptick in trades
starting Tuesday will likely lead to a SCA issuance for the
typical windy waters around Maui and the Big Island. A lingering
upper level disturbance will allow for brief periods of wet
weather for the next several days.
A series of small south and south-southwest swells will maintain
small south shore through mid-week. A slightly larger long period
south swell will further boost south shore surf up to seasonal
averages late Thursday through early this weekend.
Surf heights along north and west-facing shores will remain small
during the next 7 days. Surf heights along east-facing shores
will remain small but choppy today, then become slightly large
Tuesday onward as trade winds increase a bit.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Bohlin
AVIATION...Bohlin
MARINE...JT
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
890
FXHW60 PHFO 191302
AFDHFO
Area Forecast Discussion
National Weather Service Honolulu HI
302 AM HST Mon May 19 2025
.SYNOPSIS...
A high pressure ridge north of the Hawaiian Islands will remain
slightly weakened by a stalled frontal boundary far northwest of
the state. Expect light to moderate easterly trade winds today
becoming moderate to locally breezy from Tuesday onward. An upper
level trough and embedded lows will modulate shower activity over
the region this week. Brief periods of showers remain in the
forecast favoring the typical windward and mountain areas.
Increasing shower trends and thunderstorms are possible from
Thursday into Friday with much more stable trends returning by
this weekend.
&&
.DISCUSSION...
Starting with the water vapor satellite channel this morning we
are looking into the upper levels of the atmosphere. The water
vapor imagery shows a few weak upper lows drifting along a narrow
west to east oriented trough. The upper low that produced periods
of showers over the past few days continues to weaken and drift
northward farther away from the island chain. More stable conditions
are anticipated as a result, with lower shower coverage statewide
lasting into Wednesday. Several bands of clouds linger in the
easterly trade wind flow moving towards each island, however the
steering winds on these clouds is slightly from a more southeast
direction, driving these clouds more parallel to most island
mountains. The southeast and east slopes of the Big Island are the
exception to this rule, with some enhanced showers favored over
the Kau and Puna Districts of the island during the daytime and
evening hours.
Local radar imagery shows scattered pockets of showers moving up
into windward mountain slopes on the trade winds this morning.
These passing shower trends will show decreasing trends a few
hours after sunrise, then show increasing trends during the
overnight to early morning hours the next day. Weather balloon
soundings at 2 AM HST (12Z) show temperature inversion heights as
measured at Lihue and Hilo are roughly between 5,000 to 7,000
feet. These heights indicate a fairly typical trade wind weather
pattern with brief passing showers.
The overall weather pattern remains fairly consistent with the
subtropical ridge locked in place north of the state. A few
wrinkles in the forecast include a slight increase in trade wind
speeds from Tuesday onward as the stalled front far northwest of
the state dissipates, allowing the ridge to strengthen slightly
across the region. Another wrinkle to the forecast will occur with
the shower activity. Less shower coverage remains in the forecast
through Wednesday as the upper low drifts farther away; increasing
regional stability. By Thursday, another stronger low pressure
system moves in from the north and anchors over the western
islands by Wednesday night. Instability aloft associated with the
cold air and wind divergence surrounding this low will briefly
increasing showers on Thursday and Friday, some of these showers
may be heavy at times. Isolated thunderstorm activity is possible
although too early to add to the weather forecast grids at this
time due to uncertainties in the track and strength of this next
upper low. More stable trends return by this weekend with
temperature inversion heights in the 5,000 to 6,000 foot range,
capping shower development statewide.
Forecast confidence in this pattern can be challenging as much
depends upon our forecast guidance and our ability to correctly
predict the precise movement and intensity of these fairly weak
upper lows. However that said, forecast confidence in the
easterly trade winds is high, with moderate confidence in these
shower trends, and lower confidence in possible thunderstorm
impacts for Thursday and Friday.
&&
.AVIATION...
A hybrid moderate easterly trade wind and sea breeze pattern will
prevail today. Daytime sea breezes will develop along terrain
sheltered western slopes of each island. Cloud bands will form
over each island during the late morning to the afternoon as sea
breezes and trade winds converge. Trade winds will strengthen into
the moderate to locally breezy range from Tuesday onward as the
ridge builds in north of the state.
Brief passing showers are possible mainly over windward and
mountain areas and along converging cloud bands through Tuesday.
MVFR conditions may develop within any shower bands.
No AIRMETs are currently in effect.
&&
.MARINE...
Surface high pressure will remain anchored far northeast of the
state through mid-week, followed by another surface high building
north of the region by late-week. Moderate to locally fresh
easterly trades should keep conditions just below Small Craft
Advisory (SCA) criteria today. However, a slight uptick in trades
starting Tuesday will likely lead to a SCA issuance for the
typical windy waters around Maui and the Big Island. A lingering
upper level disturbance will allow for brief periods of wet
weather for the next several days.
A series of small south and south-southwest swells will maintain
small south shore through mid-week. A slightly larger long period
south swell will further boost south shore surf up to seasonal
averages late Thursday through early this weekend.
Surf heights along north and west-facing shores will remain small
during the next 7 days. Surf heights along east-facing shores
will remain small but choppy today, then become slightly large
Tuesday onward as trade winds increase a bit.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Bohlin
AVIATION...Bohlin
MARINE...JT
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office