2025 Indicators: SST's, MSLP, Shear, SAL, Steering, Instability (Day 16+ Climate Models)

[cycloneye]

Bryan Norcross says in this video that it will be a fuzzy season to predict if neutral ENSO dominates.

https://www.youtube.com/watch?v=mtE7tS66Ke0

[cycloneye]

 https://x.com/WorldClimateSvc/status/1905347920490004617

[DorkyMcDorkface]

That's quite a persistent -NAO dip modeled on both the EPS and GEFS into the first half of April. I'd be shocked if we don't see a rebound in MDR SSTs after this.

[cycloneye]

@AndyHazelton
This winter has had a very different trade wind regime across the Atlantic than the last two - stronger easterlies across the East tropical Atlantic and Caribbean. As a result, the East Atlantic is cooling off pretty noticeably over the last couple months. Could help cap the hurricane season some, although it's still early.

 https://x.com/AndyHazelton/status/1905958970231021838

[mixedDanilo.E]

That's quite a persistent -NAO dip modeled on both the EPS and GEFS into the first half of April. I'd be shocked if we don't see a rebound in MDR SSTs after this.
https://i.ibb.co/sdvNz8DY/download-14.png
https://i.ibb.co/Dfj2x5s3/download-13.png

It always seems like around this time as we get into April, we get a pretty substantial -NAO which then helps warm the Atlantic up significantly. We saw it in 2023, 2017 and even 2020.

[cycloneye]

MDR is still 8th warmest on record despite the cooling.

 https://x.com/BenNollWeather/status/1906683542052254171




 https://x.com/BenNollWeather/status/1906683546200408259

[cycloneye]

CanSIPS is up.

 https://x.com/dmorris9661/status/1906876729194426776




 https://x.com/dmorris9661/status/1906876731551600816

[WeatherBoy2000]

That's quite a persistent -NAO dip modeled on both the EPS and GEFS into the first half of April. I'd be shocked if we don't see a rebound in MDR SSTs after this.
https://i.ibb.co/sdvNz8DY/download-14.png
https://i.ibb.co/Dfj2x5s3/download-13.png

[DorkyMcDorkface]

 https://x.com/AndyHazelton/status/1908506261055750594

[mixedDanilo.E]

 https://x.com/daniloevan11/status/1908552307840635139

[USTropics]

We're likely to see discussions/twitter posts on wave breaking events and comparison to other seasons, and I wanted to take some time to try and explain this concept as simply as possible. I’ll start by saying wave breaking and their components is an extremely complex and relatively nuanced area in meteorology. This is something we spend a great deal of time studying in the research track at FSU (almost all of Dynamics II is devoted to just this topic). I’ll do my best to break this down as simplistically as possible, how to visualize it, why it’s important for troughs/ridges, and ultimately what wave breaking actually is.

To fully understand all of this, we first need to know what types of waves we’re dealing with and how they form.

Rossby waves are large-scale, planetary waves in the atmosphere that arise due to the Earth's rotation and the variation of the Coriolis effect with latitude (known as the beta effect). In the atmosphere, they’re most prominent in the mid-latitudes within the jet stream (think african easterly waves for hurricanes, but for the mid-latitudes instead of the tropics). There are three main factors that lead to their formation:

1. Earth’s Rotation (Coriolis Effect): As air moves south -> north, the Coriolis force deflects it to the right in the Northern Hemisphere. This has a minimum at the equator (no spin -> no cyclones) and is at a maximum in the northern latitudes.
2. Temperature gradients: The jet stream forms where cold polar air meets warmer subtropical air, creating a sharp temperature contrast. This gradient fuels the jet’s speed and sets the stage for waves.
3. Conservation of Potential Vorticity: As air moves in the jet stream, it conserves its total vorticity (a measure of rotation). When air is displaced northward or southward, it stretches or compresses vertically, changing its relative vorticity and creating wave-like undulations.

Rossby waves actually move east -> west in the northern hemisphere, but the background flow (e.g., jet stream) carries them west -> east. A good way to visualize this is:

1. Background Flow (The Train Track): Imagine the jet stream as a train track with trains moving eastward at a steady speed, say 50 mph. This represents the prevailing westerly winds that carry weather systems across the mid-latitudes.
2. Rossby Wave Propagation (The Ball): Picture someone standing on the moving train throwing a ball backward (westward) relative to the train, at a speed of 20 mph. The ball represents the Rossby wave’s westward propagation relative to the jet stream’s flow.
3. Viewer (Fixed at the Platform/Station): You’re standing still at a train station, watching this all unfold. The station is like the Earth’s surface, a fixed reference point from which you observe the net motion.

Now how it all plays out:

1. From the Train’s Perspective: On the train (within the jet stream), the ball (Rossby wave) moves westward at 20 mph relative to the passengers. This is the intrinsic westward propagation of Rossby waves due to the beta effect and vorticity dynamics.
2. From the Station’s Perspective: As the train speeds eastward at 50 mph, you, the viewer at the station, see the ball’s net motion. The ball’s speed relative to you is the train’s speed (50 mph eastward) minus the ball’s backward throw (20 mph westward), so it moves eastward at 30 mph. This is the observed eastward movement of Rossby waves when their westward propagation speed is less than the jet stream’s eastward flow.

This is a cartoon diagram of the above (we’ll get to wave breaking soon):


Why do these Rossby waves propagate westward to begin with? This gets a bit technical, but let’s take a snapshot of what is going on with each individual train as they move eastward. Below is a diagram of air particles instead of balls, and we can see how they influence one another and propagate westward:


Why are Rossby waves so important? They essentially are responsible for shaping the jet stream into alternating ridges (northward bulges, associated with warm air and high pressure) and troughs (southward dips, linked to cold air and low pressure). These features drive weather, and in practice, you’ll see Rossby waves as meanders in the jet stream on weather maps, with wavenumbers typically ranging from 4 to 8 (meaning 4-8 ridges/troughs around the globe).


Let’s take a quick pit stop. We read about short waves and long waves all the time, but what exactly are these? Let's go back to the train analogy/cartoon above and imagine someone throwing a ball again:

1. Short Waves: If the person throws the ball westward at a slower speed (e.g., 10 mph), the net eastward speed from the station is 50 - 10 = 40 mph. Short-wavelength Rossby waves propagate westward more slowly relative to the flow, so they zip eastward quickly from the ground.
2. Long Waves: If the throw is faster (e.g., 40 mph westward), the net speed is 50 - 40 = 10 mph eastward—much slower. Very long waves might even match the train’s speed (50 mph westward), appearing stationary (0 mph net) or moving westward if the throw exceeds the train’s speed. This mirrors how long Rossby waves can stall or retrograde.

We see these Rossby wave actions almost weekly. Think of a spring storm system in the U.S.: The jet stream pressure perturbation(train) races eastward at 100 mph along the Jetstream Railway. A Rossby wave (ball) propagates westward at 30 mph relative to the jet. From the ground (station), you see the wave move eastward at 70 mph, dragging a trough that spawns thunderstorms as it crosses the Plains.

So what happens when these waves get too intense or unstable? That’s where wave breaking comes in—a process that reshapes the jet stream and weather patterns. Imagine a Rossby wave as a stretched rubber band in the jet stream. Normally, it wiggles along, forming ridges and troughs:


But if it gets stretched too far—say, by a sharp temperature clash or screaming jet stream winds—it snaps or folds.


We’ll skip all the math and derivations part for why this occurs, but the end result is that omega (the wave’s frequency) can not exceed N (static stability, or buoyancy required to propagate and oscillate the wave). If the frequency does exceed N (e.g., due to a strong temperature contrast or high background shear environment being advected), it can’t hold its shape. Like an ocean wave crashing on the shore, it ‘breaks,’ twisting the jet stream into different shapes. Examples include the jet stream twisting into a cutoff high (a warm, sunny bubble) or a cutoff low (a cold, stormy pocket). For example, a cyclonic wave break might deepen a trough, park a low over the Midwest, and dump rain for days. These breaks even mess with hurricanes by boosting upper-level winds that tilt and weaken them.

In the above train cartoon, we see an anticyclonic wave breaking pattern, where the ridge stretches and folds southward, creating a cut off high. Want to know the most common reason for a cut-off low to form? Cyclonic wave breaking is where the trough elongates and folds northward, forming a cut-off low. These Rossby wave breaking events can also influence tropical cyclones by altering the wind shear profile, such as increasing upper-level westerly winds over the hurricane, tilting its structure, and weakening the vorticity envelope.

Diagram of the two different wave breaking events


For more information, check out this climate.gov article that has some really good information as well - https://www.climate.gov/news-features/b ... rns-global

[Category5Kaiju]

We're likely to see discussions/twitter posts on wave breaking events and comparison to other seasons, and I wanted to take some time to try and explain this concept as simply as possible. I’ll start by saying wave breaking and their components is an extremely complex and relatively nuanced area in meteorology. This is something we spend a great deal of time studying in the research track at FSU (almost all of Dynamics II is devoted to just this topic). I’ll do my best to break this down as simplistically as possible, how to visualize it, why it’s important for troughs/ridges, and ultimately what wave breaking actually is.

To fully understand all of this, we first need to know what types of waves we’re dealing with and how they form.

Rossby waves are large-scale, planetary waves in the atmosphere that arise due to the Earth's rotation and the variation of the Coriolis effect with latitude (known as the beta effect). In the atmosphere, they’re most prominent in the mid-latitudes within the jet stream (think african easterly waves for hurricanes, but for the mid-latitudes instead of the tropics). There are three main factors that lead to their formation:

1. Earth’s Rotation (Coriolis Effect): As air moves south -> north, the Coriolis force deflects it to the right in the Northern Hemisphere. This has a minimum at the equator (no spin -> no cyclones) and is at a maximum in the northern latitudes.
2. Temperature gradients: The jet stream forms where cold polar air meets warmer subtropical air, creating a sharp temperature contrast. This gradient fuels the jet’s speed and sets the stage for waves.
3. Conservation of Potential Vorticity: As air moves in the jet stream, it conserves its total vorticity (a measure of rotation). When air is displaced northward or southward, it stretches or compresses vertically, changing its relative vorticity and creating wave-like undulations.

Rossby waves actually move east -> west in the northern hemisphere, but the background flow (e.g., jet stream) carries them west -> east. A good way to visualize this is:

1. Background Flow (The Train Track): Imagine the jet stream as a train track with trains moving eastward at a steady speed, say 50 mph. This represents the prevailing westerly winds that carry weather systems across the mid-latitudes.
2. Rossby Wave Propagation (The Ball): Picture someone standing on the moving train throwing a ball backward (westward) relative to the train, at a speed of 20 mph. The ball represents the Rossby wave’s westward propagation relative to the jet stream’s flow.
3. Viewer (Fixed at the Platform/Station): You’re standing still at a train station, watching this all unfold. The station is like the Earth’s surface, a fixed reference point from which you observe the net motion.

Now how it all plays out:

1. From the Train’s Perspective: On the train (within the jet stream), the ball (Rossby wave) moves westward at 20 mph relative to the passengers. This is the intrinsic westward propagation of Rossby waves due to the beta effect and vorticity dynamics.
2. From the Station’s Perspective: As the train speeds eastward at 50 mph, you, the viewer at the station, see the ball’s net motion. The ball’s speed relative to you is the train’s speed (50 mph eastward) minus the ball’s backward throw (20 mph westward), so it moves eastward at 30 mph. This is the observed eastward movement of Rossby waves when their westward propagation speed is less than the jet stream’s eastward flow.

This is a cartoon diagram of the above (we’ll get to wave breaking soon):
https://i.imgur.com/Tqf0MNS.png

Why do these Rossby waves propagate westward to begin with? This gets a bit technical, but let’s take a snapshot of what is going on with each individual train as they move eastward. Below is a diagram of air particles instead of balls, and we can see how they influence one another and propagate westward:
https://i.imgur.com/TlaV8sy.png

Why are Rossby waves so important? They essentially are responsible for shaping the jet stream into alternating ridges (northward bulges, associated with warm air and high pressure) and troughs (southward dips, linked to cold air and low pressure). These features drive weather, and in practice, you’ll see Rossby waves as meanders in the jet stream on weather maps, with wavenumbers typically ranging from 4 to 8 (meaning 4-8 ridges/troughs around the globe).


Let’s take a quick pit stop. We read about short waves and long waves all the time, but what exactly are these? Let's go back to the train analogy/cartoon above and imagine someone throwing a ball again:

1. Short Waves: If the person throws the ball westward at a slower speed (e.g., 10 mph), the net eastward speed from the station is 50 - 10 = 40 mph. Short-wavelength Rossby waves propagate westward more slowly relative to the flow, so they zip eastward quickly from the ground.
2. Long Waves: If the throw is faster (e.g., 40 mph westward), the net speed is 50 - 40 = 10 mph eastward—much slower. Very long waves might even match the train’s speed (50 mph westward), appearing stationary (0 mph net) or moving westward if the throw exceeds the train’s speed. This mirrors how long Rossby waves can stall or retrograde.

We see these Rossby wave actions almost weekly. Think of a spring storm system in the U.S.: The jet stream pressure perturbation(train) races eastward at 100 mph along the Jetstream Railway. A Rossby wave (ball) propagates westward at 30 mph relative to the jet. From the ground (station), you see the wave move eastward at 70 mph, dragging a trough that spawns thunderstorms as it crosses the Plains.

So what happens when these waves get too intense or unstable? That’s where wave breaking comes in—a process that reshapes the jet stream and weather patterns. Imagine a Rossby wave as a stretched rubber band in the jet stream. Normally, it wiggles along, forming ridges and troughs:
https://i.imgur.com/6B7QmCo.png

But if it gets stretched too far—say, by a sharp temperature clash or screaming jet stream winds—it snaps or folds.
https://i.imgur.com/h6dP3b7.png

We’ll skip all the math and derivations part for why this occurs, but the end result is that omega (the wave’s frequency) can not exceed N (static stability, or buoyancy required to propagate and oscillate the wave). If the frequency does exceed N (e.g., due to a strong temperature contrast or high background shear environment being advected), it can’t hold its shape. Like an ocean wave crashing on the shore, it ‘breaks,’ twisting the jet stream into different shapes. Examples include the jet stream twisting into a cutoff high (a warm, sunny bubble) or a cutoff low (a cold, stormy pocket). For example, a cyclonic wave break might deepen a trough, park a low over the Midwest, and dump rain for days. These breaks even mess with hurricanes by boosting upper-level winds that tilt and weaken them.

In the above train cartoon, we see an anticyclonic wave breaking pattern, where the ridge stretches and folds southward, creating a cut off high. Want to know the most common reason for a cut-off low to form? Cyclonic wave breaking is where the trough elongates and folds northward, forming a cut-off low. These Rossby wave breaking events can also influence tropical cyclones by altering the wind shear profile, such as increasing upper-level westerly winds over the hurricane, tilting its structure, and weakening the vorticity envelope.

Diagram of the two different wave breaking events
https://i.imgur.com/Q3YKdAu.png

For more information, check out this climate.gov article that has some really good information as well - https://www.climate.gov/news-features/b ... rns-global

If I could give you 10 likes, I would. This is a really informative and detailed post, thank you very much for it.

[cycloneye]

April NMME run is up.

@AndyHazelton
NMME is showing a cool neutral ENSO look for Atlantic hurricane season. This is favorable for the Atlantic, but the combination of an Atlantic Niña and warm subtropics (with a near average MDR) looks to suppress activity in the deep tropics. The ITCZ also looks shifted pretty far N. Verbatim, this looks like a setup for a near to slightly above average season, with tracks probably shifted NE compared to last year. We'll see how all these pieces align.

 https://x.com/AndyHazelton/status/1909232726894321972

[TomballEd]

From today's trip to the weather twitters. Deep tropic suppression because of warm sub-tropics would seem, to me, to decrease risk of a major landfall in the Caribbean and Central/North America.

 https://x.com/AndyHazelton/status/1909232726894321972

[DorkyMcDorkface]

That's quite a persistent -NAO dip modeled on both the EPS and GEFS into the first half of April. I'd be shocked if we don't see a rebound in MDR SSTs after this.
https://i.ibb.co/sdvNz8DY/download-14.png
https://i.ibb.co/Dfj2x5s3/download-13.png

Appears to be starting now:

[cycloneye]

Andy has a video about the factors.

 https://x.com/AndyHazelton/status/1909649181276352653

[Category5Kaiju]

And here goes the tropical Atlantic warmup....

[cycloneye]

C3S model.

@OSUWXGUY
C3S seasonal modeling is out - largely confirms that the Atlantic hurricane season is likely to be near to somewhat above the recent active era level of activity

UKMET Precip Anoms for July-Sep 2025 vs 2024 - largely reflecting cooler SSTs in the tropical Atl - also +Africa

 https://x.com/OSUWXGUY/status/1910304688911626579

[Hypercane_Kyle]

It'll be interesting to see if this season continues the trend of being bimodal in nature, i.e. having a peak in June/July and a significant peak in October with below average activity in-between.

[DorkyMcDorkface]

It'll be interesting to see if this season continues the trend of being bimodal in nature, i.e. having a peak in June/July and a significant peak in October with below average activity in-between.

It's possible, but I don't think it'll be to the extent of last year. To have very little occur from late August to mid-September is exceptionally anomalous