Waviness in the tropics

Despite this being one of the most significant El Nino years we've had in a while (with the potential to become one of the strongest on record), we've still managed to have some tropical storm activity in the Atlantic.  So far there have been eight named storms, only two of which have actually reached hurricane strength.  Depressed tropical cyclone activity in the Atlantic is expected in strong El Nino years, so the small number of hurricanes so far isn't too surprising. (There's still a ways to go in the hurricane season yet, though...)

However, every few days it seems we get a new "feature" in the far eastern Atlantic that the National Hurricane Center  identifies and keeps an eye on as it moves to the west across the ocean.  These features are called "tropical waves" and they happen all the time.  The NHC forecast discussions have an entire section devoted to describing the current "tropical waves".  So just what is a "tropical wave"?

When I was much younger, I would hear them talking about tropical waves on the Weather Channel and picture something like this:

And, admittedly for a very long time, I thought that hurricanes ultimately came from these giant waves on the surface of the ocean that somehow triggered hurricanes above them.  Let's just be clear here...while a warm ocean surface plays a tremendous role in supporting hurricane development, ocean waves are not what we mean by "tropical waves".  Let's look a little higher up in the atmosphere.

To understand what makes a tropical wave, we have to understand the air pressure patterns on the earth's surface.  Some of you may be familiar with what's called the "Hadley Circulation": the large scale motions of the atmosphere that dominate the tropics.

From http://sites.psu.edu/musingsofameteorologist/2013/02/11/hadley-cells-the-foundations-of-atmospheric-circulation/

In short, near the equator it is very warm and there is a lot of convective (thunderstorm) activity.  All this warm air rising in the thunderstorms migrates away from the equator at higher altitudes before sinking again in the "subtropics"--about 30 degrees north and south of the equator.  When all that air rises near the equator, it creates lower pressure near the surface (think of all that air lifting up and away from the surface...the pressure you feel pushing down on you is lower).  Oppositely, the sinking air in the subtropics tends to create higher pressure in those regions.  Pressure again decreases as you get towards the northern mid-latitudes.  The result is a series of pressure "belts" that circle the earth in a mean sense:

These belts are an "average" condition of the atmosphere.  When we're looking at weather (particularly stormy weather), we're interested in the deviations from this average.  We have "troughs" where the pressure is lower than normal and "ridges" where the pressure is higher than normal.  This are where the idea of "waves" in the atmosphere come from---they are perturbations along these average, static belts.

Since storms are typically associated with lower pressure, we tend to look for "troughs" in the flow when we care about storms.  In the mid-latitudes (some 30-60 degrees N or S; the latitude band of the continental US), we live in a zone where there is typically higher pressure to the south and lower pressure to the north.  Therefore, our troughs tend to show up as perturbations extending down from north.  They "perturb" the average pressure pattern in a way that looks (and behaves) like a wave.  Here's an old Weather Channel map showing what this looks like in an idealized sense:

Note the "waviness" of the flow shown by the jet stream as it navigates around these areas of higher and lower pressure.  These are the "waves" we are talking about when we're talking about the atmosphere.  Not ocean waves, but pressure waves.

As I mentioned above, when we look at maps like this in the northern hemisphere mid-latitudes, our terminology makes sense.  A high pressure "ridge" looks like a "ridge" on this map, reaching up toward the north.  And a low pressure "trough", looks like a deep "trough" being dug down into the flow.  That's where this terminology comes from.

But near the tropics, remember that the pressure pattern is different.  There is lower pressure near the equator on average with higher pressure to the north.  We still look for troughs of lower pressure to cause storms---the same mechanics are at work there.  But the way troughs look is different: with low pressure to the south and high pressure to the north, a low pressure "trough" would actually be digging UP from the low pressure near the equator into the higher pressure to the north.  Here's a Wikipedia image that shows what these sorts of troughs look like:

The troughs are "upside down", but we still call them troughs.  Sometimes you'll hear them referred to as "inverted troughs".  This is one of the hardest things to explain to our Weather 101 students---a trough of low pressure doesn't always look like a "trough" on the map.  In the tropics they are upside down.

However, regardless of how they are oriented, it still makes the flow "wavy".  And that's where we get our "tropical waves".  They're still troughs of lower pressure, but they're just oriented in the opposite sense of what we are used to.  They also move from east to west, giving them another name: "easterly waves".   These areas of lower pressure extending north into the subtropics provide the seeds for eventual development of tropical storms and hurricanes, and they can also bring stormy squalls just on their own.  For weather forecasting in the tropics and subtropics, monitoring these tropical waves is essential.

On a side note, it actually is also possible for us to get "inverted" pressure troughs here in the mid-latitudes.  In fact, it's a common occurrence on the west coast in the summer, where you may have heard of the "thermal trough".  But that's an entirely different blog post...

Even the tropics are aiming the faucet for the central US

After the eastern Pacific got off to an early, rapid start with its tropical storm season (which isn't unexpected in an El Nino year), we're looking at our first tropical storm developing in the western Gulf of Mexico in the next 12 hours.  On satellite, this storm has been getting its act together:

And the National Hurricane Center thinks we'll likely see Tropical Storm Bill by tonight.  This storm is forecast to run out of ocean really fast, though, and hit the Gulf coast of Texas tonight and tomorrow.   From Weather Underground, here are the recent 18Z GFS ensemble forecasts for the track of this cyclone over the next several days.

You can see that there's pretty high agreement that the storm will cross central Texas before curving north through Oklahoma then back northeastward across Missouri and Illinois and into the midwest and mid-Atlantic states.  Of course, almost as soon as this storm hits land in Texas it will stop being a tropical cyclone and turn into an extratropical cyclone.  Though this means weaker winds, the storm will still bring a lot of moisture into the heart of the country.   Here's an animation of the satellite-derived total precipitable water in the atmosphere over the last five days.  You can see towards the end of the animation that the developing storm in the western Gulf of Mexico (with its characteristic spiraling motion) is bringing in lots of moisture towards the Texas coast.

That moisture will move inland over the next several days.  Here's the 18Z NAM forecast precipitable water for tomorrow afternoon (from HOOT):

 Lots of moisture over Texas and Oklahoma, with residual moisture from the current frontal zone  across the Ohio River Valley.  Looking ahead to Wednesday afternoon there is still a lot of moisture forecast over eastern Texas and Oklahoma.

And even by Thursday afternoon, east Texas is still looking at a ton of moisture, though we're also seeing a lot of liquid move north into Arkansas and Missouri.

Not unsurprisingly, the Weather Prediction Center (NOAA/WPC) is forecasting some intense rainfall amounts over the next three days, with a swath over 6 inches from Texas into Oklahoma and western Arkansas, and over 3 inches from Missouri into central and southern Illinois.

It's interesting to note that, despite this storm being a tropical cyclone, its ability to influence the larger-scale synoptic pattern is somewhat limited.  Notice how the storm path follows a nice, sweeping curve around the southeastern United States.  Why is that?  Because we have a relatively modest ridge of high pressure/high heights sitting over the southeastern US, as seen in this 24-hour 500mb height forecast from the NAM:

Though that ridge looks so weak (it only has a single height contour!), there's actually still more potential energy sitting in that ridge than kinetic energy in a tropical cyclone.  So, even though it seems to be a strong storm to us, our tropical storm is still a slave to the synoptic pattern, and that "weak" ridge over the southeast will "shield" that area from most of the effects of this storm.

Unfortunately, it's that same ridge that is directing the storm over areas that really don't need any more rain right now.  Here's a map of the percentage of normal precipitation during this May.  Much of Oklahoma and Texas received 200-500% of their normal precipitation for May.

Soils in that area are also saturated.  Here's a map of soil moisture percentiles as of the end of May.  Much of eastern Texas and southern Oklahoma is in the 99th percentile for soil moisture...that is, soils likely contain nearly record amounts of moisture for this time of year.

Add on top of this the fact that several major rivers throughout that corridor (and also into Missouri and Illinois) are currently at flood stage from all the rain we've already gotten recently (the orange and red dots on this map)...

And you have a recipe for a flooding disaster across a large swath of the central US.  Flood watches are already up for most of eastern Texas and eastern Oklahoma, with hydrologic outlooks suggesting at future flood watches going up throughout Arkansas, Missouri, Illinois and Indiana as well.  If you live in these areas, you need to be paying attention to the weather this week and be alert for possible flooding.  This is going to be the weather story of the week.

Forecasting a Subtropical Storm

Model forecasts have now been consistently hinting that a small, but powerful storm could form off the eastern Florida coast by the end of this week.  This storm isn't quite "tropical" but it's not quite "extratropical", so we use the term "subtropical" to describe it, as it would have characteristics of both tropical and extratropical cyclones.  Let's see what this looks like in the model forecast fields.

First an overview of the large-scale setup.  Here's the GFS forecast (from HOOT) for this Wednesday afternoon at 300mb, right before this cyclone is forecast to really get going:

Winds are shaded and heights are contoured.  We see two main "belts" of air here.  First, our friend the polar jet stream is on its way north, as is typical during the spring time.  There's a deep trough forecast over the Pacific Northwest (ending our wonderfully warm and clear weekend we had been enjoying...).  There's also a negatively-tilted ridge over the northern plains and into the upper midwest.

Further south, we have a strengthening subtropical jet stream, most visible over the eastern Pacific into Baja California as a strengthening jet streak there.  However, extending east a little further we can see that the height gradient associated with the subtropical jet, while weaker, is showing a trough over the southeastern US and Florida.  A trough there means that there must be a pocket of slightly colder air over the southeast, with some kind of gradient in temperature to the south.  We know that extratropical cyclones get their energy from temperature gradients, and we usually see those develop in association with a trough aloft.  So, seeing this setup, one might expect an extratropical cyclone to develop off the southeast coast as that trough moves east-northeastward.

Well, a cyclone is indeed forecast to develop there.  Let's zoom in now and take a look at what the deterministic ECMWF forecast (courtesy of Wunderground) from last night is saying about this developing cyclone.  Let's start with the sea-level pressure (blue contours), precipitation (colored shading), and 1000-500mb thickness (yellow contours) for 21Z Thursday:

There is a clear surface low forming off the southeast coast.  Note that the forecast precipitation is somewhat "comma" shaped: it's NOT a symmetrical circle.  Asymmetrical (comma-shaped) precipitation patterns are closely associated with extratropical cyclones.  You can even see what looks like a hint of cold and warm fronts in the precipitation: very extratropical. Tropical cyclones tend to be much more circular and symmetric.  So, again this looks more like an extratropical cyclone.

But let's look at the thickness contours (the yellow lines).  In the core of that storm, there's actually a small solid contour of thickness, at 5700m.  This is a positive thickness anomaly in the center of the storm.  What does that mean? Higher thicknesses mean that the column of air being measured is warmer on average.  So, a positive thickness anomaly would point more towards this storm being a warm-core cyclone.  Tropical cyclones are warm-core cyclones; whereas extratropical cyclones are cold-core cyclones (they tend to be colder in the center).  So, this warm-core characteristic seems more indicative of a tropical cyclone.

Let's compare this to the GFS forecast for a slightly earlier time (09Z Thursday; the GFS model develops this storm more quickly):

Here the precipitation might be slightly less clearly comma-shaped, though there is a large dry slot to the south of the low.  The warm-core idea is again shown with higher thickness values in the center of the storm.  So different models have varying degrees of warm-core-ness.

The thickness plots above are based on temperatures in the 1000mb to 500mb layer.  Let's explore temperature further by looking at the 850mb temperatures from the ECMWF forecast, which would be right in the middle of that layer:

You can see that there are definitely temperature gradients in the vicinity of the cyclone, with cooler air being pulled down from the north on the western side of the storm and warmer air being pulled north on the east side of the storm.  So the warm "core" we see in the thickness isn't as pure as we might expect from a fully tropical cyclone.  Let's check the GFS forecast, here at 21Z:

The GFS definitely has a pool of warmer air at the center of the storm, making a stronger case for a warm-core cyclone.

One tool to help decide if a cyclone is tropical or extratropical (or something else) is looking at something called tropical cyclone phase diagrams (from work by Robert Hart at Florida State).  Here's the forecast phase diagram for the cyclone in the GFS forecast:

The type of cyclone basically depends upon the thickness (the y-axis on the diagram) and the thermal wind (which is related to temperature gradients; the x-axis on the diagram).  By measuring these two things and plotting them as the cyclone evolves over time, we can see what "region" of the plot the storm falls into.  Here, the storm's trajectory (the dots along a line going from A to Z) are all in the lower right part of the chart---that "red" sector is labeled "symmetric warm-core", aka tropical cyclone.  An extratropical cyclone would be "asymmetric cold-core".  So, interestingly enough, according to this one method of looking at cyclones, they would forecast the storm forecast to develop off the southeast coast as a tropical cyclone.

As we've seen above, the latest forecasts show this particular cyclone to have aspects of both a tropical (warm-core) and extratropical (cold-core) cyclone.  So what do we do?  We call it a "subtropical" cyclone, sort of a catch-all term that describes storms that have both warm- and cold-core attributes.  More thorough definitions are at this page and this page.  If the maximum winds are less than 34 knots, it will be labeled a "subtropical depression".  Greater than that, it will become a "subtropical storm" and actually given a name from the 2015 hurricane name list by the NHC (which would make this Subtropical Storm Ana, should it get strong enough).

Regardless of which model you believe, both bring heavy rain and strong winds to the Florida, Georgia and Carolina coasts Thursday and Friday.  The ECMWF forecast has it just reaching subtropical storm strength at this time...

But there remains much uncertainty.  Not only are the GFS and ECMWF different, but the ECMWF ensemble forecasts also have a fair amount of spread.  Here's a plot comparing the ECMWF ensemble mean sea-level pressure forecast on the left with the single high-resolution deterministic forecast (what I've shown above) on the right:

While it's clear on the right that there is a strong low off the Carolinas, we don't really see that in the ensemble mean on the left panel.  What we do see in the left panel is a big blob of purple in that region, which means "greater than normal uncertainty".  In other words, different forecast runs of the ECMWF have different answers as to how strong and where exactly a storm may form off the coast, if one even forms at all.  So, we'll have to be watching all week to see what develops.

Cyclone Pam, Seattle rain records and the MJO

Last week the small island nation of Vanuatu was devastated by a major tropical cyclone: the Category-5-equivalent Cyclone Pam.  There has been terrible damage to the infrastructure on the islands, with estimates of up to 90% of the buildings in the capital of Port Vila.  Power was lost early on in the storm and is just now being restored in some places.  We can actually see this looking at the weather observations from Port Vila, here from Weather Underground:

You can see that before they lost power late on the 13th, the pressure was plummeting (down to 965 hPa) with wind speeds rapidly increasing to 45 mph.

The CIMSS Satellite blog had a post showing several satellite animations of the cyclone as it hit; I encourage you to take a look at it here.

The cyclone continued south and is giving a glancing blow to New Zealand today, where it has caused some power outages and a lot of rainfall.  Remember that in the southern hemisphere low pressure centers rotate clockwise.  Here's a satellite image from the New Zealand Met Service this morning, showing that the cyclone has basically been sheared apart, but there is still a lot of moisture with strong winds.

As The Weather Channel pointed out in an earlier blog post, Pam isn't the only cyclone we had in the western Pacific over the past week.  There were actually four active at once: Pam, which hit Vanuatu; Olwy, which is affecting the west coast of Australia; Nathan, which is hitting the York Peninsula of Austrlia, and Bavi, which is headed west towards the Philippines, but is expected to weaken before it arrives there.

Is there some uniting factor behind this burst of tropical cyclone formation?  It would appear so---the Madden-Julian Oscillation (MJO) is currently, by many measures, the strongest we have seen it in a very long time.

What is the Madden-Julian Oscillation?  The exact nature of it is still a somewhat poorly-understood topic in meteorology, despite intense research activity in recent years.  Basically, it's an area of enhanced convection (thunderstorm activity) that propagates around the equator every 30-90 days.  (See the Wikipedia article for a decent overview).  We can see this looking at a global satellite loop over the past week.  There's been a lot of cloud cover and convection moving out from Papua New Guinea and northern Australia and into the central Pacific.  You can see that as it moves east, it looks like there are tropical cyclones (Bavi and Pam) that are being "shed" from the convection.

It turns out that the basic structure of the MJO has an equatorial "heat source" being trailed by flanking "gyres" on either side of the equator.  If strong enough, these "gyres" can break away and form tropical cyclones, which is exactly what we saw last week.

How strong is the MJO right now?  Here's a diagram that tracks the strength and position of the MJO as it goes around the globe, called the Wheeler-Hendon diagram.  

You can see that there are different "zones" on the diagram, labeled with different geographical regions (the "Maritime Continent" (Indonesia, New Guinea, etc.) , "Western Pacific", "Western Hemisphere and Africa" and the "Indian Ocean".  So as the MJO moves around the globe, it's supposed to make a big circle around the diagram.  The magnitude of the MJO (how strong the convection associated with it is) is given by how far from the center you are.  The line on the plot shows where the MJO has been for the past 40 days.  You can see that for much of February (the purple part of the line, the MJO was very weak (close to the middle of the diagram) and not clearly being tracked around the globe.  But in March (the red part of the line), the MJO exploded in strength over the Maritime Continent and is now moving out over the western Pacific, getting even stronger.  This is WAY stronger than the MJO has been for a long time!  No wonder we are seeing powerful cyclones.

It turns out that an "active" MJO in the western Pacific can also contribute to heavy rain ("pineapple express") events on the west coast.  Here's a diagram from the MJO wikipedia article describing this connection:

So as the active area of convection on the equator associated with the MJO moves out over the Pacific, the moisture associated with that gets drawn northward to the west coast of North America, causing heavy precipitation.

Well, over the past two days we've had just that here in Seattle.  Here's an animated map of the total column water vapor over the past two days.  You can see the plume of moisture drawn northward from the tropics that brought heavy rain to Seattle.

So much rain fell (2.2") that Sunday was actually the second wettest March day on record in Seattle, and the single wettest day we've had since 2009 (courtesy of Scott Sistek).  It's amazing how a single weather anomaly like the MJO can contribute to both tremendous rain in Seattle and an extreme Cyclone in Vanuatu.