Record-Breaking Kelvin Wave Begins Hitting Ocean Surface

We continue to see the record breaking Kelvin Wave pushing to the surface, and anomalously warm waters are now appearing from the subsurface.

Water temperatures from the coast of Ecuador across the Equator are decidedly warmer than normal as of March 26th, as the chart above from the Climate Prediction Center shows. We see an arm of warmer than normal sea surface temperature anomalies (SSTAs) extending from about the 100W longitude line westward to roughly 140W. This noticeable warming comes as the strongest Kelvin Wave in history begins to push to the surface, in what could result in one of the stronger El Nino events on record.

The image here shows temperature anomalies on a depth by longitude layout on the bottom panel of this graphic, and the Kelvin Wave is clearly marked. As of March 31st, the water temperatures maxed out as high as 6.35 degrees Celsius above normal, confirming that this Kelvin Wave does appear to be the strongest one on record. This body of warm water provoked by the Wave is beginning to push to the surface, and this is already observed well by the extension of warm water to the surface around the 160E longitude line. I expect sea surface temperatures to continue to gradually rise in response to the agitated Kelvin Wave as it begins its trek to the surface.

We have been seeing the telltale El Nino signal in the Southern Oscillation Index for some time now. The chart above shows observed values of the Southern Oscillation Index averaged across a 30 day time period, shown since roughly December 2011. We have seen a roller-coaster pattern, as the atmosphere has been undecided on whether it will declare a La Nina or El Nino, until recently. As the chart shows, the SOI has completely tanked in recent days, to levels approaching -15. Values below -8 are considered resembling of an El Nino, while SOI observations above +8 are considered indicative of a La Nina. With the SOI being as low as it has been, there's little question in my mind that along with this Kelvin Wave, and taking into account the predicted conditions in the Equatorial Pacific in coming weeks, that we are entering into an El Nino.

A spread of global model projections of the El Nino-Southern Oscillation (ENSO) phenomenon all agree on at least a weak El Nino forming by August 2014, but even this seems a bit weak to me. I fear that model guidance is underestimating the current Kelvin Wave, and the El Nino may be a fair bit stronger by August 2014 than what is being shown here.

Updates on this unfolding situation will be provided on a weekly or bi-weekly basis, depending on the new information available.

Andrew

Thursday Severe Weather Threat Could be Season's First Outbreak

I'm looking at Thursday for the nation's first chance at a severe weather outbreak this season.

Note: There will be some technical terminology in this post. For explanations of a specific topic, rather than copying and pasting the information in a time-consuming manner, I will paste the link to an explanation alongside the topic discussed.

TwisterData

We look to see an upper level low emerge over the Central Plains by Thursday afternoon, taking on an apparent negative tilt by the time we reach Thursday evening. The presence of a negative tilt, depicted in the 500mb vorticity image above as the maxima vorticity values pushing to the southeast, will add to the intensity of these storms, as it is known that negatively-tilted storms define the mature stage of a storm system, priming the atmosphere for a major severe weather event. (More on negatively-tilted storms in middle of this post). As the storm digs into the Plains, we will see ridge formation in the Central and East US, indicating the open flow of moisture and instability across the East, concentrated particularly in the Oklahoma, Arkansas, Missouri, and Louisiana/Texas areas.

TwisterData

A look at the jet stream for Thursday evening confirms the potential severity of the event, as we see a split jet flow across the United States. The jet stream splits over the same areas we mentioned above, as the subtropical jet stream is forced south along the Gulf Coast and the Pacific jet is pushed north into the Upper Midwest. This split indicates the presence of divergence over our severe weather area, highlighted below by the Storm Prediction Center. With divergence, we see air rising strongly into the middle and upper layers of the atmosphere, helping to form thunderstorms. (More on divergence on bottom of this post)

Storm Prediction Center outlook for Thursday.
Outlined area denotes enhances severe weather threat.

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Skew-T chart for northern Oklahoma

The latest run of the NAM model has come in quite a bit more tornadic than what other guidance suggests. This image is valid at 1 AM Central local time for early Thursday morning, not Thursday night, which we have been talking about up to this point. There is a severe weather risk on Wednesday night, and this skew-t chart shows it well. By 1 AM, we see just over 2000 j/kg of CAPE, a substantial amount of instability. We also observe the EHI, or Energy-Helicity Index to be at a whopping 7.4 for this time period. This would be a very dangerous situation, but it's not being talked about as much. The reason for that is because it looks like our capping inversion may hold through the night and kill off the risk. As the skew-t shows, we see the red temperature line not really curving to the left much from the surface to 700 millibars. This means temperatures want to stay warm, and that limits thunderstorm formation. It's possible this does end up being a substantial severe weather event, but I'm not seeing too much activity for Wednesday night. The big story is Thursday night.

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TwisterData

Back to Thursday night, lower level winds look to be roaring just southeast of the storm center, as this forecast 700mb wind speed chart shows. These elevated lower level winds extend all the way down to the 925 millibar level, which isn't too high above the surface (a couple thousand feet, roughly). These lower level winds should act to strengthen the lower level jet stream, shown well on that 700mb chart above, which will then act with the rest of the environment to provoke this severe weather event.

My thoughts follow the Storm Prediction Center's outlook above, with the primary threats being hail, damaging winds, and an isolated tornado.

Andrew

April 3rd Potentially Major Severe Weather Outlook

Confidence is increasing in a potentially major severe weather event on April 3rd.

This is an update to yesterday's post on this severe threat. A full update will come tomorrow.

The Storm Prediction Center has outlined the areas of northeast Texas, eastern Oklahoma, Arkansas, southwest Missouri, and southeast Kansas for a severe weather risk on Thursday, April 3rd. 

An upper level low will push eastward into the Southwestern states at the beginning of the workweek, reaching far northern Baja California by Wednesday evening, as the 500mb relative vorticity and height contour map shows above. We see that the maximum vorticity is not 'tilting' in any particular way, meaning it isn't pointing towards the southwest or southeast. Instead, we see the contour lines rather symmetrical around the upper level low, signifying a neutrally-tilted storm system.

USA Today

As the graphic shows, there are two types of tilts to a trough- positive and negative. The positive tilt trough sees the jet stream bending towards the southwest, as the Energy Pocket, also known here as the 500mb maximum vorticity, pushes in that direction. The negatively tilted trough indicates the system has reached maturity, as the vorticity maximum now pulls the system to the southeast. The mature storm now produces more vigorous storms, hence why we are concerned more when a negatively tilted trough comes around, compared to a positively-tilted trough.

When looking back to the GFS model forecast at the top of this post, we can now confirm that the storm is at a neutral tilt. This means it is stronger than the positively-tilted storm, but not at the mature level that is defined as a negative tilt. Because the upper level trough is of a neutral tilt, we still have to be concerned about the formation of a capping inversion that must be eroded before the storms can commence.

Example of a capping inversion

This graph might look complicated, but we're only going to look at the circled portion. The red line represents temperatures, and the green line depicts dewpoint. The lines bend and move as you move up the graph, as they show temperatures and dewpoints with height- the height legend is located on the left side of this chart, in hPa (interchangeable with millibars). If we start at the surface, at the bottom of the graph, we see how temperatures begin to decrease with height. This indicates instability, as warm air rises and cool air sinks. However, we suddenly arrive at a point in the atmosphere, around 850 millibars, when the temperature warms quickly and the dewpoint drops. This means the air from the surface that had been rising can no longer rise, because temperatures have now warmed above what they were a bit lower below this circled portion. Thus, thunderstorms cannot form. This phenomenon is the capping inversion. The capping inversion can be broken, as the warm temperatures in the circled portion are cooled down. When they cool down enough, the capping inversion is broken and thunderstorms can now freely form. A 'broken cap' is shown well in this example graph below.

Example of no capping inversion

In this graph, we no longer see the sudden warming of temperatures. All we see is a steady cooling in temperatures throughout the troposphere, which tells us the cap has broken, and thunderstorms are free to form.

There actually is a very slight capping inversion in this graph, too. Can you find it?

If you guessed it was located at the very bottom of the red line, you're correct. There is a slight inversion at the surface as temperatures warm a bit before rapidly cooling. The red line breaking to warmer temperatures only slightly signifies a weak cap that should be easily broken.

With our neutrally-tilted storm system, we're going to need to break the cap, but when that does happen (which it will), the fireworks will begin.

Note: Two images will be displayed below. There will be the jet stream forecast on top, with an example graphic on bottom. The top graphic (forecast jet stream) will be discussed first, before we shift to a discussion about the different regions of the jet stream, which is when the example graphic will be used. Bear in mind the example graphic is NOT a current forecast.

Forecasted jet stream for April 3rd
Divergence circled, more on divergence will be discussed below.

EXAMPLE GRAPHIC for the topic discussed below.
NOT A CURRENT FORECAST.

The top image of the two above shows the GFS jet stream forecast for the evening of April 3rd, when we expect this severe weather event to occur. There is an area of divergence that I circled, on the right exit area of the jet stream (continue reading for explanations on those terms). In the bottom example image, I separated the jet stream into four sections. We have the 'Left Entrance' region, on the bottom left part of this diagram; the 'Right Entrance' region in the bottom right; the 'Left Exit' region in the top left, and the 'Right Exit' region in the top right area. The severe weather event looks to be located over the Right Exit region, and being in the Right Exit region is a big deal.

In the Right Exit region, we see divergence aloft, meaning air is being pushed up and outward, as the diagram above shows. This divergence acts as a helper for the formation of convection, including (but not limited to) thunderstorms. Even in winter, being in an area of divergence allows for the formation of snow. If the divergence is strong enough, thundersnow can occur as well (with other atmospheric conditions cooperating, of course). For our severe weather event here, with the divergence centered in the Right Exit region, I'm closely watching Oklahoma, Arkansas, and portions of Missouri and Texas for severe weather this Thursday, April 3rd.

Andrew 

April 9-15 Multiple Potentially Significant Storm Systems

I'm seeing the threat arise for not one, but two potentially significant storm systems.

Tropical Tidbits

The GFS model has been consistently bringing in a strong upper level low into Japan around April 4th, beginning to attain a negative tilt on the image above, valid for the afternoon of April 4th (for more information on negatively-tilted storms, please click on this link to see the post published yesterday on this topic). There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. If we take the April 4th day and extrapolate it out 6-10 days, we arrive at the April 10-14 timeframe for what could be a hefty storm. I say it could be strong, as the strength of these East Asian systems has been reflected in the resultant United States storm . For instance, a strong storm over Japan does usually result in a strong storm in the US 6-10 days later, and that's what we're looking to see in this April 10-15 timeframe.

But we're not just looking for one system. This time, there are indications we could see two systems.

Tropical Tidbits

About a full day after the original system moves out from Japan, we see another swath of significantly below-normal heights enter Japan. The GFS image above, now valid for April 5th, reflects this, and we can see our first storm system that was discussed above now located just west of the ridge in the Bering Sea. This second storm system is kind of a tricky one. I'm watching closely here, as it could end up being one storm with residual cold weather just hanging behind. However, this forecast says we are in for two storm systems, and since we're entering spring, these strong storm systems can create nasty severe weather. For that reason, I'll err on the side of caution and highlight two storms in this post, but do realize that this may switch back to one significant storm.

The pattern I had highlighted earlier last week, which showed how the Northeast was at the most risk, is now a bit more hazy than when we last analyzed this timeframe. Model guidance is no longer as favorable for an East Coast impact, but rather than drop that region from a potential impact zone, I'll still tentatively keep the Central and East US in line for this storm. We should know much more about what this storm(s) could do in about 4 or 5 days from today.

As you can tell, there's a lot of uncertainty. Let's sum up what we do know.

- There is the potential for at least one significant storm system around the April 9-15 period.
- Severe weather does look to be a potential factor in this timeframe.
- Cooler and unsettled weather can be anticipated for this timeframe.

Andrew

April 2nd Severe Weather Event Forecast

It's looking more likely that we'll see a severe weather event over the southern Plains.

An upper level low will push eastward into the Southwestern states at the beginning of the workweek, reaching far northern Baja California by Wednesday evening, as the 500mb relative vorticity and height contour map shows above. We see that the maximum vorticity is not 'tilting' in any particular way, meaning it isn't pointing towards the southwest or southeast. Instead, we see the contour lines rather symmetrical around the upper level low, signifying a neutrally-tilted storm system.

USA Today

As the graphic shows, there are two types of tilts to a trough- positive and negative. The positive tilt trough sees the jet stream bending towards the southwest, as the Energy Pocket, also known here as the 500mb maximum vorticity, pushes in that direction. The negatively tilted trough indicates the system has reached maturity, as the vorticity maximum now pulls the system to the southeast. The mature storm now produces more vigorous storms, hence why we are concerned more when a negatively tilted trough comes around, compared to a positively-tilted trough.

When looking back to the GFS model forecast at the top of this post, we can now confirm that the storm is at a neutral tilt. This means it is stronger than the positively-tilted storm, but not at the mature level that is defined as a negative tilt. Because the upper level trough is of a neutral tilt, we still have to be concerned about the formation of a capping inversion that must be eroded before the storms can commence.

Example of a capping inversion

This graph might look complicated, but we're only going to look at the circled portion. The red line represents temperatures, and the green line depicts dewpoint. The lines bend and move as you move up the graph, as they show temperatures and dewpoints with height- the height legend is located on the left side of this chart, in hPa (interchangeable with millibars). If we start at the surface, at the bottom of the graph, we see how temperatures begin to decrease with height. This indicates instability, as warm air rises and cool air sinks. However, we suddenly arrive at a point in the atmosphere, around 850 millibars, when the temperature warms quickly and the dewpoint drops. This means the air from the surface that had been rising can no longer rise, because temperatures have now warmed above what they were a bit lower below this circled portion. Thus, thunderstorms cannot form. This phenomenon is the capping inversion. The capping inversion can be broken, as the warm temperatures in the circled portion are cooled down. When they cool down enough, the capping inversion is broken and thunderstorms can now freely form. A 'broken cap' is shown well in this example graph below.

Example of no capping inversion

In this graph, we no longer see the sudden warming of temperatures. All we see is a steady cooling in temperatures throughout the troposphere, which tells us the cap has broken, and thunderstorms are free to form.

There actually is a very slight capping inversion in this graph, too. Can you find it?

If you guessed it was located at the very bottom of the red line, you're correct. There is a slight inversion at the surface as temperatures warm a bit before rapidly cooling. The red line breaking to warmer temperatures only slightly signifies a weak cap that should be easily broken.

With our neutrally-tilted storm system, we're going to need to break the cap, but when that does happen (which it will), the fireworks will begin.

This image shows the GFS jet stream forecast for the evening of April 2nd, when we expect this severe weather event to occur. In this image, I separated the jet stream into four sections. We have the 'Left Entrance' region, on the bottom left part of this diagram; the 'Right Entrance' region in the bottom right; the 'Left Exit' region in the top left, and the 'Right Exit' region in the top right area. The severe weather event looks to be located over the Right Exit region, and being in the Right Exit region is a big deal.

In the Right Exit region, we see divergence aloft, meaning air is being pushed up and outward, as the diagram above shows. This divergence acts as a helper for the formation of convection, including (but not limited to) thunderstorms. Even in winter, being in an area of divergence allows for the formation of snow. If the divergence is strong enough, thundersnow can occur as well (with other atmospheric conditions cooperating, of course). For our severe weather event here, with the divergence centered in the Right Exit region, I'm closely watching Oklahoma, Arkansas, and portions of Missouri and Texas for severe weather this Wednesday, April 2nd. The Storm Prediction Center had outlined that area yesterday, but revoked the outlook today due to model disagreement. Despite this, the indications are there that we will see the season's first formidable severe weather event on April 2nd.

Andrew