Sunday, April 26, 2015

A Final Farewell

(For some context, please read the most recent post, A Final Notice to the Viewers).

On April 15th, I made a decision to move from one stage of my life to the next, by announcing that I would be ceasing operations here at The Weather Centre on our five-year-anniversary, April 26th 2015. In that process, I finally lifted the veil over my age, my (lack of) credentials, and how I got started on this blog in the first place.

The response I got was very much the most overwhelming and inspiring collection of comments I have ever seen anywhere on the Internet, and possibly at any time in my life. As I read and re-read your comments, I started to really realize what The Weather Centre has become. It is not a blog; we've built it into a community unlike any other. For five years, we all contributed to teaching each other (myself included) a little more about the weather with each post, and that's something I'm extremely proud of.

As I sit here, writing what will be the final post on The Weather Centre blog, I'm experiencing a wide range of emotions. I can still remember the feverish nights spent during the April 2011 tornado outbreak, posting tornado warnings on this blog, not being able to keep up with all the warnings that kept coming out. I can still remember rushing home from school on a daily basis to check the weather models and whip up a new post on an impending winter storm. But the thing I remember most is how fun it's been, and how sad I am to see it go. However, as the saying goes:

"Don't cry because it's over, smile because it happened."
- Dr. Seuss

As I close the book on this chapter of my life and move on to a new one, I can't help but smile.

Thank you all for such an incredible five years. This has become something more than I could have imagined in my wildest dreams, and you've all been the driving force behind it. There is nothing else I can say but thank you.

For the final time,
Andrew Racki

Wednesday, April 15, 2015

A Final Notice To The Viewers

This is quite likely the most difficult post I've had to write in the last five years of running The Weather Centre, and I can assure you it wasn't easy actually sitting down to hammer it out, either. After all, this has been my livelihood for so long, I can't really remember what occupied the majority of my time before this.

That's why it makes me sad to announce that The Weather Centre blog will close permanently on our five-year anniversary: April 26th, 2015.

When I started The Weather Centre on April 26th, 2010, I wasn't much of a forecaster. At the time, I made the blog solely because I had an interest in weather, and because there happened to be a severe weather event on the way on April 30th of that year. My initial posts, if you dive back to look at them (example here, here, and here), were simply maps with some crude MS Paint drawings on them if even that, describing the upcoming pattern for that late April set-up.

As a matter of fact, I wasn't even much of a person yet. I started The Weather Centre blog when I was 12 years old, just rounding out 7th grade. In case you're wondering, yes, you've been receiving the musings of a teenage aspiring meteorologist for the last five years. I'll get to why I never revealed my age a little later in this post, but I'd like to go back even further.

Many of you who have read this blog likely got an interest in weather from a rather traumatic weather event. For some, it may have been seeing a tornado or hurricane on TV, or perhaps being in a tornado or a hurricane. For me, it was having The Weather Channel on the television for hours on end, starting when I was just about 2 years old. My father has always shared my interest in meteorology, but I took it to a different level, to the point of where we are today. I wanted to keep learning; I wanted to know why it stormed in some areas, and why it didn't storm in others. I wanted to know how tornadoes formed, and how they could turn a new house into splinters of wood in under a minute. I wanted to understand the hurricane, how a massive storm system could drop over a foot of rain on coastal locations, with sustained winds in excess of 100 miles per hour. 
So I did keep learning. It just wasn't in 'the conventional way'. You hear teachers in schools these days berating Wikipedia, and using the internet in general to learn. I'm living proof that such an attitude could not be more detrimental to learning. Just about everything I've discussed on this blog, from the Typhoon Rule to the El Nino to the Sudden Stratospheric Warming, I've learned over the internet. As I grew older, yes, I began to buy more meteorological books, print out scholastic journals, etc., but never feel guilty for searching online to find an answer to your math problem or something of the like.

I want to come back to my age. Over the past five years, I've had more than a few people ask me how old I am, or what my credentials are in the forecasting world. To be quite frank, I never answered those questions, because I've grown to learn that it really does not matter. I see degreed meteorologists publicly shaking their heads on Twitter, Facebook, or on other public social media at something a kid just like me may have done. One particular controversy has always been teenagers posting those ECMWF snowfall maps, which has become a pretty real problem for operational forecasters, but I digress. Nothing infuriates me more than when I see those operational forecasters putting down someone online, with no credentials, who just wants to be able to forecast weather like the pros. Why do I get so distraught over that? Take a look at this clip from The Incredibles movie, and you might see what I mean. I should add those forecasters who will publicly shame amateur forecasters are in the minority; the great majority of degreed forecasters show significant respect towards the younger generation that will follow in their footsteps.

"Not every superhero has powers, you know. You can be super without them."

Before I close up shop here, I want to go over the achievements we as a community have accomplished. 

- In five years, The Weather Centre has accumulated over 5,650,000 page views. I can safely say I would never have dreamed that so many people would have wanted to read my weather discussions, and it is such an incredible honor to have run such a successful blog.

- The Weather Centre was featured and/or mentioned on the Toronto Star, the New York Times, the Huffington Post, NPR, Yahoo, and countless other meteorological and news entities. Every one of them made me more proud to do what I do.

- We formed a community of over 3,700 Facebook fans, and close to 1,300 Twitter followers. I have found that you all are the best group of weather enthusiasts I have come across on the Internet, and it is an immense privilege to say that. I am forever grateful to have had the opportunity to converse with all of you.

- Lastly, and perhaps most importantly, we proved that weather can be easy to understand. Over these last five years, I've received dozens of comments from viewers across the world, thanking me for making complicated weather phenomena normally learned in college, easy to understand.

So, for the last time, I want to extend a thank you. I want to thank you, the viewer, reading this right now, for supporting not only this blog, but me for the last five years. Thanks in large part to The Weather Centre, I have grown into a person I never thought I would become, and I'm incredibly excited to see where my life takes me next. I'm leaving not out of selfishness, but because this chapter of my life is naturally coming to a close. In the next several months, I'm going off to college to start my studies to become a degreed meteorologist, my life's dream. In addition, research projects that I began this past winter are quickly showing signs of verification. One of them, forecasting the EF-strength of tornadoes before they form, is looking very promising, and I'll need to devote a significant amount of time to improving my research, my studies, and everything else that makes life worth living. The one thing I ask of you is that you do what I've done. Go out and teach others about the weather; there are so many people that want to understand the weather but find everything too complicated. This blog provides a great resource for you to continue on.

Once again, thank you for these last five years. They've been the best five years I've ever experienced, and I'm incredibly proud to say we've accomplished the goal that's been in place since I first started The Weather Centre: to show others that weather can be easy to understand, to learn, and to share with others.

The last couple of weeks before April 26th will be mainly cleaning up some things, probably issuing some sort of winter prediction for 2015-2016 (it's only fitting, given our preliminary winter forecasts used to be issued in early June), and enjoying the ride. You'll still find me tweeting about the weather even after we close, and perhaps an occasional post on the Facebook page as well; you can follow the Twitter account at, and our Facebook page at


Andrew Racki

Saturday, April 11, 2015

Long Range Climate Models Show Cold, Snowy SST Pattern for 2015-2016 Winter

I'll bite the bullet and be 'that one person' who starts discussing next winter far too early. Long range climate models are showing a sea surface temperature anomaly prognosis that could be conducive to another cold and snowy winter, particularly along the East US.

Climate Prediction Center
The graphic above shows mean sea surface temperature anomalies from seven different long range models, valid for November 2015. There are a few areas of interest on the chart above. We'll begin in the Gulf of Alaska and work our way south from there.

In the Gulf of Alaska, the ensemble mean of models tells us that the warm pool we have seen the last two winters will only solidify itself for a third consecutive winter, with temperature anomalies definitively above normal in that area.

So what would that warm pool in the Gulf of Alaska mean for us? The chart above shows the correlation between the Pacific Decadal Oscillation, and temperatures in the United States in the winter. For example, if the Pacific Decadal Oscillation (PDO) were negative (which would be signified by cold waters in the Gulf of Alaska), the negative correlation area over the South and East US tells us temperatures would be warm there. Similarly, temperatures would be colder than normal in the positive correlation area.
As we head into the coming winter, it is expected that the pool of warm waters over the Gulf of Alaska will persist, as climate models above suggest. Using our chart from the ESRL, a positive PDO correlates to below-normal temperatures in the South and East, and warmer than normal temperatures in the West. This is not a good preliminary outlook for those in the Southwest undergoing a devastating drought.

The other key item we see on this prognosis is a large swath of above-normal sea surface temperatures along the Equator in the Pacific. This is the El Nino phenomenon.

In a typical El Nino winter, warmer than normal conditions will prevail along the south Alaskan shore into western Canada and the Pacific Northwest. Wetter and cooler than normal conditions are then favored in the Southwest, South-central, and Southeast United States, all the way up along the East Coast, though that area is not depicted with those anomalies. Drier conditions then prevail in the Ohio Valley.

Last season, there was talk of an El Nino brewing for the winter, which it did end up doing, but very late in the season. Part of the reason the El Nino didn't evolve as eagerly as forecasted was likely how Equatorial Pacific waters were rather cool in the spring before last winter, which is not a good breeding ground for the El Nino. This spring, however, we are already under a weak El Nino, and climate models expect that to intensify as we push into the fall. If this happens, the forecast for next winter could once again be one of snow and chilly weather for the East.

To summarize:

- Climate models are indicating portions of the South and East US may see favorable conditions for another cold and snowy/wet winter in 2015-2016.


Monday, April 6, 2015

Thursday, April 9th Potentially Widespread Severe Weather Outlook

This is the latest outlook (as of Monday, April 6th) for Thursday's potentially widespread severe weather.

Storm Prediction Center
(click all images to enlarge)
The above image shows the Storm Prediction Center outlook for Thursday, April 6th.
The following areas have been outlooked for a 15% chance of severe weather within 25 miles of any given point:

- extreme southern Wisconsin
- extreme southwestern Michigan
- Indiana
- northern and southern Illinois
- extreme eastern Iowa
- eastern Missouri
- western Kentucky
- western Tennessee
- Arkansas
- extreme northwest Mississippi
- northeast Texas
- northern Louisiana
- southeast Oklahoma

The 15% chance of severe weather, put into words, generally suggests that there is a noticeable risk for severe weather in the areas mentioned. Residents in those areas should keep an eye on the forecast as the date approaches, and watch for inclement weather when the event arrives.

The SPC has outlined the following regions for a 30% chance of severe weather within 25 miles of any given point:

- Illinois
- eastern Missouri

The 30% chance of severe weather, put into words, indicates that there is a relatively good chance that severe weather will strike the areas mentioned above. In addition, there is a risk for more significant severe weather. Residents in those areas should continue to keep an eye on the forecast for this date, and closely monitor all watches and warnings when the event date arrives.

Weather Prediction Center
We now turn our attention to the forecasted surface map on the morning of Thursday, April 9th. At this point, we see a rather potent set-up over the nation. A strong low pressure system will be moving northeast into the Midwest, dragging the warm front northward as it does so. The trailing cold front will provide the focal point for thunderstorm formation inside the warm sector.

The NAM model is expecting that we will see over 3000 joules per kilogram of CAPE form by late afternoon over Iowa, Wisconsin, and Illinois. Convective Available Potential Energy, or CAPE, is the parameter that defines the amount of instability in the atmosphere. In short, higher instability means that the air is more buoyant- it is able to rise easier, and thus create thunderstorms. It is uncommon to see CAPE values in excess of 3000 j/kg, particularly this far north in the Midwest, so it is rather noteworthy.

The next thing we want to look at is the wind field in the lower levels of the atmosphere. Here, we have the NAM model's projected 700 millibar wind speeds on the evening of Thursday, April 9th. We can clearly see the storm in question over Iowa and Minnesota, but what's the most eye-catching is the jet streak rounding the base of the trough earlier on, now pushing out into the Chicagoland/Milwaukee metro areas. As this happens, it should increase the wind shear for when storms begin to enter those same areas a few hours later. Summed up, this increases the tornado threat for the Illinois/Wisconsin/Missouri areas, likely why the SPC has a 30% risk over those regions.

For the tornado threat...

The Weather Channel's Dr. Greg Forbes has put out a rather ominous forecast using his TORCON indicator, which predicts the chance of a tornado within 50 miles of any given point.

Severe thunderstorms in south WI, central and south MI, northwest
OH, IN, IL, east IA, MO (except northwest corner), southeast KS,
east and south-central OK, AR, west KY, west TN, northwest MS,
northwest LA, central and northeast TX. TORCON - 4 south WI,
south MI, north IL, north IN; 3 - MO, rest of IL and IN; 2 to 3
rest of area above
A TORCON of 4 has been assigned to southern Wisconsin, southern Michigan, northern Illinois, and northern Indiana. That means that there's roughly a four in ten chance that a tornado will touch down within 50 miles of any given point; that's a pretty high number.

All of the factors that support tornado formation appear to be in place. There will be buoyancy/CAPE (though the abundance of that instability is still in question), there will not be a temperature inversion aloft to prevent storm formation, the cloud bases should be low to the ground, and there will be abundant wind shear and helicity in the atmosphere. Plugging all of this into my own research (described in more depth in a previous post), the environment appears conducive for EF2 to EF3 tornadoes in that north IL/south WI corridor, if supercells can form and produce tornadoes. This is all subject to change, but there's certainly the risk of some tornadic activity for the aforementioned areas.

To summarize:

- There is an increasing threat of widespread severe weather on Thursday, April 9th.
- The most intense severe weather looks to focus on Illinois and Missouri.
- An appreciable tornado threat may exist over southern Wisconsin into northern Illinois.
- Crunching numbers indicates that IF tornadoes do form in the aforementioned areas, they may be of EF2 to EF3 strength.
- Uncertainty still exists with this event.


Analysis of the Potential Tornado Outbreak on Wednesday, April 8th

This is a post that will detail an analysis of the potential tornado outbreak on Wednesday, April 8th. If you've been watching my discussions over the past several weeks, you've noticed how I've used my 'tornado research' to look at severe weather situations. Today, you'll get the first in-depth look at that research, and why I believe significant tornadogenesis (EF3+) is likely on Wednesday.

First, we'll go over the Storm Prediction Center outlook and frontal positions for Wednesday.

Storm Prediction Center
(click all images to enlarge)
The Storm Prediction Center's outlook for Wednesday shows a Slight risk of severe weather over southern Iowa, extreme southeast Nebraska, west-central Illinois, much of Missouri, eastern and central Kansas, and central Oklahoma. It is expected that these areas will see the brunt of the severe weather threat, with a lesser severe weather threat present in the Marginal risk (shown as MRGL) area.
There is a portion of the Storm Prediction Center's text outlook that I'd like to share; for some of you, it may be a little complicated, so I've underlined the parts that sum up the paragraph best.

I underlined how the SPC foresees the rapid development of storms with any storms that are able to develop (we'll discuss that a bit later on), and how the storms would quickly turn supercellular. Additionally, locally damaging winds and very large hail is expected. Perhaps the most ominous sentence in this discussion is the last one, where the SPC indicates that any storms which do develop are expected to have significant severe weather associated with them. That's some pretty alarming wording, especially two days in advance of the event. We'll work through why this is as we go on down the post.

Weather Prediction Center
Let's now go over the forecasted surface map for Wednesday morning, about 12 hours in advance of storm activity. We see a low pressure system in the Oklahoma panhandle, with a dry line extending south into Texas. A stationary front then extends northeastward into central Illinois, where another low pressure system sits. It is expected that this frontal boundary will drift north as the day goes on, and will aide in severe weather development.

Now that we know how this threat will set up, let's go over the forecasted severe weather parameters on Wednesday evening.

(click all images to enlarge)
The most critical ingredient to look at, to ensure that we have thunderstorms to begin with, is forecasted Convective Available Potential Energy, or CAPE. The higher the CAPE value, the more buoyant the air is, meaning the air is able to rise easier, and form thunderstorms. In this forecast, off of the NAM model (which is notorious for exaggerating forecasted CAPE/instability), we see values on the order of over 3000 joules per kilogram of buoyancy extending from northern Texas, through central Oklahoma, and into central Kansas. Lesser, but still prominent instability then exists across the eastern portions of the latter two states. CAPE values of over 3000 j/kg are not exactly common, so when they come up on a forecast, one tends to take note. Overall, this is a very favorable environment for thunderstorms... right? Not exactly, but we'll visit that a little later.

Next, let's see how the upper air pattern looks, for the same Wednesday evening timeframe as the CAPE graphic we checked out. We see the Pacific jet stream arcing over the ridge along the western coast of North America, contributing to that jet streak rounding the base of the trough in the Rockies. What really grabs my attention, however, is the roaring subtropical jet stream entering Mexico and Baja California. That jet stream is barreling into North America at over 100 knots, and will be a big factor for Wednesday's tornado potential. We see an arm of the jet stream nosing into Oklahoma by Wednesday evening, which also sets up an area of divergence, as noted by the low wind speeds, over southern Kansas and northern Oklahoma. It's no coincidence that the greatest threat for tornadoes lies within that area of divergence.

Let's now turn our attention to the lower level jet stream, once again valid for Wednesday evening. Here, we see wind speeds in excess of 45 knots over central Oklahoma and southeastern Kansas, right where the best instability is. This lower level jet will be able to advect the warm, moist air from the Gulf, as well as the drier air from the west to create this potentially severe environment. This kind of set-up is a consistently favorable one for severe weather in the Central US.

There is a caveat to this severe weather situation. Model guidance is indicating that all of the wind shear, instability, and helicity in the atmosphere will have to contend with a warm layer above the surface, located just over 5,000 feet above the ground. The Storm Prediction Center appears optimistic that storms will be able to break through this warm layer and allow for scattered storms to form. If storms are able to form, they have the potential to become quite dangerous, threatening life and property, as will be shown below. For now, I'll go with the SPC, but this temperature inversion could become an issue down the road.

Now that we've evaluated all of the basic ingredients for thunderstorm and tornado formation, let's move on to a couple less conventional forecasting methods.

We begin with the forecasted CWASP chart from the GFS model, valid on Wednesday evening. In a nutshell, the CWASP parameter is able to diagnose tornado favorability from a number of indices, combined into one. As the image tells you, percentage values over 65% tend to see the probability of significant tornadoes rise. The forecasted CWASP chart for Wednesday evening peaks at over 85% in eastern Kansas, with significant values over 80% extending across Missouri and Oklahoma, even a bit into Texas. This is the first 'red flag', per se, that Wednesday evening could be a pretty tornadic environment for any storms that form.

A method I like to look at, but not necessarily put full trust in, is analog forecasting. In this case, the CIPS agency analyzes the forecasted conditions for a given date, and spits out the top 15 days where the atmospheric pattern was similar to those forecasted conditions. This chart is valid for the morning of April 9th, and paints a pretty ominous picture. Here, all hail, damaging wind, and tornado reports from all top 15 analog dates are combined into one picture, to give a view of where severe weather is most favored. As the text shows at the top of the page, 102 significant (EF3+) and 122 long-track tornadoes occurred in regimes similar to the one forecasted on Wednesday into Thursday. Again, I would not put my full trust in this, since there remains considerable uncertainty with regards to the entire event, as well as my own precautions with using this analog forecasting, but you get the gist of how ominous this is.

Alright, now that we've gotten past all of the computer modeling forecasts, it's time to go over why I think significant tornadogenesis (EF3+) is likely with storms that break through on Wednesday, as the SPC suggests.

This past winter, I began toying around with tornado data over the past decade or so, as well as the conditions they formed in. Fast forward a few months, and I was able to make a method that had great success in hind-casting tornado strength, before the storm cell and tornado even formed. I began applying it to this season, and it has performed well so far. Seeing as my research has been performing well, this severe weather event could be potentially dangerous, and there's a need to get the word out early and often, I figured now is the best time to share the outlook with you.

Using the NAM model (which tends to exaggerate severe weather parameters), the environment over Oklahoma City, OK is favorable for tornadoes of EF5 strength on Wednesday evening. Once again, that's from the NAM, notorious for exaggerating severe weather. When I ran it for the more-level-headed GFS model, it predicted a tornado strength of EF3 to EF4 for the same area. Another mathematical model I developed indicated a pretty high chance of significant tornadogenesis, with a strength of EF3 or greater, if storms do form.
The big caveat is that this model cannot predict if storms will form; it says that if storms do form and tornadoes are produced, that is the strength that the tornado should be. It's all new research, but has performed well in initial real-time applications.

Thus, if storms are able to break through the temperature inversion, the environment (as of the forecasts at this typing) is favorable for tornadoes to form and be of EF3+ strength.

To summarize:

- A potentially significant severe weather event is forecasted over central Kansas and northern Oklahoma on Wednesday, April 8th.
- Any storms that manage to form in these areas have a rather high potential to produce significant severe weather, including strong tornadoes.
- The environment is supportive for tornadoes to be of EF3 strength, or higher.
- Considerable uncertainty still exists with this event, and it must be monitored in coming hours and days.
- Those in the SPC highlighted risk at the top of this page should review severe weather action plans, in the event that this severe weather potential comes to fruition.


Sunday, April 5, 2015

Wednesday, April 8th Potentially Tornadic Severe Weather Outlook

There appears to be a rising threat of tornadoes on Wednesday, April 8th, for portions of the Plains.

Storm Prediction Center
(click all images to enlarge)
Since our post on this particular day's threat yesterday, the Storm Prediction Center has elevated its wording for this day, indicating that the threat has increased. Today, we see a 15% chance of severe weather within 25 miles of any given point over central Oklahoma, a sliver of north-central Texas, much of central and eastern Kansas, the majority of Missouri, and west-central Illinois. Compared to yesterday's risk area, this 15% threat area has expanded over the Plains and a bit into the Midwest, indicating the increasing confidence in a severe weather event.

Let's take a look at what factors will be playing into this severe weather threat.

Weather Prediction Center
This chart shows the forecasted surface map on Wednesday morning. We see an elongated stationary front extending from the northern Rockies into southern Kansas, central Missouri, and all the way to the Mid-Atlantic. A low pressure system is seen in the Texas panhandle, with an associated dry line across western Texas. Another low pressure system is also moving towards the Plains, shown in Utah in this graphic. By the evening hours, it is expected that this stationary front will lift north into the areas outlooked for severe weather.

Now that we see what will ignite this severe weather threat, let's start checking out the forecasted severe weather parameters.

We begin with the forecasted Convective Available Potential Energy (CAPE) values for the evening of Wednesday, April 8th. The presence of CAPE, the buoyancy of air, is critical for any thunderstorm development. CAPE values indicate the ease of which warm air at the surface can rise and create thunderstorms. Higher CAPE values, in excess of 2000 joules per kilogram, can support thunderstorms at a severe level; values over 4000 joules per kilogram are considered uncommon, and can support explosive thunderstorm development.

In this forecast, a narrow band of high buoyancy stretches from the central Oklahoma/Texas border, almost due northward to central Kansas. The highest values in this corridor appear to be around 3000 joules per kilogram, indicating potentially intense thunderstorm development. It's no coincidence that the highest instability values are forecasted within that risk area put out by the Storm Prediction Center, as well as lower, but still significant CAPE values to the east.

We'll now take a look at the upper air flow for the same Wednesday evening timeframe as that CAPE image we just analyzed. We clearly see the strong upper level trough over the Rockies, with a jet streak rounding the base of that trough in Colorado, New Mexico, and Arizona. Of more importance is the merging of the two jet streams over the Plains. We see the Pacific jet stream contributing a bit, with some weaker winds pushing southward to the east of that ridge along the west coast of North America, even though the northern jet is primarily in Canada. The subtropical jet stream then takes control, as it comes in to Baja California and northward into the southern Plains. We see a jet streak (embedded area of higher wind speeds) over western Texas, with the severe weather risk area just to the east of this streak. That's not a coincidence, either; tornadoes tend to form when a jet streak is displaced just west of the given region, meaning tornadoes may be favored in the eastern half of Oklahoma and eastern Kansas, as well as other states also in this SPC outline.

Finally, we'll take a look at the forecasted CWASP parameter for Wednesday evening. The CWASP parameter gives an estimation, in percentage form, of the likelihood of tornado formation. When the forecast exceeds 65%, significant tornadoes become possible. This forecast has values over 85% in eastern Kansas and central Missouri, indicating a very tornadic environment is present. If convection does form (as the Storm Prediction Center has appeared less enthusiastic about storm coverage on this day than on Thursday), we could see multiple tornado-producing storm cells.

To wrap up, this chart shows how tornadic Wednesday could be. This is a composite image of the top 15 analogs with environments similar to the forecasted environment on Wednesday evening. We see an abundance of tornado reports over Oklahoma and Kansas when a similar set-up presented itself, only confirming this threat.

To summarize:

- The risk for severe weather appears to be growing on Wednesday evening, especially over Kansas and Oklahoma.
- If currently forecasted conditions come to fruition, multiple tornadic storm cells could erupt over the aforementioned regions.
- Considerable uncertainty still exists.


Saturday, April 4, 2015

April 9th, 2015 Potentially Significant Severe Weather Outlook

There is an anomalously high risk for severe weather on Thursday, April 9th.

Storm Prediction Center
(click all images to enlarge)
The above image shows the Storm Prediction Center's long range outlook chart over the Day 4 to Day 8 (3 to 7 days from today) period. This particular map is valid for Thursday, April 9th, and it shows a very ominous forecast.
The SPC has outlined the following regions for a 15% chance of severe weather within 25 miles of any given point:

- extreme southern Wisconsin
- eastern Iowa
- northern and southern Illinois
- western Indiana
- northwest and extreme southeast Missouri
- east-central and southern Arkansas
- southeast Kansas
- central Oklahoma
- northeast Texas

The 15% chance of severe weather, put into words, generally suggests that there is a noticeable risk for severe weather in the areas mentioned. Residents in those areas should keep an eye on the forecast as the date approaches, and watch for inclement weather when the event arrives.

The SPC has outlined the following regions for a 30% chance of severe weather within 25 miles of any given point:

- central and southern Illinois
- much of Missouri
- all of northwest Arkansas
- extreme southeast Kansas
- eastern Oklahoma

The 30% chance of severe weather, put into words, indicates that there is a relatively good chance that severe weather will strike the areas mentioned above. In addition, there is a risk for more significant severe weather. Residents in those areas should continue to keep an eye on the forecast for this date, and closely monitor all watches and warnings when the event date arrives.

Weather Prediction Center
Let's now identify the factors playing into this severe weather threat. Looking at a surface map for the morning of Thursday, April 9th, we see an elongated stationary front which has drifted northward, now displaced over Nebraska into southern Minnesota and southern Wisconsin, diving into central Iowa and Kentucky. We also see a broad area of low pressure centered in Kansas, with a warm front draped across Missouri, a trough along the Oklahoma/Arkansas border, and a dry line in western Oklahoma and central Texas. All of these items will play a role in the severe weather risk for this day. It is expected that the broad area of low pressure will meander its way northward as the day progresses, bringing the warm sector further north as well. Going off of this map alone, any tornado threat would likely be centered along the warm front, which could move as far north as central Illinois and central Iowa.

Now that we know what will be causing this severe weather risk, let's now look at forecasted severe weather parameters.

The first item that must be looked at is Convective Available Potential Energy, or CAPE. This parameter defines the amount of instability in the atmosphere. In short, higher instability means that the air is more buoyant- it is able to rise easier, and thus create thunderstorms. On the evening of April 9th, we see a broad sector of elevated instability on the order of 2000 to 3000 joules per kilogram extending from Iowa to southern Texas. Higher instability values are found in Texas and Oklahoma, where the dry line is expected to produce more instability-based storms than those further to the north.

The next chart that must be looked at is the upper air flow, shown for the same timeframe as the instability chart above. The jet stream forecast shows a merged subtropical jet stream entering along Baja California, along with the jet streak produced rounding the base of the trough in the Rockies. This will likely produce an environment favorable for severe weather just east of the combined jet stream, particularly in the 30% risk areas described above. If you look closely, you'll also see an area of divergence, where the contour lines seem to bulge away from southern Wisconsin and northern Illinois. This indicates rising air, favorable for thunderstorm formation.

To summarize:

- A severe weather threat exists across the Midwest and Southern Plains on Thursday, April 9th.
- A more enhanced zone of severe weather is present in Illinois, Missouri, Arkansas, and Oklahoma.
- This could be a large-scale severe weather event, and must be monitored in coming days.
- Appreciable uncertainty still exists with the spatial coverage and extent of this event.


April 8th, 2015 Severe Weather Outlook

There is an elevated risk for severe weather on Wednesday, April 8th.

Storm Prediction Center
(click all images to enlarge)
The Storm Prediction Center has posted a 15% risk for severe weather within 25 miles of any given point for Oklahoma, eastern Kansas, and much of Missouri. If you're familiar with the Storm Prediction Center's Day 1 through Day 3 outlooks, you might be wondering why you don't see the 'Slight' or 'Enhanced' demarcations on this chart. In the long range (beyond Day 3), confidence can be high enough to outline an area for 15% or 30% risk of severe weather, but because it is so long-range, that's about as confident as these outlooks can get.

Weather Prediction Center
The first thing we want to do is look at the surface map to identify what features will be inciting this severe weather event over the aforementioned states. On the morning of Wednesday, April 8th, we see an elongated stationary front, stretching from western Kansas across southern Missouri, meeting up with a low pressure system in southern Indiana. We also see another storm system in western Oklahoma with a small warm front and associated trough in Texas. If you were to click through different days to get a context for where these boundaries are moving, you would find that the stationary front will be drifting north to cover Kansas and Missouri by evening on Wednesday. This means that the stationary front should cause the severe weather over Kansas and Missouri, while that smaller system in western Oklahoma combines forces with another system dropping out of Colorado to produce a severe weather threat in Oklahoma.

Now that we know what we're looking at, let's start checking out model forecasts for our severe weather parameters.

The first thing to look at is the forecasted instability, since instability (the action of warm air rising as cool air surrounds it) is necessary for all thunderstorm formation. This chart above shows the Convective Available Potential Energy (CAPE, the meteorological definition of instability) forecast for the evening of April 8th. We notice a corridor of high instability on the order of 3000+ joules per kilogram extending across eastern Kansas and central Oklahoma. It is expected that a boundary, potentially a dry line, will be available to conjure up such instability. We see lower, but still notable instability values over Missouri. Here, the stationary front should be able to fire up thunderstorms as well, some of them potentially being severe.

Also helpful is looking at the wind pattern aloft; here, the 300 millibar jet-stream wind forecast is displayed for the same timeframe as our graphic above. For this chart, we see a large trough dipping in the Western US, and allowing a sharp jet streak to form around the base of this trough, in Arizona and New Mexico. Additionally, we see the subtropical jet stream poking in to Baja California, and even providing a jet streak (a band of stronger wind speeds than the surrounding air) in western Texas and Oklahoma. The jet streak nosing into Kansas and Oklahoma will aid in severe weather development, especially with the positioning of the jet streak just west of the axis of highest instability in those states. In some research I've done this past winter, tornado formation can be favored in the areas directly east of a jet streak; in this case, that favors the Oklahoma, Missouri, and Kansas regions.

To summarize:

- A severe weather threat is unfolding on Wednesday, April 8th for eastern Kansas, Oklahoma, and much of Missouri.
- As of now, model forecasts are showing a potentially tornadic environment.
- Very large hail could be a prominent threat in Kansas and Oklahoma.
- All of this is subject to change.