Saturday, March 9, 2013

2013 Official Severe Weather Outlook

Edit on March 9, 10:15 PM: Added stratospheric warming to the mix, does not affect final forecast.

This is the 2013 Official Severe Weather Outlook, made by The Weather Centre. The following describes the predicted set-up for March through June 2013, using unbiased information and no hype.

We're going to start out with my analog package for this timeframe. I used years from my 2013 Revised Severe Weather Outlook, but cut out some years that, after closer analysis, were not as close to what we're seeing right now. I ended up using 1965, 1968 and 2009 for my analog years, and the first image shown above details 500 millibar height anomalies for March through June, essentially the main months of the severe weather season. Cold colors signify low pressure, while warmer colors tend to indicate high pressure. Looking at this map, we see a large swath of blues and purples in the West US. This does mean that we would be seeing a stormy pattern over that region this severe weather season if my analog package works out. The darkest purples are centered in the Southern Rockies, and that is where I put the 'L', signifying the low pressure. Further to the east, these very below normal height anomalies suddenly end in the Midwest and arc up into southern Canada. Close analysis under the letter H would reveal a small spot of green, and this means general high pressure in that region, hence the 'H'. This analog package pattern is a textbook example of the negative Pacific North American (PNA) index. The negative PNA involves low pressure in the West. If we follow Newton's Third Law of Motion, which dictates that every action has an equal and opposite reaction, the East US must then have high pressure form, and that is what the negative PNA does. This high pressure in the East US then forms what we call the Ring of Fire. If you remember last summer, we saw a large high pressure system sit stagnant over the Plains, while thunderstorms ran over parts of the Plains and Midwest for multiple days. This is the 'fire' part of the Ring of Fire, because the thunderstorms are moving along the ring of high pressure. I could definitely see a similar set-up for the Midwest and Great Lakes if this analog package verified in the coming severe weather season. Considering we have seen a negative PNA pattern for much of the winter, it's quite possible that the -PNA regime would continue into the spring, and this would help out verification chances for this analog package.

Continuing with the analogs, we move to zonal wind anomalies at the jet stream level. For those of you unfamiliar with the term, zonal wind just means from west-to-east. If you hear your local weatherman say we are entering a zonal pattern, it means the jet stream isn't really being disrupted by storms or high pressure, it's just moving in a fairly straight west-to-east line. Therefore, zonal winds are typically the best to identify the jet stream, so we are using this in our analog package. I highlighted two areas of above normal winds- one in the Pacific-Baja California-Plains, and one in the waters just offshore Canada. They are both marked clearly with arrows. The top one is labeled as the Pacific jet stream. As the name implies, it comes from the Pacific and is the main jet stream in the winter that separates warm and cold air. The subtropical jet stream, commonly abbreviated to the STJ, comes from lower latitudes and can be strengthened when conditions are favorable in the winter for Nor'easters. In the severe weather season, an active STJ is almost necessary for severe weather. Looking at the two jet streams, we see they actually come together in the Midwest and Great Lakes right over major cities like Chicago, Detroit, Madison and even in extreme south central Canada. When these two jet streams merge, severe weather is abound to happen if the rest of the atmosphere cooperates. If you were to ask me, does the merging of these two jet streams increase the chances of severe weather in these areas, I would say yes. The cities I mentioned a few sentences ago would certainly be under the gun if this analog package worked out. Something I have been heralding is the shifting of Tornado Alley more to the east this spring, into the Midwest and eastern portions of the Plains- this would be part of that shift eastward.

Now that we've used the past to predict the future, let's look at the present.

We start with the Gulf of Mexico. In the spring, it is essential for storm systems to gather rich, humid air from the Gulf of Mexico in order to ignite a severe weather event. Looking at the latest temperature anomalies over the Gulf of Mexico, we can see widespread warmer than normal sea surface temperatures, particularly in the western and eastern thirds of the body of water. The central portion of the Gulf isn't as impressive. Regardless, we can see a clear trend of warmer than normal waters. These warmer than normal waters means more available moisture and instability, both key ingredients for thunderstorms. Thus, compared to normal, it would be a reasonable assumption to say that this above normal sea surface temperature trend in the Gulf of Mexico may aid in an enhanced severe weather season for the parts of the United States I will show below.

Another piece of the puzzle is the stratosphere. Shown above are temperature anomalies in Celsius over the January-February-March 2013 period. Only a portion of the image is colored in because we have just started March. If we take a look towards January, we see a very abrupt warming event take place throughout the heavy majority of the stratosphere. This warming event was a prolonged event with prolonged effects that continued to propagate down to the surface until mid February, as the yellows show in the above image. To this day, the stratospheric polar vortex has not recovered from this historic warming event, and I do not expect it to fully recover for the remainder of the winter and spring. As the image also indicates, there was something called a negative QBO in effect, that still remains to this day. The negative QBO, or Quasi-Biennial Oscillation, involves wind anomalies in the stratosphere. Basically, if you have negative zonal wind anomalies in a certain part of the stratosphere, the QBO is considered negative. These negative zonal wind anomalies are anti-polar vortex, so they weaken the vortex and thus delay any potential strengthening. If we see this negative QBO stay through the Spring, (which I'm hesitant to believe) cold weather hitting the US would be more likely than without the negative QBO.

We are also looking at the drought for this severe weather season. The ongoing drought does exactly the opposite to the effects the additional Gulf moisture has. The drought makes the land so dry that evaporation cannot occur. Thus, when storm systems enter the region highlighted in reds and maroon colors, they will not be able to grab moisture from the ground and cannot produce as strong of storms as would have been produced if the drought was not in place. Also, the drought creates warmer than normal weather over the Plains. Because little evaporation is occurring, clouds struggle to form. This lack of cloud cover enables the sun to shine on in a vicious cycle that supports long-term droughts. The warmer than normal temperatures can then create something called a baroclinic zone, which just means a temperature gradient. That is, the warmer, drier air can create its own air mass that can collide with the more temperate air mass out east. When storm systems plow through, it would not be uncommon to see these conflicting temperature masses enhance severe weather potentials.

Another important factor that was first noticed by weatheradvance.com is the presence of Northern Hemisphere snow cover. As we know, a temperature gradient is necessary for severe weather to form. Cold fronts are sharp temperature gradients as well. Looking at the chart above, we see that we have recently seen above normal snow cover for the month of February that has remained stagnant at moderately above normal levels since mid February. The presence of snow cover anywhere not only provides a great opportunity for kids to have fun, it lowers temperatures. If we continue to see above normal snow cover in Canada going into April and May, I find it very plausible that the temperature gradient between the South and North US will be increased. This comes as a result of that snow cover in Canada allowing cold air to flow south and above normal Gulf of Mexico temperatures providing a base for above normal temperatures and moisture in the South. The drought will also allow for increased temperatures as the lack of moisture allows for a lack of clouds, as I explained above. Some of the worst severe weather events have happened in the presence of above normal snow cover in the Arctic. It is also valid to say that some of the worst severe weather events have happened without this snow cover, but the general idea I'm trying to convey is that snow cover equals colder temperatures equals a larger temperature gradient, which means a larger threat for severe weather.

The stratosphere is also something to watch at this moment. Shown above is an animation of temperature anomalies at the 10 millibar level. As you can see, strong warming has developed in the Himalayan Mountain Range over the past few weeks. This practice of stratospheric warming originating as a result of mountains is not an uncommon event. Mountains naturally force air higher into the atmosphere, and it is very easy for this air to be pushed high enough to reach the stratosphere, especially in the most significant northern hemisphere mountain range in the world, the Himalayas. In the past week, this swath of above normal temperatures in the upper stratosphere (10mb) has really flared up and began to move north and east towards the Bering Sea and then into the Arctic. This track is very typical of sudden stratospheric warmings, I see little reason to argue why the warm mass of air would not do this. However, as the last few frames will show you, the warm air mass has stagnated and flattened out on the northern edge, kind of retrograding back west into Europe. At this point in time, I'm remaining optimistic that this warm mass will propagate into the Arctic Circle and initiate a full-blown sudden stratospheric warming. I have a hard time believing it will dissipate (although that solution is very much on the table).

If the warm air mass does get into the Arctic, we can expect an even more active spring. The mass of warm air over the Arctic will result in high pressure winning its spot in that same domain, resulting in a pattern that favors cold air masses taking a shot at the United States. This, in turn, results in a more severe weather-prone atmosphere through a variety of other things the sudden stratospheric warming can do, but things we will not discuss today. The point is, if (keyword is if) this swath of warm air is able to blossom into the Arctic, you can expect a more amplified severe weather season than even I may be predicting.

Now that we've examined the past and present, let's go to the future and see what forecasts say for this severe weather season.


There's another weather phenomenon called the AAM, which means Atmospheric Angular Momentum. This is one of the more complicated and physics-based indices that I am watching these days, so I'll just give a nutshell description of what the AAM can show. In a positive AAM, the jet stream can be enhanced, and a negative AAM can show a weakened jet stream. The image above shows the predicted AAM anomalies on top, with latitudes shown on the left side of the image. The 30N-60N latitude area falls within the domain of the United States and into Canada, so that is where we want to look for the AAM anomalies. The bottom part of the image shows the predicted AAM values from 4 separate CFS (the long range American model) ensembles. I outlined a section of slightly above normal AAM anomalies in the top half of this image to show that the jet stream could be enhanced here. If we look towards the latitudes, we see that the outlined portion falls between the 30N and 60N lines, which covers territory from the Mexico/US border to the midsection of Canada. Now, if you were to look even closer, you would see that the highest AAM anomalies are more centered towards that 30N-40N region. This would put the enhanced jet stream south of the Great Lakes and North Plains. This would seem more logical than having an enhanced jet stream across the entire continent of North America. This enhancement of the AAM comes along from April into the end of May, so that could be the time when we see some good severe weather in the nation. Of course, severe weather can strike at any time, but a stronger jet stream can amplify chances for severe weather.

This is a forecast from one of the long range forecast models in the Climate Prediction Center, showing 500 millibar height anomalies. As we discussed in the analog package section, reds show above normal heights (high pressure) while blues show below normal height anomalies, which signifies low pressure. As we look up towards North America, we see above normal heights in the North Pacific towards (but not in) the Gulf of Alaska. This fits in nicely with my analogs, which show that high pressure area in the lefthand corner of the image. Also shown is a wide swath of above normal heights in northern Canada and Greenland. This depiction of above normal heights near Greenland is called the negative North Atlantic Oscillation (NAO). The NAO has a positive and negative phase. When there is high (low) pressure over Greenland, the NAO is said to be negative (positive). As a result, low (high) pressure forms over the Eastern US, and the subtropical jet stream, which helps Nor'easters form, is strengthened (weakened). Thus, when the NAO is negative, one can anticipate better chances for cold and storms in the East US, while the positive NAO brings about warmer weather. The presence of a negative NAO tells me the subtropical jet stream previously mentioned above will be enhanced, thus adding to the severe weather threat in the South US. Looking into the United States, we see an area of above normal height anomalies in the Southeast. Again, my analogs showed this, and this will divert the storm track and active subtropical jet stream north through the Midwest and Ohio Valley (also shown in the first analog image). The negative height anomalies in the Pacific Northwest and West Coast in general in the analog package are reciprocated in this model's forecast, shown by the light blue contour on the west coast of North America.

I'm feeling pretty confident in my outlook shown above. Based on what appears to be a very reasonable analog package, combined with long range model forecasts, analysis of the ongoing drought and offshore water conditions, I anticipate an above normal area of severe weather extending from the South Plains into the Midwest. There is a light red region and a dark red region which show above normal risk and the highest risk, respectively. The highest risk area encompasses major cities like Chicago, St. Louis, Little Rock, Tulsa, Dallas and Austin. The lighter red region of above normal risk covers territory more into the Southeast and Ohio Valley. I anticipate below normal severe weather across parts of the Plains, especially in northeast Colorado, Nebraska, South Dakota and northwest Kansas. These areas are most likely to receive less severe weather reports than their seasonal average as the weather pattern does not favor an active weather season across that region.

Andrew

22 comments:

Eric said...

Andrew, I love this post, extremely informative and interesting, however I do want to shed some light on the ongoing stratospheric warming event, and in fact, if you go back into the stratospheric temperature archive and look at all of the years with late season warming events, and then you look at the following tornado seasons, there is certainly a notable increase in tornado activity and large outbreaks. Take 1999 for example, late stratospheric warming event, then in April and May a series of outbreaks followed, including the extremely powerful Moore Tornado of May 3, 1999 with the fastest wind speed ever measured on earth at 318 MPH. Then, you can also go back and look at April 2011 and see there was a warming event at the beginning of April, and of course we all now what followed as far as severe weather in that month.

Andrew said...

Eric: I was actually going to put it in there, but recently the warming event has started to flatten out and stagnate. That, combined with how prone I am to forgetting things, ended up excluding the stratosphere in this post. But yes, SSW's certainly are big players in the severe weather season, no doubt about that.

Eric said...

@ Andrew
Well, I'll include it in my new post, but I actually think the culprit behind the warming in the first place is the MJO, and you'll note how it came across the Indian Ocean and into the western Pacific as we get towards the 20th or so, at the exact same time period this warming event was initiated over the Himalayas, as you've described whereas air rushes over the mountainous terrain, it is "squeezed" and forced upwards and as it comes down the other side of the mountains, the air near the surface sinks, while the air well above the surface continues along steady or even rising to a certain degree. This creates a diversion of air along a propagating wave of energy near earth's surface, and since air is spreading out, this allows for the air molecules within that given air mass to move more freely about, and since air molecules have more space to move about, this is indicative of warming.

Eric said...

Now, I don't find it just coincidence that this warming event happened and the MJO was moving quickly across the Indian Ocean and into the western Pacific, (where the Himalayas are located), and since the MJO is associated with rising air and latent heat release, this causes a tendency for the atmosphere above to warm in response (supporting that Kelvin Wave theory I developed with SSW in December), however, the MJO slowed its progression east, and its no coincidence the warming event has slowed, but the reason the MJO slowed progression east just goes back to the global ocean temp anomalies, and when you look at the western Pacific, it is much warmer in comparison to the Indian and the central and eastern Pacific, and considering warmer ocean, warms the air above and induced rising motion, it makes sense that the MJO has stalled, which led to a slowdown in the warming event. However, when you look at the deep tropical Atlantic and note the very warm waters (warmer than the west Pacific where the MJO is currently located) induced by the -NAO enforcing weaker than normal trade winds, you have to say to yourself, that the MJO must at some point come east towards the Atlantic.

Eric said...

In doing so, would force a major Kelvin Wave, and -SOI regime (which I showed in my most recent post a correlation to tornado activity in April 2011), and would kickstart the ongoing warming event, and not to mention the fact that with the sunspot cycles crashing low again over the last several days (as noted by the last several years with a increase in warming events likely tied to a weak and quiet sun) to now with the official NOAA sunspot number below 60, and with above normal snowpack over the northern hemisphere in place, the ingredients are certainly there for this warming event to reintensify once again in the medium to long ranges, which just would raise my concerns for major tornado outbreaks

Anonymous said...

Iowa looks to be having (the central plains) an average spring & rainfall it does seem?
Which to me means cold & rainy…am I right? I do not like that at all….I get we need
moisture, but, we could also use some sun & heat! I am so depressed!
Thank you so much Andrew for all your hard work I really appreciate you!
Even all the cold & snow & rain you bring! I still appreciate you! Thank you!
bree

Greg Carbin Jr. said...

Very good, accurate depiction! We here at the SPC headquarters in Norman, OK are going right along with you. The recent warming in the stratosphere will likely have at least some role in the robust severe weather season. Some of our worst outbreaks have happened during years where stratospheric warming was present, and even when it was winding down, affects can still linger for weeks, even months. The pattern doesn't rebuild overnight. So those from the southern Great Lakes into the Mississippi River Valley are the ones most favored for the most robust, most frequent severe weather this season. As soon as we slide our way into April, that looks to really get going strong. Not looking for too much severe weather in the short term, at least nothing in terms of a major outbreak. Those in the Chicago area, the Quad Cities, Indianapolis and St. Louis need to be especailly watchful as those areas seem to be the target zone no matter how you slice it. It's been a long time since we've seen the greatest severe threat in these areas, and what the plains usually see could be shifting into these areas, and it is critical to be sure the public is alerted as swiftly as possible. We don't want to csuse panic, but sometimes being scared is what can save your life. Ignoring and not caring about the weather can endanger your life! Again, great update. Lets hope we don't get it too bad. We get this gut wrenching feeling already, and you know exactly what we mean! We know what happens in major outbreaks, and it is very sad and largely preventable. Take care, Andrew.

-Greg Carbin Jr.

Andrew said...

Eric and Greg: Always appreciate the invaluable insight you both provide. I did update this outlook to include the stratosphere- while it did not change my forecast graphic, the chances for a more amplified severe weather season are boosted, as you two alluded to in your comments. Keep the comments coming, the more the merrier!

Best regards,
Andrew

Eric said...

@Andrew
Thank you, of course I am going to be closely monitoring the SOI, and according to some of the forecasts I am seeing, it looks like there could be a major kelvin Wave and -SOI crash towards week 3 & 4, and if my theory from December is right about the Kelvin Wave and stratospheric warming event relationship, this could be exactly when what the stratospheric warming event needs to get going, which of course would not be good, considering that April 2011 also started out with a major -SOI crash and warming event, will be interesting to see how this unfolds.

Anonymous said...

Thank you. What an excellent write up. I follow severe weather every spring, and it is nice to know what to expect. This kind of foreknowledge leads to preparedness, and preparedness can lead to lives being saved. I don't always understand the complexities of weather, but I can grasp this enough to know that a potentially active season is on tap for the Great Lakes Region. I live in the Chicago area, and it looks like we are, if not in the "bullseye" very close to it. Again, excellent work.

Thinkingwritingguy said...

Thank you very much for the outlook. As a resident of Northeast Illinois, this outlook has been very helpful as well as an eye-opener. I have many contacts in Public Safety and will be forwarding the URL to them. Having this kind of foreknowledge will aid us all in being ready for the severe weather season. Again, excellent work.

Anonymous said...

Will you be doing personal forecasts sometime in the future? I know it's looking like Michigan could be under the gun this spring, but I'm curious as to how things will turn out in my local area.

Andrew said...

Anonymous at 7:10: Thank you very much for the appreciation, I'm very glad you like it!

Anonymous at 2:48: Due to the number of visitors at this blog nowadays, personal forecasts have ceased.

Anonymous said...

Well if you can't do personal forecasts, can you please do regional forecasts like you do with the winter forecast? So people can not only see that they are under the gun for severe thunderstorms in general, but specific types of them like hail. Also if you do this you should add sort of a forecast for how many high risk days each area will have. Just my input!

Andrew said...

There's no way to predict types of severe weather this far out, and regional forecasts require massive amounts of time I do not have at the moment.

Anonymous said...

Well I understand if you don't have time. Your first statement, however, is quite debatable. We know at this point what patterns favor what severe weather. For instance, we know that a massive high pressure system that sits itself down in the Southeast is very conducive to damaging wind producing MCS's that travel around the flank of that high. Highly meridonial jet streams are favorable for tornado outbreaks, and high heat with relatively little atmospheric support is conducive for pulse severe weather, i.e. hail and winds. In your analogs, you can look for such patterns.

Andrew said...

Anonymous at 6:27: You cannot forecast above normal probabilities of hail or damaging winds- each event is different. Tornadoes do have some characteristics that you can use to predict them, but I am not willing to put out a forecast on individual types because of both time and how nearly impossible it is to accurately predict them.

Anonymous said...

Well as I said, it is just a forecast. You don't need to do it, but the SPC has identified many patterns conducive to storm forecasting. In fact, papers have been published on it. If you can look at the pressure anomalies/jetstream analogs, you can get a reasonable understanding of the most likely threats. Also simply using climatology in general is incredibly useful. For instance, we know that Chicago is much more likely to see damaging straight-line winds vs. hail and tornadoes.

Anonymous said...

Highly meridional jet streams/upper level troughs are generally not as favorable for tornado outbreaks as somewhat lower-amplitude, yet still vigorous upper level disturbances (ala 4/3/74, 5/4/03, 3/21/52 and 4/27/11). The are some exceptions to this rule, particularly in the Plains (like 3/28/07, 5/5/07 and 5/23/08), but strongly meridional upper level winds can lead to problems with the deep layer wind profile (more specifically a back-veer-back configuration) that can be problematic when looking for discrete tornadic supercells. It is one of the reasons why May 24th, 2011 (El Reno/Chickasha/Goldsby) didn't go off in Kansas like it did closer to the OKC metro area, even when some of the other parameters were projected to be just as impressive the day prior.

Anonymous said...

What could Virginia get out of this track? Usually we get the worst when it comes from the South to the North. Can we still get anything from your predicted track?

Anonymous said...

You may not have but southern VA and NC in the prediction for a reason but here is my argument. Looking at the models, it looks like the South will be on the gun with severe weather in late april and may. During june however, the storm track looks like it shifts northward and eastward. (as usual) This would give storms to the Tennesee valley region. And finally in august the area spreads eastward and westward. This puts western PA aswell as VA and NC at a higher severe threat. Are you sure we won't get above avg. severe? Or was it that we would be below avg. until august? Im 12 and I plan on becoming a meteorologist whenn I grow up. Please answer, Justin.

Mike said...

Got a question for you Andrew, you mentioned areas in Southcentral Canada could be in for severe weather this spring, does it look like areas in Southern Manitoba & Winnipeg,MB,Canada could be in for severe weather this spring given the jet stream position's? Let me know please as I wasn't sure if you were talking about Southern Ontario or Manitoba. thx....

Mike from Twitter @SouthMBWeather