Saturday, March 9, 2013

Accumulating Snowfall in Plains, Midwest

Heavy accumulating snowfall looks to be on the way for parts of the Plains and Midwest in the next few days.

A storm system currently moving out of the Rockies will move northeast through the southern Plains and heart of the Midwest to deliver accumulating snowfall to the central/northern Plains and Upper Midwest. High resolution model forecasts, like the one shown above, depict snowfall reaching beyond the 12 inch mark in Iowa and portions of Nebraska. For anyone who saw yesterday's post concerning that banding in Nebraska, I find it to be no surprise that forecasts have changed yet again and shifted this heavy band. Models never handle these isolated bands of heavy snow well. The purple areas express snowfall forecasts of 6-12 inches of snow, and this looks to be in store for Wisconsin into the heart of Iowa, parts of Kansas and portions of Nebraska. Lighter snow in the 3-6 inch range will fall in the blue regions, and even lighter amounts look to be in store for the light gray colors. Winter weather alerts are posted below.

Dark Blue: Winter Storm Watch
Light Purple: Winter Weather Advisory
Pink: Winter Storm Warning
Red-orange: Blizzard Warning


Thunderstorms Firing in Texas

Radar imagery indicates isolated thunderstorms are firing in central Texas, with a more linear structure developing in north central Texas.

Discussion: Strong lower level frontogenesis is enhancing thunderstorm development over Texas, with two areas of weather evolving. The first area, in north central Texas, involves linear thunderstorm development with a primarily damaging wind threat. Second area is focused in south central Texas, where isolated thunderstorms are blossoming. We find the highest tornado threat centered in this second area at this time. Strong moisture convergence, particularly over the second area, is helping to form and sustain these isolated thunderstorms. Short range model forecasts indicate the thunderstorms will continue to fire, and most likely shift into a more linear phase as shown in the first area. I do think that the tornado threat will persist for the next hour or two before the primary threat becomes damaging wind and hail.


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 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.