Wednesday, July 30, 2014

Fall 2014 Seasonal Forecast

This is the outlook for Fall 2014.

We will begin by analyzing the temperature outlook from a trusted long-range ensemble guidance system.

For the August-September-October period, the ensemble system shows an outlook of warmer than normal temperatures across the Western US and portions of the Northeast, with darker shadings indicating higher confidence in that forecast anomaly. The warmest temperatures appear to be centered over Montana and Idaho in this outlook, as well as portions of New York and towards Maine. The big story here, however, is the anomalous cold shown in this outlook. We see below normal temperature anomalies extending from the Canada/US border, all the way down to the US/Mexico border. The core of the cold looks to be across the Plains and Midwest, with higher confidence in cold weather displayed by the dark blues. These dark blues look to be most prevalent in the Midwest, Plains, Great Lakes, Ohio Valley and Mid-Atlantic.

If we fast-forward to the October-November-December temperature forecast, we find a similar outlook as the one we just analyzed for the August-September-October period. In this forecast graphic, we see cold weather forecasted across most of the nation. The cold extends from the Pacific Coast to the Carolinas, with the deepest cold weather displayed over the North and Central Plains, as well as the Southwest US. Most of the coastal Eastern Seaboard looks to be in slightly above normal temperatures, though confidence in this is rather low.

After a cold winter, spring and summer, a cold outlook for fall is something that makes sense. We are still seeing that persistent pool of warm waters in the northeast Pacific, which has allowed these seasons to be so chilly. As a result, we should be wary of a chilly fall this year, something that could continue into the winter.

Let's now examine the precipitation outlook.

The precipitation outlook for August-September-October from this long range ensemble system has a variety of conditions across the country. We see a general dry trend over the Southwest, unfortunately worsening the already-catastrophic drought in California and other states. The Pacific Northwest is showing a variable signal. The North Plains are exhibiting above-normal precipitation anomalies in rather high confidence for this timeframe, and this trend continues into the Midwest and Great Lakes. A stark dry trend is observed in the Southern Plains with enhanced confidence, and a similar story can be found in the upper Midwest. In the East US, we see mainly variable conditions that cannot let us determine a particular forecast for this area. Beyond this forecast period, the precipitation outlook exhibits too low of confidence to determine a thorough forecast.

Because we don't know what the El Nino situation will do for this fall and winter, it's hard to see how reliable this precipitation outlook is. The drought conditions in the Southwest should allow for a general dry trend there by process of a feedback loop, unless we see a sustained El Nino arise. I do believe we see conditions in the Midwest and Great Lakes a bit drier than they are projected to be, with conditions along the East Coast averaging around normal or slightly above normal.


Monday, July 28, 2014

Updated Thoughts on 2014-2015 Winter Outlook

This post will show my personal thoughts on the upcoming winter. In the past, I've put out posts focusing on one or two mechanisms that may influence the winter, but this article will show you what my personal thoughts are.

We're going to begin with a discussion focusing on sea surface temperature anomalies. We've gone over this quite a bit so far this summer, but I haven't really put my two cents into those posts.

Looking at this chart, I see a few things to keep an eye on. Primarily, the warm pool in the northeast Pacific must be watched closely. In the winter of 2013-2014, we saw persistent ridges of high pressure forming along the West Coast, leading to strong upper level lows dropping south into the Central and East US. This pattern allowed extreme cold to be pumped mercilessly into the United States. The warm pool has not dissipated from last winter to this winter, so my concern for another cold winter automatically rises.
I believe that mechanisms upstream (to the west) of the United States influence the weather pattern more than mechanisms more downstream (to the east), primarily because the upstream factors are almost guaranteed to affect our weather based on their location. A prime example is that ridging due to the warm pool in the northeast Pacific last winter. This allowed for a cold winter. However, the lack of persistent ridging over Greenland last winter would have typically argued for a warmer winter, especially in the East US. Did this happen? No, we still received record-breaking cold. You can point to the upstream location of that warm pool of water as a reason why we didn't see warmer weather prevail. All in all, my point is this winter has substantial potential to be cold once again. If those warm waters persist in the northeast Pacific into the winter months, it's probably a good bet you'll need those extra blankets to keep warm for December through February.

Something that has me concerned, however, is the large presence of warmer than normal water anomalies around Greenland and in the waters around northeast Canada, all the way to Europe. What could happen this winter is that we see persistent ridging in the northeast Pacific, but also in the northern Atlantic. From there, the question shifts to if we will see a persistent upper level low in North America, or not. The lack of an upper level low would likely place the US under predominantly zonal flow, something that would result in a warm winter. However, the presence of a strong upper level low could easily make for relentless cold in portions of central and eastern North America. This isn't subtracting from the overall likelihood of a cool winter based on the water anomalies in the northeast Pacific, but it is something to keep an eye on.

In a way, we haven't really seen last winter's pattern leave us. The image above shows 500mb wind speeds in the shaded regions, with wind barbs and pressure contours spread across the image. This graphic is valid for early this afternoon, on July 28, 2014. Even though it's July, you might notice some similarities to the upper air flow from January 2014. We still see strong ridging over the Western US, and an anomalous upper level low in Canada allowing cold air to dig into the North US. If this pattern continues into the fall months, the chances for a cold winter increase greatly. This is something that will have to be monitored closely.

Refresh this page if the animation stops looping.
The big discussion in the weather world revolves around the situation unfolding in the Pacific. Earlier this year, a record-breaking Kelvin Wave brought about the prediction of a moderate or strong El Nino event for this winter, with calls for Super El Nino-like conditions also being heard (admittedly, from me, too). The Kelvin Wave did hit the surface and brought about significant warming to the eastern Pacific waters. However, as we progressed into summer, these warm water anomalies seemed to evaporate overnight, the culprit seen as the band of below-normal waters about 100 meters down from the surface in the animation above. As of now, we are counting on the mass of slightly above-normal temperature waters in the western part of the animation to push east and hit the surface to make another push at an El Nino. It remains to be seen how this will all play out. One possibility is that the warm waters can hit the surface and induce a weak El Nino event. Another possibility is that the cold waters eat away at the warm waters and eliminate the chances for an El Nino this winter.

My current thoughts on the matter are that we are likely to see a weak El Nino this winter. The dissipation of the strong Kelvin Wave has left us with nothing to support a moderate or strong El Nino, so the most likely scenario is a weak El Nino, if we are to see one at all. I'm uncertain as to how those cold water anomalies will react to when/if the warm waters push east; it'll be something to monitor closely this fall. When all is set and done, however, I'm supporting a weak El Nino. A weak El Nino set-up would resemble something like this, as the image below shows.


To summarize:

I expect that we see a cooler than normal winter for many across the North US, primarily the Great Lakes, Upper Midwest, and Northeast. This comes as a result of the warm waters in the northeast Pacific, as well as the expectation for a weak El Nino. Temperatures should be warmer in the West US once again. I do think we see the drought in the Southwest US ease up at least a bit, but until we can nail down the presence (or lack thereof) of an El Nino, it's up in the air.
Precipitation looks to be on the above-average side around the Great Lakes again, thanks to lake effect snowfall, with the same outlook pegged for the Northeast. This comes from the warm waters off the East Coast likely interacting with cold air from Canada to produce the chance of precipitation. Conditions in the South Plains and Gulf Coast are likely to be slightly wetter than normal, with the opposite anomaly predicted for the North Plains. The Midwest and Ohio Valley ought to see around normal anomalies, as things look right now.

Bear in mind this is not my final forecast. It is merely an update with my personal thoughts, in advance of the 2014-2015 Official Winter Forecast to be issued in October.


Saturday, July 26, 2014

Extensive Damaging Wind Episode Expected Today

A severe weather episode, comprised of tornadoes, large hail, and potentially extensive damaging winds, will take place today.

The Storm Prediction Center has issued a Moderate Risk of severe weather for a portion of the Midwest, including much of central and southern Illinois and southwest Indiana. A Slight Risk of severe weather extends from extreme southeast South Dakota to far western West Virginia. It is expected that a swath of severe thunderstorms, capable of damaging winds, as well as an isolated tornado, will traverse the aforementioned risk areas. The Moderate Risk outline highlights areas where the threat is greatest.

The above outlook is superimposed on the latest infrared satellite imagery on the graphic shown here. On this graphic, we see a complex of thunderstorms moving into Iowa from South Dakota, sending cloud cover off to the east into the Great Lakes, likely limiting any severe threat that was there to begin with. We also see clouds in the western half of the Slight Risk area, and this is something that has been limiting instability so far today. Unless that cloud cover evaporates fairly quickly, I am concerned that severe weather will not be favored for Iowa, Nebraska, Missouri, and western Illinois. On the other hand, the Moderate Risk area is experiencing only sporadic cloud cover, with much of that area cloud-free. This re-assures me that severe weather is still expected today, and this is confirmed by rising instability values over that cloud-free zone.

Those in the Moderate Risk area today should prepare for the following:
• An isolated tornado or two.
• Large hail, possibly damaging to vehicles and property left outside.
• Damaging winds, potentially on a large scale, with significant damage possible in some areas.


Wednesday, July 23, 2014

Current Water Temperatures Support Southeast Ridge Next Winter

Current sea surface temperature anomalies are supportive of the dreaded Southeast Ridge making an appearance next winter.

The image above shows sea surface temperature anomalies from July 11, 2014 to July 18 2014, projected across North America. In this image, we want to focus on the anomalies around the Southeast United States and up through the East Coast.

We see a general presence of slightly to well above normal sea surface temperature anomalies when examining the waters to the west and east of Florida. In those regions, we can observe widespread anomalies over one degree K above normal. Looking up along the East Coast, we see these warmer than normal anomalies still present, though not as intense as those near Florida and the Bahamas.

While this feature may not be as significant when compared to those features in the Pacific, it does hold substantial weight when analyzing the expected climate of the United States this winter. Depending on a multitude of conditions in the atmosphere, there is the potential for a semi-permanent ridge of high pressure to form in the Southeastern United States during the cold season. The durability and strength of this ridge varies on a case-by-case basis, but its presence can never be overlooked when it appears. The ridge allows the storm track to be deflected northward, sometimes bringing winter storms into the Plains if the ridge is strong, and other times into the Midwest and Ohio Valley if the ridge is weaker. What is clear, however, is that this phenomenon spells bad news for winter weather fans residing along the East Coast. This ridge almost always delivers warm, quiet, snow-less weather to those in the East. Unfortunately, if the latter description sounds like you, the chances of these conditions arising this winter have been raised.

The winter weather fans in the East Coast may not want to hear this news, but those in the Plains and Midwest should thoroughly enjoy this latest news, even if it may just be a flash in the pan. We'll have to see how the El Nino situation evolves this fall, hence the flash in the pan reference.


Tuesday, July 22, 2014

Weak Upwelling, Hostile Environment Could Spell Weak El Nino Ahead

A weak episode of upwelling in the eastern Pacific, combined with a continued hostile environment with respect to El Nino formation, could indicate that a weak El Nino may be in store for the future, rather than previous projections of moderate and strong El Ninos.

Michael Ventrice
Shown above is what the concepts of upwelling and downwelling look like. Imagine, for a moment, the eastern Pacific is completely flat, no perturbations or disruptions in the surface or underwater currents. Imagine that the western portion of the water begins to rise. In the Pacific, Kelvin Waves can propagate from west to east along the Equator, bringing about a rise in sea surface temperatures and in the actual water height. As a result, we see water levels in the east drop ever so slightly, likely not even recognizable. As time progresses, in the Pacific, the Kelvin Wave will eventually push east, and the warm waters will push to the surface. As the third panel shows, this sort of motion is referred to as "upwelling", where the subsurface waters push to the surface. In response, a body of water nearby (in this case, the Central Pacific) will exhibit "downwelling" characteristics, where water temperatures will either warm or cool, the opposite temperature anomaly as the anomaly involved in the upwelling incident.

If that was confusing, don't worry, we can explain it better below.

The image above shows the anomalous depth of the 20ยบ Celsius isotherm below the surface in the eastern Pacific. In layman terms, positive anomalies on the chart above mean warmer than normal waters, while negative anomalies mean colder than normal waters. Check out how we've seen a series of cold and warm episodes across the Pacific in the last year. In October 2013, we saw cooler than normal water temperatures shift east with time (hence the slanting down and east with time (left legend) and direction (bottom legend)). In an interesting correlation of how upwelling episodes seem to determine the strength of the following downwelling episode, we saw warmer than normal waters follow quickly in its footsteps around November 2013. This was a classic example of upwelling and downwelling. We saw upwelling occur with the below normal waters in October, as cooler waters were brought from underwater to the surface, and the resultant downwelling episode occurred in November, when surface-originated waters were forced underwater and eastward. We saw an even stronger occurrence of this in January 2014, when upwelling occurred, and then our historic Kelvin Wave induced the upwelling in February and March 2014.

So, it would only be natural to expect an even stronger upwelling phase now, right? Wrong. Looking at that chart above, now that our Kelvin Wave has passed, we see barely any evidence of sustained upwelling. If we consider that the strength of the upwelling episodes (cool anomalies) could actually predict the strength of the downwelling episodes (warm anomalies), one might think that the upcoming downwelling episode may result in more of a weak El Nino than one of a stronger magnitude. Although the correlation discussed isn't exactly how the upwelling/downwelling episodes work, it's worth seeing if such a correlation might be even remotely successful at predicting the upcoming El Nino.

The El Nino has been difficult to come by. This most recent Kelvin Wave, expected to bring us that strong El Nino, couldn't hold its ground and ended up dissipating. Add to that the atmosphere never exhibited El Nino characteristics, and it's all-around bad luck. Some change is on the way, however.

Kyle MacRitchie
The image above shows the long range forecasted MJO phase from the CFS model, from Kyle MacRitchie. This forecast has the Madden Julian Oscillation moving into Phases 8, 1 and 2 by the early and middle parts of August. In simpler terms, this forecast suggests that we will see enhanced convection over the western portion of Oceania and around the Indian Ocean in August. Why is this important? When we see tropical convection in those areas, the atmosphere can respond by pushing this convection eastward into the open Pacific, possibly as far as into South America. If this convection can reach the Indian Ocean and around Oceania, it can create westerly winds from that area into the east Pacific, setting up a favorable environment for El Nino formation. If the convection can actually move into South America, the potential of El Nino-like conditions forming greatly rises. The gist of all of this is, the tables could be turning in favor of an El Nino, after a long time of suffering quite an uphill battle.

To summarize, the upwelling-downwelling pattern we discussed earlier tells us that only a weak El Nino could be in the cards down the road. However, after examining the projected placement of tropical convection in the Indian Ocean, El Nino formation could actually be favored, even if it's only favoring a weak or possibly moderate El Nino (I would place my bets on the former option, however).