Tuesday, September 30, 2014

Weak Stratospheric Warmings Suggest Turbulent Winter Ahead

The first substantial stratospheric warming event appears to be underway in the Northern Hemisphere, and could be a signal for what's ahead this winter.

The animation above shows temperature anomalies at the 10 millibar level of the stratosphere across the world over the past month. The Southern Hemisphere has been on the receiving end of more than one powerful sudden stratospheric warming (SSW) event, but we'll focus our attention on the Northern Hemisphere for now.

Take a look at what's been happening over Canada and into Greenland in recent days. We've been seeing warmer than normal temperature anomalies push northward from Canada into Greenland, and now across the Arctic Circle. In the winter, when these events happen, outbreaks of cold air may be expected at lower latitudes approximately 2-4 weeks after the event. This warming is pretty minor, so I wouldn't expect any significant cold to come from it, but there's more to this whole process.

If we look at the same animation, now at the 50 millibar level of the stratosphere (generally considered the middle of the stratosphere), we still see that swath of warmth pushing north from Canada into Greenland, and actually extending westward into the Pacific as the warmth is shoved into the Arctic. The presence of this warmth across the upper and middle stratosphere means that this isn't just a fluke warming, but in terms of an early-season stratospheric warming event, this one's pretty substantial.

All of this may be hinting at what kind of winter we'll see this season. If the stratosphere is already rather vulnerable to warm air invasions, it could mean that the polar vortex, which spans the troposphere and stratosphere, may be weakened significantly this winter, possibly leading to more frequent wintry outbreaks. The chance of this polar vortex weakening is raised by the expectation for stratospheric winds to be unfavorable for the formation of a strong polar vortex.

More on this will be analyzed in the near future.


Sunday, September 28, 2014

Above-Normal Stratospheric Temperatures May Signal Blocky Winter Ahead

Warmer than normal temperatures in the lower stratosphere are raising the possibility of a "blocky" winter ahead.

The above image shows a pretty daunting image of the stratosphere, so let's decipher it. The red line shows observed temperatures at the 70 millibar level of the stratosphere, between the 65N and 90N latitude lines. The dashed green line illustrates average temperatures for any given time, while the gray outlines give an indication of the record high and low stratospheric temperatures for any given time period over the past few decades of records.

Gazing over the image above, specifically around the most recent records of temperatures, we find ourselves on the above-normal side, with that red line bursting up into above-normal territory more than once in the last few weeks. This year's warmth at the 70mb level looks to be a bit more prevalent than that of last year, as you can see just to the left of the center of this image.

Why is this important to the coming winter? Above-normal temperatures in the stratosphere allow for a higher threat for persistent high pressure to form over the Arctic Circle, and general upper-latitude area. The polar vortex, a strong low pressure system of cold air located across the troposphere and stratosphere, can be strengthened during times when the stratosphere is colder than normal, and weakened when warmth prevails.

When this warmth prevails, strong bodies of high pressure can punch north from the lower latitudes into the Arctic, disrupting the polar vortex. When this happens, fragments of the vortex can break off and be sent to the lower latitudes (as was seen last winter), or the whole vortex can be shunted down south. If the high pressure sticks around in the Arctic for long periods of time, it can be referred to as "blocking" high pressure, for the way it "blocks" the pattern from flowing east to west around the globe, since the high pressure remains stagnant and backs everything up.

The risk for sustained high pressure is increased as we see above-normal temperatures affecting the surface of the Arctic. The graphic above shows us observed temperatures north of the 80th parallel from the first day of the year to present day. Over the summer, we saw sustained below-normal temperatures, as evidenced by the red line being below the green average-temperature line, and I had discussed this as showing a potential cold air build-up for the upcoming winter.

Things certainly have changed since then! In the past couple of months, temperatures north of the 80th parallel have stayed predominantly above-normal, only reaching the average line a few times. The most recent observation shows temperatures diving, but the anomaly is still above-normal for this time of year.

For winter weather fans and warm weather fans, this is a double-edged sword.

On one hand, the lack of colder-than-normal air means not as much cold air may be available for the coming winter, barring a regime shift that would cool down the upper latitudes. Such a scenario could then mean a general 'warm-ish' feel for the coming winter, where cold weather would still occur (since it's the winter season), but the intensity of cold air may not be that extreme.

On the other hand, the warmth both at the surface and into the lower stratosphere suggest a pretty elevated risk of that blocking high pressure this coming winter. All of this warm air in the Arctic has a pretty decent chance of depleting the polar vortex this winter, and this risk is increased even further per some items that will be discussed in our Official 2014-2015 Winter Forecast on October 11th at 12:00 PM Central Time.

To summarize, above-normal temperatures across both the stratosphere and troposphere appear to be raising the risk of blocking high pressure in the upper latitudes this winter, which could then raise the risk of a cold winter in North America.


Saturday, September 27, 2014

Persistent Cold Pattern Expected to Succeed Current Warmth

Another bout of cold weather is expected to replace the nice weather that most of the United States is currently feeling.

The first image we'll analyze above is the North American Ensemble Forecast System, or NAEFS 500mb height forecast. The contour lines show isobars at the 500mb level, with colored regions showing the degree of uncertainty, or "spread" among the ensemble members.

The graphic above, valid October 3rd, shows a deep trough pushing south into the Central US. We can see this well by the NAEFS projected depression of 500mb height contours. A glance around North America tells us this troughing pattern looks to be supported by two areas of ridging. One body of high pressure looks to be positioned in the West US, in a more suppressed version of last winter's ridging episodes, while the other ridge appears to be positioned along the Southeast and East Coast, evident by the arcing motion of the 500mb height contours.

Something we will have to monitor in the days leading up to this outlook of October 3rd is the high uncertainty over the Plains with this trough. The NAEFS outlook shows greens and yellows encompassing the expected placement of the trough, which means ensemble members aren't so sure this will actually end up happening. However, as the trough is actually showing up on this image, it means more members support it than not, a positive sign for this trough being able to form.

This next ensemble projection, from the Canadian ensemble system, shows 500mb height contours valid on October 6th. In agreement with the NAEFS ensemble forecast, we still see that trough centered over the Plains and Midwest, now trying to push a bit east into the East Coast. Sustained ridging is still highlighted over the West US, and a bit still present in the Southeast. A look at the individual members of this ensemble mean image shows only two of the twenty members do not show a large trough in the Central US on this date. Therefore, confidence is rather high that this cooler pattern will play out.

Lastly, we'll take a look at the ECMWF ensembles' outlook for October 3rd, the same time period as the NAEFS graphical outlook. In this forecast, we see a large trough placed over the Central US, echoing the projections made by the NAEFS and CMC ensembles. Adding to the similarities is the rather large spread / degree of uncertainty among ensemble members, as the deeper purple colors on the right-hand panel above show over the Plains. Because it's still showing up in the ensemble mean images here, I would keep confidence in this cold outlook for the start of October, but still be wary for changes in the outlook in the near future.

To summarize, model guidance is agreeing on the potential for a cold blast to kick off October, but individual projections are exhibiting varying degrees of uncertainty as to just how intense the cold may be.


Friday, September 26, 2014

CFS Weeklies Determining Upcoming Winter Storm Track

The weekly outlook version of the CFS model, as well as 45-day precipitation forecasts from its four ensemble members, appear to be starting to figure out the upcoming winter storm track.

The image above shows two weeks' precipitation anomalies, one panel for each week. The top panel shows the first week's precipitation anomaly outlook, from September 26th to October 2nd, while the bottom panel shows anomalies for the October 3rd through 9th period. In the top panel, we see a general dry spell expected to linger throughout the East US, as strong ridging induced by a stormy West keeps weather systems at bay. It is this stormy West that will lead to above-normal precipitation anomalies in this same timeframe.

For the second week, valid from October 3rd to October 9th, we see the pattern change. Below-normal precipitation anomalies take hold in portions of the Western US, most notably the Pacific Northwest. Above-normal precipitation anomalies then take hold in the Central United States, including the Gulf Coast. These wet conditions will hit this region in a crucial time, as the new Lezak Recurring Cycle, or LRC, begins to take shape. I'll have more on that later on in this post.

This next graphic is the same as the one we just analyzed, but now valid for the Week 3 and Week 4 period. The top panel shows the precipitation anomaly forecast from October 10th to October 16th, while the bottom panel shows the anomalies from October 17th to October 23rd. During the first week, the latest CFS outlook has those below-normal precipitation anomalies persisting in the Pacific Northwest, now also being reciprocated in the Ohio Valley and South Plains. Wetter than normal conditions do prevail in the Northern Plains and Upper Midwest, through that's the only area that experiences such an anomaly throughout the country.

By the fourth week, we once again see above-normal precipitation anomalies return to the Midwest and Great Lakes, stretching into the Mid-Atlantic. Variable conditions look to exist across the Plains and most of the West, save for the Pacific Northwest.

All of this is pretty important to us, as this looks to occur during the month of October, when the new Lezak Recurring Cycle forms. Something I've discussed on here more than a few times is the concept of the Lezak Recurring Cycle, or LRC. The LRC was developed by meteorologist Gary Lezak, and discusses the idea that weather patterns which develop in October leave a 'footprint' of sorts that is repeated in a regular interval, between 40-60 days through the winter and following spring. In other words, the weather patterns that develop in October repeat themselves for the better chunk of the next year.

If we take that description of the LRC and look back at those panels of precipitation anomalies for the month of October, the general idea is that parts of the Midwest, Great Lakes, North Plains, and even East Coast could be in for a snowy winter.

Let's keep this model analysis going with additional (but different) CFS products.

This is the first of four images from the WxCaster site. In this graphic, we see the projected accumulated precipitation from one of four CFS ensemble members, all valid from today to November 9th. In other words, this image shows total precipitation from today to the ninth of November. Using our LRC guidelines, this should help capture most of the 'new' precipitation pattern as it unfolds in October and into November.

On this first graphic, this particular CFS member foresees a rather dry October and early November for the Midwest and Great Lakes, in opposition to the 16-member weekly panels we analyzed earlier. Instead, the heaviest precipitation is shunted north, into the Central and Northern Plains, or east into the Eastern Seaboard. Just going by this graphic alone, I would favor a snowy winter for the East Coast and Plains. Let's see what the other members say.

This second member shows a much different story than the first one. In this projection, the heaviest precipitation over the next 45 days is dropped on the Midwest and Great Lakes, in a similar fashion as the CFS weekly panels earlier in this post. The Northeast also would see a very wet October and November.

We now have two opposing ensemble members. We can't draw a conclusion from them just yet, so let's move ahead to the next member.

This third member is definitely more toned down than what the second member showed, but is more in line with that second member's geographical location of the anomalies compared to the first member. In this graphic, we see substantial precipitation extending from the South Plains into the Midwest and Ohio Valley, all the way into the Mid-Atlantic. This outlook does look similar to those weekly panels we discussed.

Our fourth and final member we will analyze takes the middle ground between these ensemble projections. According to this outlook, one track of very wet conditions for October and November would set up in the Plains, while another would hit the Midwest and Great Lakes. I might also be able to see a third area of wet conditions in the Mid-Atlantic and up the East Coast. Put it all together and this member definitely foresees an active LRC pattern ahead.

To summarize, while there are certainly discrepancies among model outlooks, and accuracy isn't that high with long range forecasts to begin with, it seems a trend favoring a wet October for parts of the Plains, Midwest, and East Coast may be in order. If this verifies, it could lead to a more secure winter storm track, quite possibly a snowy one.


Thursday, September 25, 2014

Long Range Guidance: Atmosphere Primed For Intense Winter

Long range guidance appears to be favoring the Madden-Julian Oscillation (MJO) to remain in its latter phases this winter, something that could make this winter a brutal one.

Let's first define what the Madden-Julian Oscillation is. The MJO is comprised of the placement of enhanced tropical convection over certain parts of the Equatorial Pacific and Indian Ocean. The chart above shows Outgoing Longwave Radiation (OLR) anomalies for each of the eight phases of the MJO; each phase is numbered in the bottom left corner. Blue shadings indicate negative OLR anomalies, which translates to enhanced convection in the area. Similarly, yellows depict positive OLR values, which indicate suppressed tropical convection. You can see how, as the phases progress, the enhanced convection in blues shifts eastward. Phase 1 begins with storminess nearly due south of India, while Phase 8 has that same storminess well east-northeast of Australia.

Let's put what we've learned to the test. The image above shows the precipitation anomaly forecast from the JAMSTEC model, valid over the upcoming winter months of December, January and February. Instead of focusing in on North America, we're going to concentrate on the anomalies over the western Pacific. In this forecast, well above normal precipitation anomalies are targeting the region just north and east of Australia. This is a big clue to us that the enhanced tropical convection we discussed earlier will be centered in those green anomalies.

If we compare the placement of the above-normal precipitation anomalies on the JAMSTEC model to the average negative OLR placement for each MJO phase in the first image of this post, it looks like this graphic is suggesting a predominantly-Phase 6 MJO for this winter. It's expected that the MJO won't remain solely in Phase 6 for the whole winter, but this model says it should stay in Phase 6 more often than not.

This next image is nearly identical to the one we analyzed from the JAMSTEC model. This graphic is from the CFS model, and once again shows projected precipitation anomalies over the December-January-February period for the coming winter. As in the JAMSTEC image, green indicates above-average precipitation, while orange depicts below-normal precipitation.

According to this image, we see a swath of above-normal to well-above-normal rainfall anomalies positioned well to the northeast of Australia. If we do a quick check of the composite image that we first discussed to kick off this post, it's apparent that the CFS model is favoring a mainly-Phase 8 winter.

This final image seems daunting at first, but it's relatively easy to decipher. We'll examine the bottom image for this post, though some of you more experienced weather enthusiasts may know what the top panel means as well.

The bottom panel shows the Japan Meteorological Agency (JMA) three month rainfall outlook over the tropics and mid-latitude. In this graphic, yellows and oranges depict below-normal precipitation, while blues indicate above normal precipitation. According to the JMA, the precipitation anomalies are positioned northeast of Australia, almost a middle ground between the JAMSTEC and CFS models. A glance at the OLR composite and comparing it to the JMA forecast, the MJO appears to be favored to stick around Phases 6 and 7, maybe a bit into Phase 8.

Let's now go over what these different phases mean for the US during the winter.

The image above, from AmericanWx, shows temperature anomalies over the Northern Hemisphere during all amplitudes for Phase 6 MJO events during January (I chose the month as a middle ground, since there were no three-month composites for December-January-February). The image does show cold weather in the western part of Canada, and warm weather in the east. This could translate to warm weather in the East US and chilly weather in the West during Phase 6 MJO events, but we can't make a confirmation on that, since we just don't see it on this chart.

If we now take a look at 500mb height anomalies during a Phase 8 MJO event, we see a much different story than that associated with a Phase 6 event. During Phase 8 events, strong ridging evolves over the Western portion of North America, with strong troughing in the East US and Central US. This translates to warm weather in the west, and cold weather in the east. The anomalies are much more decisive than those with the Phase 6 chart, and if the CFS projection verified from earlier in this post, it's possible another brutal winter might be in store.

To summarize, long range model guidance is hinting at the atmosphere being primed for a pretty harsh winter in coming months, if the Madden-Julian Oscillation cooperates.

More on how the atmosphere is becoming primed in a future post...


Wednesday, September 24, 2014

Updated JAMSTEC Model Supports Cold, Snowy Winter

The latest run of the JAMSTEC long range model came in last night, and it's still calling for a heavy-duty winter for millions across the country.

The graphic above shows the temperature anomaly forecast for the December-January-February period of 2014-2015. In this image, focusing on North America, we see an outlook very similar to what I had been pondering a handful of weeks ago. In this outlook, much of the south half of Canada is inundated with below-normal temperatures, which then intensifies and extends into the Northern US.

Interestingly enough, these below normal anomalies only hit the Plains, Great Lakes, Midwest, and eastern Rockies. For all other areas, save for the Southeast, temperature anomalies are actually above normal. The projection for the Eastern Seaboard includes a warmer than normal winter, though not a "blowtorch" winter, per se. The Western US also looks to see a warm winter, according to this outlook.

For precipitation, zooming in on North America, we do see a well-above normal precipitation trend over the Ohio Valley. This trend continues into the Northeast and Mid-Atlantic, even backtracking through the Great Lakes and Midwest. Such a wet forecast is also observed in the Southwest, where rain would certainly be welcome. The Pacific Northwest may then see slightly below normal conditions.

The piece that puzzles me (pun intended) in this forecast is the swath of above-normal precipitation anomalies across the Ohio Valley. The West US is exhibiting typical El Nino characteristics, with dry conditions in the Pacific Northwest and wet conditions in the Southwest. However, more La Nina-like conditions are observed in the East, with the above-average precipitation forecast over the Ohio Valley. It remains to be seen how reliable this outlook is, since this is the minority opinion out of the global forecast models right now.

To summarize, the new JAMSTEC is supporting a cold and snowy winter for much of the Central and East US, possibly spoiled by a warm winter in the Eastern Seaboard.


Monday, September 22, 2014

Long Range Lookout: Warm End To September, Chilly Start To October

Model guidance is hinting at the idea of a warm end to October, before a chilly October opener enters the scene.

The two-panel image above shows 500mb height anomalies over North America on the left, with the spread among individual ensemble members on the right. For the image on the left, depressions of contour lines, as well as normalized standard deviations in the shaded areas. On the left panel, darker shades of purple indicate higher and higher uncertainty among ensemble members for what will happen in a given area.

According to the graphic above, valid September 28th, a strong trough looks to be pushing into the Western US, forcing high pressure to build in the Central and East US. Warm and dry weather can be expected from this event, while the West may experience its first period of cold and stormy weather in some time. This ridging of high pressure over the Central and Eastern US should allow for warmer than normal conditions to persist through the end of the month.

By the first week of October, the story changes. The image above shows another ensemble "spread" forecast over the 500mb level, now comprised of multiple North American model guidance systems, and now valid for October 1st. In this forecast, we see 500mb contours collapsing south over the Plains and Rockies, as this trough tries to push east. The ridge we discussed earlier is now weakened and suppressed to the south, as it begins to lose control of the Central and Eastern US.

While all of this is playing out, there's a very interesting story playing out in the Pacific. If we take a look at the northeast Pacific region, in the top-left part of this graphic, we see a large swath of yellows and reds, meaning high uncertainty over the area. The ensemble average for this area shows stormy weather in the Bering Sea, but those red colors mean not all ensemble members agree with this prognosis. The downstream effects of this uncertainty in the Pacific are shown in the Western US, as the stormy weather in the Bering Sea/Northeast Pacific produces high pressure over the Rockies, where some uncertainty is shown by the swath of green colors in that region.

To summarize, a warm end to September is expected, but model guidance is indicating a return to cold weather may be expected to kick off October, when a strong trough tries to push into the Central US.


Saturday, September 20, 2014

JAMSTEC Model Predicting Harsh Winter Ahead

A popular weather model, the JAMSTEC climate model, is forecasting a harsh winter ahead... but will it verify?

The image above shows the December-January-February temperature anomaly forecast for the winter of 2014-2015, with the forecast made in August 2014. This forecast is made up of 27 ensemble members, which can enhance its credibility, so long as the forecast is within reason.

In this image, we can see strong below-normal temperature anomalies across nearly the entire United States. Canada and Alaska look to see above to well-above normal temperature anomalies, with the anomalies maximized over the far northern reaches of the country. Last winter, the core of the cold was displaced in the north-central US, but if this model is correct, it would be shifted into the Southern Plains.

On the precipitation side, the JAMSTEC model is forecasting a swath of above-normal anomalies to affect the Southeast US, even stretching west into the Texas/New Mexico region. These wet conditions look to extend up the East Coast, almost in typical El Nino fashion. The Pacific Northwest would see a rather dry winter if this solution were to verify, while the Southwest would see slightly below-normal to average precipitation anomalies.

Even though this model certainly is painting an ominous picture, is it a realistic one?

The graphic above once again shows an outlook for the December-January-February period, but now depicts sea surface temperature (SST) anomalies . In this image, I can identify two areas of concern, as far the forecast potentially not verifying.

First off, we see a moderate to borderline-strong El Nino evolving in the Pacific. SST anomalies of 1.2 to 1.5 degrees Celsius above normal might be a bit too strong for this winter, as I'm currently expecting only a weak to possibly-moderate El Nino. It would be quite a stretch for a strong El Nino to hit this winter.

Second, we see a pool of deep negative SST anomalies in the waters just south and west of Greenland, while well above-normal anomalies flourish on either side of the land mass just north of the cold pool. There isn't as much reasoning behind this one as there is a gut feeling; the alignment of SST anomalies looks odd to me, and I'm not so sure I would put stock in this forecast.

To summarize, the JAMSTEC model is indeed forecasting a very ominous winter to once again hit the United States. However, concerns with the sea surface temperature anomaly forecast for the coming winter could mean the forecast may be flawed.


Wednesday, September 17, 2014

New LRC Winter Pattern Projected To Be Snowy

Long range climate models are in agreement that the month of October may feature above-normal precipitation in the North and East US, which could play into a very snowy pattern later on in the winter LRC cycles.

What we'll do first is analyze four precipitation forecasts, each one slightly different from the other based on their initialization and model foundations, from the CFS model. Shown above is the first member of the CFS suite, showing precipitation accumulated over the next 45 days. The end of this forecast period puts us at the very end of October, on Halloween night.

In this forecast, we see a swath of substantial precipitation values extending from the Plains into the Upper Midwest, giving the Great Lakes some of those wet conditions as well. A very dry forecast is noted in the southern Midwest, Ohio Valley, and into the Gulf Coast. We then see somewhat-dry conditions into the Mid-Atlantic, with wet conditions again present in the Northeast.

The next 45-day precipitation forecast from the CFS ensemble members shows a pretty similar story across the board as the first forecast image. In this new projection, we see what appears to be a swath of wetter-than-normal conditions extending from the Southern Plains and into the Midwest, once again hitting the Upper Midwest and Great Lakes the hardest. In a twist, we now see the Eastern Seaboard receiving similarly-heavy precipitation totals, quite a difference from the first ensemble member. Overall, once again, it appears a wet end to September and most of October would be expected for the Upper Midwest.

This third ensemble member from the CFS once again retains a similar projection as the first two members for the next 45 days, but differs in the East. For the Central US, heavy precipitation values are displaced across most of the Midwest and west-central Great Lakes, extending back into the Plains. In this forecast, the Mid-Atlantic is dry, in a similarity to the first ensemble member. The Northeast then appears very wet, all in all looking like a combination of the dry scenario from the first member and wet scenario from the second member.

This fourth member I specifically saved for last, since it appears to be most radical with its forecast in the Midwest (that should be taken as a caveat as well). In this final forecast member, we see very heavy precipitation values over the month of October and late September in the entire Midwest and most of the Great Lakes, stretching down south into the Plains, and even hitting the Ohio Valley in the process. The Eastern Seaboard is forecasted to be predominantly dry, save for the New England region. This could mean that the wet-East projection from the second member we analyzed may be a false forecast, but that's not something we'll investigate right now.

So, why should we care what these four members say about precipitation in the next month and a half?

Because we could be staring right at our winter precipitation pattern.

Something I've discussed on here more than a few times is the concept of the Lezak Recurring Cycle, or LRC. The LRC was developed by meteorologist Gary Lezak, and discusses the idea that weather patterns which develop in October leave a 'footprint' of sorts that is repeated in a regular interval, between 40-60 days through the winter and following spring. In other words, the weather patterns that develop in October repeat themselves for the better chunk of the next year.

So, if we take the four members above to be true (which is NOT something I'd advise to do; I'm showing them in collaboration with additional forecasts below), the Upper Midwest, Midwest, Great Lakes, Plains, and other parts of the East could be in for a very snowy winter.

In the image above, we see an average of precipitation anomalies over the month of October, compiled from eight global forecasting models. In this graphic, we see a familiar trend. The Plains are projected to receive above normal precipitation, leading directly into the Midwest and Great Lakes, as well as the New England area. Does this look familiar? Because it's nearly identical to some of the projections we were viewing earlier in this post.

Based on the average of these eight global model projections, as well as the four CFS ensemble members we dissected earlier, it's safe to say the current trend favors a very wet October for the Midwest (particularly the Upper Midwest), portions of the Great Lakes, Plains, and Northeast. If these projections end up verifying (again, there are many long range caveats associated with this), then the aforementioned areas may want to prepare for a pretty snowy winter, so long as the LRC cooperates.

Since we don't have as many projections for temperature as we do for precipitation, I won't discuss temperature projections for the LRC in-depth right now. However, based on the average of the eight climate models, October could be a warm month, which might then transfer over into a warm winter. I personally don't agree with the diagnosis for now, but the possibility will be re-examined later on this fall.


Sunday, September 14, 2014

The Story Behind The Polar Vortex

Ever since last winter, the polar vortex has taken over as the go-to thought as soon as the prospect of cold weather is brought up. Let's take this time today to go over what the polar vortex actually is, and disprove some inaccuracies I've seen swirling around the world.

1. The Polar Vortex Exists Year-Round
The phrase 'polar vortex' is used to describe a strong low pressure system (vortex) that is semipermanently placed over the Arctic region (polar). In the summer, the vortex significantly weakens, and is replaced by high pressure. During the fall months, however, the vortex gains strength yet again and low pressure dominates the Arctic as the polar vortex gears up for winter.

2. The Polar Vortex Extends into the Stratosphere, Not Just the Troposphere

The image above shows the 10-day forecast from the GFS model in the stratosphere, at the 30 millibar level. That's pretty high up the sky when you consider us humans are at the 1000 millibar level. In this forecast, we see relatively weak low pressure establishing itself over the Arctic Circle, with weak high pressure surrounding it. That low pressure body is the "birth" of the polar vortex for this cold season, as it begins to come back to life and strengthen for the winter.

Some people assume the polar vortex is no different than your average storm system that brings snow to your neighborhood. The vortex is actually quite different. In order for it to be the polar vortex, it must exist at both the tropospheric and stratospheric levels. That's how big and controlling it is in the atmosphere.

3. There is only ONE Polar Vortex

This is something I've been hearing ever since January 2014, when this polar vortex business really took off. The belief is that there is more than one polar vortex in the atmosphere. This might stem from the aforementioned idea that some believe the polar vortex is no different than your average storm system, but regardless of its origins, the belief is false.

Take a look at the image above. In this image, we see a view of 500mb height anomalies across the Northern Hemisphere on January 7th, 2014. Warm colors correspond to high pressure, which brings warmth and generally quiet conditions. Blues and purples indicate the presence of low pressure, which permits stormy and chilly weather. Just taking a glance over this map, we see one strong upper level low that seems to be the strongest one in the entire hemisphere. That upper level low, seen scraping the northern US and inundating Canada, is the tropospheric version of the polar vortex. As you can see, there is only one low as strong as the one in North America, hence there is only one polar vortex. Those other low pressure systems are just disturbances compared to the polar vortex.

To give you a better idea of how there's only one polar vortex, let's take a look at the stratosphere on January 7th, 2014. In this image, we see what appears to be two strong areas of negative height anomalies, being pinched by two bodies of ridging/high pressure on either side of the globe. If you thought that this image shows two polar vortexes, you are incorrect!
What's actually happening is the two bodies of high pressure are trying to tear apart the polar vortex, something that happens rather often each year. The difference between other years and this past year is that the ridging forced the main part of the polar vortex into North America; that other area of negative height anomalies in Asia is just a weaker part of the single polar vortex that was torn off of the main body. Some of you more experienced weather enthusiasts may know this type of set-up as Wave-2 stratospheric activity, as two bodies of high pressure push into the Arctic.

To close out this post, here's a stellar graphic made by the folks at the National Weather Service office in New York in early January, when the hype over the polar vortex was in full swing.

To summarize:

• The Polar Vortex exists year-round, varying in strength. It does not (and will not) "come back" for the winter, as it's already present and cannot "come back".

• The Polar Vortex exists across both the troposphere (where we live) and the stratosphere (where Felix Baumgartner jumped from his capsule in October 2012); it does not exist at the surface.

• There is only one polar vortex; our world would be much different if more than one polar vortex existed.

Additional points of summary are included in the NWS graphic above.


SOI Continues To Plummet As El Nino Emerges

The Southern Oscillation Index, a key indicator for the emergence of an El Nino or La Nina, continues to free-fall as our El Nino looks to finally come to the surface.

The image above shows a 30-day average of the SOI values from January 2012 to the present day. The Southern Oscillation Index is calculated by examining the pressure differences between Tahiti and Darwin, Australia. When values drop below -8, conditions are considered optimal for El Nino formation. Values above +8 are considered optimal for La Nina conditions.

 As the chart shows, we had been seeing predominantly-positive anomalies this past spring and early summer, which is likely a reason why the the historic Kelvin Wave never brought about the Super El Nino so many (myself included) had discussed. However, beginning right around the start of July, we saw SOI values tank, and they've remained below zero for the entire period since. Values have gone both above and below the El Nino 'benchmark' of -8 since that time, but have remained below zero the entire time.

Due to potential copyright infringement concerns, I will refrain from posting a forecast of the SOI I came across yesterday. What it shows, however, is continued negative SOI values for the next few weeks, and possibly beyond.

The animation above shows a depth-by-longitude animation of equatorial temperature anomalies, in Celsius, since mid-July to present day. In this animation, we see the Kelvin Wave-induced warm waters spreading east and pushing to the surface as they do so, now just beginning to hit the surface.

The SOI being negative indicates that trade winds along the Equator and areas in the neighborhood of the Central Pacific have shifted to a state that will allow this warmth to stay and ferment on the surface, likely into an El Nino. Back in April, with the historic Kelvin Wave warmth, trade winds wee unfavorable, and prohibited the warmth from sustaining itself on the surface.

In coming days and weeks, we'll likely see this winter's El Nino bring itself to the surface, likely at a weak, potentially moderate strength. I'll have more on the projected El Nino strength in the near future.


Saturday, September 13, 2014

September 27-October 1 Potential Storm System

The September 27th to October 1st period is being monitored for a potential storm system.

Tropical Tidbits
The image above shows 500mb height anomalies over the West Pacific area on the afternoon of September 21. In this image, blues indicate the presence of below-normal height anomalies, which can then lead to cooler than normal temperatures, as well as stormy conditions. Warm colors are associated with positive height anomalies, which generally bring warm and quiet conditions.

As we see in the image above, a trough is moving through Japan on the 21st, according to the latest GFS Ensembles forecast. We see negative height anomalies across the entire country of Japan, but the real point of interest is the depression in the contour lines. This depression indicates a trough moving through the area, and that's what may provide us with the chance for a storm system in the September 27th - October 1st period.

Forecasted teleconnections can give us an idea of where this potential storm may end up.

On the top-left, we see the forecast for the Pacific-North American (PNA) index. We see that at the end of the forecast period, on September 27th, the PNA is projected to be descending from a strong positive state. This tells us that ridging in place across the Western US may be dissipating by the time this potential storm rolls around. Typically a positive PNA means the Midwest and Ohio Valley are most favored for storm systems, but it could be more for the Plains if the PNA is descending to neutral or negative territory.

Additional updates will be provided in the near future.


Long Range Regional Outlook (Ohio Valley): September 20-October 12

This is the latest Long Range Regional Outlook forecast for the Ohio Valley area, valid for September 20th through October 12th.

CMC Ensembles
I have elected to use a combination of the CMC and ECMWF ensembles for this outlook, after finding the GFS ensembles to be in disagreement with the two aforementioned systems, which do hold a consensus.

The image above shows the CMC ensemble mean 500mb height contours on the date of September 22nd. In this forecast, depressions in the contours indicate troughing (cold and unsettled weather) while arcing motions in the contours depict ridging, which results in warm and relatively quiet weather. In the graphic shown above, we see a broad depression in the contours across the Plains and into the Midwest, though we then see slight ridging emerging over the Ohio Valley. This would tell me that the forecast period of September 20th to October 12th likely opens with some seasonable to above-normal temperatures, before colder weather moves in.

ECMWF Ensembles
Now shown above is a two-panel forecast from the ECMWF ensemble system. We see forecasted 500mb height contours and anomalies (with legend on the right) in the left-hand panel, with the ensemble 'spread' on the right image. An ensemble 'spread' indicates the degree of uncertainty among individual ensemble members on a particular area and its forecast. For example, we see deep purples on the right panel over the Great Lakes, which the left panel says will experience troughing (due to the depressed contours). This tells us that the ensembles are uncertain as to how likely this particular factor is to actually occur, and results in a wider spread of ensembles.

In the image above, valid for September 21st, we see a similar layout as that of the CMC ensembles. We see suppressed ridging over the Western US, created by some stormy weather in the Gulf of Alaska (see green shadings of below-normal height anomalies). This ridge in the West results in cooler weather for the Plains and Midwest, as the CMC ensembles depicted, but that then leads to some slight ridging over the Ohio Valley. This jives well altogether with the CMC ensembles.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Ohio Valley, we might expect to see some cold weather, though I wouldn't call it a 'cold blast'. In this forecast, the upper level low looks displaced north enough that the core of the cold would likely stay in Canada. This fits in with the CMC and ECMWF ensemble projections of a weak trough moving east from the Plains and Midwest (as we discussed earlier), and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect some chilly weather in the Ohio Valley around the September 27th - October 1st period.

After going over the factors listed above, the temperature and precipitation outlooks for the Ohio Valley over the September 20th - October 12th period are as follows:

Temperature Outlook:

The Weather Centre
Temperature Outlook
Temperatures for the Ohio Valley are expected to remain around average to slightly below-average, due to a warm start to the forecast period and a few weak cool shots.

Precipitation Outlook:

The Weather Centre
Precipitation Outlook
The precipitation outlook calls for predominantly below-normal precipitation over the forecast period, due to somewhat dry signals over the Japan region for the next week or two, among other factors.

The next Long Range Regional Outlook will be published Saturday, September 20th.


Friday, September 12, 2014

Long Range Regional Outlook (Northeast & Mid-Atlantic): September 20-October 12

This is the premiere post of the Long Range Regional Outlooks, this one valid for the Northeast over the September 20th to October 12th period.

After examining long range ensemble guidance, a split became evident between the ECMWF/CMC ensembles, as well as the GFS Ensembles. Have elected to side with the ECMWF/CMC projections, as GFS Ensembles are generally going against the grain of the other two ensemble sets, as well as my own thoughts.

The CMC ensemble mean 500mb geopotential height forecast is shown above. As a general rule of thumb, depressions in the contour lines tend to indicate troughing and cold/stormy weather, while arcing of the contour lines indicates ridging of high pressure, as well as warm/quiet weather. In the image above, we see 500mb height contours valid for September 20th. We see a rather zonal flow (non-wavy pattern) set-up over the northeast Pacific, with the jet stream pushing into the west coast of North America. This is in contrast to the GFS ensembles, which predicted deep troughing in the Gulf of Alaska. There is some slight ridging in the Southwest, leading to some weak troughing across Central and Eastern US areas.

The ECMWF ensembles are shown above, with the 500mb geopotential height anomalies and contours shown on the left panel, as well as the 'spread' (degree of disagreement among the ensemble members) for this forecast of September 21st on the right. This forecast is similar to the CMC ensemble projection above, as we see the Pacific jet stream pushing into the Western part of North America.

According to the height anomalies and contours, we do see some substantial troughing in the Gulf of Alaska, though it is not to the degree of the GFS ensembles. We then see some weak ridging in the Southwest, as was also seen with the CMC ensembles, though it is suppressed. Some weak troughing evolves in the Central US before weak ridging in the East.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Northeast, we might expect to see the northern parts of the region get some substantial cold around a September 23rd - 27th period. This fits in with the CMC and ECMWF ensemble projections, and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect continued cold in the Northeast around the September 27th - October 1st period.

After going over all of the factors above, as well as extra analyzations not mentioned in this post, the outlook for the September 20th to October 12th period over the Northeast is as follows.

Temperature Outlook:

The Weather Centre
Temperature Outlook
Precipitation Outlook

The Weather Centre
Precipitation Outlook
I expect the Northeast to remain predominantly below normal for temperatures in the aforementioned time period, while precipitation should end up a bit below normal, close to neutral.