Long-Range ECMWF ENS Control Member Shows Frigid January; Will it Verify?

 The latest long-range ensembles from the European forecasting agency came in the other day, and while on the whole they produced a generally milder outlook, we did see a surprise forecast from the 'control' member of these ensembles - a remarkably frigid January.

Before we get into the good stuff, let's review what that means.

The ECMWF agency is known for their namesake weather model, but they also provide a critical forecasting tool with their ensemble system. In an ensemble forecast, an initial forecast model is taken, tweaked ever so slightly, and then re-ran. That's one ensemble 'member'. Then, they take that initial data again, tweak it ever so slightly in a different way, and re-run it again. That's another ensemble member, and it continues. The ECMWF's ensemble system has 51 different members - they are all different versions of the ECMWF weather model, but each one has slightly different starting conditions in order to produce a different forecast. 

You might ask why one would do that, since it sounds a lot like all we're doing is intentionally changing the initial conditions and letting it run free. That doesn't sound very productive, does it? In reality, the value comes from how similar all of these 51 members are at each point in time. For example, if all 51 ensemble members showed a winter storm in the Midwest ten days from today, that would be a big boost to forecast confidence. Why? Because if all 51 members have that storm happening, it means no matter what the weather conditions are right now (i.e. even if they're tweaked in all kinds of ways), we still end up with a winter storm on the horizon. As a result, ensemble forecasting is a potent part of any forecaster's toolbox.

But each ensemble set - whether it's the ECMWF ensembles, the American (GFS) ensembles, or any other set - has one member that is different from the rest. This member's initial conditions are the best possible data we have, and all the other members are those slightly-tweaked versions of this one. We call this special ensemble member the control. Indeed, the control member is the raw forecast taken strictly using initial data. Nothing is tweaked, nothing is altered, just the raw initial conditions. Every other ensemble member takes that control member data, tweaks it a bit, and re-runs it, like I described above. 

One may argue that this makes the control member more attention-worthy, since it only uses the best initial conditions data we have and isn't artificially tweaked. Whether that argument has merit is something I have yet to verify on my own.

In any event, now that we understand why an ensemble's 'control' member is worth monitoring, let's get down to the good stuff: the reason why this post was made.

Attached above from WeatherBell (all images on this post are from WeatherBell) is a look at the seven-day average temperature anomaly from the ECMWF ensemble control member over the December 25th - January 1st timeframe. We see that the control member anticipates a deep and formidable reservoir of below-normal temperatures in western Canada, which begins bleeding south and east into the United States as December turns to January. Indeed, seven-day average anomalies of more than 25 degrees Fahrenheit below normal are nothing to shake a stick at!

The outlook continues into mid-January below:

From January 11th - 18th, temperatures are shown by the ECMWF ensemble control member to be much colder than normal across almost all of the country, particularly east of the Front Range and especially in the Ohio Valley. With seven-day average temperatures seen at almost 20 degrees F below normal in parts of Ohio, Indiana and Kentucky, it's clear that the ECMWF Control member expects the Arctic gates to burst open in the first month of the new year.

Teleconnections accordingly show a breakdown in the tropospheric polar vortex as judged by a collapse in the Arctic Oscillation in the blue line (control member's forecast)...

... and a similar plunge in the North Atlantic Oscillation (NAO) from the control member allows that frigid Canadian cold to plow eastwards...

Now, that's all good and fun to look at, but this post wasn't written to get all the cold weather fans out there hyped up. Indeed, as the above two teleconnection charts show, the control member is an extreme outlier in the late-December and early-January period, and the average of all ensemble members just barely turns the NAO negative at the end of this month. The mean doesn't even turn negative at all for the Arctic Oscillation!

My takeaway is this: we're seeing some noteworthy cold signals for the end of December and start of January from the ECMWF control and the CFS ensemble system (a different analysis of that coming later today), but we can't rely on models that far out. To me, it's just not smart forecasting to use them for anything aside from large grains of salt, if that makes sense. Instead, I'm focusing on a changing global weather regime as the Madden-Julian Oscillation changes into colder phases for the end of December and through January. That, I feel, is where the real intrigue lies for Arctic outbreak risks as we enter the new year.

Stay tuned for that aforementioned post later today.

To Summarize:

• The ECMWF ensemble's 'control' member - often seen as the most accurate member of the ensemble group - is expecting much-below-normal temperatures from the end of December through much of January in the U.S.
• At this stage, while it's intriguing, I would not take this signal as anything more than a large grain of salt
• There is more value in monitoring the changing global pattern as the new year kicks off, and a post discussing that will be published a little later today.

Andrew

Brief Pattern Change Expected Next Week

A change in the current weather pattern, albeit a brief one, is expected next week.

PSU
Image Valid: January 12

Ensemble forecasts see strong ridging building over Alaska and extreme northwest Canada, extending from northern Siberia all the way across the Arctic Circle to the waters immediately south of Greenland. Consequentially, this will force the tropospheric polar vortex to lower latitudes, and it just so happens that ensemble guidance sees a piece of this vortex will dip south into Canada, and will exert its influence on the United States.
It's important to note that the tropospheric polar vortex will not enter the United States, it will stay well north of the Canada/US border. Cooler air will still result, but extreme cold should not be expected right now.

PSU
Image valid: January 15

By January 15th, the pattern is progressive enough so that the piece of the tropospheric polar vortex shifts east into eastern Canada. We also see strong ridging dominating the Arctic Circle, which will drop the Arctic Oscillation (AO) index well into negative territory, as we'll see a little later on in this post. Note the strong trough modeled over the Gulf of Alaska- this will maintain a positive phase of the East Pacific Oscillation (EPO). While I won't discuss the EPO in this post, that trough in the Gulf of Alaska is what will return our pattern to a more seasonal, and possibly warmer than normal pattern after this brief change to chillier temperatures.

Above 3 images from CPC

Just a brief look at the teleconnection forecasts, we see the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) both expected to drop into negative territory. This is a sign for cooler than normal weather, and as we saw earlier in this post, ensemble guidance believes a brief shot of colder weather will hit the Central and East US. The Pacific-North American (PNA) index is expected to stay positive, possibly heading more towards  neutral territory in the medium-long range, but generally should remain positive. This is likely a result of that trough in the Gulf of Alaska. That trough will keep the pattern progressive, which means the ridge that forms over the West US as a result (the positive PNA indicator) will likely drift east and spread warmer weather east.

To Summarize:

- A brief flip to colder weather is expected next week, mainly between a January 10-15 period.
- Warmer weather is expected to return after January 15th.

Andrew

January 9-12 Potential Winter Storm

There appears to be the risk for a storm system to impact the East US, particularly the Northeast, in the January 9-12 timeframe.

Tropical Tidbits

We'll first go over the forecast of the storm, and then get into it at a more analytical level. This forecast panel shows the 18z GFS forecast of precipitation type and intensity, as well as mean sea level pressure contours and 1000-500mb thickness values for the early morning of January 11th. We see a strong storm system just offshore the Northeast, producing widespread moderate to heavy rain right along the coast from New Jersey on northeastward, with light snow located in inland New York into east Pennsylvania. This solution could give copious amounts of rain and/or snow to the East Coast.

As expected, the internet is biting hard on this storm, given it could be the first real wintry system for the East this winter. Not to burst anyone's bubble, but below explains why I'm not expecting a real snow threat with this storm, and why not to bite on any forecasts yet.

Tropical Tidbits

This graphic shows us the 500-millibar vorticity values, forecasted for 12z (6 AM CST) today, Monday. That red box, all the way out near the Aleutian Islands, encompasses our storm system. That's right, the system that people are looking at is currently hundreds of miles away, closer to Russia than mainland United States. That's the first sign that we shouldn't take any model guidance at face value right now. Heck, the latest 00z GFS run came in and showed the storm pushing well out to sea, not even affecting most of the East US. I just chose to show that 18z GFS graphic so I could show what I'm talking about, and to make a few points, like this one.

Additionally, check out all the pieces of energy out ahead of our main system. If you count closely, just looking at this graphic, I can identify at least three separate storm systems downstream of our system of interest near the Aleutian Islands. Three systems is a lot for model guidance to sort out, and it doesn't help that one of those systems is also well out at sea, not even close to the radiosonde network which would help model accuracy. In sum, models are going to suffer a lot with this storm, both with the distance the storm is from the U.S., and how many systems are downstream of our storm itself.

Tropical Tidbits

This next point is specifically for the snow enthusiasts in the Northeast already getting excited for this storm. This is the 850-millibar temperature forecast for the morning of January 9th. At this point in time, our system is located somewhere in the South Plains, though it is not well-shown on this map. That's alright though, because we're looking at this image for the temperature spread across the Central and East US. All colors green and warmer are temperatures above freezing at this layer of the atmosphere (about 5,000 feet off the ground). That means that most areas east of the Mississippi River, save for Maine, Vermont, and New Hampshire, should be comfortably above freezing in the couple of days, even several hours prior to the storm actually beginning to impact the East (if it does at all).

For a good snow set-up, no matter where you may live, you want an established cold air mass before the storm begins. We saw that guideline in play with our last storm system, which dumped copious amounts of snow in the Plains into Iowa and Wisconsin, even laying down a few inches of sleet near Chicagoland. It was all that sleet and freezing rain that fell because there was no antecedent cold air mass. This go-around, we also will be lacking a pre-established cold air mass, and that really concerns me with snow prospects.

CPC

While I'm not set on the idea of a snowstorm, I am open to the concept of this actually being a storm for the Northeast. The North Atlantic Oscillation (NAO) will be negative when this storm occurs, albeit only slightly negative. This should encourage the storm to curve up the coast, though just how much 'encouragement' there is remains in question. It's completely plausible the storm goes straight out to sea, as the 00z GFS portrayed, and it's also possible it curves up the coast. I'm not confident in it going one way or the other right now, but both solutions are plausible.

To Summarize:

- There is the chance for a wintry storm system in the Northeast between the January 9-12 timeframe.
- Personally, I see a very realistic chance that this storm produces more rain than snow as a result of no antecedent cold air mass.
- Model guidance should not be trusted this far out.
- This storm could plausibly go out to sea rather than up the coast.

Andrew 

December 26-29 Significant Winter Storm

There is growing concern that a potentially-historic snowstorm may impact the Southern Plains, with lesser, but still-substantial amounts to the north and east in the December 26-29 timeframe.

Tropical Tidbits

The system of interest is currently in the Gulf of Alaska, pushing towards the Pacific Northwest as of posting time. We see the higher vorticity values in the circled region, indicating the presence of the trough. Looking over the map right now, the pattern downstream (to the east) of this energy is rather zonal (not wavy; flow is west-to-east), which would keep this storm from going too far north. However, the flow immediately upstream (to the west) of this piece of energy is rather meridional (wavy; flow can appear to move in a north-south direction at times), which will likely play a role in the storm track later on. For now though, we can only confirm the presence of our storm system in the Gulf of Alaska.

Tropical Tidbits

The first snowfall map we'll analyze is the 00z GFS model, the most recent as of the time I am writing this post (it's currently 1:11 AM on Thursday, sleep is not my friend right now). The latest GFS shows amounts in the 6-14" range across northern, western and central Texas, with scattered swaths of 14-20", particularly in far western Texas into New Mexico, and into north-central Texas. A narrow, but intense band of amounts exceeding 24" is projected onto the southwest portion of Oklahoma into north-central Texas. Amounts in the range of 12-18" are then outlooked in much of central and northern Oklahoma, before amounts taper off to the 5-10" range from southeast Kansas through Chicago. The energy then appears to transfer east along the East Coast, dropping significant amounts in the Northeast, with New York, Massachusetts, Vermont, and New Hampshire among the states projected to receive up to 18".
This model has been quite variable with snow amounts, but has maintained consistency with the storm actually occurring, and confidence is increasing in the storm occurring in at least some format.

Tropical Tidbits

Next, we'll take a look at the Canadian GEM model forecast for this system. This model paints a different scenario, expressing the variability that still exists with this event. This model drops a general 6-18" swath across northern, western and central Texas, with the highest amounts in the far northern and western regions. Oklahoma also experiences significant snows, with a general 6-12" swath for much of the state, and even a 24" bullseye on the Oklahoma/Texas border just southeast of the Oklahoma Panhandle. Amounts then diminish to a 6-12" range in northern Missouri to northern Illinois and Indiana, southern Michigan, and boosting amounts slightly as the energy hits the Northeast.

While I'm not buying into any particular snowfall solution just yet, I am advocating a particular track. Next, I'll explain why I believe we will see the heavy snow axis fall where it is currently projected, or even further south and east than the above maps show.

Both images from CPC

The first two items to discuss are the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). The AO measures the strength of the tropospheric polar vortex- a positive AO indicates a stronger than normal polar vortex, while a negative AO indicates a weaker polar vortex. The positive phase of the AO tends to promote a more zonal flow, generally warmer than normal and less stormy. The positive phase of the NAO works similarly, promoting warmer weather in the eastern 2/3rds of the country, and zonal flow. Both of these indices are forecasted to maintain positive strength when this storm comes around, telling me that the storm will be hard-pressed to push northward like earlier model runs had suggested.

CPC

We're also looking at a positive Pacific-North American (PNA) index for when this storm is forecasted to hit. The positive phase of the PNA promotes high pressure in the west, and low pressure in the east. It's typically conducive for storms in the Midwest and Ohio Valley for this reason, and can also lead to these transfer events, which give the Northeast the threat for stormy weather as well. A typical positive PNA set up is shown below, and we are expecting this positive PNA set-up for the storm's occurrence.

NCSU

But the real kicker is a piece of energy that, in a way, is both the least and most important part of this storm.

Tropical Tidbits

The above image shows the 500mb vorticity map, forecasted by the 00z GFS for 84 hours out. We see our storm now well developed and centered in far northern Mexico, beginning to push eastward into the southern Plains. However, the point of discussion here is that area of higher vorticity in the northern Plains and Midwest, and it is this item that tells me we could see the storm track stay to the south.

That band of higher vorticity values is an area of confluence aloft. Confluence is the act of air essentially rushing into a given space- similarly, diffluence is the act of air evacuating a given space. In the atmosphere, confluence aloft can result in high pressure at the surface, which acts to force storm systems away (in this case, further to the south). It is this that completes my belief that the storm will take this more eastern track and give cities like Oklahoma City, perhaps Dallas, Chicago, Detroit, and much of the Northeast a solid snow after Christmas.

To summarize:

- A significant winter storm is forecasted to hit a large portion of the country in the December 26-29 timeframe.
- Snowfall amounts in excess of 12" cannot be ruled out for the Southern Plains, including Texas and Oklahoma.
- Snowfall amounts in excess of 6" cannot be ruled out for portions of the Midwest and Great Lakes, including Chicago and Detroit.
- Snowfall amounts in excess of 12" cannot be ruled out for the Northeast.
- High uncertainty still exists with this system.

Andrew

Early Signals For Pattern Change in Early January

There are emerging signals for a change to a colder and snowier pattern in the early days of January 2016.

PSU

The image above shows the GFS Ensemble mean 500mb height anomaly forecast for hour 384- the infamous fantasy land, end of the run panel. The ensembles are expecting ridging to build up along the west coast of North America, with troughing taking over in the Bering Sea and Arctic Circle. This kind of pattern would likely promote an overall-cooler set-up than the one we're currently in, which will feature well above normal temperatures on Christmas Day.

You all know I wouldn't post a 384-hour graphic without good reason, so let's dive in.

CPC

The ensemble forecasts out of the Climate Prediction Center for the Pacific North American (PNA) index show a dip to negative values as we head to Christmas, but then rising to neutral, and even positive values to kick off 2016. The majority of members are in positive territory by the start of the new year. The relative consensus of most ensemble members on a neutral or positive value of the PNA is encouraging for winter weather fans. Positive PNA values typically result in colder and snowier weather for the Central and East US, while negative PNA values result in warmer weather.
Some of you may ask why it's been so warm even though the PNA was positive in December- that can be attributed to the El Nino, producing enhanced convection in a part of the Equatorial Pacific conducive for warm weather here in the U.S., however that's a topic for another post.

ESRL

A grand overview of four key teleconnections reveals this improving pattern (improving, at least in the eyes of winter weather fans). We see a continued positive EPO through the forecast timeframe- if you're more knowledgeable, you'll recall that the positive phase of the East Pacific Oscillation (EPO) encourages colder weather in the eastern 2/3rds of the country, while the negative phase encourages warmth. The WPO (West Pacific Oscillation) closely follows those guidelines as well.
As noted earlier, we see the forecasted PNA rising to at least neutral values in the long range near the start of 2016.

If all of these forecasted indices and teleconnections verify on an as-is basis (which, I may add, is rather unlikely), we could expect a cooler pattern in the Central and East US. This would be mitigated by the positive phases of the Arctic Oscillation (shown in the first image by negative height anomalies across the Arctic Circle) and North Atlantic Oscillation (shown in the first image by weak troughing over Greenland), both of which tend to discourage persistence of colder weather.

To summarize:

- There are indications that a pattern change may be on the way for the start of 2016.
- Model guidance will change drastically, and this is nowhere near set in stone. However, if this does verify to a certain degree, a cooler pattern may be on the horizon.

Andrew

Long Range Outlook (Made January 28, 2015)

This is the long range outlook post, made January 28th, 2015, valid for the next 7-31 days.

We'll begin with a look at what's happened in the last week, as well as a verification check on the calls made in our last long range outlook post.

ESRL

Over the last week, we saw rather persistent ridging building over the Western United States into southwest Canada. This forced the development of some colder weather in eastern Canada and the northeast United States. The tropospheric polar vortex continued to be in a disrupted state, with multiple bodies of ridging being forced into the Arctic Circle.

In our last outlook, we predicted a period of cooler than normal weather for much of the Central and East US. That call didn't verify as well as we hoped, with warmth taking hold in the Northern Plains, as well as some rather mild conditions across the Southeast. However, the general idea for cold being maximized in the Northeast did work out well.

JMA

We'll now go over tropical forcing across the globe, and show how our pattern has been affected by it.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

We've begun to see enhanced convection make its way across the waters due south of Eurasia, as our new Madden-Julian Oscillation (MJO) wave forms. This convection is currently located west of India when viewing it from a longitudinal aspect. Such positioning of convection usually favors a colder and stormier pattern in the East US, as was evidenced by the recent blizzard that hit parts of the coastal Northeast. We're still seeing enhanced tropical convection west of the 180 degree longitude line, and this may have led to the unexpected warmth that messed with our call for a cool period this past week. As this convection moves east and dies off, the newly-developing body of convection should take control.

Now that we've discussed what has happened, let's start to go over what will happen.

ESRL

The image above shows 500mb geopotential height anomalies over the Northern Hemisphere, where blues and purples depict negative height anomalies, usually indicative of cold/stormy weather. Similarly, greens and reds indicate the presence of positive height anomalies, a precursor to warmth and generally quiet weather. In this panel of the ESRL ensemble forecast, valid for 9 days from today, we see that the persistent lobe of the tropospheric polar vortex has retracted north into Greenland, a classic positive North Atlantic Oscillation (NAO) signal. However, due to the blossoming ridge in the West, stormy weather still continues in the nation's midsection. This does not last long, however. This forecast matches our projection in the last Long Range Outlook for a brief cold spell around February 6-7.

ESRL

By the 336-hour forecast mark, ensembles are in agreement that the positive NAO will permit that ridge in the West US to bleed east, resulting in a warmer than normal pattern for much of the country. Negative height anomalies are shown along the Gulf Coast, but an unfavorable set-up upstream (to the west) of the United States means that warmth should prevail. This matches our earlier call for warmth after about the first week of February. A consolidated tropospheric polar vortex certainly does not help matters for winter weather fans, either.

Now that we've looked over the next 7-14 days, let's start looking out even further.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of February 2nd. Note the presence of a trough just to the east Japan, with a very powerful ridge just to the west of the country, arguably influencing the nation more than the trough. This trough appears to be a storm system that may impact us here in the United States down the road, just before this ridge may take hold, but that potential will be investigated in a later post. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a general warm period, possibly cooler in the East, around February 8th - 12th.

Tropical Tidbits

Long range models have been hinting at the idea that a strong upper level low or deepening trough will slide into Japan around February 6th or 7th. For now, ensembles are taking a more progressive and generally weaker approach with this potential, as is expected. For now, we'll watch a February 12th - 16th period for cooler weather, throwing a wrench into our outlook from the last post.

ESRL

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA includes a decrease in positive values as of right now, before a spike back well into positive territory in the long range. This works well with the ESRL ensembles we were analyzing earlier. The NAO forecast generally stays positive in the long range, a red flag for that positive PNA ridge to shift east into the Central and East US. A dip to negative territory does occur in the January-February transition, which may create a storm threat (upcoming posts will address this), but nothing too serious. Both the WPO and EPO are negative in the long range, which is good for winter weather fans. However, the consensus has been for this -EPO/WPO regime to weaken heading beyond the 16 day period, which may only enhance the threat for warmer weather in the middle of February.

Finally, let's use the skills learned in our OLR analysis earlier, to forecast the long range.

CPC

As in the JMA graphic, cooler colors define negative outgoing longwave radiation anomalies, which mean the presence of thunderstorm activity. Warm colors depict positive OLR anomalies, otherwise known as suppressed storm activity. In the 6-10 day forecast panel, we begin to see our new MJO wave really strengthen and define itself as it moves east, just south of the Indian subcontinent. Notice in this same panel that our current MJO wave near the 180 degree longitude line is nearly dissipated by this forecast time period. At the 11-15 day forecast mark, the MJO wave has strengthened considerably, and has now switched to pro-warmth phases. These pro-warmth phases can generally be identified when -OLR anomalies move east of the Indian subcontinent, and this should happen even beyond the 11-15 day forecast period. Beyond then, confidence is too low to accurately produce a forecast.

To summarize:

- A period of rather chilly weather should kick off the month of February.
- A brief period of seasonal to cool weather is expected to impact the Central and East US around February 6-7.
- In mid-February, the atmosphere is in favor of a warm pattern. However, disagreement from the Typhoon Rule means this part of the month could really go either way. We will re-examine this in the next long range post.
- An early look at late February continues to show a predominantly warmth-favoring pattern.

Andrew

Atmospheric Trifecta Preparing to Deliver Cold, Snowy January

A trio of atmospheric signals are gearing up for what could be a rather cold, snowy January.

Research I completed last night showed significant (10"+) snowstorms in the Midwest are most favored under the negative phase of the Arctic Oscillation, the negative phase of the East Pacific Oscillation, the negative phase of the North Atlantic Oscillation, and the positive phase of the Pacific-North American index. We look to have at least three of these factors locking down the atmosphere to round out December and kick off 2015.

Tropical Tidbits

The image above shows the GFS ensembles' forecasted 500mb geopotential height anomalies over the Northern Hemisphere, valid on the evening of December 26th. On this graphic, we see a prevailing negative EPO, as highlighted in this graphic with ridging over the Gulf of Alaska. A negative NAO has also emerged just west of Greenland, combining with ridging in Eurasia to provoke a negative AO regime. I'm watching for a positive PNA signal, but as a ridge is passing over the region, I'm content on acknowledging a trifecta for now.

It doesn't end there, however...

Tropical Tidbits

By December 30th, we find more than a few big developments have taken place. We see our negative EPO ridge has blossomed into a massive swath of high pressure, now extending along the West Coast into Alaska... and then some. Ridging over Greenland still exists, paving the way for a continued negative NAO and negative AO tandem. As a result, we continue to observe below-normal anomalies in the East US, signaling a very cold period to end December and start January.

My thinking is we'll see those below-normal height anomalies slowly progress west, as a west-based negative NAO (where the ridge is displaced to the west of Greenland) typically favors cold weather more into the Central US. This would be amplified/supported by the strong -EPO ridge, slowly evolving into a +PNA signal, it appears.

Now, onto the snow...

Tropical Tidbits

The image above shows the GFS ensembles projection on December 28th, showing jet stream wind speed values. There are a few things to note here.
First off, check out the extended Pacific jet stream. We're seeing that occur in the very near future, and it's no coincidence that this extended jet stream into North America is happening at the same time some storm threats are arising. The extended jet allows for cyclogenesis in the Northern Pacific, where energy may be shunted east into the United States, leading to those winter storm chances.
Second, we see a notable subtropical jet stream (STJ), as depicted by the green colors extending from the northeast Pacific into Mexico and the South US. With a negative NAO regime in place, as well as an enhanced subtropical jet stream, it's possible the East Coast could get rocking and rolling with these storm chances.

As for the Central US, I alluded to the below normal height anomalies pushing east. As far as snowfall impacts, clipper systems look to make their return, as the strong ridge pushing into Alaska produces a sustained northwest flow pattern (where the winds are out of the northwest). This could lead to not only episodes of snow in the North US, but also lake effect snow episodes, as all Great Lakes are currently well below last year's ice levels at this same time.

To summarize:

- The atmosphere is preparing to shift from a generally mild December pattern to a rather harsh January pattern.
- Sustained cold weather is likely for most of the nation east of the Rockies.
- Enhanced chances of snowstorms will be seen both along the East Coast, into the Central US (depending on individual storm tracks, of course).
- Buckle up, things are about to get fun.

Andrew

New, Successful Analog Guidance Predicts Warm December, Cold January

A new system of analog guidance I composed in recent days, showing success in the limited number of trial runs I have conducted, is forecasting a predominantly warm month of December, followed by a cold January.

ESRL

The image above shows analog guidance's temperature forecast for the month of December. As the image shows, millions across the country would average out over a degree (Celsius) above normal, primarily in the Plains and Midwest. Only the Southwest would see a colder than normal start to winter.

Now, how accurate is this analog guidance? I just got this system up on its feet last night, and these are the first results of the forecast. Some development is still needed, but that will come with time. I've noticed the analogs seem to understand the general pattern (i.e. negative Arctic Oscillation, positive PNA, etc), but still do struggle with less-synoptic features. For now, I'm presenting the data here, allowing it to verify and examine how it does later in January.

ESRL

In January, my analogs see the mass of cold air being stored in Canada on the last image bulldozing southward, bringing below-normal temperature anomalies to much of the nation, especially the northern Plains. The South then looks to see slightly above normal temperatures. Oddly enough, this pattern is surprisingly La Nina-like, even though I made sure to factor in El Nino analogs.

ESRL

The 500mb height forecast anomalies for January show sustained ridging over the Arctic, forcing what appears to be the brunt of the Polar Vortex south into western North America. This then bleeds east, allowing for those chilly temperature anomalies in the Central US.

I'll end this post with a look at the analog forecast for the stratospheric portion of the polar vortex, since this image only shows the tropospheric version.

ESRL

If you look closely, you'll see the polar vortex has been nearly obliterated...

To summarize:

- A new set of successful analog years is predicting a warm month in December, followed by a cold month in January.

Andrew

Extended Forecast Discussion (Part 2)

This is the second part of my Extended Forecast discussion, and will focus on the 2-3 week forecast range. A third part will be made for the final 4th week forecast this weekend.

For Part 1, click here.

AAM Transport
ESRL

The image here shows atmospheric angular momentum transport anomalies over the past few months. We want to focus on the anomalies in the top-right corner, the last few days of recording. On this graphic, we see a swath of yellows and oranges seeming to push diagonally upward along the image. Those yellows and oranges define positive AAM transport anomalies, and they are beginning their long-awaited movement to the north, which will throw a big wrench in our weather pattern. 

According to the oranges and yellows on the screen, the positive AAM anomaly transport values have made it as far north as about the 40N parallel, and they are strengthening. Because positive AAM anomalies generally can mean an enhanced jet stream, we might expect high pressure ridging to build northward with time as well, as the jet stream pushes north with the +AAM transport anomalies. This is already being observed with warm spells beginning to intrude upon the Central US.

This spells a predominantly warm weather period in the next few weeks, as this strong jet stream will eventually make it to the Arctic and keep the Arctic Oscillation (AO), and by extension the North Atlantic Oscillation (NAO) positive. Such a positive AO, which helps induce warm weather, is already on the forecast from the Climate Prediction Center, as shown below. 

CPC AO

The spaghetti-like alignment of the red ensemble forecast members for the Arctic Oscillation shows uncertainty associated with this pattern, as the very strong AAM transport anomalies push poleward, but eventually the ensembles should even out to a positive AO forecast over the next two or so weeks.

We can expand on the AAM for a more global look to this piece of the forecast.

ESRL
Relative AAM

Take a look at the chart above. Here, we see anomalies of the relative Atmospheric Angular Momentum (AAM) over the last few months, with a globally-averaged line on the bottom panel. If we look closely at conditions over the past few weeks, we can just barely see those yellow splotches of positive AAM anomalies that seem to be drifting in an upward-diagonal direction, meaning they're moving poleward. You wouldn't suppose this has to do with the AAM transport anomalies moving poleward too, would you?...

After the AAM transport anomalies make their move into the Arctic, guidance has been consistent on the idea of the AAM dropping well into La Nina-like negative territory. If this should happen, we would likely move into Stage 1 of the GWO stages. The stages, and their descriptions from the ESRL, are listed below.

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.

I italicized the Stage 1 description, as we would likely be heading for Stage 1 so long as projections remain consistent in coming days. Let's analyze that Stage 1 description as best we can. Reading through it, we notice that in Stage 1, the AAM anomaly is negative. With the forecast for the AAM dropping deep into negative territory, we can cross that off our checklist. Note how the description includes troughs in the Western USA, with a ridge over the Southeast. Model guidance has been latching onto that idea for a while now, as this shot of the GFS Ensembles 500mb height anomaly forecast valid for November 1st shows.

PSU

Lastly, check out the final sentence in that Stage 1 description. "High impact weather is favored across the Plains". 

Huh?

Well, let's decipher this. Stage 1 features the aforementioned troughs in the West US. If it also features a ridge in the Southeast, we can make a relatively safe assumption that they're talking about a predominantly-negative PNA pattern.

NCSU

The image above shows a typical negative PNA pattern across North America. Note the downsloping jet stream entering the West Coast; that tells us of troughing/storminess in the Western USA, just like we saw in the Stage 1 description. Conveniently, we also see a strong ridge positioned in the Southeast, oddly enough just like what we read in the Stage 1 description. Coincidence? No. 
Lastly, look where those two primary jet stream-like features lead- right into the Plains. That confirms our suspicions of a negative PNA pattern in coming weeks as we transition to that GWO Stage 1, placing the primary storm track over the Plains, and maybe a bit into the Rockies and Midwest at times.

SOHO

I want to briefly touch on what the Sun is doing right now. The chart above shows a sunspot identification image, where oranges and blacks indicate the presence of sunspots. Notice that large complex of sunspots in the center of the sun. This complex has been rotating to face Earth for the last week or so, and has finally gotten the Earth in its direct line of sight. The sunspot complex will be rotating away with time, but if it happens to expel a Coronal Mass Ejection (CME) or other expulsion of energy, our weather could turn warm rather quickly within the next month. For now, it's something to monitor.

JMA

The graphic above shows Outgoing Longwave Radiation (OLR) anomalies across the tropics regions, in a global view. Here, we can identify areas of tropical forcing, where enhanced or suppressed convection may be driving or enhancing our weather pattern here at home. Glancing over this image, we do find a swath of negative OLR anomalies, indicating enhanced thunderstorm activity, placed south of India and a bit southwest of the subcontinent as well. This swath of convection has an entirely different meaning for our weather pattern, which will be discussed later on in this discussion. But for now, we'll analyze it as the Madden-Julian Oscillation (MJO).

The MJO states that enhanced or suppressed tropical convection over certain parts of the Equatorial Pacific basin or Indian Ocean can have different effects on the global weather pattern. In this case, when comparing observed negative OLR anomalies in the image above with the Bureau of Meteorology's average negative OLR placement for each MJO phase, we can determine what MJO phase we appear to be in. See if you can figure it out on your own.

BOM

If you guessed Phase 2 into Phase 3, you're right! Phase 2 and 3 MJO events typically see enhanced tropical convection just south of India, very similar to the enhanced tropical convection we're seeing now in that JMA graphic above.

Now that we know what phase we're in, let's see if we can identify how this phase MJO is driving our weather pattern.

Americanwx

The image above shows typical 500mb height anomalies for Phase 3 MJO events in the month of October. Blues indicate stormy and cool weather, while greens, yellows and reds highlight warm and quiet weather. In "normal" October Phase 3 MJO events (the word 'normal' placed in quotes, since no MJO phase is really ever 'normal'), we tend to see high pressure extending from the West US into the Central US, with a swath of negative height anomalies from Greenland to the East Coast. Interestingly enough, we're currently seeing that same scenario play out, though not as defined to the naked eye as you might want it to be. Regardless, we are seeing that progressive ridge formation from the West extending into the Central US, with some stormy East US weather as a result. Though we might not make it to Phase 3 of the MJO, and the MJO isn't the only part of these negative OLR anomalies, we're certainly observing Phase 3-like conditions. Consequentially, if this continues, we might expect a continuation of such warm spells in the next few weeks or so in the US, as this may be enhanced with the strong +AAM transport anomalies driving north, and the resultant positive AO. 

That little section above was primarily from the first part of this Extended Forecast Discussion. Now that we're into Part 2, however, we can finally discuss the other use behind those negative OLR anomalies: The Hadley Cell.

Michael Ventrice

The chart above shows 14 day-averaged omega anomalies on a pressure-by-longitude cross-section. Though it may seem daunting at first, it's not all that complicated. The line marked with the '0' is the Equator, while the 30S and 30N demarcations are lines of latitude. On this chart, we see a swath of blues and purples extending from the surface (~1000 hPa) to about the jet stream level (200hPa). Checking the legend at the bottom of the graph, we recognize this to be a swath of negative omega values. If you remember what the concept of omega usage is, negative omega values tend to indicate rising air, while positive values indicate sinking air.

Interestingly enough, we're looking right at a cross-section of the Hadley Cell.

This example image above, depicting the Hadley Cell, shows how this rising and sinking air works. We will see warm Equatorial air rising, as the negative omega values depict, usually rising into the upper atmosphere. From there, the air cannot rise further, so it is carried northward (or southward, depending on hemisphere) to around the upper regions of the lower latitudes, i.e. 30N or 30S. From there, the warm Equatorial air has cooled, and now sinks back to the lower atmosphere. This is how we identify the positive omega values on the chart above. From there, the cool air is carried southward (or northward, again varying with hemisphere) back to the Equator to start the process over again.

Applying that example graph to the omega chart, we can roughly make out where the Hadley Cell is currently placed. We see a body of sinking air around 40N, where the circulation has been weakening in recent days, with another swath of sinking air possibly forming such a cell in about the 20N region. The Hadley Cell circulation at the 30N area was much more defined in this graph a few days ago, shown below.

Old chart showing the defined Hadley Circulation Cell from the Equator to about 40N.

As the very-knowledgeable blizzardof96 pointed out with respect to this Hadley Cell, its placement around 40N is making it difficult for troughs to form in the area (you Typhoon Rule enthusiasts may recognize this as a warm signal for the US). However, with the cell apparently weakening, and model guidance already re-introducing troughs to East Asia in the near-future, it looks like our week 2-3 forecast is turning cooler.

Summarization

- Very strong positive AAM transport anomalies are moving poleward, meaning warm weather can be expected for the next week or two, before the anomalies hit the Arctic and sustain a positive Arctic Oscillation.
- The positive Arctic Oscillation will then provide the chance for warmer weather for the next few weeks, depending on how long that positive AO can be sustained.
- Dropping AAM anomalies after the passage of the positive AAM transport anomalies suggests a GWO Stage 1 alignment, supportive of a cold West US, stormy Plains, and warm East US in the 2-3 week forecast.
- The Hadley cell dissipation is likely to allow troughs/storms to return to East Asia, setting up the Typhoon Rule for a gradual return to stormy/cooler weather in that 2-3 week timeframe.

Look for the final installment, Part 3, to debut this weekend.

Andrew