Long Range Outlook (Made June 9, 2019)

This is a Long Range Outlook made for publication on June 9th. The forecast period will cover the remainder of June. Click on any image to enlarge it.

Observed 500-millibar geopotential height anomalies as of 1pm Central, June 8th.
Source: Tropical Tidbits

A look around the Northern Hemisphere on the afternoon of June 8th shows a rather-busy pattern over North America, with an upper level low positioned over northwest Canada and a secondary trough dropping into the Pacific Northwest while still being influenced by the first system. A strong ridge maximized over the northern Great Lakes into southern Canada contrasts with a closed low positioned in the Southeast U.S., forming a Rex Block-looking pattern despite there not being any actual blocking event taking place. Indeed, even downstream of this pattern into Newfoundland, the flow is rather amplified with an upper level low grinding against a strong ridge that is firmly entrenched into Greenland. Upstream of the United States, weak ridging is evident in the northeast Pacific, helping to diffuse the Pacific jet stream in the process. A strong storm system is seen south of the Aleutian Islands, with additional disturbed weather skirting along the northern Pacific.

Forecasted 500-millibar geopotential height anomalies valid 7am Central, June 12th.
Source: Tropical Tidbits

Even within the five-day forecast window, the evolution of the flow over the Northern Hemisphere becomes rather complex rather quickly. The ridge and closed low mentioned in the eastern half of North America are seen drifting eastward, with model guidance suggesting the ridge will propagate northeastward into Greenland to maximize positive 500mb height anomalies. Additionally, the observed weak ridging in the northeast Pacific is expected to strengthen and entrench itself across the western coast of the continent, thereby setting up a pattern representative of the positive phase of the Pacific-North American (PNA) oscillation, seen by ridging along the West Coast, as well as the negative phase of the North Atlantic Oscillation (NAO), seen by ridging over Greenland.

The result of these two features, as well as a messy picture for the West Pacific Oscillation and East Pacific Oscillation (EPO), is a likelihood of cooler than normal temperatures across the eastern two-thirds of the United States, as well as into Canada. The positive PNA and negative NAO will act to direct air from northern Canada down into the United States. Indeed, the five-day average 2-meters-above-the-surface temperature anomalies below, through the period ending June 14th, shows a substantial and widespread period of below-normal temperatures for the majority of the country.

Forecasted 5-day average ~surface temperature anomalies, valid through the period ending 1pm June 14th.
Source: Tropical Tidbits

Model guidance is showing strong confidence in the evolution of strong ridging over both western North America and Greenland as well:

NCEP Relative Measure of Predictability for 500-millibar values at forecast hour 96 (June 11th 7pm).
Source: NCEP

Given the high confidence in the projected pattern through the five-day forecast timeframe, it does appear that colder-than-normal temperatures are likely for the eastern two-thirds of the country through at least June 14th.

Forecasted 500-millibar geopotential height anomalies valid 7am Central, June 18th.
Source: Tropical Tidbits

Ensemble model guidance is in agreement that the strong ridge over Greenland (again, the negative-NAO pattern) will remain in place through at least the day 10 forecast period, as shown above by the ECMWF (European model) ensembles. Upstream of the contiguous United States, too, model guidance does not anticipate any significant changes in the aggregate pattern, with predictions of ridging continuing along the west coast of North America as an upper level low situates itself just south of Alaska.

Let's now turn to teleconnections and oscillations to ascertain if model guidance is correct in depicting the forecast into the end of June.

Forecasted states of the Pacific-North American (PNA) index, top-left; North Atlantic Oscillation (NAO), top-right; Western Pacific Oscillation (WPO), bottom-left; and the Eastern Pacific Oscillation (EPO), bottom-right.
Source: Earth System Research Laboratory (ESRL)

Viewing model-forecasted teleconnections over the next two weeks or so, we find a broad pattern similar to what was described earlier, with the positive PNA and negative NAO expected to dominate the outlook for North American weather throughout the forecast period. Guidance does weaken the PNA significantly from a strong-positive state to a slight-negative state by June 20th. I personally don't see this as remarkably likely when compared to a forecast that has the PNA remaining positive (even if modestly so) through the period, and I'll discuss why shortly.

As noted earlier, the WPO and EPO are not expected to be strong enough to materially disrupt the +PNA/-NAO combination, with the moderately-positive West Pacific Oscillation forecast actually a boon for helping to sustain the ridge along the western coast of North America, and thereby sustain the +PNA. Model guidance sees the EPO as hovering essentially around levels too weak to have a definitive impact on the weather pattern.

I'd like to briefly discuss a second reason - in addition to the expected positive WPO - why I believe the ridge along the western coast of North America (the +PNA) will sustain longer than guidance indicates here.

Madden-Julian Oscillation (MJO) phase space diagram forecast, from the ECMWF model.
Source: Climate Prediction Center and ensembles.

For a refresher on the concept of the Madden-Julian Oscillation and how to dissect a phase-space diagram, feel free to read this post from earlier. The European model and its ensembles anticipate the MJO staying in a low-grade Phase 3 state over the short-term, before shifting into Phase 4, strengthening somewhat, and then potentially weakening into the 'circle of death'. Given how model guidance in that linked post had expected us to be in the 'circle of death' at the time of this writing, rather than in Phase 3, we will only trust the forecast to take us from a low-grade Phase 3 to a low-grade Phase 4 through the end of June.

We are able to see how the atmosphere typically acts when the MJO is in Phase 4 for the month of June below.

500-millibar geopotential height anomalies when the MJO is in Phase 4 during the month of June.
Source: American Weather

When the MJO is in Phase 4 during the month of June, positive height anomalies tend to be favored over the Gulf of Alaska into parts of the Aleutian Islands, with another area of positive height anomalies over Greenland. Below-normal anomalies are then painted across a good swath of the eastern two-thirds of North America.

If you're thinking that this Phase 4 composite above looks remarkably similar to the forecasted pattern beginning around June 11th, as discussed earlier in this post, you aren't the only one. It doesn't seem coincidental to me that model guidance maintains the +PNA/-NAO pattern through much of the remainder of June, particularly since the MJO moving into Phase 4 would support exactly this kind of pattern to continue.
It is possible that guidance thinks the PNA will weaken and even reverse into a negative state by late June simply because the models are - correctly - trained to forecast the state of a teleconnection by comparing the forecasted pattern with a "textbook" pattern. For example, scientists have developed what the atmosphere should look like when the PNA is said to be positive, and thus models compare their forecast to that "ideal" pattern to determine what state the PNA will be in. However, the atmosphere is not binary like that - in other words, even though the PNA might be forecasted to be negative, the pattern itself may still resemble a positive PNA. Let's discuss this further.

Forecasted 500-millibar geopotential height anomalies valid 7am Central, June 21st, with annotations.
Source of image: Tropical Tidbits
Source of annotations: Author

Shown above is the 500-millibar height anomaly forecast from the GFS Ensembles, valid the morning of June 21st. This is the purported date by which the PNA is forecasted to turn negative, as per the four-panel graphic examined earlier in this article. Looking at the broad forecasted pattern, however, shows the pitfalls that simply taking the index forecast & going with it can present.

First, we recognize why guidance thinks the PNA will flip from strong-positive to a negative state: there is an expectation that troughing will develop along and off the coast of western North America, a part of which I have circled in red. To be sure, stormy activity in this region is indeed what a negative-PNA consists of, so the models are not at fault here. The issue we want to resolve is a lack of context that looking only at forecasts of the indexes results in.

When broadening out beyond the trough along the western coast of the continent, the ridge that is expected to send the PNA into strong positive territory is seen, now having retrograded slightly into the Gulf of Alaska. The movement of the ridge to the west is indeed why troughing is seen developing along the western coast. But take a look at the way the ridge extends into Alaska to see what I mean by 'lack of context'. Instead of merely topping off around Anchorage, the positive anomalies extend well into Alaska, and join up with another ridge based in Canada, the combination of which then pushes into the Arctic Circle. That doesn't mean much for the trough along the west coast of the continent, but it's certainly something we need to note, and here's why.

Take a look at Greenland again. The strong ridge is still forecasted to be positioned over Greenland, keeping the negative-NAO in place, as expected by the four-panel teleconnection graphic from above. Big deal, so what? So, the negative NAO pattern keeps colder than normal conditions in place over the Northeast U.S., Great Lakes and into the Mid-Atlantic. But with that strong ridge in Alaska and Canada, I find it more plausible that the lobe of colder weather will also affect the Midwest, parts of the Plains, and the Ohio Valley. Why? That ridge in western Canada will team up with the Greenland ridge to force the primary lobe of cold air to the south, as is already seen in the graphic above.

But it's the fact that the Gulf of Alaska ridge extends its influence into the Canadian ridge that makes me think those regions will also be affected by cold air. The alignment of the Gulf of Alaska ridge suggests it shouldn't be difficult at all for that ridge to push northeastward into Canada, where the other ridge is based. If/when this happens, the trough along the western coast of North America will be suppressed, and perhaps even displaced to the south towards the Southwest U.S., which could then allow the ridge to build back into the southwest part of Canada or even into the Pacific Northwest again.

All in all, that would allow a more positive-PNA-esque pattern to emerge (with respect to cold air hitting the mid-section of the country and not just the Northeast), even though by definition we should be in a neutral or even negative-PNA state, since that trough will still likely stick around, just a little farther south.

Whether this scenario actually plays out is something only time will tell, of course, but I wanted to emphasize this because when looking at teleconnection forecasts, you need to also know the *context*. I have gotten burned plenty of times by only going off the teleconnection forecasts (for example, only seeing a -NAO or +PNA and basing the forecast off that) and not understanding the context for that forecast (for example, the +PNA might not be as sturdy as the teleconnection forecast shows because of a strong upper level low just to the west of North America, or something along those lines).

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All told, this provides a good basis for my outlook for the remainder of June.

- Through the middle of June, I expect colder than normal conditions to evolve for the eastern two-thirds of the country. This cooler weather will be widespread, with areas only west of the Front Range likely to see seasonal to above-normal temperatures. In terms of precipitation, any severe weather threats should stay confined to the southern portion of the country, with the jet stream and broader pattern alignment unfavorable for any severe weather outbreaks at this time.

- From the middle of June to the end of June, I expect a continuation of this cooler than normal pattern, though with a reprieve coming for the Plains and perhaps parts of the Midwest as a trough builds in to parts of western North America to return some parts of the central U.S. to more seasonal conditions. However, firmly cooler than normal conditions should persist for the eastern third of the country, especially in the Northeast. Any seasonal conditions will be pockmarked by cooler bouts of weather for those in the Midwest and Great Lakes into the Ohio Valley, though again the coldest conditions should stay further east. Storm chances should increase for the central part of the country with a return to more seasonal weather, but zonal flow aloft and the lack of a ridge in the Southeast should dissuade any significant severe threats from hitting the Central U.S., and likely from hitting the country in general.

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

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

Special Post: 2015-2016 Winter Pattern Beginning to Form?

In today's special post, we're looking at how the upcoming winter pattern may be giving a sneak peak at what it will do this cold season.

ESRL

The above two-panel image shows daily sea surface temperature anomalies for September 16th on the left, with the 500-millibar height anomalies for September 17th shown on the right. I've discussed the importance of realizing an ocean-atmosphere relationship for some time, and this latest evidence is only adding credibility to that relationship.

I've highlighted two areas of interest on the right-most panel. From Alaska, south into the British Columbia and Pacific Northwest regions, negative height anomalies were observed on September 17th. In the other highlighted region, positive height anomalies were extending from the Bering Sea into the waters north of Hawaii. Although not shown, we also saw a closed upper level low placed due south of that ridge in the Bering Sea, forming a Rex Block pattern.

It is no coincidence that sea surface temperature anomalies reflect this atmospheric pattern. We see a swath of above-normal to well-above-normal water temperatures across the North Pacific, with a body of below-normal SST anomalies just south of the Aleutian Islands, extending back towards Japan. In addition, we also see a cooling of water temperatures over the last month in the Gulf of Alaska / Northeast Pacific, as the graphic below shows.

BoM
Change in water temperatures over the last calendar month

In addition to this pattern showing up in the past, model guidance insists it is here to stay, at least for the next 10 days.

PSU

The GFS ensemble mean 500-millibar height anomaly forecast for 132 hours out is shown above on the left panel. We see a set-up very similar to what we have been seeing, with a Rex Block orientation in the North Pacific, a deep trough in the Gulf of Alaska, and a stagnant ridge in the Central and East US.

PSU

Even in the very long range, over 10 days out, the GFS ensembles still show a signal for ridging in the Bering Sea, a trough in the Pacific Northwest, and a ridge in the Central US. While accuracy at this long range timeframe is quite low, the pattern has a decent chance of locking up like this if it verifies 5 days down the road.

While we may see sea surface temperatures change drastically over the next few months, I'm getting the feeling that we're closing in on our winter pattern, and it could resemble something like the pattern shown above.
This current pattern, and the one forecasted over the next two weeks, is not unlike a modified strong El Nino, as shown in this link (click here), which I've been discussing as a favored set-up for the winter (a modified textbook El Nino pattern). Whether it sticks around and/or returns during the winter remains to be seen, but I find it plausible we enter into this pattern for the winter. This would bring about a warm winter for much of the Central and East US.

To Summarize:

- The current weather pattern is forecasted to continue over at least the next two weeks.
- This pattern is following sea surface temperature anomalies over the Pacific, making it more likely to maintain.
- This pattern resembles a modified Strong El Nino pattern, and could stick around for the winter months.

Andrew

Pattern Change Imminent for North America

Welcome back to The Weather Centre! We are now resuming winter weather posting, which means we're back in business for the next several months! We're kicking off this winter with a pattern change update for North America.

It is expected that a pattern change will impact the United States in the next week or two, leading to substantial air mass shifts across North America.

ESRL

Over the past week or so, we've seen a persistent, high-amplitude weather pattern over the Northern Hemisphere, particularly in the North Pacific. Deep negative height anomalies have been situated west of the Aleutian Islands, as well as along the coast of western Canada. A strong ridge has maintained position over the Aleutian Islands and across the Bering Sea. This volatile pattern has wedged the United States into a rather stagnant pattern, with warm weather over-spreading the Central United States, making pushes north and east as time has gone on.

This pattern is about to change, as we see two main playmakers rile up the atmosphere.

Tropical Tidbits

The first will be the shifting of that deep trough along the west coast of Canada south into California, and then moving north and east into Montana, as this GFS Ensemble forecast shows on the morning of this coming Sunday. Locally, this means warm weather continues for the Plains, with that ridge still not budging, as well as warmer weather in the Midwest and Northeast. However, as that trough moves eastward, we will start to see the jet stream become more zonal.

Tropical Tidbits

By Wednesday morning, we see significant changes to our atmospheric mid-levels. We now see a dominant zonal flow of the jet stream, as the smoothness of height contours over North America shows. Some slight ridging remains in place along the East Coast in the United States, but this too will be shunted away in due time, as the zonal flow is maintained by the strong storm system in the immediate vicinity of Greenland (some of you more knowledgeable weather enthusiasts may recognize this as a pattern similar to that of a +NAO wintertime pattern).

Tropical Tidbits

By Friday morning, a week from this post's publishing, we see a complete flip in the pattern from where we are currently. Strong ridging has taken over along the western coastline of North America, maximized in the Pacific Northwest into the British Columbia region. In response, similar to a +PNA pattern in the winter, we see cooler, stormier weather working its way into the Central US and East US. A note for any severe weather fanatics still watching the fall tornado season threat, I wouldn't be surprised to see some storms anywhere from Oklahoma to the North Plains under this northwest flow.

There is another factor that we will have to watch in the next couple weeks, and that is tropical storm Ignacio.

RAL/UCAR

Ignacio is currently located in the middle of the Pacific, due south of the westernmost Aleutian Islands. The image above shows forecasted storm tracks from all available GFS Ensemble members. The consensus among these members, as well as other statistical forecasting models, is to have Ignacio curve north and east, making another 'landfall' as a post-tropical system in British Columbia. From there, already-low confidence gets significantly lower, as ensemble tracks take this storm anywhere from the Arctic Circle vicinity (as shown by member AP17), or the Lake Michigan region (as per members AP10, 12, and 14).

As far as impacts, we can split it up into a number of talking points. For one, this will drastically lower forecast model skill over the next two weeks. While it isn't necessarily uncommon for post-tropical systems to make their way into the mid-latitudes, forecast models almost always see a drop in verification due to this occurrence. Therefore, long-range forecasts over the next several weeks are subject to anomalously worse verification rates.
Secondly, as Ignacio was once a Category 4 hurricane, we could see a strong storm system develop over land here in North America. Local impacts may include substantial rainfall, possibly to the point of severe weather potential, but that all depends on where the storm tracks, and if it even makes it to North America to begin with.

To summarize:

- A pattern change will unfold across North America over the next 7-14 days.
- A heat wave will impact the East US, to be replaced by cooler weather about a week from today.
- Post-tropical Ignacio will hinder forecast model accuracy, and may also bring about anomalous weather conditions in the U.S. in the next 10-20 days.
- Cooler weather will prevail in the Central US in the next 7-14 days.

Andrew

Long Range Outlook for March, Early April 2015

This is the Long Range Outlook for the period of March 2015 into early April 2015.

Paul Roundy

The above image is pretty complicated, but bear with me and it won't be so bad. This chart shows a longitude-by-time forecast for tropical convection anomalies (colored shadings) between 7.5º North and 7.5º South latitude, centered on the Equator. Blue colors correspond to enhanced tropical convection (thunderstorms), while yellows are indicative of suppressed convection. There are a lot of lines here, but the only ones we need to worry about are the red lines, which show the active phase of the Madden-Julian Oscillation in solid (dashed is the suppressed 'phase'), as well as the Kelvin Wave, which is in thin pink.

We currently see an MJO wave evolving from about 75 degrees East, as the red lines in that longitudinal sector show. It is expected that this wave will strengthen in the next week or two as an Equatorial Rossby Wave moves over the region, shown as a light-green delineation. The placement of tropical convection in this part of the world is equivalent to a Phase 4 MJO event. As shown in the image below, temperatures during a Phase 4 event will tend to lean warmer than normal for much of the country, but a deep snowpack across much of the North and Central parts of the US tells me this warm-up could very well be muted around March 12th.

CPC

A Kelvin Wave then forms around that March 12th period, and this convectively-coupled Kelvin Wave traverses the Pacific in time for the final days of the month. This very progressive tropical forcing theme tells me the country is likely in for some roller coaster weather patterns for the remainder of the month, despite the MJO wave signal only shifting eastward slowly in contrast to the Kelvin Wave movement. This particular chart has us reaching Phase 5 of the Madden-Julian Oscillation by the final days of March, which would typically favor a cooler than normal pattern for the Northern US.

However, this cold outlook for the end of March is working in contrast to a projected pattern shift in the Pacific, in which ensemble guidance sees a trough positioning itself along the west coast of North America to permit warmer weather in the Central and East US. With warm signals coming from the Typhoon Rule as Japan undergoes a generally-warm pattern in the next two weeks-plus, and tropical forcing not really being effective this past month due to such dominance from the Pacific pattern, I'll opt for a warmer outlook to close March.

To summarize:

- An overall cool pattern should remain in place through about mid-March.
- Model guidance is then indicating, at the very least, the synoptic (overall) weather pattern will undergo some changes for the last half of March. I see this as being a good shot for a warmer pattern.
- This warmer pattern could continue into early April.

Andrew

Long Range Ensembles Predict Incoming Spring-Like Weather

Long range ensembles are indicating a much warmer weather pattern is on the way for much of the country that has been held hostage by brutal cold in the last month.

PSU
Click to enlarge

The above image shows 500mb geopotential height anomalies from the GFS ensembles, projected over the Northern Hemisphere and valid for March 1st. In this graphic, we see a massive ridge of high pressure, the same one which has allowed for such a cold and snowy February, meandering westward over the north-central Pacific, as opposed to the northeast Pacific and Gulf of Alaska. This comes as a strong trough drops into the Southwest and Baja California. In response, with the strong upper level low previously positioned over Greenland, now over Europe, a ridge is able to form over the Central and East US. It looks like this ridge will be suppressed a bit, but warmer weather is on the way for those in the Plains, Midwest, Ohio Valley, and Northeast, among other areas.

PSU

By March 7th, ensembles are indicating the ridge remains in the north-central Pacific, but an upper level low has now developed in the Gulf of Alaska. Unfortunately, this then results in a ridge forming over the Western US, as the graphic above shows. Consequentially, the semi-permanent Arctic long wave trough (better than using 'polar vortex', apparently) is being pushed back into North America, setting up another period of cold weather for the Central and East US. This would likely last for a handful of days, right between February 5-10 or so.

But... finally... finally!... relief comes.

PSU

In the long-long range, to March 11th, ensembles see our ridge forcing itself north in the north Pacific into the Aleutian Islands, along the jet stream, where the "polar vortex" is stationed just off to the north. The net result for us is another trough setting up over the western section of North America. You weather enthusiasts know this as a negative Pacific-North American (PNA) pattern, with a trough in the west and warmth in the East.

This looks to be our new pattern, going strictly by these ensembles. Stormy weather in the West with predominantly warm weather out east.

To summarize:

- A brief warm-up is expected in the final days of February and early days of March.
- Another cold blast can be expected around March 7th.
- The pattern may then shift to a warmer set-up for the middle half of March.

Andrew 

Long Range Outlook (Made February 19, 2015)

This is the updated long range outlook, made on February 19th, and valid for late February into the first half of March.

Tropical Tidbits
Click to enlarge

We're currently seeing a portion of the semi-permanent Arctic low pressure vortex intruding into the United States, with anomalously low 500mb geopotential height values appearing in the colored shadings and analysis in the graphic above. This anomaly has been forced by a strong ridge pushing into Alaska from the northeast Pacific and west coast of North America. Over the next few days, cold air intrusions should continue as this ridge flexes its muscles and persuades additional Arctic air masses to collapse into the lower latitudes.

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 positive PNA and negative EPO have worked in tandem to indicate this strong ridge blossoming into the Gulf of Alaska and general northeast Pacific. In the upcoming couple of weeks, model guidance is indicating we see the ridge shift further offshore to the west, as the PNA dip from positive to negative shows. Over time, ensemble guidance is telling us that this ridge could keep shifting west, resulting in the EPO actually moving positive for the first time in a while. The positive EPO signal, however, is weak, and will have to be watched for a 'false positive', both literally and metaphorically. If this forecast does verify, we can expect a warm-up in the early days of March.

Tropical Tidbits

Ensembles indicate we will see a ridge of high pressure pushing into Japan on the morning of February 21, as the graphic above shows. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, we can anticipate a warm-up for millions that have undergone a brutal February in the final days of the month, likely into early March. From there, confidence decreases, but a return to an average or cooler than normal pattern may be expected.

You'll notice I haven't used tropical forcing in this post; that's because the ridge in the West is just so overpowering that the Madden Julian Oscillation can't do much of anything.

To summarize:

- A cold pattern is expected to round out February.
- A warm-up is expected for the last days of the month into early March.
- The first half of March should be characterized by a generally average to slightly below-average temperature pattern. There are hints of some stormy weather in early March.

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