Sunday, June 9, 2019

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).


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.