NOAA is watching for a possible stratospheric disruption, but its November 7 to 15 outlook keeps Illinois and the Great Lakes warm under a positive-PNA pattern. A polar vortex disruption changes the odds later. It does not promise a cold burst next week.
On November 1, NOAA’s Climate Prediction Center (CPC) favored above-normal temperatures for Illinois, the Great Lakes and the rest of the Midwest for both November 7 to 11 and November 9 to 15. The strongest warm signal, with a probability above 80 percent, was over the Northern Plains. A day earlier, CPC’s monthly discussion said model forecasts show the potential for a stratospheric polar vortex disruption and a possible transition into a sudden stratospheric warming (SSW) the following week.
Those two statements are not in conflict, and the gap between them is the story. A weakening stratospheric polar vortex is the kind of event whose effects, by CPC’s account, can last up to two months. It does not by itself deliver cold air to Chicago next week, and the official forecast for the next two weeks is built on a different pattern.
| What is happening | What is not established |
|---|---|
| Model guidance signals a potential stratospheric polar-vortex disruption. CPC’s 31 October discussion cites model forecasts of a disruption and a possible transition into an SSW the following week. | A major SSW has not been observed as of that discussion. It is a forecast, and CPC says “potential” and “possible.” |
| The Midwest is favored warm through mid-November. Above-normal temperatures are the favored category for Illinois and the other Midwest states for November 7 to 11 and 9 to 15. | That the rest of the month, or the winter, will be warm. The outlooks end on November 15, and CPC’s monthly discussion flags added uncertainty for the eastern U.S. |
| A later negative-AO response is plausible. CPC says SSW impacts usually appear as a negative AO and that models already show one. | That a Midwest Arctic outbreak is imminent. No CPC product in this analysis forecasts one, and CPC says impacts can take up to two months. |
CPC issued its 6-10 day and 8-14 day prognostic discussions at 3 p.m. Eastern on November 1. Above normal is the favored temperature category for Illinois, Wisconsin, Michigan, Iowa, Minnesota, Missouri, Indiana, Ohio and the Dakotas in both windows. CPC also favors below-normal precipitation for much of the Great Lakes and the Mississippi Valley. Forecast confidence is rated 3 on a scale of 1 to 5 for each window, which CPC labels average, and it cites fairly good agreement among the model height forecasts but conflicting or weak precipitation signals.
| State | November 7 to 11 temperature | November 9 to 15 temperature | Precipitation (6-10 day, 8-14 day) |
|---|---|---|---|
| Illinois | Above normal | Above normal | Below, below |
| Wisconsin | Above normal | Above normal | Below, below |
| Michigan | Above normal | Above normal | Below, below |
| Minnesota | Above normal | Above normal | Below, below |
| Iowa | Above normal | Above normal | Below, near median |
| Missouri | Above normal | Above normal | Below, below |
These are categories, not magnitudes. Above normal is the favored outcome, not a certainty, and CPC’s state tables do not give a probability for each Midwest state. The text singles out the Northern Plains, with probabilities above 80 percent in the 6-10 day outlook and the highest confidence in the 8-14 day outlook.
CPC describes a mean 500-hPa flow broadly consistent with the positive phase of the Pacific-North American pattern, known as the PNA. A deep, slow-moving trough sits over the northeastern Pacific south of mainland Alaska, with a second trough near the Aleutians sliding southeastward to reinforce it. Downstream, anomalous ridging over the western U.S. moves east toward the High Plains, and above-normal heights consolidate over central Canada and the Northern Plains by the end of the 6-10 day period. In the 8-14 day period CPC expects the Pacific trough to strengthen and the ridge to stay consolidated over central Canada and the Northern Plains.
The ridge is only part of the map. In the southern stream, CPC forecasts a trough over the southern Plains early in the period that deepens toward the lower Mississippi Valley and the Southeast and reaches the East Coast around day 10. The text does not claim the ridge covers the whole Midwest; the warm forecast for Illinois and the Great Lakes comes from CPC’s state and map products rather than from a sentence about the Midwest in the discussion. CPC’s monthly discussion also calls the ongoing El Niño the dominant teleconnection behind the forecast and expands above-normal temperature probabilities of 50 to 60 percent across the northern tier.
In its 31 October discussion CPC wrote that model forecasts indicate the potential for a stratospheric polar vortex disruption and a possible transition into an SSW the following week. It calls a November SSW very rare and says that, should one occur, its impacts can be seen for up to two months, usually as a negative phase of the Arctic Oscillation (AO), which it says can already be seen in models. CPC does not say an SSW has occurred.
CPC’s sudden stratospheric warming monitoring page plots the 10 hPa zonal-mean wind from GFS and GEFS analyses and forecasts. The point for now is simple: the event remains forecast, not observed, and what the monitoring plots show on any given day decides which sentence in the table above applies.
Three things stand between a disrupted vortex and cold air at the surface. The disruption has to become an observed major warming, the signal has to propagate downward into the troposphere, and the resulting pattern has to deliver Arctic air to a particular region. CPC allows for impacts of up to two months after an SSW.
The competing influence is the one CPC names. A negative AO, which an SSW tends to bring, runs against the forecast PNA phase, which is more in line with El Niño. CPC says the two compete in the eastern U.S. and add some uncertainty there. It does not name the Midwest in that sentence, and it does not say the competition will resolve toward cold.
It does not show that the polar vortex event is overhyped: a November SSW would be rare and its effects could matter for weeks. It does not show that the rest of November or the winter will be warm. The outlooks end on November 15, the ridge is a forecast, and CPC rates confidence as average. It does not show what SSW impacts will be for any particular city. We read CPC’s products as issued on November 1 and October 31, and CPC updates the 6-10 and 8-14 day outlooks daily, so what is current may differ by the time you read this.
CPC’s 6-10 and 8-14 day outlooks and their discussions are posted daily, and CPC’s sudden stratospheric warming monitoring page plots the 10 hPa wind at 60N from the GFS analysis and forecasts. Compare each new outlook with the state tables above. The AO index and CPC’s Week 3-4 outlook update on November 6 are the later checkpoints.
Our reading. For November 7 to 15, CPC’s forecast is built on the PNA pattern, and in that forecast the ridge is winning the first round. A disrupted polar vortex changes the odds for later in the month and into winter, and CPC says as much, but it has not been observed yet and CPC does not promise a Midwest cold burst. The test is the three checkpoints above.