Short Answer

More than 2 inches of rain in Chicago by the end of September 2026 is considered likely by the market (99.0%). Official NWS data, however, shows cumulative rainfall at 1.75 inches as of September 20, 2026, and the Climate Prediction Center forecasts below-normal precipitation for the remainder of September, which significantly reduces the likelihood of reaching higher thresholds.

1. Market Behavior & Drivers

Market probability surged mid-month based on weather reporting. A 31.0 percentage point spike on September 17 coincided with significant thunderstorms and heavy rainfall. The price climbed further to 97.0% on September 19 following widespread news of multi-day downpours and flood advisories over that weekend.
The price then corrected downward on September 20. This 12.0 percentage point drop appears to be a reaction to official meteorological data showing actual rainfall totals were less severe than weather reports may have suggested. As of September 21, cumulative precipitation for the month was below normal. An earlier 20.0 point spike on September 15 lacks a clear news driver, though it followed forecasts for a "wet weekend".
  • Reaching above 2 inches appears likely, with 1.75 inches recorded at O'Hare as of September 20, 2026.
  • Surpassing 3 inches is improbable given the below-normal precipitation forecast for the remainder of September.
  • Accumulating above 4 inches has low probability, compounded by below-normal precipitation forecasts for September.

Who Wins and Why

Outcome Market Model Why
Above 3 inches 76.0% 95.8% Accumulating 1.25 additional inches is highly improbable given forecasts for below-normal precipitation.
Above 4 inches 5.0% 24.8% Forecasts for below-normal precipitation further diminish the likelihood of accumulating 2.25 additional inches.
Above 5 inches 1.0% 5.0% Below-normal precipitation forecasts significantly reduce the likelihood of reaching these higher thresholds.
Above 2 inches 99.0% 100.0% Cumulative rainfall is 1.75 inches, with below-normal precipitation forecast for the remainder of September.
Above 6 inches 2.0% 2.0% Below-normal precipitation forecasts significantly reduce the likelihood of reaching these higher thresholds.

Current Context

Chicago precipitation trails normal for September's first three weeks. As of September 21, 2026, Chicago's cumulative rainfall remains below the monthly normal. The National Weather Service records total precipitation for the first three weeks of September 2026 at approximately 1.90 inches at Midway Airport [1][2] and 0.26 inches at Chicago Executive Airport [1][2]. As of September 20, O'Hare Airport recorded 1.75 inches [1][2][3]. The monthly normal precipitation for Chicago in September, based on the 1991-2020 period, is 3.19 inches [4].
Localized, light rain has characterized September's precipitation patterns. Precipitation patterns have featured scattered, light rain showers across the area [1][2]. A notable daily rainfall of 1.79 inches occurred at Midway on September 17, 2026 [1][2]. No snowfall has been recorded in Chicago for September 2026 as of September 21, 2026 [1][2].
September began with an abnormally hot and dry period. The month started with an abnormally hot and humid period from September 1 to September 4, 2026 [5]. Temperatures reached 90 degrees Fahrenheit or above for four consecutive days [5]. During this time, the Chicago area largely remained south of major thunderstorm activity occurring along the periphery of a heat dome [5].
Sources (5)
  1. 1National Weather Servicepreview-forecast.weather.gov
  2. 2National Weather Serviceforecast.weather.gov
  3. 3National Weather Serviceforecast.weather.gov
  4. 4September Normals for Temperature and Precipitation for Chicagoweather.gov
  5. 5September 1-4, 2026: Abnormally Hot and Humid Start to Septemberweather.gov

2. Price Chart

Historical Price (Probability)

Outcome probability
Date

3. Significant Price Movements

Notable price changes detected in the chart, along with research into what caused each movement.

Outcome: Above 4 inches

📉 September 21, 2026: 16.0pp drop

Price decreased from 21.0% to 5.0%

What happened: The primary driver of the price drop was the release of official climate data on or around September 21, 2026, which reported Chicago's month-to-date precipitation through September 20, 2026, was between 1.75 inches [1] and 2.96 inches [2]. Despite heavy rainfall on September 19-20 contributing approximately 1.74 inches to Chicago's total [3], these accumulated figures were significantly below the 4-inch outcome target for the month and even below the normal September total of 3.19 inches [4]. This traditional news and data release coincided with the price move, leading market participants to perceive the "Above 4 inches" outcome as substantially less likely. Social media was not identified as a primary driver based on the provided information.

📉 September 20, 2026: 61.0pp drop

Price decreased from 82.0% to 21.0%

What happened: The primary driver for the 61.0 percentage point drop was the actual meteorological data released on or around September 20, 2026. Chicago-O'Hare International Airport recorded 1.21 inches of precipitation, significantly below the "Above 4 inches" threshold [2][5]. While heavy rain impacted the area, widespread totals were generally below 4 inches, an extreme amount for a single day [3][6][4]. This objective weather data directly reduced the perceived probability of the "Above 4 inches" outcome. Based on the provided information, social media activity was irrelevant.

📈 September 19, 2026: 22.0pp spike

Price increased from 60.0% to 82.0%

What happened: The 22.0 percentage point price spike was primarily driven by widespread traditional news reports of significant, multi-day heavy rainfall and thunderstorms affecting the Chicago area over the weekend of September 19-20, 2026, which included flood advisories [3][7]. This general reporting of severe weather likely created anticipatory sentiment among market participants, increasing perceived odds for the "Above 4 inches" outcome. However, official rainfall totals for September 19, 2026, were significantly lower, recording less than 2 inches in most areas and well below the 4-inch threshold [3][2][1][8][9][10]. Social media activity cannot be assessed as a primary driver, contributing accelerant, or noise due to a lack of provided information.

Outcome: Above 3 inches

📈 September 17, 2026: 31.0pp spike

Price increased from 58.0% to 89.0%

What happened: The primary driver of the 31.0 percentage point spike in the "Above 3 inches" prediction market was likely a combination of widespread news of heavy rainfall and potential confusion with past events. On September 17, 2026, the Chicago area experienced significant thunderstorms, heavy rainfall, and flood advisories, leading to flight delays [11][12][13][14]. However, official rainfall totals at major Chicago airports, such as Midway, were below the 3-inch threshold (1.79 inches), although some areas like Aurora reported around 3 inches [11][12][14]. It appears probable that market participants either overestimated the totals from the 2026 storms or confused the event with a distinct September 17, 2023, occurrence which did feature significant flash flooding and 3-plus inch rainfall in the Chicago area [15]. As no relevant social media activity was found, it was irrelevant to this market movement.

📈 September 15, 2026: 20.0pp spike

Price increased from 43.0% to 63.0%

What happened: There is no evidence of social media activity or specific traditional news directly correlating to a 20.0 percentage point spike for "Above 3 inches" of rain on September 15, 2026. While initial rainfall occurred on September 15 and forecasts mentioned a "wet weekend" [16][17][18][19], actual daily precipitation was only 0.09 inches [20][9][21][1][10]. The significant heavy rain event (2-3 inches) occurred later, on September 19-20, 2026 [22][23][24]. Therefore, the market movement appears disconnected from immediate, accurate weather conditions for the specified date and outcome, and social media was irrelevant.
Sources (24)
  1. 1National Weather Serviceforecast.weather.gov
  2. 2CLI from KLOTkamala.cod.edu
  3. 3Here are the latest rainfall totals from the Chicago area – NBC Chicagonbcchicago.com
  4. 4September Normals for Temperature and Precipitation for Chicagoweather.gov
  5. 5CLI from KLOTkamala.cod.edu
  6. 6September Precipitation Rankings for Chicagoweather.gov
  7. 7Chicago under flood advisory, rain cancels Chicago River Swim - Chicago Sun-Timeschicago.suntimes.com
  8. 8National Weather Servicemarine.weather.gov
  9. 9National Weather Servicepreview-forecast.weather.gov
  10. 10National Weather Serviceforecast.weather.gov
  11. 11Morning storms blamed for nearly 1,000 delays at Chicago airports - Chicago Sun-Timeschicago.suntimes.com
  12. 12National Weather Serviceforecast.weather.gov
  13. 13Ground stop issued for O’Hare, Midway due to thunderstorms – NBC Chicagonbcchicago.com
  14. 14Illinois, Indiana Minor Flood Advisory | Effective 09/17/2026 05:35 | GovOneStopgovonestop.com
  15. 15September 17, 2023: Significant Flash Flooding in the Near South Suburbs of Chicagoweather.gov
  16. 16Chicago weather: Rain and storms threaten morning commute – NBC Chicagonbcchicago.com
  17. 17Chicago’s Last Summer Weekend Could Be — Surprise — Yet Another Wet Oneblockclubchicago.org
  18. 18Rain returns to Chicagoland after unusually dry start to September | FOX 32 Chicagofox32chicago.com
  19. 19CLI from KLOTkamala.cod.edu
  20. 20National Weather Serviceforecast.weather.gov
  21. 21IEM:: ARR Data Calendar for Sep 2026mesonet.agron.iastate.edu
  22. 22Chicago weather: Heavy rain, thunderstorms expected this weekendnbcchicago.com
  23. 23Chicago Airports Cancel Flights as Storms Raise Flood Risk - Bloombergbloomberg.com
  24. 24Chicago weather: Heavy rains Sunday morningbancodeprofissionais.com

4. Market Data

Contract Snapshot

For the "Rain in Chicago this month? Above 3 inches" contract, a YES resolution triggers if total rainfall exceeds 3 inches, and a NO resolution triggers if it is 3 inches or less. The maximum payout date for this contract is Sep 30, 2026. As of September 4th, an amendment to the contract terms includes The Weather Company as a source agency for settlement data.

Available Contracts

Market options and current pricing

Outcome bucket Yes (price) No (price) Last trade probability
Above 2 inches $1.00 $0.01 99%
Above 1 inch $1.00 $0.01 98%
Above 3 inches $0.82 $0.23 76%
Above 4 inches $0.08 $0.93 5%
Above 6 inches $0.05 $1.00 2%
Above 5 inches $0.02 $1.00 1%
Above 7 inches $0.01 $1.00 1%

Market Discussion

Preliminary data confirms rain in Chicago during September 2026, with Chicago-Midway reporting a total monthly precipitation of 1.90 inches and Chicago Executive Airport recording 0.26 inches [1][2]. The month was notably warmer than normal, with an early September heat dome largely suppressing thunderstorm activity [1][2][3].

Sources (3)
  1. 1National Weather Servicepreview-forecast.weather.gov
  2. 2National Weather Serviceforecast.weather.gov
  3. 3September 1-4, 2026: Abnormally Hot and Humid Start to Septemberpreview.weather.gov

5. Trust Index

Octagon Trust Index Kalshi 77 Good
How it adds up
Integrity80% of score82Good
Trade quality20% of score55Caution

Includes the cost to trade: a $1,000 order can't be filled here because the order book is too thin.

Trust score77Good

Weighted blend with hard caps — a critically weak safety pillar, or a severe trading anomaly, caps the total regardless of the rest. Full methodology · About the Trust Index

Trust profile
Integrity4 screens run · 6 don't apply
Market integrity
77Good

Outcome is hard to influence

No suspicious activity on any screen

Screened clean
Large tradesNo single trade dominates the tape. Clear
Thin-volume movesMoves carried volume Clear
Size patternsNo distribution read Clear
2 screens not run
Outcome control · doesn't apply to this market type
Thin-leg conviction · doesn't apply to this market type
Info fairness
80Good

Structural prior — no screens apply

No suspicious activity on any screen

Screened clean
Information exposureNo concentrated group can know the outcome materially early. Clear
4 screens not run
Move vs. catalyst · doesn't apply to this market type
Longshot repricing · doesn't apply to this market type
Pre-broadcast certainty · doesn't apply to this market type
Cross-venue timing · doesn't apply to this market type
Resolution quality
92Strong

Resolution mechanism is dependable

No suspicious activity on any screen

Resolution rules read: Clear

Liquidity
74Tradeable

Fills fine at modest size

7 contracts · worst first

Above 6 inches4
Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Above 5 inches5
Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Above 4 inches24
Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Above 7 inches31
Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Above 3 inches76
Tradeable · high confidence

Typical bar range 4.8% of price over 22 traded bars

Above 2 inches83
Deep · low confidence

Light recent trading — size may not fill near the screen price.

Above 1 inch—
No recent trading to measure

No recent trading to measure

Move quality
41Mixed

Some moves lacked volume

7 contracts · worst first

Above 3 inches39
Mixed · high confidence

Volume opposes the move over 6h

Above 7 inches64
Mostly confirmed · high confidence

Volume is light/neutral for the move over 6h

Above 4 inches66
Mostly confirmed · high confidence

Volume opposes the move over 6h

Above 6 inches71
Confirmed · medium confidence

Volume confirms the move over 24h

Above 5 inches78
Confirmed · high confidence

Volume confirms the move over 6h

Above 2 inches—
No significant move (6h)

Price was stable over the last 6 hours.

Above 1 inch—
Only 0 candle closes

Only 0 candle closes

Rule clarity
100Clear

Counts toward Resolution quality, not here

7 contracts · worst first

Above 2 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 1 inch100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 3 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 4 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 6 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 5 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 7 inches100
Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Trade quality
55Caution

Order book is thin

Tradable, but expect execution cost beyond small sizes.

Outcome quality 69 (notional-weighted across 6 scored legs)

$100 order: 8.3c slippage (~10% of a 79.5c contract)

$1,000 order: book too thin to fill

$10,000 order: book too thin to fill

Above 2 inches83Market scoreLiquidity83Move quality–Rule clarity100
Why 83
Liquidity
83 · Deep · low confidence

Light recent trading — size may not fill near the screen price.

Move quality
No significant move (6h)

Price was stable over the last 6 hours.

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 1 inch–Market scoreLiquidity–Move quality–Rule clarity100
Why this score
Liquidity
No recent trading to measure

No recent trading to measure

Move quality
Only 0 candle closes

Only 0 candle closes

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 3 inchesPrimary65Market scoreLiquidity76Move quality39Rule clarity100
Why 65
Liquidity
76 · Tradeable · high confidence

Typical bar range 4.8% of price over 22 traded bars

Move quality
39 · Mixed · high confidence

Volume opposes the move over 6h

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 4 inches24Market scoreLiquidity24Move quality66Rule clarity100
Why 24
Liquidity
24 · Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Move quality
66 · Mostly confirmed · high confidence

Volume opposes the move over 6h

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 6 inches4Market scoreLiquidity4Move quality71Rule clarity100
Why 4
Liquidity
4 · Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Move quality
71 · Confirmed · medium confidence

Volume confirms the move over 24h

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 5 inches5Market scoreLiquidity5Move quality78Rule clarity100
Why 5
Liquidity
5 · Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Move quality
78 · Confirmed · high confidence

Volume confirms the move over 6h

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear
Above 7 inches31Market scoreLiquidity31Move quality64Rule clarity100
Why 31
Liquidity
31 · Very thin · low confidence

Light recent trading — size may not fill near the screen price.

Move quality
64 · Mostly confirmed · high confidence

Volume is light/neutral for the move over 6h

Rule clarity
100 · Clear · high confidence

All checks pass

objective outcome · Clearofficial source · Cleartime boundary · Clear

underwriting_v1.4 · computed Sep 21, 2026

6. What upcoming weather systems are forecast by the NWS and CPC for late September 2026, and what is their potential to push Chicago's monthly rainfall total above the 3.19-inch average?

Sep 2026 Rainfall (as of Sep 20)4.19 inches [1][2][3]
Normal Sep Monthly Average3.19 inches [1][2][3]
Late Sep 2026 Precipitation ForecastBelow-normal [4][5][6]
Chicago has already exceeded its average September rainfall total. As of September 20, 2026, Chicago has recorded at least 4.19 inches of rainfall for the month, which already surpasses the normal monthly average of 3.19 inches [1][2][3]. Significant rainfall events contributed to this total, notably 3.11 inches on September 17 and an additional 0.54 inches on September 20 [1]. This early accumulation pushed the monthly total above historical averages, addressing the initial question even before the final week of the month [1].
Late September forecasts predict below-normal precipitation for Chicago. The Climate Prediction Center forecasts a tilt toward below-normal precipitation for the Great Lakes region, including Chicago, for late September 2026 (September 23–29) [4][5][6]. This forecast is attributed to an anticipated Omega-like pattern with a strong ridge, leading to surface high pressure [4][5][6]. Given that the monthly total has already exceeded the average, the forecast for below-normal precipitation in the remaining days does not indicate a potential to push the total above the average, as it has already been surpassed [1][4][5][6]. Prediction markets also indicate a 92.0% probability that Chicago will receive above 1 inch of rain for the entire month of September 2026 [7].
Sources (7)
  1. 1IEM:: ARR Data Calendar for Sep 2026mesonet.agron.iastate.edu
  2. 2September Normals for Temperature and Precipitation for Chicagoweather.gov
  3. 3September Precipitation Rankings for Chicagoweather.gov
  4. 4Climate Prediction Center - 6-10 and 8-14 Day Prognostic Discussionscpc.ncep.noaa.gov
  5. 5Climate Prediction Center - 6-10 and 8-14 Day Prognostic Discussionscpc.ncep.noaa.gov
  6. 6Probabilistic Hazards Outlookcpc.ncep.noaa.gov
  7. 7Trader | Paritypredictparity.com

7. How does Chicago's cumulative rainfall as of September 21, 2026, compare against the 1991-2020 NWS climatological normal for this point in the month?

September 2026 Cumulative Rainfall2.96 inches (as of Sept 20, 2026) [1]
September 2026 Climatological Normal2.16 inches (as of Sept 20, 2026) [1]
September 2026 Rainfall Above Normal0.80 inches (as of Sept 20, 2026) [1]
Chicago's cumulative September rainfall exceeds the climatological normal. As of September 20, 2026, Chicago recorded 2.96 inches of cumulative rainfall for the month. This amount was 0.80 inches above the 1991-2020 National Weather Service (NWS) climatological normal of 2.16 inches for the same period [1].
Current precipitation trends indicate above-average conditions for the year. The 1991-2020 NWS climatological normal for expected cumulative precipitation in Chicago (O'Hare) through September 20 is approximately 2.05 inches [2][3]. For the entire month of September, the 1991-2020 NWS climatological normal for precipitation is 3.19 inches [2][3]. Furthermore, Chicago's year-to-date precipitation as of September 19, 2026, measured 32.72 inches, which was 3.96 inches above the normal of 28.76 inches for that specific period [4].
Sources (4)
  1. 1CLI from KLOTkamala.cod.edu
  2. 2Chicago (O'Hare) 1991-2020 Climate Normalsweather.gov
  3. 3September Normals for Temperature and Precipitation for Chicagoweather.gov
  4. 4National Weather Serviceforecast.weather.gov

8. How do the September 2026 rainfall totals from Chicago's O'Hare and Midway airports compare, and what does historical NWS data show about their typical variance?

Preliminary Sep 2026 Rainfall - Midway1.90 inches (as of Sep 19, 2026) [1]
Avg Sep Precipitation (1981-2010)Midway 3.31 inches, O'Hare 3.21 inches [2][3][4]
Sep Precipitation Climate Normal (1991-2020)3.19 inches [5]
Preliminary September 2026 rainfall data shows 1.90 inches for Midway. As of September 19, 2026, preliminary rainfall for Chicago-Midway Airport totaled 1.90 inches for the month of September [1]. The available information does not yet include the September 2026 rainfall total for Chicago O'Hare Airport. Separately, Chicago Executive Airport recorded a preliminary total of 0.26 inches for September 2026 [6]. Furthermore, a prediction market on September 21, 2026, assigned a 92% probability that Chicago would accumulate over 1 inch of rain in September 2026 [7].
Historically, average September precipitation shows minor differences between airports. Average September precipitation has been notably similar between Chicago O'Hare and Midway airports. Based on 1981-2010 climate normals, Chicago Midway typically received approximately 3.31 inches, while Chicago O'Hare averaged about 3.21 inches [2][3][4]. The broader climate normal for total monthly precipitation in September for Chicago, based on 1991-2020 data, is 3.19 inches [5].
Sources (7)
  1. 1National Weather Servicepreview-forecast.weather.gov
  2. 2Compare the Climate and Weather at Chicago Midway International Airport, Chicago O'Hare International Airport, and DuPage County Airport - Weather Sparkdeploy.weatherspark.com
  3. 3Chicago O'Hare International Airport Climate Normals | Illinois State Climatologist | Illinois State Water Survey | Prairie Research Institutestateclimatologist.isws.illinois.edu
  4. 4Chicago Midway Climate Normals | Illinois State Climatologist | Illinois State Water Survey | Prairie Research Institutestateclimatologist.isws.illinois.edu
  5. 5September Normals for Temperature and Precipitation for Chicagoweather.gov
  6. 6National Weather Serviceforecast.weather.gov
  7. 7Trader | Paritypredictparity.com

9. What specific rainfall events, per NWS daily reports, correspond with the major market price movements between September 15 and September 20, 2026?

September 19 Precip (NWS O'Hare)0.86 inches [1]
24-hr Rainfall Sep 16-17 (NWS Midway)1.80 inches [2]
September 17 Precip (IEM Chicago/Aurora)3.11 inches [3]
Multiple rainfall events occurred in Chicago during mid-September 2026. While specific NWS daily reports for September 15 to September 20, 2026, do not contain daily precipitation totals for that period, other NWS data sources indicate several rainfall occurrences. The NWS climate summary for Chicago/O’Hare reported 0.86 inches of precipitation, including thunderstorms and heavy rain, on September 19, 2026 [1]. Additionally, preliminary NWS Midway September 2026 climate data indicated a greatest 24-hour rainfall total of 1.80 inches spanning September 16–17 [2]. An NWS forecast-maintenance product for September 19 also described widespread rain showers across the Chicago area [4].
Significant precipitation totals were recorded from September 16 to 19. The Iowa Environmental Mesonet (IEM) Chicago/Aurora ASOS calendar further corroborated these reports, recording 0.06 inches on September 16, 3.11 inches on September 17, and 1.36 inches on September 19 [3]. These observations highlight heavy rainfall around September 16–17 and another substantial event on September 19. The prediction market for 'Rain in Chicago in Sep 2026?' (condition 'Above 1 inch') was trading at 92.0 cents as of September 21, 2026, suggesting a high market expectation for total September rainfall to exceed one inch [5].
No direct correlation links rainfall to specific market movements. The provided facts do not contain information connecting these identified rainfall events to specific major market price movements within the specified September 15-20 window.
Sources (5)
  1. 1National Weather Serviceforecast.weather.gov
  2. 2National Weather Servicepreview-forecast.weather.gov
  3. 3IEM:: ARR Data Calendar for Sep 2026mesonet.agron.iastate.edu
  4. 4National Weather Serviceforecast.weather.gov
  5. 5Trader | Paritypredictparity.com

10. Based on NOAA's latest diagnostics, what is the current El Niño-Southern Oscillation (ENSO) phase and how does it typically influence September precipitation in the Great Lakes region?

Current ENSO PhaseEl Niño, strengthening [1][2][3]
Probability of Very Strong EventGreater than 90% (Northern Hemisphere fall and winter 2026-2027) [1][2][3]
NOAA El Niño AdvisoryIssued on September 10, 2026 [4][1][5][6][7][8][9][2]
El Niño is strengthening, with a high probability of becoming very strong. As of September 2026, the El Niño-Southern Oscillation (ENSO) phase is El Niño and it is currently strengthening. The NOAA's Climate Prediction Center (CPC) issued an El Niño Advisory on September 10, 2026, confirming its presence and intensification in the equatorial Pacific [4][1][5][6][7][8][9][2]. Sea surface temperature anomalies in the eastern equatorial Pacific have already exceeded +3.0°C [1][2][3], and there is a greater than 90% probability that this will develop into a very strong El Niño event during the Northern Hemisphere fall and winter of 2026-2027 [1][2][3].
El Niño's September precipitation influence is indirect for the Great Lakes. While ENSO significantly affects seasonal precipitation and temperature patterns across the United States, its most prominent impacts occur during the cool season, specifically November through March, in regions like the Great Lakes and Ohio Valley [10][11]. Consequently, El Niño's influence on September precipitation in the Great Lakes region is less direct and does not exhibit the same strong, consistent teleconnection patterns typically observed in the winter months [10][11].
Sources (11)
  1. 1https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.txtcpc.ncep.noaa.gov
  2. 2Current Status and Predictionscpc.ncep.noaa.gov
  3. 3Rare Strong PDO Negative El Niño Arrives - September 19, 2026econcurrents.substack.com
  4. 4Climate Prediction Center: ENSO Diagnostic Discussioncpc.ncep.noaa.gov
  5. 5Current Status and Predictionscpc.ncep.noaa.gov
  6. 6Global Ocean Monitoring: Recent Evolution, Current Status, andcpc.ncep.noaa.gov
  7. 7Synopsis: ENSO-neutral conditions may transition to La Niña cocpc.ncep.noaa.gov
  8. 8Climate Prediction Center: ENSO Diagnostic Discussioncpc.ncep.noaa.gov
  9. 9Climate Prediction Center: ENSO Diagnostic Discussioncpc.ncep.noaa.gov
  10. 10El Nino & the Lower Great Lakes and Ohio Valleyweather.gov
  11. 11ENSO Precipitation and Temperature Forecasts in the North American Multi-Model Ensemble: Composite Analysis and Validationweather.gov

11. What Could Change the Odds

Key Catalysts

Prediction platforms, including Kalshi and Polymarket, host active markets for short-term weather outcomes like daily rain and temperature, as well as long-term climate predictions such as global temperature anomalies [1][2]. These markets are utilized for both risk management and speculative trading [1][2].
Key catalysts for climate-related predictions include official data releases from agencies like NASA/NOAA, changes in El Niño/La Niña status, and extreme weather events [3][4]. These factors trigger high volatility in betting odds for monthly or yearly outcomes [3][4].
Recent precipitation in Chicago illustrates how extreme weather events can act as catalysts. As of September 20, 2026, Midway Airport recorded 1.90 inches of rain for the month to date, with a heavy rainfall event occurring on September 19-20, 2026 [5][6][7][8]. Chicago-O'Hare had recorded 1.75 inches for September 2026 as of September 19, 2026, which was below the normal of 2.05 inches for that period [5]. Local climatological data for September 2026 reported monthly precipitation totals of 0.26 inches at Chicago Executive Airport and 1.90 inches at Chicago-Midway [9][6].

Key Dates & Catalysts

  • Expiration: October 16, 2026
  • Closes: October 01, 2026
Sources (9)
  1. 1Temperature check: alarm as prediction markets lean into weather...theguardian.com
  2. 2Climate Prediction Markets & Weather Odds | Kalshikalshi.com
  3. 3September 2026 Temperature Increase (ºC)polymarket.com
  4. 4Will 2026 be the hottest year ever? | Prediction Markets | Coinbasecoinbase.com
  5. 5National Weather Serviceforecast.weather.gov
  6. 6National Weather Servicepreview-forecast.weather.gov
  7. 7A soggy, sloppy, soaked ending for Chicago's summer | WGN Weekly Climate Reportwgntv.com
  8. 8Here are the latest rainfall totals from the Chicago area – NBC Chicagonbcchicago.com
  9. 9National Weather Serviceforecast.weather.gov

13. Historical Resolutions

Historical Resolutions: 20 markets in this series

Outcomes: 10 resolved YES, 10 resolved NO

Recent resolutions:

  • KXRAINCHIM-26AUG-10: NO (Sep 01, 2026)
  • KXRAINCHIM-26AUG-9: NO (Sep 01, 2026)
  • KXRAINCHIM-26AUG-8: NO (Sep 01, 2026)
  • KXRAINCHIM-26JUL-10: NO (Aug 01, 2026)
  • KXRAINCHIM-26JUL-9: NO (Aug 01, 2026)