Understanding NWS Data & Reports

What is a DSM? (Daily Summary Message)

The Daily Summary Message (DSM) from the National Weather Service (NWS) provides a concise summary of key weather conditions observed up to a certain point in the day. It's a public communication tool for current weather and significant changes.

Key Elements Often Included:

High and Low Temperatures: Recorded high/low temperatures for the period covered so far. Crucially for traders, DSMs often report the highest temperature observed up to the report's issuance time.
Precipitation Information: Details on rain, snow, etc.
Wind and Conditions: Wind speed/direction, general conditions (sunny, cloudy).
Warnings and Advisories: Active weather alerts.
Significant Weather Events: Notes on storms, temperature extremes, etc.

(See the Market Bots page for specific DSM issuance times per city).

What is a CLI? (Climatological Report)

The Climatological Report (Daily), or CLI, is the official daily summary used by platforms like Kalshi to resolve temperature markets. It provides the final, quality-controlled high and low temperatures for the official reporting period (usually 12:00 AM to 11:59 PM Local Standard Time).

Important Considerations:

Official Source: This is the key document for market settlement.
Quality Control: Data in the CLI has undergone NWS review. Data seen in real-time (like Time Series) is preliminary and subject to change. Faulty sensor readings might be adjusted or removed in the final CLI.
Timing: The final CLI covering the full day is typically issued sometime after the reporting period ends. Mid-day or preliminary CLIs exist but are not used for final settlement.

(See the City Resources page for links to CLI reports and the summary table for daily counts).

ASOS Temperature Rounding Process

Automated Surface Observing Systems (ASOS) continuously measure temperature, but the way data is reported can introduce rounding differences, particularly between hourly and 5-minute reports.

Continuous Measurement: ASOS records 1-minute temperature averages.
Hourly Reporting Rounding: For official hourly reports (METARs, often seen around :51-:54 past the hour), the temperature is typically rounded to the nearest 0.1°F before internal conversion to Celsius for transmission. When converted back to Fahrenheit for display, only minor rounding differences may occur. These are generally more precise.
5-Minute Reporting Rounding (Common on Time Series): For stations providing 5-minute interval data, ASOS often rounds the 1-minute average to the nearest whole degree Fahrenheit first before converting to Celsius. This initial rounding to the whole degree introduces larger potential discrepancies compared to the actual measured value.

Key Impact: Hourly reports generally offer higher precision. The values seen in 5-minute NWS Time Series data have undergone rounding to the nearest whole degree F first, creating ambiguity about the *exact* temperature observed during that minute. This is why understanding the NWS Timeseries Rounding Formula is important.

NWS Timeseries Rounding Formula Explained

The temperature values displayed in NWS Time Series lists and graphs often undergo a specific conversion and rounding process, leading to potential ambiguity, especially for 5-minute data points. Here's the typical process described by the community:

The OMO is originally recorded in a whole F°
The OMO F° gets converted to C°
The C° gets rounded to the nearest whole C°
The rounded C° gets converted back to F°
The F° gets rounded to the nearest F°

* Step 4 is what gets displayed publicly on the NWS Timeseries Graph
* Step 5 is what gets displayed publicly on the NWS Timeseries List

Why this matters: A displayed value (e.g., 70°F on the list) could potentially originate from an actual measurement slightly different (e.g., 69.5°F or 70.4°F) due to this multi-step rounding process involving Celsius conversion. Understanding this helps interpret the potential range of the true temperature.

NWS Timeseries Guide: Understanding Reading Types

The NWS Time Series display provides valuable data but contains different types of readings with varying precision:

F° Hourly (e.g., XX:51 - XX:54): These are official hourly observations. They generally do not have the same rounding ambiguity as 5-minute readings and represent the temperature for that specific minute more accurately.
F° 5-Mins: Readings reported every 5 minutes (e.g., XX:00, XX:05, XX:10...). These do contain rounding ambiguity due to the process described above (rounding to whole F°, converting to C°, rounding C°, converting back to F°). Do not assume this is the exact temperature; use the rounding formula concept to understand the possible range.
F° OMO (One-Minute Observation): Temperatures are measured every minute, but most are not publicly displayed on the standard NWS Timeseries page. Accessing these often requires specific methods (like the OMO Bot described on the Market Bots page).
F° 6-Hourly Max: A maximum temperature reported within specific hourly readings (around 00Z, 06Z, 12Z, 18Z) covering the previous 6-hour period. This value incorporates OMOs. (See 6 Hour Bot info).
F° 24-Hourly Max: The maximum temperature for the previous 24 hours, often included in the hourly reading around 12:51-12:54 LST.
F° SPECI: Special observations generated between standard reporting times, usually triggered by significant changes in weather conditions (like visibility). These SPECI reports often contain exact temperature readings without the standard 5-minute rounding ambiguity.

Key Takeaway: Master the NWS Timeseries Rounding Formula concept to better interpret 5-minute data points and gain an edge.

What Are Weather Models? (NWP)

Numerical Weather Prediction (NWP) models are complex computer simulations that use mathematics and physics to forecast weather.

Data Input: They start with current atmospheric conditions gathered from weather stations, satellites, balloons, and radar.
Simulation: Equations governing atmospheric physics are applied to simulate changes in temperature, pressure, wind, and moisture over time.
Grid System: Calculations are performed on a 3D grid representing the Earth's atmosphere.
Processing Power: Supercomputers run trillions of calculations to generate forecasts.
Model Variety: Different models exist (e.g., GFS, ECMWF, NAM, HRRR) with varying geographical coverage (global vs. regional) and forecast lengths.
Forecaster Analysis: Meteorologists analyze outputs from multiple models to assess confidence and produce forecasts, as no single model is perfect.
Accuracy & Updates: Short-term forecasts (hours to a few days) are generally more reliable than long-term ones. Models are constantly updated with new data to improve accuracy.