The live KMDW temperature of record - NWS CLI basis for Chicago

What the KMDW daily high is

The daily high is the highest air temperature recorded at the KMDW ASOS sensor during a defined day window, reported in whole degrees Fahrenheit. The sensor reports in tenths of a degree Celsius; that value is converted and rounded half-up, which is the convention the National Weather Service uses in its published climate products. A reading of 17.8 °C settles as 64 °F.

How we read temperature — the precision ladder

Not all temperature data is equally precise, and the difference decides contracts. KMDW data arrives in four grades, and this page treats each accordingly:

• 1-minute / 5-minute reports — fastest, but coded in whole degrees Celsius. One coded degree spans 1.8 °F, wider than the 1 °F a contract settles on, so these values are shown as a range. Speed only, never a settlement figure.
• Hourly METAR remark (T group) — tenths of a degree Celsius, the first settlement-grade evidence and the value NWS carries into its climate products.
• DSM (Daily Summary Message) — the provisional official high, issued shortly after local midnight.
• CLI (Climatological Report) — the gold standard and final settlement value, computed from raw sensor data with no additional rounding layers.

The ladder matters because public temperatures are rounded more than once. A sensor recording 77.6 °F can surface on an app as 78.8 °F — rounded to 78 °F, converted to 26 °C, converted back — while the official summary settles at 78 °F. The inflated figure is a mirage created by compounding rounding. That is why a live reading here appears as a range until the tenth-degree remark confirms it, and why a coarse peak can never raise the settled maximum.

The settlement source

NWS CLI — the official NWS Climatological Report daily maximum, measured over the fixed Local Standard Time climate day and reported in whole degrees Fahrenheit.

Why this page and not the weather app on your phone

A consumer app shows a rounded forecast for a city-sized polygon, refreshed on its own schedule. A contract settles on one sensor, one day window, and one rounding rule. This page reads that single sensor directly, applies the exact day boundary each venue uses, and rounds the way the published climate product does — so the figure you see is the figure the market resolves against, not an approximation of it.

How the nowcast band is built

The projection starts from the official NWS hourly gridpoint forecast for KMDW, shifted by its own verified error against the last few hours of actual ASOS observations. That bias-corrected peak is blended with the running observed maximum, weighted by how much of the day's heating is still ahead, so the forecast leads in the morning and the observation takes over after the peak. With no forecast available the CLI product's normal max for the date is used instead, and the page says so. The band is sized from measured forecast error, and the running maximum is a hard floor: the day's high can stay flat or move up, never down. The nowcast is a model output, not an official forecast product. The 7-day outlook strip lower on the page is different: it shows unmodified weather.gov gridpoint highs with no bias correction, so its first cell can disagree with today's projection above.

Data provenance and latency

Live observations are taken from the KMDW official airport sensor over a low-latency relay that delivers the reports faster than the public feeds. The station transmits every 5 minutes; those routine reports code temperature in whole degrees Celsius, and only the hourly report carries the remark group with tenths of a degree. This page reads the remark group whenever it is present and, when it is not, shows the range of whole-degree answers the coded value allows rather than a decimal it cannot support. Every reading passes quality control for spikes, frozen sensors, and implausible values before it can set a running maximum; rejected readings are counted on the card. The official CLI product is read only to confirm settlement after the fact, never as a live observation source.

How we know the number is right

Accuracy here is measured, not asserted. A verification harness replays this exact pipeline over recent completed climate days and scores it against the MAXIMUM row of the published NWS Climatological Report, with the official ASOS one-minute archive as a second reference. The resulting exact-match rate, mean absolute error, signed bias, and per-day table are published on the Accuracy panel above and refreshed whenever the harness runs. Consumer weather apps publish no such comparison, because they are modelling a city polygon rather than reproducing one sensor's climate record.

Free and independent

CLI is free to use: no account, no signup, no paywall, no rate limits on the page itself. It is not affiliated with, endorsed by, or operated on behalf of any exchange, broker, or weather agency, so the methodology above is the only thing shaping the numbers.