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The State Climatologist Office for Illinois says the all time low temperature of Illinois was -36 °F, with a report of -37 °F in 2009 being disqualified.

This morning, the station Sinnissippi Point reported:

6:00 AM -37.6 °F
6:15 AM -37.8 °F
6:30 AM -38.3 °F
6:45 AM -38.5 °F
7:00 AM -38.4 °F
7:15 AM -38.9 °F
7:30 AM -38.7 °F
7:45 AM -39 °F
8:00 AM -37.7 °F

Galena was slightly lower reporting:

6:48 AM -39 °F
6:53 AM -39 °F
6:59 AM -39.2 °F
7:04 AM -39 °F
7:09 AM -39.2 °F
7:14 AM -39 °F
7:19 AM -39 °F

and other stations (such as North Fork Plum River: -36.8 °F; Hawkeye Observatory: 37.4 °F) are reporting temperatures near -37 °F.

Was a new record set this day? Or is there some reason to disqualify or consider unofficial today's reports.

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The record is unlikely to change, at least based on the particular measurements provided in the initial question.

According to the Illinois State Climatologist Office at the link you provided, the -37°F temperature record was discarded for lack of quality assurance: "However, that observation came from a small airport station designed for aviation purposes and was not part of the climate network in Illinois. As a result, the data were not always archived and no quality control procedures were applied to the data."

From the URLs in your question, the recent weather observations you refer to exist within Weather Underground's network of personal weather stations. In other words, they are often relatively inexpensive sensors that are owned and maintained by private individuals, and their measurements will not have the accuracy required to be included in the official temperature records for the state. The actual temperature at the Sinnissippi Point station may or may not have been colder than Illinois' verified lowest temperature, but without additional info (e.g. knowledge of specific instruments/sites and operator, as implied in the answer by @DavePhD) the official record will not change based on that measurement.

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    $\begingroup$ My understanding is that the surface observational data assimilated into operational forecast models typically comes from only a set of "official" stations with at least some known maintenance/quality control criteria, not the set of all possible weather stations. That's mainly based on my recollection of what I've learned from the US forecast model community, and based on results from some searching (e.g. the UK community: rmets.onlinelibrary.wiley.com/doi/full/10.1002/qj.2372) it seems to hold up at least to some extent. But that's probably a topic worthy of its own entire question. $\endgroup$ – dplmmr Jan 31 at 17:18
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    $\begingroup$ This report of -35 °F wrh.noaa.gov/mesowest/… looks more official. But that doesn't specify what the minimum was, only reports at 20 minute intervals. $\endgroup$ – DavePhD Jan 31 at 17:41
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    $\begingroup$ That looks like the same class of Automated Weather Observing System (AWOS) mentioned by the State Climatologist Office for the -37 °F measurement, and the reports around that time don't have any error codes. So a site like that could conceivably produce an official record low temperature if it passed a more detailed quality control analysis (although in this case I wouldn't be surprised if it doesn't get any special attention since it wouldn't quite make a major new record). $\endgroup$ – dplmmr Jan 31 at 17:50
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    $\begingroup$ @gansub it's worth noting, the new IBM global modeling effort is supposed to more heavily use such independent sensors more... will thus be something interesting to compare against and see if it is helpful or harmful to forecasts $\endgroup$ – JeopardyTempest Feb 1 at 16:45
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According to the Washington Post:

The cold snap is smashing all-time records in Northern Illinois. The state could have a new low temperature record as of Thursday morning, after Mount Carroll plummeted to minus-38 Thursday morning. The temperature was measured by a trained weather observer, according to the National Weather Service. The data will be examined by the Weather Service and the state climatologist to be certified as Illinois' lowest temperature.

National Weather Service lists -38 °F as the low at Mount Carroll here:

https://w2.weather.gov/climate/xmacis.php?wfo=dvn

Mount Carroll held the record from 1930 to 1999.

Update from https://stateclimatologist.web.illinois.edu/2019/03/06/il/:

After a comprehensive review, the State Climate Extremes Committee (SCEC) unanimously voted to validate the -38 degrees reading as the new official state record minimum temperature. This committee ensures that the observation is meteorologically plausible, is within a range that the reporting instrument can detect, and that the instrument is in proper working order.

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  • $\begingroup$ Very interesting, and I'll be curious to see what comes of that! I also edited a couple points in my own answer to clarify that these general networks of personal weather stations could include more reliable data, additional information is needed for quantitative use of any particular measurements. $\endgroup$ – dplmmr Jan 31 at 18:38
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    $\begingroup$ @dplmmr in the OP, two of the stations used Davis Vantage Vue and one used AcuRite Pro Weather Center instruments $\endgroup$ – DavePhD Jan 31 at 18:41
  • $\begingroup$ Good info, I'd assume the characteristics will be checked, along with things like siting location that have a big impact on the measurements (with some confidence in things like that given by the owner being a trained observer). $\endgroup$ – dplmmr Jan 31 at 19:15
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    $\begingroup$ @dplmmr if the state climatologist considers a measurement to be a possible state record, this committee is supposed to investigate ncdc.noaa.gov/extremes/scec $\endgroup$ – DavePhD Jan 31 at 21:50
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    $\begingroup$ @dplmmr mycarrollcountynews.com/news/… that's the latest info $\endgroup$ – DavePhD Feb 6 at 16:47

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