NOAA replied with the below feedback:
Our GHCN-Daily POC Matt Menne has provided the following feedback in italics to your questions regarding Neshaminy Falls, PA's and other stations' data values and GHCN-Daily's QC of them:
"Please see my responses to the questions below (in italics):
1. Did NOAA really run a spatial consistency test on the TMIN value on October 2 for Neshaminy Falls?
Yes, the checks are run daily on all data automatically. As long as the minimum data requirements are met for a QC test, the test is applied a) If the answer is no, why not?
2. If a spatial consistency test was done and it passed QA does this mean regardless of the fact the value had a difference from nearby weather stations that exceeded 8 degrees Celsius is this due to the way the math behind the spatial consistency tests (both regression and corroboration) are structured?
As noted in Durre et al. 2010, (Table 4), the absolute value of the residual for the spatial regression check needs to exceed 8 deg C and the normalized residual must exceed 4.0 standard deviations using neighboring values for the day before, the day of and the day after the value being tested in order to fail the test. Likewise the spatial collaration test compares anomalies on the day before, the day of and the day after the value in question and the anomalies must differ by 10 deg C. If these conditions are not met, the value will pass the test.
a) In light of the examples above would it make sense to modify the math behind the spatial consistency tests?
The spatial checks are conservative by design to avoid flagging valid observations. Altering the thresholds would inevitably lead to an increase in legitimate values failing the checks. While we would like to catch only errors, the nature of these observations is such that no set of checks can be assured to flag only errors. The spatial consistency checks are particularly susceptible to flagging too many legitimate values, which is why the thresholds are set to be conservative.
3. Why did the July 11 example from South Jersey Regional airport fail the spatial consistency test yet the similar October 2 example from Neshaminy Falls did not? It is all a question of exceeding or not exceeding the thresholds described in Durre et a. 2010 (Table 4), assuming the criteria for having the minimum number of neighboring observations are met.
4. Do you think since the eastern United States had hot 90 degree plus Fahrenheit afternoon temperatures on October 2 that somehow that TMIN of 52 by Neshaminy Falls should have been caught by some test since it so "obviously" wrong?
We would certainly like to flag all errors, however the observational day for COOP observers is sometimes somewhat irregular if the observer is unable to read the thermometers at exactly the same time each day. This is why many of the daily quality checks consider a 3-day window centered on the day in question. Note in this example, that a front was nearby separating the very warm air in the mid-Atlantic from a cooler air mass in New England, and that much cooler temperatures followed on Oct. 3, which was likely a factor in the value passing the spatial check. In a case like this, NOAA's Datzilla process can be used to flag a value based on additional scrutiny and forensics.
5. Furthermore, not counting the unarchived November 1 TMAX of 73 and the December 11 TMIN of 15 for Neshaminy Falls that are obviously wrong I have 2 other recent archived examples from Neshaminy Falls. September 15 TMIN of 54 Fahrenheit (not sure about this one since it did rain).
October 1 TMIN of 53 degrees Fahrenheit (that is obviously wrong for the same reasons stated above for October 2).
For September 15 and October 1 for Neshaminy Falls the difference from the 2 nearby Trenton Mercer airport and South Jersey Regional airport weather stations was only 7-9 degrees Fahrenheit. Would it make "prudent" sense to make the test threshold even tighter or will this lead to more false positives/more workload/not feasible with budget constraints/etc. ?
Based on extensive testing and years of user feedback, we have opted for the thresholds in place that prevent over-flagging the data but require the occasional need to submit a "Datzilla" data ticket to requiring us to flag or unflag a particular value.
6. Also in regards to (4) note as stated there were two days in a row (October 1 and October 2) that the TMIN values were obviously wrong for Neshaminy Falls yet that was not enough to cause either the spatial regression or the spatial corroboration check tests to fail for at least one of those days?
As the response noted for the first question (3), is all a question of exceeding or not exceeding the thresholds described in Durre et a. 2010 (Table 4), assuming the criteria for having the minimum number of neighboring observations are met.
7. Finally note for South Jersey Regional airport the inaccurate TMIN of 28 degrees Fahrenheit on September 27 appropriately failed the spatial consistency test. (Neshaminy Falls and Trenton Mercer airport were 47 degrees and 50 degrees Fahrenheit respectively).
However the South Jersey Regional airport inaccurate TMAX of 94 degrees Fahrenheit on July 23, 2019 DID NOT fail the spatial consistency test.
It is an inaccurate value -- Neshaminy Falls And Trenton Mercer airport reported respectively 76 and 75 degrees Fahrenheit on that day (and thus the difference is at least 10 degrees Celsius for both cases).
Notice too no hourly temperature values were reported for that entire day for this ASOS station so there must have been some issue going on: https://www7.ncdc.noaa.gov/CDO/cdoselect.cmd
Regardless, since a TMAX of 94 was submitted why did it not fail the spatial consistency test?
Again, it is all a question of exceeding or not exceeding the thresholds described in Durre et a. 2010 (Table 4), assuming the criteria for having the minimum number of neighboring observations are met.
In Matt's reply to your Question #4 and #5, he has mentioned that we (NCEI) have the ability to flag (or unflag if it applies) a data value; doing so has to be based on supporting evidence to do so. For more recent data values such as these for the past data-year of 2019, we very much depend on the National Weather Service (NWS) and its Forecast Office at NWS-Mt. Holly, NJ (WFO-PHI) to inform us via the proper channels of whether these actions are necessary. We're going to depend on their expertise in the case of these stations' questioned data temperature values to do so, if needed.
We (NCEI) very much depend on NWS to verify and properly document via Station-Level Metadata Updates that the equipment and observing practices at each station meets the standards of the well established NWS Directives pertaining to the data observing network in question (COOP, ASOS, etc.). Having said that, overall issues with the data observing practices with the COOP Site at Neshaminy Falls, PA (GHCND ID: USC00366194; COOP ID #366194) and the other stations in this general local area that you've mentioned here should be further discussed with NWS-Mount Holly, NJ (WFO-PHI) and NWS Eastern Region HQ all of whom are cc'd on this email reply.
I hope Matt's answers to your questions and our further advisement help your understanding of GHCN-Daily's QC of NCEI's archived data. We greatly appreciate your email inquiry and have a great day!