Thread by @pmarshwx

Tweet Storm Warning! What follows is the history of SPC categorical outlook labels as told by me. There may be some deficiencies in the specific details but the overall sentiment is as best I know it to be. (1/21)

SPC originally had three categories: Slight, Moderate, and High. These were geared toward meteorologists and emergency managers. The ordering made sense. (2/21)

Probabilities were then introduced to underpin the categories. The idea was that by underpinning the categories, users could have a more consistent expectation of what would happen. (3/21)

It worked. Emergency managers started/continued developing action plans based on the categorical outlooks/labels. (4/21)

One complaint we regularly got was that the range of outcomes within the Slight risk were too broad. Put another way, the underlying probability space for Slight risk was too great. (5/21)

SPC decided to split Slight into two categories: Slight and Enhanced-Slight to mitigate this concern. (6/21)

At the same time we decided to drop the "See Text" labels and make those a stand-alone category so that it could be parsed without the need to parse all the underlying probability forecasts. (7/21)

Nothing was changed with our forecasts as they were supposed to be driven by the probabilities. What changed was that we added additional resolution to the categories to better reflect the resolution of the underlying probabilities. (8/21)

This left SPC with the following challenge: Pick a word that fit below Slight. (9/21)

After examining survey feedback, no obvious word was found that unambiguously fit below Slight. Marginal appeared to fit best and was chosen. (10/21)

Now, remember, the original premise was to not change the existing labels, merely add additional resolution to the existing categorical forecast. (11/21)

This left an ordering of:
0: Thunder
1: Marginal
2: Slight
3: Enhanced-Slight
4: Moderate
5: High (12/21)

For reasons I am not 100% clear on (most/all of this predates my working at SPC), the compound "Enhanced-Slight" was not viable. (13/21)

Since the original survey work had evaluated additional words (including enhanced) and no clear-cut better word was identified in the survey results, Enhanced-Slight was shortened to Enhanced. (14/21)

Now, it was known that this ordering was not perfect, but it met the original objective of leaving Slight, Moderate, & High -- and their probabilistic underpinnings -- alone. (15/21)

This allowed those who had built up actions associated with the long-term labels to keep their actions. (16/21)

To mitigate the other concerns a conscious effort was undertook to add numbering to the labels. (17/21)

Additionally, we revamped our website so that the outlook color associated with the highest categorical outlook is prominently displayed throughout the main website page. (18/21)

The idea was to establish a relationship with the words, colors, and numbers so that long-time users became comfortable with any combination of communication approaches. In other words, EMs could key off levels 4 or 5, not Moderate/High. (19/21)

Part of this effort was also undertaken so that we could one day undertake a complete revamp of our labels and switch to an ordering that the weather enterprise and social science all agree works best. (20/21)

Man, this was way longer electronically than in my head. (21/21)