Severe thunderstorm(+) Climatology and Trends

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Climatology

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Trend

Trends are inherently far noisier: focus on coherent spatial patterns, not the value in any single county.

both maps share one view

Methods

Sources. Hail, tornado and thunderstorm wind come from the NOAA/NCEI Storm Events Database, using county-type records (CZ_TYPE = C). Freezing rain and peak wind come from NOAA/NCEI Integrated Surface Database hourly observations at ASOS stations. The remark FZRANO (freezing-rain sensor not operational) is not counted as freezing rain, and a peak-wind group with a three-digit speed (100 kt or more) is treated as a coding error.
Locate each report. Storm Events reports are assigned to a county by STATE_FIPS + CZ_FIPS rather than by county name, which avoids name collisions and independent cities. Connecticut is drawn with its eight historical counties, because Storm Events still files reports under them rather than the planning regions the Census adopted in 2022. Puerto Rico reports are included from 2009, the first year they carry municipio codes (earlier ones use forecast zones). Station hazards stay at their station.
Apply the threshold. Each report is scored against the selected size or intensity cut. Thresholds are cumulative — a 2″ hailstone also counts as a 1″ one.
Collapse to hazard days. Count distinct calendar dates on which a county — or a station — saw at least one qualifying report. One storm producing forty reports in a county on one afternoon is one hazard day.
Screen the station records. A station-year below 90% complete is treated as no observation and dropped, not as zero days. That is the opposite of the county rule below, and both are right: every county exists every year, a station does not. QC stations further restricts to the set of DelPizzo et al. (2025), who kept only sites with an overnight manual observer. Freezing rain opens on that QC'd subset, because automated present-weather sensors report FZRA too erratically to trend; the button adds the rest back if you want to see the difference. Peak wind is measured differently and is not screened this way, so it opens on all stations.
Store those counts per county (or station) × year × month × threshold. Everything below is computed live in the browser from that table.
Climatology map. Mean annual hazard days over the selected period. For counties a year with no days is a real zero and is counted in the denominator; for stations, unobserved years are excluded rather than counted as zero.
Trend map. Ordinary least squares on the annual counts, reported per decade. A county (or station) is left grey unless it clears the same three coverage gates as the 2° grid page: enough years carrying a day (at least 8, or 30% of the period), those years spanning at least 70% of the period, and at least 2 of them in each half. The annual and monthly panels apply the same gates. Significance is a two-sided t test on the slope; stations reaching p < 0.05 are ringed.
Annual and monthly panels. For a selected county, that county alone. For a state, calendar dates on which anywhere in the state qualified — deliberately not the sum of county-days, since one storm can cross twenty counties in an afternoon. For stations with none selected, the mean over those observed in every year of the period, so a station entering or leaving the record cannot masquerade as a trend.
Smoothing. The two county maps are drawn through a light Gaussian blur, sigma 2°, on by default and switchable to Raw. Measured over the real county centroids that means half the weight comes from within 230 km and 90% from within 381 km, with a hard cutoff at 444 km, so a point smears into a blob roughly 520 km across and a median of 286 counties contribute to each value. That is a firm blur: it resolves the broad corridors rather than county-scale structure, and the peak hail value drops from about 15 to 6 days/yr. Switch to Raw to see the underlying county numbers. The 2° grid page uses the same sigma. It is area-weighted — without that, Georgia's 159 small counties would outvote a single large western one covering the same ground — and normalised, so only counties that have a value contribute and coastal counties are not dragged towards zero by the empty space beyond them. It never fills gaps or changes which counties are drawn, and it never touches the annual or monthly panels. Because it is a neighbourhood average, the drawn value is not the county's own — hover reports both when they differ. Station hazards and derecho events are not smoothed: those are points and single storms, not fields.
Colour scales. Set from the data on screen: robust upper percentiles for climatology, symmetric about zero for trend.
Derecho is the same county climatology and trend as thunderstorm wind, but counting only wind reports that belong to a derecho. A county scores a derecho event when it lay inside the coherent wind swath of a definitive derecho in the SPC archive — NEXRAD-confirmed cases only, so the record begins in 1996. It is counted in events per year rather than hazard days, because a derecho is one coherent storm rather than a category of report. Each event is dated by the swath's own start (UTC) rather than each report's local date: half these swaths cross a date boundary, and dating by report counted one storm twice in any county hit either side of midnight. Two swaths that start on the same date count once in a county both cross, so nationally the 96 definitive swaths fall on 93 dates. A report belongs to a swath when it falls inside the archive's UTC window, keeps pace along the track with the clock, lies no farther from either endpoint than the swath's length plus 100 km (the paper's three tests), and — our addition — lies within 300 km of the straight start-to-end track line, which keeps separate storms far off to the side out of the swath. Values are small by construction — the busiest counties see about one derecho every two years — so read the map as a corridor map, not a frequency map, and treat trends over a record this short with the caution the yellow note above the trend panel describes.
Derecho events work differently — they are single storms, not a climatology, so that mode shows one map and no trend. The swaths come from the SPC archive of Squitieri, Wade and Jirak (2026), the first comprehensive US derecho archive: 184 events from 1956 to 2025. The archive publishes a UTC start and end time and start/end coordinates for every swath, so the track drawn in solid magenta is theirs. Wind reports are screened by the paper's own three tests: they must fall inside the published UTC window, their spatial progress along the track must match their temporal progress (|SP − TP| under 0.5 for short swaths, 0.25 above 1000 km), and they must lie no farther from either endpoint than the swath's length plus 100 km. We add a fourth: within 300 km of the straight start-to-end track line. Surviving reports are then linked into a coherent swath by growing disks of 25 km or more around each until one connected group holds 90% of them, and the dashed outline is the boundary of that union — the paper's wind-swath polygon, not a convex hull. Reports that fail a screen stay on the map in grey; about 7% of reports in-window do. A swath can legitimately draw as more than one piece. The Confidence menu exposes the archive's tiers: 96 definitive cases confirmed against NEXRAD (1996–2025), 48 likely and 16 possible pre-NEXRAD cases reconstructed from radar summary charts and paper storm reports, 13 further NEXRAD-era likely cases, and 11 suspected hybrids the paper deliberately keeps separate. The default is the 144 the paper headlines. Report counts here run well above those quoted in the derecho literature, which works from SPC filtered reports that thin near-duplicate reports of a single gust. A few swaths show a track but no reports: four fall in June–July 1993, which are missing outright from the wind file, and any others are too recent for the Storm Events data used here.