This document analyzes extreme precipitation events in the Gulf region from 1949-2017, with a focus on Hurricane Harvey. It examines rainfall totals from Harvey and other events using grid boxes of different sizes. Harvey had rainfall exceed 50 inches in some locations over multiple days. The analysis ranks the top 100 rainfall events and finds that 24 of these were caused by tropical cyclones, while most others were associated with fronts. It discusses the challenges of analyzing extreme precipitation at different spatial and temporal scales, from individual thunderstorms to hemispheric weather patterns.

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Kenneth E. Kunkel
NOAA Cooperative Institute for Climate and Satellites, North
Carolina State University
Historical Perspective on
Hurricane Harvey Rainfall

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Hurricane Harvey
• One focus of working group
o Late August 2017
o Multi-day rainfall exceeded 50 inches in some
locations
o This reached Probable Maximum Precipitation
levels
o Massive flooding

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Physics and Statistics/Math
• Phenomenology-centric view of extreme
precipitation events

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Meteorology of Observed Extremes
Dominated by large systems

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Extreme Precipitation – climate analysis
• Define an overlapping grid of cells separated by 1/10 degree in latitude and 1/10
degree in longitude covering longitudes 80–100°W and latitudes 25–35 °N (large
blue box on following figure).
• Within the grid, consider all possible 2-degree by 2-degree boxes (all boxes like the
red box in the figure). This represents an approximate area of 40,000 km2.
• Compute daily precipitation for 1949-present as a simple average of all stations in
each box. All boxes that are wholly or partly over water are not included in this
analysis.
• For each grid box, identify top 5-day precipitation totals.
• Pool everything together and identify the top 100 events for 1949–2017 across the
entire region, ignoring those that overlap in time or space with larger event.
• Rank and plot these.
• Also did same analysis on other grid sizes from 1° to 3°

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Grid box lattice

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Ranking of 2° x 2° events
Harvey

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2° x 2° event locations

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Ranking of 1° x 1° events
Harvey

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1° x 1° event locations

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Ranking of 3° x 3° events
Harvey

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3° x 3° event locations

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Meteorological Causes
• For 2° to 2° “top 100” events:
– 24 were caused by tropical cyclones
– Most of rest were associated with fronts

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Monthly Distribution - 2° to 2° events

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Top 100 Gulf Region Events

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• Complex temporal and spatial coherence and
variability of extreme precipitation events –
– Individual thunderstorm cells – hour, a few km
– Thunderstorm complexes – a few hours, tens-100+ km
– Spiral rain bands in hurricanes – a few hours, tens-
100+ km
– Low pressure wave – day, 100s of km
– Hurricanes – day, 100s of km
– Synoptic low pressure system – days, 1000+ km
– Hemispheric jet stream wave patterns – weeks, 1000s
of km
The Challenge
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• Weather system based inquiry of historical
trends and future changes
– Statistical trend significance tests
– Automated tools
– How do we incorporate this approach into the
standard statistical approaches for estimating
design values
Research
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Basic Elements of PMP

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Annual Maximum 24-hr Upward Motion

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Questions?