harshithkethavath/FloodPrint-FFW-Polygons-WS2024

FloodPrint: NWS Flash Flood Warning Polygons (Warm Season 2024)

Part of the FloodPrint Collection — a CONUS-scale dataset for NWS Flash Flood Warning verification research.

Dataset Summary

This dataset contains NWS Flash Flood Warning (FFW) polygons for the contiguous United States (CONUS) covering the 2024 warm season (April–September 2024). Each record represents a single issued warning polygon — the forecaster's best estimate of the geographic area at risk at the time of issuance.

This dataset is the Phase 2 output of the FloodPrint pipeline.

Dataset Details

Property	Value
Record count	~3,700
Geographic coverage	CONUS
Temporal coverage	April 1 – September 30, 2024
Spatial representation	Polygon geometries (Shapely, WGS84 / EPSG:4326)
Source	Iowa Environmental Mesonet (IEM) WWA Archive
File format	Parquet (geometry stored as WKB)

Data Fields

Column	Type	Description
eventid	str	Unique NWS event identifier for the warning
wfo	str	3-letter NWS Weather Forecast Office that issued the warning (e.g. GSP, LWX)
issue_utc	datetime64[UTC]	Warning issuance time (UTC)
expire_utc	datetime64[UTC]	Warning expiration time (UTC)
geometry	geometry (Polygon)	Warning polygon boundary in WGS84 (EPSG:4326)

Reading geometry from Parquet

GeoPandas drops geometry metadata when reading Parquet directly. Use:

import pandas as pd
import geopandas as gpd
from shapely import wkb

df = pd.read_parquet("ffw_polygons_ws2024.parquet")
df["geometry"] = df["geometry"].apply(wkb.loads)
gdf = gpd.GeoDataFrame(df, geometry="geometry", crs="EPSG:4326")

Data Collection and Quality Control

Warning polygons were fetched from the IEM Warning/Watch/Advisory (WWA) archive, chunked by month to avoid API timeouts. The following quality control steps were applied:

• Type filter: Only Flash Flood Warning (FFW) event type retained
• Zero-duration removal: Warnings where issue_utc == expire_utc removed (data artifacts)
• Area bounds check: Polygons with area below 1 km² or above 50,000 km² removed
• Original polygon only: Where a warning was amended or cancelled, the original issued polygon is retained, consistent with standard NWS verification methodology

Design Note: Original vs. Amended Polygons

NWS forecasters can refine or cancel warnings via Follow-up Statements after initial issuance. This dataset uses the original issued polygon throughout, not any amended version. This is the standard approach in NWS verification methodology and ensures consistency with what was communicated to the public at decision time.

Known Limitations

• Archive gaps: IEM archives are generally complete back to 2008, but occasional gaps exist for specific dates or WFOs. Any missing warnings would cause a slight undercount of the true issued warning population.
• Polygon complexity: Some polygons are non-convex (e.g., following river corridors or terrain). Geometry is stored as-is; simplification is left to the downstream user.

Source

All warning polygons sourced from the Iowa Environmental Mesonet (IEM) WWA Archive:

https://mesonet.agron.iastate.edu/request/gis/warning_by_year.phtml

IEM data is freely and publicly available. This dataset redistributes a cleaned and filtered subset.

Citation

If you use this dataset, please cite the FloodPrint project and the IEM data source:

@misc{floodprint2026,
  title     = {FloodPrint: Spatiotemporal verification of NWS Flash Flood Warnings across CONUS for the 2024 warm season},
  author    = {Harshith Kethavath},
  year      = {2026},
  publisher = {GitHub},
  url       = {https://github.com/harshithkethavath/FloodPrint}
}

Original data source: Iowa Environmental Mesonet (IEM), Iowa State University. https://mesonet.agron.iastate.edu

License

Creative Commons Attribution 4.0 International (CC BY 4.0) — source data is publicly available from NOAA/IEM with no use restrictions.