IEM Daily Bulletin for Sep 17 2025

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 17 September 2025

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Sep 16 2:11 PM	iem,main	📝 Sundry updates per review	Link
Sep 16 3:00 PM	pyiem,main	🗑️ Mark pyiem.util database stuff for removal	Link

Daily Feature

Monday SPS Polygons
Date: 16 Sep 05:30 AM
Votes: Good: 10   Bad: 0 Abstain: 0

An increase in humidity along with very warm temperatures helped to fire afternoon thunderstorms over Iowa on Monday. Most of these storms stayed below severe levels (less than 58 MPH gusts and one inch hail). The National Weather Service issued a number of Special Weather Statements (abbreviated SPS) to cover these sub-severe storms. The NWS SPS product can be issued for a specific polygon region to highlight a sub-county region under the highest threat from the storm. The featured map presents an IEM accounting of the number of NWS issued SPS products containing a polygon on Monday. The IEM offers a number of tools for these SPS products, including a GIS Download, a search tool, and the raw text which can be downloaded in bulk as well.

The featured media can be generated on-demand here

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NWS Watch/Warning Summary for 12 AM 16 Sep 2025 - 12 AM 17 Sep 2025 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	1	0	0	0	0	0	0	0
Svr Tstorm	130	21	2	0	18	1	2	1
Flash Flood	18	2	0	0	1	0	2	---

ARX = LaCrosse, WI DVN = Davenport, IA DMX = Des Moines, IA OAX = Omaha, NE FSD = Sioux Falls, SD

IEM Cow Report

SVR+TOR Warnings Issued: 131 Verified:  48 [36.6%]
Polygon Size Versus County Size            [17.1%]
Average Perimeter Ratio                    [21.2%]
Percentage of Warned Area Verified (15km)  [20.3%]
Average Storm Based Warning Size           [930 sq km]
Probability of Detection(higher is better) [0.71]
False Alarm Ratio (lower is better)        [0.63]
Critical Success Index (higher is better)  [0.32]