IEM Daily Bulletin for Sep 19 2025

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 19 September 2025

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Sep 18 10:30 AM	iem,main	🐛 Address feature style issues, etc	Link
Sep 18 10:31 AM	iem,main	⬆️ Update ruff	Link
Sep 18 3:12 PM	iem,main	✅ Improve coverage	Link
Sep 18 3:12 PM	iem,main	⬆️ Update ruff	Link
Sep 18 3:25 PM	iem,main	💚 Address issue found in CI	Link
Sep 18 7:52 AM	pywwa,main	🚀 Update to new AWX API	Link

Daily Feature

Weak Winds Aloft
Date: 18 Sep 05:30 AM
Votes: Good: 8   Bad: 0 Abstain: 0

Showers and thunderstorms have frequented Iowa over these past few days. Those of you that frequently monitor NWS RADAR may have noticed how strangely these storms have moved, or rather barely moved. The lack of movement is due to very weak winds aloft as illustrated by the featured chart. The chart presents wind speeds from the Wednesday evening NWS Davenport sounding (0 UTC 18 Sep is 7 PM 17 Sep CDT) expressed as percentile values compared against other 0 UTC soundings from the site during the month of September. For example, the 850 hPa (a few thousand feet aloft) wind speed value of 1.5 m/s (~3 MPH) ranks at the 1.2 percentile, so roughly the lowest value over a three year period. So while the atmosphere over Iowa has been unstable with plenty of available moisture, the weak winds aloft have caused the storms to be pulse and not move much at all. Some of the storms were able to attach to some surface or boundary aloft, which allows them to organize a bit more and produce gusty winds and/or hail. The slow movement has also lead to localized heavy rainfall events. You may notice the larger wind speed values above 100 hPa (stratosphere) and wonder why it is different. In some regards, what happens above the tropopause (boundary between troposphere and stratosphere) is detached from what happens below. Additionally, these values can be misleading when the troposphere is anonymously warm or cold and the mandatory levels occur at significantly different altitudes aloft. Anyway, the moral of the storm is to blame the weak winds for the strange storm activity!

The featured media can be generated on-demand here

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NWS Watch/Warning Summary for 12 AM 18 Sep 2025 - 12 AM 19 Sep 2025 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	0	0	0	0	0	0	0	0
Svr Tstorm	66	0	0	0	0	0	0	1
Flash Flood	32	0	0	0	0	0	0	---

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

IEM Cow Report

SVR+TOR Warnings Issued:  66 Verified:  15 [22.7%]
Polygon Size Versus County Size            [8.0%]
Average Perimeter Ratio                    [22.5%]
Percentage of Warned Area Verified (15km)  [13.9%]
Average Storm Based Warning Size           [932 sq km]
Probability of Detection(higher is better) [0.78]
False Alarm Ratio (lower is better)        [0.77]
Critical Success Index (higher is better)  [0.21]