IEM Daily Bulletin for Apr 17 2026

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 17 April 2026

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Apr 16 9:58 PM	iem,main	🔨 Attempt autoculling of discontinued thresholds https://mesonet.agron.iastate.edu/plotting/auto/?q=230	Link
Apr 16 9:59 PM	iem,main	⬆️ Update ruff	Link

Daily Feature

Evening Thunderstorm Forecast
Date: 16 Apr 05:30 AM
Votes: Good: 11   Bad: 1 Abstain: 0

Wednesday saw multiple rounds of severe weather impact Iowa with a number of locations receiving significant and damaging hail. After a main line of storms developed over central Iowa during the afternoon hours and pushed into Eastern Iowa, there was some optimism that the severe weather was done for the evening over central Iowa. Instead, more storms developed just before sunset and brought another round of hail to central Iowa. The featured "postage stamp" plot takes a look at one forecast model's (HRRR) simulated RADAR reflectivity forecast valid 8 PM Wednesday (1 UTC on 16 April 2026). The upper left plot is the "verification" / observed RADAR mosaic produced by the IEM. The other 15 panels are time lagged sequential forecasts from the HRRR, since the HRRR produces a new forecast every hour. The HRRR model is one of the main tools in the near-term weather forecast toolbox, but follows the adage of all models being wrong but some are useful. Some of the earlier forecast runs picked up on the development of storms over central Iowa, but others had near no storms forecast at 8 PM.

The featured media can be generated on-demand here

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NWS Watch/Warning Summary for 12 AM 16 Apr 2026 - 12 AM 17 Apr 2026 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	2	0	0	0	0	0	0	0
Svr Tstorm	101	0	0	0	0	0	0	1
Flash Flood	6	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: 103 Verified:  50 [48.5%]
Polygon Size Versus County Size            [23.8%]
Average Perimeter Ratio                    [35.8%]
Percentage of Warned Area Verified (15km)  [26.2%]
Average Storm Based Warning Size           [1151 sq km]
Probability of Detection(higher is better) [0.80]
False Alarm Ratio (lower is better)        [0.51]
Critical Success Index (higher is better)  [0.43]

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