IEM Daily Bulletin for May 21 2026

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 21 May 2026

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
May 20 9:48 PM	iem,main	✨ Add percentage missing estimate to 1min precip Sadly, the NCEI 1 minute data is incomplete, so provide an estimate on how much is missing for autoplot totalling precp during severe weather. https://mesonet.agron.iastate.edu/plotting/auto/?q=222	Link
May 20 10:12 PM	iem,main	🎨 Improve navbar rendering on mobile	Link
May 20 4:19 PM	pyiem,main	⬆️ Update precommits	Link
May 20 4:20 PM	pyiem,main	🎨 Address navbar ally / clicking issues	Link
May 20 3:58 PM	iem-web-services,main	⬆️ Update ruff precommit	Link
May 20 3:59 PM	iem-web-services,main	🎨 Modernize and add test for spc_watch_outline	Link
May 20 4:02 PM	iem-web-services,main	💚 Address code review	Link

Daily Feature

Upcoming GDD Forecast
Date: 20 May 05:30 AM
Votes: Good: 7   Bad: 0 Abstain: 0

For mid May, Tuesday was a bit chilly with high temperatures struggling to get out of the 50s. Such chilly temperatures imply a meager growing degree day accumulation, which is ever important this time of year to promote crop development. The featured chart presents daily growing degree days for Des Moines with the observed value on Tuesday plotted along with two model forecasts, a crude daily average accumulation (short daily lines) and the associated accumulated values. The NWS NDFD forecast does go out as far as the GFS deterministic model, so that is why the red bars stop on the 26th. The chart nicely shows that we'll have four more days to go with below average GDDs prior to about a week of warmer weather taking us into June.

The featured media can be generated on-demand here

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NWS Watch/Warning Summary for 12 AM 20 May 2026 - 12 AM 21 May 2026 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	98	0	0	0	0	0	0	4
Flash Flood	11	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:  98 Verified:  52 [53.1%]
Polygon Size Versus County Size            [19.0%]
Average Perimeter Ratio                    [12.5%]
Percentage of Warned Area Verified (15km)  [23.6%]
Average Storm Based Warning Size           [1368 sq km]
Probability of Detection(higher is better) [0.80]
False Alarm Ratio (lower is better)        [0.47]
Critical Success Index (higher is better)  [0.47]

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