IEM Daily Bulletin for Jun 18 2026

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 18 June 2026

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Jun 17 11:15 PM	iem,main	📝 Sundry updates per review	Link
Jun 17 7:28 AM	pyiem,main	🚀 Require matplotlib 3.11 for testing due to font changes	Link
Jun 17 7:31 AM	pyiem,main	💚 Update custom_background test artifact This has local dev laptop variances :(	Link

Daily Feature

Clear skies and overnight precip
Date: 17 Jun 05:30 AM
Votes: Good: 14   Bad: 1 Abstain: 0

Overnight Monday into Tuesday morning was interesting as you may have noticed clear skies when the sun set and then clear skies again when the sun rose, but then you realized that it rained overnight! Such was the case for Ottumwa. The airport weather station reported clear skies at 1 AM and at 7 AM with 0.09 inches falling during the 5 AM hour along with thunder reported. So the subject of today's daily feature attempts to look into how frequently total precipitation of at least 0.05 inches prior to 7 AM is observed by a given 1 and 7 AM cloud coverage observation for Ottumwa. There are caveats galore with such an analyses including many complexities with how automated stations report cloud coverage and changes with observation equipment and techniques over the years. The plot was limited to dates after the year 2000 as an attempt to limit some of these changes. All of that aside, the frequencies shown are the number of days with at least 0.05 inches of precipitation for a given cloud coverage combination. The 1 AM Clear + 7 AM Clear combination comes in at 2.5% frequency, so it is somewhat rare but perhaps not exceptionally rare situation. There are some interesting relationships to tease out from the plot including the increased precipitation frequencies when cloud cover amount increases between 1 and 7 AM, which intuitively makes sense.

NWS Watch/Warning Summary for 12 AM 17 Jun 2026 - 12 AM 18 Jun 2026 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	60	2	3	0	0	0	0	7
Svr Tstorm	134	25	0	15	10	0	6	3
Flash Flood	26	4	0	6	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: 189 Verified:  59 [31.2%]
Polygon Size Versus County Size            [30.3%]
Average Perimeter Ratio                    [29.8%]
Percentage of Warned Area Verified (15km)  [18.0%]
Average Storm Based Warning Size           [1612 sq km]
Probability of Detection(higher is better) [0.77]
False Alarm Ratio (lower is better)        [0.69]
Critical Success Index (higher is better)  [0.29]

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