IEM Daily Bulletin for Sep 5 2025

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 05 September 2025

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Sep 3 5:32 PM	iem,main	📝 Sundry updates per review	Link
Sep 4 10:56 AM	iem,main	✅ Improve coverage	Link
Sep 4 10:18 AM	iem-database,main	✅ Add iemrasters test data	Link
Sep 4 10:24 AM	iem-database,main	🐛 Fix permissions	Link

Daily Feature

Hour of High Temp
Date: 04 Sep 05:20 AM
Votes: Good: 12   Bad: 0 Abstain: 0

A tell-tale sign of the seasons changing is the passage of robust cold fronts with legitimate air mass changes as what happened on Wednesday. For Estherville (NW Iowa), the high temperature for Wednesday was set during the early morning hours well before sunrise. This situation is somewhat rare for September with only a handful of other dates since 2010 having a similar occurrence. The featured chart looks into the hourly frequency by week of the year for the high temperature to occur at based on NWS CLI-mate reporting sites (read: mostly automated airport weather stations). The chart uses a logarithmic scale as the frequency counts during the mid-afternoon hours dwarf any other hour of the day. During the cold season, the influence of air masses can be seen with increased overnight frequencies than during the warm season. Additionally, you can see enhanced frequencies near midnight during the cold season as frontal passages will lead to multi-day warming or cooling events. The lowest frequencies are found during the pre-dawn hours during the warm season as solar heating is the strongest and cold-air advection during the daytime is not sufficient to counter.

NWS Watch/Warning Summary for 12 AM 4 Sep 2025 - 12 AM 5 Sep 2025 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	3	0	0	0	0	0	0	0
Svr Tstorm	71	0	0	0	0	0	4	0
Flash Flood	24	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:  73 Verified:  39 [53.4%]
Polygon Size Versus County Size            [21.2%]
Average Perimeter Ratio                    [27.0%]
Percentage of Warned Area Verified (15km)  [22.1%]
Average Storm Based Warning Size           [1451 sq km]
Probability of Detection(higher is better) [0.85]
False Alarm Ratio (lower is better)        [0.47]
Critical Success Index (higher is better)  [0.49]