I've received an e-mail from a friend who was wondering when Red Admirals would be arriving in the upper Midwest this spring. He had not seen any postings of that species on the Wisconsin Butterflies recent sightings page, or on iNaturalist, and asked me how unusual it is to not see Red Admirals this far into the season.
The first Red Admirals in central Iowa have been later than average this year (the average arrival time here is in mid-April, but the first one can arrive as early as late March to as late as mid-May). There have been several possible reasons for this:
1) It has been a generally cold spring until now.
2) Whenever there were warmer temperatures above 60 F with southerly winds, the air was very dry. They prefer not only warmth and winds from the south, but also sufficient humidity, with dew points above 55 F.
3) The jet stream for the last few weeks was passing from central Texas through Mississippi, Alabama, and Georgia and then into South Carolina. This pattern kept the warm, humid Gulf air well to our south, and brought several weeks of rainy, stormy weather to those states while we in the upper Midwest stayed unusually dry.
All three of these factors changed yesterday, April 26. A loop in the jet stream moved northward, which allowed southerly surface winds to bring warm, moist air into our area. Not only did the temperature rise rapidly into the low 80s, but the dew point also gradually rose from 41 F before sunrise to 56 F in mid-afternoon. So yesterday was the first day this year that was favorable for Red Admirals in our area to be active and to migrate.
This spring I've been out checking the Iowa State University campus for Red Admirals each sunny or partly sunny afternoon above 60 F since late March, without finding any. Likewise, there were none on April 25 - it was still too cool and dry. But when I went there yesterday, I found the first one of the season, perching on and patrolling around the sunny west wall of Catt Hall. It was a pleasant surprise, but one also anticipated due to the favorable conditions. Now, as spring continues and the weather becomes more favorable, we can expect more Red Admirals to be arriving in the upper Midwest.
Here is a dew-point map for 1 p.m. Central Daylight Time, April 26, showing the warm, moist, southerly air flow into the upper Midwest (attribution: RAL Real-Time Weather Data, National Center for Atmospheric Research, weather.rap.ucar.edu/surface):
Also note the dry line running from southwest Texas to western Kansas. Here the moist air from the Gulf of Mexico meets dry air from the desert southwestern states. Thus this line is likely to be the farthest possible western extent of any ongoing Red Admiral migration at this time. The dry line is also notorious for spawning severe thunderstorms and tornadoes along the moist side of the line.
-- Royce Bitzer