The gradual shortening of the daylength although typically the Ipomoea alba do not like cold temperatures , but you might get lucky and see some blooms before the cooler temperatures cause the I.alba to shut down...

I guess I'm confused, because I thought these guys were more warm weather bloomers, when the days are longer and the nights shorter.

ivysmon - The full answer to your question involves more than just day-length but other factors that can have a modulating effect on the effect of the shortened day-length.

There are many factors which can influence the natural rhythms and cycles of biological systems , many factors are known and many factors remain to be adequately explored and further elucidated.

Generally , the Ipomoea alba is associated with semi-tropical to tropical climate zones but even in these areas the initiation of flowering usually occurs after the Summer Solstice of June 21st when the day-lengths start to get shorter...

The earth is not perfectly round and the orbit around the sun is not a circle but more elliptical , thus there are differences in the type of wavelengths of light that the plants receive when the earth is at different positions in the orbit around the sun...and plants can become attuned to these wavelength differences...as what may seem as 'subtle' to our typical perspective may be critical to other life forms...

There are additional factors that can influence how much the plant responds to the signal from the short days and the overall ambient temperature is one such factor , so that Ipomoea alba will usually flower in the warmer zones sooner than in the colder zones...it flowers in the colder zones when the days are so short that the signal from the short days to initiate flowering is increasingly strong as per the plants response.

The short day response is also not completely written in stone since plants at or very near the equator receive very much the same amount of light all year long and plants in these areas will often adapt to other environmental factors to synchronize their flowering...e.g., the migration of certain animals or pollinators...

The full scope of all factors involved is beyond the scope of my reply here but I would like to assure you that basically the shortening of the day-length is the main factor that initiates flowering...

The is an additional factor involved here and that is the particular strain of Ipomoea alba that your seeds originate from because seeds from a very tropical area may not flower as well as seeds from plants that are adequately adapted to your particular local environment and unfortunately this factor is frequently disregarded.

Plants which have gradually adapted to cooler locations will do better in similar locations...

I recommend trying to locate seeds from plants that have adapted to your local environment for as many generations as possible , as seeds from these plants will be best adapted to your location....also try to get seeds from the 1st plants that flower rather than later flowering plants and perpetuate this characteristic (!)...thereby helping Mother Nature to fine tune the plants to your preferences (!)

There are a few Forums devoted to Morning Glories and it may be that if your interest is intrigued enough you might find your way to a Morning Glory Forum where the details of many different MG's may be explored in more depth...

I hope that my attempt to answer may prove to be helpful to your interest at some point...

best regards,

Ron

P.S. - Feel free to private message me if you are interested to explore the topic in more depth and / or other potential options

Here is a link that might be useful: Morning Glories can respond differently to variables