Appears serious. You are correct that Eastern or Canadian Hemlock (Tsuga canadensis) does not tolerate high heat well. Nor does it tolerate drought or urban conditions. Still, it's hard to imagine a week of hot weather in the sun would cause this much needle drop in such a short amount of time. I am unable to diagnose from this vantage point as there are a number of possible reasons.

Hemlocks don't tolerate poor drainage well, so if you have clay soil that does not include a way for excess water to drain off, that could be an issue.

Hemlock Wooly Adelgid is of major concern in Michigan. Inspect your trees for this pest. See the information in the link below regarding this pest of hemlocks in Michigan:

https://www.canr.msu.edu/news/losing_most_of_michigans_eastern_hemlock_resource_is_a_real_possibility

There are other potential pests and diseases, including hemlock scale, bagworm, needle rust mite, woolly adelgid and needle blight.

As the name might imply, Canadian Hemlock is only suited to cooler, northern region. You should avoid locations exposed to dry winter or summer winds; partial shade is best. This species can be very shade tolerant, but the foliage becomes open in denser shade. Soils should be uniformly moist throughout the year. Eastern hemlock (Tsuga canadensis) requires acidic conditions, with a pH range of 4.2 to 5.7.

If you have not done so, you may wish to consider conducting a soil test to determine your soil's pH level. Self-mailer soil test kits from the Michigan State University Plant and Pest Diagnostic Lab are available. You can obtain them at the link below. Full instructions are there and also come with the kits. The cost is $25. The Lab will make recommendations for any needed soil amendments. 

https://shop.msu.edu/product_p/bulletin-e3154.htm

Hemlocks are not pollution tolerant and suffer considerable damage from exposure to atmospheric pollutants. Like all evergreens, they are subject to desiccation (winter burn) from drying winter winds. They do not go dormant in winter. Once the soil freezes, they cannot uptake moisture from frozen soil. That fact, coupled with winter winds drying moisture within the plant, can cause this desiccation. All evergreens should be thoroughly hydrated going into winter; water them thoroughly in late fall before the ground freezes. Record cold temps this past winter could also be a factor.

Your hemlocks obviously are valuable to you, so you may wish to consult a certified arborist. An arborist is a certified and generally insured expert in trees and their problems. He or she can examine your tree and make recommendations regarding the health of the tree, any needed judicial pruning, and whether or not (in his opinion) it should be removed. You can find a certified arborist in your area here:

http://www.treesaregood.org/

Click on "Find an Arborist" at the top of the page; then click on United States from the pull-down menu. Then enter your State and City. This should bring up a list of certified arborists nearest to you. Or you may be able to locate one under "Tree Services" in your local Yellow Pages.

Generally, arborist evaluations are not expensive and the best way to get the needed positive diagnosis.

Good luck!