Good questions, Benjamin (in Seattle).
Dozens of excellent "migrant traps" for watching warblers and other songbirds exist in the lower 48 states in the USA.
I'll mention a few here: (courtesy of http://www.birding.com/top200hotspots.asp)
There's many other excellent options beyond the ones I note below. Which ones would you add to my list?
Kennesaw Mountain National Battlefield Park, Georgia
33.91 N 84.61 W
The mile-long road to the top of the "mountain" should yield about 20 warbler species in late April. On weekends, you can ride a shuttle bus to the top. Good trails cover most of this park located about 20 miles northwest of Atlanta.
Cape May, NJ
38.56 N 74.57 W
Hawks "funnel" into Cape May each fall, making this the best spot on the East Coast for raptors. Fantastic for warblers and other migrating birds in spring and fall. One of the top 10 spots in North America.
Central Park, New York City
40.47 N 73.58 W
Birds? In New York City? During spring migration, Central Park is a welcomed island of green trees in the middle of a concrete desert. Warblers, Tanagers, Grosbeaks (and maybe a Rock Dove).
Crane Creek/Magee Marsh/Ottawa NWR
41.37 N 83.09 W
Spring migration here may be even better than Point Pelee -- and two hours closer if you live in Ohio! Go visit the Oak Openings and Irwin Prairie on the west side of Toledo as well.
41.56 N 82.31 W
This tip of Ontario extends into Lake Erie, forming a welcome site for migrating birds in May and a natural "funnel" in the fall. Warblers in the spring are everywhere. Watch the flight of Monarch butterflies and huge flocks of Blue Jays in the fall. Considered by most as one of the Top 10 birding spots in North America.
Devil's Lake State Park, Wisconsin
43.42 N 89.73 W
Great scenery and a mix of northern and southern birds can be found here. For worm-eating Warbler, try nearby Baxter's Hollow Preserve. The International Crane Foundation is located just north of here in Baraboo.
As for when warbler migration begins during the spring, the range of dates vary by latitude and, often, annually, based on weather patterns.
In general (and to oversimplify), warbler migration begins in Florida in March (and becomes obvious by April) while southern Wisconsin, for example, attracts warblers in abundance by the last week of April (though it more typically peaks in the first or second week of May). Point Pelee (noted above) is often best visited during the initial days of May while upper Michigan usually peaks with warbler activity during the third and fourth weeks of May.
That's not to say warbler migration is absent prior to March in Florida or prior to May in Wisconsin. Early warbler visitors are present in both areas (e.g., LA Waterthrush in FL; Yellow-rumped and Palm Warbler in WI, among other species).
But, again, in general, warbler migration is best considered an April and May phenomenon in most lower 48 USA states.