Summer is basically in full swing already. My guess is a couple showery days in late May then again in mid June then nothing but light drizzle on a handful of mornings til September.
Certainly the worst on record for the PNW/western Canada. Nothing remotely close with the "heat domes" since then, of course.
The deadliest heatwave was likely the 2003 European one, which killed somewhere between 50-80k people.
Next would be Russia in 2010, with 55k or so. But that comes with an asterisk as a significant percentage came from smoke inhalation due to wildfires.
In 1911, France had a heatwave which killed over 40k.
For the US, deadliest would be July 1936 with around 5k, mostly in the middle of the country.
We’re not talking about a permanent Niño in this hypothetical. Just a return to something more like what we saw during the 1400s - 1600s (during the initial descent into the LIA) which was Niño/+PDO leaning on balance. Very possible it was a quasi-inverse expression of the modern ENSO dipole, with multiyear Niños and single year Niñas, based on some high resolution proxies and modeling.
Also it appears that either ENSO amplitude was generally reduced, or there were prolonged stretches of neutral/very low amplitude ENSO imbedded in there.