The mercury is making a valiant run into the mid 70s this afternoon. Sunlight in late September isn't strong enough to warm us up all by itself, a helper breeze out of the south is needed to drag warmer air this far north. Here's the catch: The southerly wind is ahead of a cold front which will cool the city down for the remainder of the week.
The sunny start to the afternoon has a good chance of being replaced with a shower or thunderstorm. The front should arrive late this evening, killing the chance of substantial rain. Tomorrow's high will be in the upper 60s with the slight chance of an afternoon shower from a passing upper-level low.
Cooler still on Wednesday and Thursdays with highs only in the lower 60s. The temperature is likely to dip into the 40s on either Thursday or Friday morning for the first time since mid-May. The weekend currently promises to be warmer and wetter.