The first 29 days of September have been warm and dry, and it hasn't rained in more than two weeks, but that is about to change in a very dramatic fashion. Huge amounts of tropical moisture are streaming up from a disturbance in the Gulf of Mexico ahead of a strong cold front coming down from Canada. As the front moves toward the East Coast over the next couple of days it is going to wring that moisture out of the air.
The heaviest rain will fall upstate where several inches are expected and a flood watch is in effect. Here in the city showers and thunderstorms could start as early as this afternoon are are likely to persist through the day tomorrow. September rainfall is two inches below normal in Central Park as of this morning but the month could easily wind up wetter than average by midnight tomorrow.
Much cooler weather is in store after the front passes late tomorrow. While Wednesday should warm to the lower 70s, leaving us with, by far, the warmest September ever, high temperatures on Thursday and Friday will probably stick to the upper 50s. It hasn't been that cold since early June. There's also a good chance for showers on both days as the cold front gets pushed back toward land and moisture from Tropical Storm Joaquin begins to arrive.
There is a lot of uncertainty to this weekend's forecast. That uncertainty is tied to the path taken by Joaquin. There is not yet a strong consensus among the forecast models or human forecasters on how the storm will behave, but with a ridge of high pressure building over the western Atlantic Joaquin has the potential bring high winds and dump tremendous amounts of rain over a wide area this weekend and into early next week. With the ground already saturated from rains earlier in the week the Northeast could see widespread flooding.