Now that mid-September is upon us South Floridians, we can look forward to the end of the summer “wet” or high humidity season in the next month. It’s easy to get jealous of the northerners this time of year, relaxing in the comfy autumn 60s and 70s, while the Miami dew point remains stubbornly stuck around 72 and the overnight low hovers around 77. Even the locals tell me they are sick of the humidity by this point, but the good news is that the first cold front should be here soon–bringing the first snowflakes of the year to our friends up north and diffusing to a nice refreshing breeze by the time it makes it down to Florida.

What defines the start of Miami’s winter/dry season?

The National Weather Service in Miami, FL is the official record keeper of wet/dry season statistics. A great overview can be found at the following website. According to the official definition (which is somewhat subjective), the “wet” summer season is characterized by:

-Overnight low temperatures in the mid-70s F

-Dew point temperature consistently above 70 F

-A near-daily occurrence of rain showers and/or thunderstorms

The end of the wet season was generally determined to occur when the dew point first dropped into at least the lower-60s, which usually coincides with decreased precipitation and minimum low temperatures dropping below 70. Of course, the humid, rainy weather can reoccur briefly at any time during the winter after wet season has ended, especially in October or early November. A tropical system such as the remnants of Hurricane Rina in 2011 can also bring brief heavy rains after wet season is over. But once that first real cold front makes it to Miami, dry weather becomes the norm instead of the exception.

What is the average date of the end of wet season?

For Miami (using a 42-year study period)

Median date: October 17

Earliest date: September 24, 1983

Latest date: November 5, 1995

Around two-thirds of the total rainfall in Miami occurs during the 5-month rainy season, although this fraction varies from 50% to 90%.

When will dry season start this year?

Short answer: not in the next two weeks.

Long answer: based on climatology, we are probably still about a month away from the start of dry season, although we could also get lucky and experience a brief period of drier weather before then.

There are a few methods available for estimating the specific date. My favorite tool is a surface dew point model forecast map that is available from the College of DuPage numerical model output website. To see the map, click on GFS -> Surface -> 30mb mean dew point. The map is also shows the surface streamlines, which makes it easy to locate cold fronts. The first image below shows the GFS model forecast for next Tuesday (Sept 18):

The map shows a typical late-September pattern across the eastern US. A cold front stretches right along the Ohio Valley, which is easy to pinpoint by identifying the sharp dew point gradient. The streamlines indicate winds from the northwest behind the cold front. Florida remains in southerly flow with dew points in the low 70s, or typical wet season weather.

So, is this the cold front that ends wet season? Not likely, judging by the forecast below for two days later, September 20th:

On the bright side, the dew point gradient indicating the front has made it down to the Florida panhandle. However, the streamlines no longer point from the northeast, instead they indicate westerly winds, blowing parallel to the frontal boundary. In other words, the front has stalled out and is now a stationary front. It isn’t going to get any farther south. Furthermore, a moist return flow has already built up again over the central plains. The tropical easterlies prevail over South Florida.

At this time it is simply too early in the season for a cold front to make it down to South Florida. There are several reasons for this but it basically amounts to the fact that the polar regions are not cold enough yet, so the polar jet and mid-latitude storm track remain too far north and too weak to generate the instability needed to drive fronts down to Florida. In a few weeks that will change as each weather system gets a little colder and stronger than the previous one.

Long-term dry season prognosis:

When anticipating longer-term conditions on a monthly to seasonal scale, longer term climactic predictors such as the El Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) can be used to forecast conditions for the rest of the dry season. With El Nino conditions expected this winter, the dry season may end up being wetter than normal. However, that relationship is worthy of an in-depth discussion in a future post!