Since the Fall season unofficially began after Labor Day, it has rained on at least one day of every single weekend. The only exception was the weekend of September 30th through October 1st; however, you'll probably remember that on the Friday preceding that weekend, former Tropical Storm Ophelia helped to fuel record-breaking rainfall in spots like Park Slope, Brooklyn (9.80"), while most of the Tri-State saw at least 5 inches of rain, more than a month's worth, in just one day. The incredible deluge led to public transit shutdowns and absolutely nightmarish scenes in the Five Boroughs and beyond.
The calendars below show September and October (so far); the days marked in blue indicate when more than two-tenths of an inch of rain fell during that day. The second half of September was awfully soggy, and October has been drier, but the common thread is weekend rain.
The blue boxes indicate days in September when NYC's Central Park recorded at least one tenth of an inch of rain.
(Data from the National Weather Service office in Upton, NY)
The blue boxes indicate days in October when NYC's Central Park recorded at least one tenth of an inch of rain.
(Data from the National Weather Service office in Upton, NY)
And the forecast for this weekend calls for more rain on both Friday and Saturday. What the heck is going on?!?
The forecast for Friday through Sunday from the National Weather Service office in Upton, NY on
Thursday afternoon, October 19th.
This may seem like an unfortunate coincidence, to have seven consecutive weekends with rain (or flooding-related cleanup), but it's not as flukey as it seems. A series of storm systems at the synoptic scale (don't worry, I'll explain) have moved across the United States at a fairly consistent speed, and it's going to take a break in this pattern to bring us a completely dry weekend.
A typical planetary wave pattern from a vantage point above the North Pole.
Image courtesy of NOAA
Synoptic-scale systems comprise the largest size category in weather. A synoptic system can easily span over 1,000 miles. Weathermakers such as Nor'Easters are synoptic in scale, as are dry weather features like a dome of high pressure, such as the Bermuda High we're familiar with in the summertime. These systems move with the jet stream; surface low pressure systems (again, like Nor'Easters) are found in troughs, whereas high pressure areas are usually centered in ridges. The image above shows a typical trough-and-ridge pattern from the vantage point above the North Pole; you'll recognize the continental United States under trough #2.
Notice that the pattern was similar last weekend, when we got nearly an inch of rain in Central Park on Saturday, October 14th. One of the dips in the wave, or troughs, had moved through the Tri-State, carrying a large storm system with it.
Along with their size, synoptic-scale weather systems are relatively slow-moving. They last for days to weeks; in comparison to weather features like a single thunderstorm, which has a time scale spanning minutes to hours, synoptic systems last a long time. Imagine synoptic systems are like the elephants of the weather world; powerful and large, but not very nimble. In this analogy, thunderstorms are like jackrabbits. They're fast and can change direction quickly, but they cut a small path.
Forecast surface analysis for Friday, October 20th from the Weather Prediction Center.
Notice that the area of green has a long barbed blue line (cold front)
draped through it; the length of this line is an indication of a synoptic-scale system.
Why is this relevant to our persistent pattern of wet weekends? Because, along with being unable to change speed or direction quickly, these large storm systems consequently tend to develop a regular cadence for the observer at a single spot- in this case, that spot is the Tri-State area. It will take a shift in this cadence, which could occur in the form of the development of a blocking pattern, to create a new rhythm in the atmosphere. Eventually, the pattern will change, even if it's because of the change in seasons, a lowering sun angle, and a natural shift of the polar jet stream southward. But if the texts I've been getting from my mother are any indication, my guess is that many people in the Tri-State are hoping for an earlier end to this pesky pattern. The good news is that it has been drier so far in October than it was last month, and the Climate Prediction Center believes that trend will continue.
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