Upslope Showers on Sunday

An interesting weather phenomenon occured Sunday afternoon across northwestern Connecticut. It happens all the time but it’s nonetheless interesting. A somewhat unstable atmosphere along with a light easterly wind has allowed showers to develop along the ridge line on the Hartford/Litchfield County line.

The hills in Burlington, New Hartford, Canton, and Granby are over 1000 feet in elevation. There’s a 900 foot rise from the Connecticut River to the summit of the ridge. An easterly wind flow is forcing air to rise from the valley to up and over the ridge. This is called upslope. A schematic of this phenomenon is on the left courtesy of COMET.

This can increase snowfall or rainfall amounts in a storm or can even initiate thunderstorms. Today storms were initiated from upslope effects with very weak large scale forcing with small, localized effects, in essence, taking over.

In this situation the air that’s being blown by the wind from east to west has nowhere to go but up once it hits the ridge west of Hartford. The rising air cools and condenses and can tap into the atmospheric instability and continue to rise.

Here’s a snapshot of the radar this afternoon from weathertap. You can see a couple showers from north of Westfield, Mass and west of Hartford in the upslope regime. If you were to loop this image you’d see the showers basically staying in the same place which shows they are being driven by upslope winds.

Towns that frequently are in an upslope area tend to get much more precipitation then valley locations. The reason is that when the wind comes down hills and into a valley it sinks and dries. Upslope is how some mountains on the west coast of the United States can average 1000 inches of snow in a winter while areas to the east, on the other side of the mountain are deserts (eastern Washington, eastern Oregon, and Nevada).

This is happening all across the northeast today. The east slopes of the Berkshires and Litchfield Hills are seeing precipitation as are the east slopes of the Catskills and Worcester Hills in Massachusetts. The west slopes of the Berskhires/Litchfield Hills (towns like Pittsfield, Great Barrington, Salisbury, Sharon) are dry as they’re on the “downslope” side of the hills today. Notice on the satellite image below from UCAR the areas that are cloud-free.


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