The ‘first major synoptic system of the season’ is coming, forecasters say. But what does it mean?

• A cold front will approach the region and move through over the weekend
• It will bring rain that could persist through part of the day on Saturday
• Sunday likely will be our coldest day since May

BETHLEHEM, Pa. — Been longing for cooler, autumn-like weather? Well, the wait's almost over, forecasters say.

“The first major synoptic system of the season will impact the region beginning Friday and over the weekend,” the National Weather Service said in its latest forecast discussion.

What does that mean, exactly?

Synoptic, or large-scale weather systems, span distances of a few hundred to several thousand miles and include features that have a lifespan of a few days to a week or more. They include:

• High pressure systems
• Low pressure systems
• Air masses
• Mid-latitude and extratropical storms
• Tropical storms and hurricanes

When you watch your local weather forecast, you are seeing synoptic scale meteorology presented on a weather map.

It differs from mesoscale meteorology, which includes events that are smaller in size, last a day or less and impact areas on a regional or local scale — such as thunderstorms or tornadoes.

How will the system come together?

A cold front off to our west is expected to amplify as it crosses the Great Lakes on Friday and begins to approach our region. Meanwhile, Tropical Storm Phillipe will be out in the Atlantic racing north.

“That’s going to be a player in what’s going to happen with this frontal boundary" cold front, EPAWA meteorologist Bobby Martrich said in his Thursday video forecast.

“We knew the front was coming, it’s just a matter of timing,” Martrich said, predicting the movements of Phillipe would ultimately slow the approach of the front into our area.

Phillipe "is not going to affect our area at all, but it will slow the boundary up a little bit, so this is now something that’s going to continue into Saturday — but I don’t think it’s an all-day deal,” Martrich said.

What are the expected impacts?

This is the “first fall front of the season” and will please those who have been longing for cooler temperatures, the weather service said.

Current guidance shows rain spreading into the region beginning Friday afternoon or evening — perhaps with a few rumbles of thunder — and the coverage and intensity of rain increasing overnight into Saturday.

The cold front should push through Saturday morning or early afternoon, with rain persisting until the front passes.

The greatest rainfall amounts of a half-inch to an inch of rain are currently forecast along and north of Interstate 78, though Martrich said there should be further clarity on the arrival and departure of precipitation on Friday.

Winds are expected to increase Saturday night and Sunday, perhaps into the 15 to 25 mph range, with gusts near 30 to 35 mph.

Sunday likely will be the coolest day for the region since early May, the weather service said.