Autumn officially begins Tuesday, and it will start to feel like it in the Lehigh Valley over the coming days as cooler weather sets in.

First off, a beautiful Saturday is in store for the region, with mostly sunny skies and a high temperature in the low 70s, according to the National Weather Service.

The sunshine will be short-lived, as Sunday is expected to bring showers and possible thunderstorms as a cold front moves in. Rain showers are forecast to arrive in the early hours of Sunday morning, with up to 1.5 inches of rain possible through Monday, according to the weather service.

Thunderstorms are also possible Sunday evening. The risk of excessive rainfall is marginal, but the weather service warns there could be localized flash flooding, especially in urban areas prone to poor drainage.

The rain should wrap up overnight Sunday into Monday, giving way to the coolest weather of the season so far to start the new week.

While Monday’s high temperature is expected to be around 70, on Tuesday, Wednesday and Thursday it will only be in the low 60s, according to the weather service forecast.

Here’s a look at the extended forecast:

Saturday: Mostly sunny, with a high near 74. Northeast wind around 5 mph becoming calm in the afternoon.

Saturday night: A chance of showers, mainly between 2 a.m. and 5 a.m. Mostly cloudy, with a low around 58. Southeast wind around 5 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Sunday: Showers, with thunderstorms also possible after 5 p.m. High near 71. Southeast wind around 5 mph. Chance of precipitation is 90%. New rainfall amounts between a quarter and half of an inch possible.

Sunday Night: Showers and thunderstorms before 8 p.m., then showers likely between 8 p.m. and 2 a.m. Low around 56. Southwest wind around 5 mph becoming north after midnight. Chance of precipitation is 80%. New precipitation amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Monday: Partly sunny, with a high near 69. Northeast wind 5 to 10 mph.

Monday Night: A chance of showers after 2 a.m. Mostly cloudy, with a low around 49. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Tuesday: A chance of showers. Mostly cloudy, with a high near 63. Chance of precipitation is 30%.

Tuesday Night: Mostly cloudy, with a low around 48.

Wednesday: A chance of showers after 8 a.m. Mostly cloudy, with a high near 63. Chance of precipitation is 30%.

Wednesday Night: A chance of showers. Mostly cloudy, with a low around 48. Chance of precipitation is 30%.

Thursday: A chance of showers. Mostly cloudy, with a high near 63. Chance of precipitation is 30%.

Thursday Night: A chance of showers. Mostly cloudy, with a low around 48. Chance of precipitation is 30%.

Friday: Partly sunny, with a high near 66.