The National Weather service is warning of a fairly good chance of locally heavy rainfall within any storms this afternoon and tonight in St. Lawrence County.

The chance of rain is put at 40 percent this afternoon, rising to 60 percent tonight.

Highs today will top out in the mid 80s, and overnight lows will be in the mid 60s.

A slight chance of rain will persist Thursday, put at 20 percent, rising to 40 percent at night. Highs are expected in the low to mid 80s and lows around 60.

Friday will feature sunny skies, dry weather and near normal temperatures.

A frontal passage tonight will result in a slight downward trend in temperatures and dewpoints to finish out the week, returning to near normal by the weekend.

The NWS forecast for the St. Lawrence Valley and northwestern Adirondacks:

• This Afternoon: A 40 percent chance of showers and thunderstorms. Some of the storms could produce heavy rain. Cloudy, with a high near 84. West wind around 5 mph. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

• Tonight: Showers and thunderstorms likely before midnight, then a chance of showers. Some of the storms could produce heavy rain. Patchy fog after 3am. Otherwise, mostly cloudy, with a low around 67. Northeast wind around 5 mph becoming light and variable. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

• Thursday: A slight chance of showers and thunderstorms before 11am, then a slight chance of showers and thunderstorms after 2pm. Patchy fog before 9am. Otherwise, partly sunny, with a high near 83. Southwest wind 5 to 8 mph. Chance of precipitation is 20%.

• Thursday Night: A 40 percent chance of showers and thunderstorms before midnight. Mostly cloudy, with a low around 61. Southwest wind 5 to 8 mph. New precipitation amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

• Friday: Sunny, with a high near 78. Calm wind becoming northwest around 6 mph in the morning.

• Friday Night: Mostly clear, with a low around 56. Northwest wind around 5 mph becoming calm in the evening.

• Saturday: Mostly sunny, with a high near 81. Calm wind becoming northeast around 5 mph in the afternoon.

• Saturday Night: Partly cloudy, with a low around 61. Northeast wind around 5 mph becoming calm in the evening.

• Sunday: Mostly sunny, with a high near 84. Calm wind becoming northeast around 5 mph in the afternoon.

• Sunday Night: Partly cloudy, with a low around 64. Light and variable wind.

• Monday: Partly sunny, with a high near 84. Light east wind.

• Monday Night: Showers likely. Mostly cloudy, with a low around 66. Light east wind. Chance of precipitation is 60%.

• Tuesday: Showers likely. Mostly cloudy, with a high near 80. Southeast wind around 6 mph becoming southwest in the afternoon. Chance of precipitation is 70%.