The National Weather Service has posted a winter storm warning for heavy snow in the St. Lawrence Valley in effect from 8 a.m. Wednesday to 8 p.m. Thursday.

The warning means severe winter weather conditions are expected with significant amounts of snow, now expected to be between 8 and 12 inches.

That warning also means the snow will make travel dangerous.

Light snow will develop Wednesday morning, “but once it starts it will come on quickly,” said one bulletin from the NWS.

It will increase in intensity by Wednesday night, and begin to taper off from west to east going into Thursday morning.

And temperatures will plummet into the single digits as the storm passes.

Temperatures today in the St. Lawrence Valley are expected to be mild, with high around 40, and the sky will be cloudy before tomorrow’s snowstorm.

The snow could begin overnight tonight, and lows will be in the mid-20s.

Temperatures tomorrow will drop to below 20 by late afternoon. Winds could gust as high as 30 mph.

Tomorrow night the low is expected to be around seven degrees, and gusty winds will continue.

You can track the storm through the resources on the NorthCountryNow.com weather page.

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

• Today: Mostly cloudy, with a high near 41. West wind 9 to 11 mph.

• Tonight: A 30 percent chance of snow, mainly after 4am. Cloudy, with a low around 26. West wind around 11 mph. New snow accumulation of less than a half inch possible.

• Wednesday: Snow, mainly after 7am. The snow could be heavy at times. Temperature falling to around 19 by 5pm. West wind 11 to 13 mph, with gusts as high as 29 mph. Chance of precipitation is 100%. New snow accumulation of 3 to 7 inches possible.

• Wednesday Night: Snow. The snow could be heavy at times. Low around 7. Wind chill values as low as -3. West wind around 13 mph, with gusts as high as 33 mph. Chance of precipitation is 100%. New snow accumulation of 3 to 7 inches possible.

• Thursday: Snow likely, mainly before 2pm. Mostly cloudy, with a high near 17. West wind around 13 mph, with gusts as high as 26 mph. Chance of precipitation is 70%. New snow accumulation of less than one inch possible.

• Thursday Night: A 30 percent chance of snow showers before 8pm. Partly cloudy, with a low around 1. Southwest wind 8 to 13 mph.

• Friday: A 30 percent chance of snow showers. Mostly cloudy, with a high near 33. South wind around 9 mph.

• Friday Night: Snow showers likely. Cloudy, with a low around 29. South wind around 10 mph. Chance of precipitation is 60%.

• Saturday: A chance of rain and snow showers. Mostly cloudy, with a high near 38. West wind around 9 mph. Chance of precipitation is 50%.

• Saturday Night: A chance of snow showers. Mostly cloudy, with a low around 15. West wind 6 to 8 mph. Chance of precipitation is 30%.

• Sunday: Partly sunny, with a high near 24. West wind around 8 mph.

• Sunday Night: Mostly cloudy, with a low around 4. North wind 5 to 8 mph.

• Monday: Partly sunny, with a high near 22. Northwest wind 3 to 5 mph.