Heavy mountain snow with brief snow squalls across thenorthwestern U.S. through tonight into Tuesday.Moderate to heavy snow from North Dakota into northern Minnesota onTuesday.Severe thunderstorms possible Tuesday evening/night from the Midwest tothe Ohio Valley/Great Lakes.Record warmth engulfing much of the mid-section of the country will bedrastically colder by Wednesday with sub-zero will chills across thenorthern Plains and upper Midwest.The stage is set for a dome of arctic air that has been locked up inCanada to be unleashed into the United States through the next couple ofdays.  An amplifying upper-level trough that is instrumental in unleashingthe arctic air mass in Canada for the past week will first push a coldfront rapidly across the northwestern U.S. through tonight.  The frontwill sweep across the region with rapidly falling temperatures, gustywinds, and brief periods of snow squalls across the interior PacificNorthwest, reaching into the northern Rockies by Tuesday morning. Meanwhile, the arctic air will be surging into the northern Plains inearnest tonight.  A low pressure system that forms along the front willorganize a swath of moderate snow across North Dakota through tonight,becoming heavy snow Tuesday morning across northern Minnesota.  The lowpressure system is forecast to intensify rapidly and move across the GreatLakes Tuesday night into Wednesday morning, bringing a period of snowacross the region behind the storm center.Much of the mid-section of the country has been dominated by a dry andwarm air mass under the influence of an expansive high pressure systemlately.  A large area of record warm temperatures is engulfing much of themid-section of the country this afternoon.  As the arctic air massadvances into the United States through the next couple of days,drastically colder temperatures will be ushered down the Plains onTuesday, and then across the Great Lakes, the Ohio Valley toward the DeepSouth and the East Coast by Wednesday evening, with sub-zero will chillsacross the northern Plains by tonight reaching into the upper MidwestWednesday morning.  In addition, the cold front is forecast to become verypotent, with showers and thunderstorms rapidly forming over the OhioValley into the lower Great Lakes Tuesday evening along with warm andgusty winds.  Some of these thunderstorms are expected to become severe aswell.  The winds behind the front will become very strong and gusty fromthe north and northwest with rapidly falling temperatures surging down thePlains states and up into the Great Lakes through Wednesday.Following the heavy mountain snows and snow squalls tonight, precipitationis expected to gradually become lighter and more scattered throughWednesday.  However, moisture from the next Pacific system is forecast toreach the Pacific Northwest by Tuesday night with coastal rain andincreasingly heavy snow reaching into the Cascades on Wednesday. Meanwhile, some rain could reach southern Arizona and southern New Mexicolater on Wednesday when a lingering Pacific cutoff system finally makesits way into Baja California.