Severe weather threat returns to the northern and central Plains onTuesday, followed by severe weather and heavy rain/flash flooding threatsfarther down the central Plains on Wednesday......More high-elevation snows expected across the Pacific Northwest intothe northern and central Rockies for the next couple of days......Warmer than average temperatures across the southern tier states to theMidwest and Mid-Atlantic but cold and windy across the Northwest...The weather pattern that keeps sending fast-moving disturbances across thewestern U.S. will continue to form new low pressure systems in quicksuccession over the mid-section of the country through the next couple ofdays.  The latest low pressure system that brought severe weather andflash flood emergencies over eastern Texas, together with widespread coldrain across the northern U.S., to wet snow near the Canadian border, willweaken as it tracks northeast across the Great Lakes into southern Canadaon Tuesday.  Meanwhile, the heaviest thunderstorms and severe weatherassociated with the trailing front had moved off the Louisiana coast. Less intense showers and thunderstorms are expected to push eastwardacross the Mid-South, Tennessee and Ohio Valleys toward the Appalachiansand New England on Tuesday, before becoming more widely scattered alongthe Eastern Seaboard into Wednesday morning.  Snowfall on the order of6-12 inches are forecast to accumulate over parts of the Cascades andnorthern Rockies increasing to nearly 2 feet over the higher elevations ofGlacier National Park.As the surge of cold air pushes through the Northwest on Tuesday with theleading cold front reaches the northern and central Plains, a couple oflow pressure systems are forecast to develop.  The low pressure systemthat will move across the upper Midwest will bring a threat of severethunderstorms across the northern and central Plains ahead of the trailingcold front on Tuesday.  Another low pressure system will develop over thecentral Plains on Tuesday, this system will bring a higher threat ofsevere weather and more widespread heavy rain threat farther south intothe central Plains later on Wednesday as the system intensifies.Temperature-wise, fast-moving cold upper troughs propagating through theNorthwest and Northern Plains/Upper Midwest will keep things cooler thanaverage today.  More cold air will push through the Pacific Northwest onWednesday.  Meanwhile, a warming trend will send afternoon temperatures ashigh as the lower 90s for the next couple of afternoons across the centralto southern High Plains.  Temperatures will also be well above averagealong the East Coast on Tuesday followed by a slight cool down onWednesday as a back-door cold front dips south from New England.