WEATHER (The following text accompanies the weather map for Sunday, Oct. 11, sent to computer graphics clients of The New York Times News Service. For information, call: 1-888-603-1036.) A wintry chill will grip areas from the northern Rockies to the northern Plains as the jet stream dips unusually far to the south. Low pressure crossing the Snake River Valley will bring periods of snow to northern Wyoming and southern Montana. A few inches of snow will blanket the region by evening, with more than 6 inches expected in a few places. Clouds and scattered showers will surround a front draped from the Gulf Coast to the Southeastern seaboard on Sunday. South of the front, sun and summery heat will persist throughout Florida. Temperatures will peak near record levels in parts of the peninsula, with highs reaching the mid-90s in a few locations. Canadian high pressure will bring sun and cool weather to the Midwest and Northeast. Scattered frosts and freezes will occur over much of the Northeast late Sunday night. The West Coast will be dry and seasonably cool. FOCUS: OCTOBER SHIELDhurricane quickly diminishes in size. which is nearly surrounded by warm water, remains at risk. The state of Texas is relatively shielded from tropical storms at this time of year. Since modern records began in the 1940s, only one hurricane, Jerry of 1989, has crossed the Texas coast in the month of October. 