Friday, 11:30 a.m.
We've seen a couple of forays of cold air into the country since the beginning of September. So far this month, though, it has been pretty warm in much of the nation. That's all about to change, as a deepening upper-level trough digs into the Ohio and Tennessee valleys, and allows a series of chilly air masses to fan out across the Plains into the South and East.
Up to now, most of the chillier air masses have failed to reach the East Coast in any meaningful fashion. If you look at the wide view of the 500mb pattern, as forecast by the 12z Oct. 18 NAM for this evening, you can see pretty easily why:
The strong ridge off the Southeast coast is diverting most of the disturbances northeastward, thereby limiting how much cooler air can crawl over the Appalachians into the East. The biggest push to date is happening now in the wake of the most recent upper-level disturbance and attendant cold front, as dew points have dropped dramatically into the 40s today in much of New England and the mid-Atlantic states down into North Carolina. The air itself is a little cooler than it was yesterday in many of these areas, but the main difference is in the amount of moisture in the atmosphere.
The next upstream disturbance delivered a slushy inch or two of snow to Denver early today, and as it sweeps across the Plains, it will cause some showers to break out from the western Gulf coast toward the lower Mississippi Valley this afternoon and tonight. At the same time, another feature will roll across the Midwest, carving out an area of low pressure along the next cold front that will spread mainly rain across the Ohio Valley into the eastern Great Lakes tomorrow.
By Sunday afternoon, these features will more or less combine into one that rockets across New England, drawing some cooler air over the mountains into the East. They will also have the impact of trimming back the Southeast ridge a little. With a clear sky and light winds expected Sunday night, many places will be chilly enough to have a touch of frost over the interior of the mid-Atlantic and New England.
Here's the 500mb forecast for Sunday afternoon:
And the accompanying surface map for Sunday night:
The next disturbance will already be digging across the Midwest, with another low moving into the Great Lakes and a cold front arcing from it through the Midwest back into the central Plains. The air mass behind it will be even colder, and this time it will have much better momentum to get not only to the Appalachians, but to the Gulf Coast, the East Coast and down into Florida. The upper-level ridge will be beaten down even more, as the 0z Oct. 18 GFS 500mb forecast for Wednesday evening depicts:
What follows for the rest of the week into next weekend for the entire eastern half of the country will be the coldest period of weather since last spring. One area of high pressure building into the South Wednesday night and Thursday will bring the initial shot of chill through the pattern. A second surface high will follow into the northern Plains Thursday, then into the Ohio and Tennessee valleys Saturday then into the East Sunday. Underneath the high, it will be quite chilly at night, with temperatures slow to rebound in the afternoons.
Furthermore, there will also be some lake-effect issues in around the Great Lakes. It's a bit early to start predicting any kind of accumulation for any given site, especially in light of how warm the Great Lakes are at this point. However, there will be some snow in the air from parts of the Midwest to the eastern Great Lakes and northern Appalachians during the second half of next week.
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