Joe Lundberg

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A Gradual Progression to Colder Weather

October 16, 2013; 11:14 AM

Wednesday, 11:59 a.m.

Traces of Octave's moisture are still being felt from Texas to the Tennessee Valley at this hour. That eastern Pacific tropical storm has long since dissipated, but not without leaving a decided mark. Several inches of rain pelted parts of Texas and Arkansas over the past couple of days, and that rain can clearly be traced to Octave. Even before the storm made landfall, southwest winds aloft were carrying the storm's moisture across Mexico into Texas and points downstream. With a front moving into place at the same time, a natural overrunning boundary developed to enhance the rainfall. Flash flood watches remain in effect for northeastern Texas, several parishes in northwestern Louisiana and most of southern Arkansas.

While this has been going on, an upper-level disturbance has been rolling around in Colorado, producing some snow in the ski areas and even a few snow showers south of Colorado Springs to the New Mexico border. That feature will roll out of the southern and eastern Rockies and across the southern Plains this afternoon and evening, as depicted by the 12z Oct. 16 NAM 500mb forecast:

This feature will pull the last of the moisture out of Texas and across the lower Mississippi Valley tonight. A wave of low pressure will then form along the front later tonight over the Tennessee Valley. The combination of the upper-level disturbance, a developing surface low, an overrunning surface and tropical moisture will lead to an enhancement of rain with some stronger thunderstorms tonight across northern Mississippi into northern Alabama and southern and eastern sections of Tennessee. That enhancement will ride the spine of the Appalachians tomorrow, keeping it mild east of the mountains yet again, despite little sunshine. From there, the surface storm will rocket across southern New England tomorrow night with rain, then quickly exit through the Gulf of Maine and Nova Scotia on Friday.

Behind the storm, the most immediate impact will be to dry out the Eastern Seaboard. The almost constant onshore flow for the past nine days will finally be completely erased, and with some sunshine, temperatures really won't be too much lower than tomorrow despite an air mass change. The big change will be Friday night, when a clear sky will allow temperatures to finally drop back to near normal.

Throughout all of this, the cool air will still be piling up west of the Appalachians. With the presence of the upper-level ridge over the Bahamas and South Florida, however, the core of the cool air will be thwarted in its attempt to get over the Appalachians to the East Coast. Cooler, yes, but really no worse than normal. Look at the 6z Oct. 16 GFS ensemble 2-meter temperature anomalies for Friday:

Now look at the 500mb forecast for Saturday morning:

A feature that drops down the back side of the building or digging upper-level trough will scrape the Front Range of the Rockies late tomorrow night into Friday with a burst of rain and snow, then dive into the southern Plains to produce a relatively small area of rain in Kansas and Oklahoma. A weak wave of low pressure will form Friday night along the next cold front in the pattern, and as it darts for the eastern Great Lakes Saturday, it will bring clouds and rain along for the ride, along with more chilly air. Once again, though, that upper-level ridge off the Florida coast will prevent the worst of the chill from reaching the East Coast, though Sunday will be a little cooler no doubt:

In fact, the coolest air will hold off in the East until late next week. There will be another warmup at midweek ahead of an even stronger upper-level trough. Only then, when that upper-level ridge breaks down finally, will there be a decided chill in the air from New England to the Carolinas as we move into the final week of October.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.