Wednesday, November 25, 2015

Southern Flooding Biggest Threat From Thanksgiving Weekend Storm

Forecasts call for three to eight inches of rain
in a huge area of the southern Plains through the
weekend (depicted in red on map) 
We're still on track to have a stormy Thanksgiving weekend in much of the western and central United States, with a variety pack of snow, wind, ice, sleet, snow, thunderstorms and especially flooding.

The storm has already been making its way through the far western United States, with more than a foot of snow in parts of California's Sierra Nevada mountains, which frankly need the moisture.

To give a sense of how energetic this storm is, the community of Bend, Oregon received about 14 inches of snow, and four inches of that came within three hours, says The Weather Channel.

Bend is on the eastern side of the Cascades, which normally block moisture from places like Bend. The place is semi-arid. On average Bend gets about two feet of snow in an entire winter, and they're about two thirds there with just one storm.

Eastern skiers are salivating over the Rockies, because they've already gotten some snowstorms and many resorts in the northern and central Rockies can expect another 12 to 18 inches of snow out of this.

Where I live in Vermont, we got just a dusting of snow yesterday and no significant snow is in the forecast through the weekend. Skiers and riders here are dying for a taste of the Rockies, let me tell you.

The storm will emerge out into the Great Plains later today and especially Thanksgiving, and that's where the real trouble begins.

And it's to a great extent Sandra's fault.

Who the hell is Sandra?

Glad you asked. Sandra is a late season hurricane that formed this week off the western coast of Mexico. It's very late season. According to Dr. Jeff Masters in his Weather Underground blog, only six tropical systems since 1949 have formed in the eastern Pacific after November 18. Two of them were this year!

Rick formed a week ago out there there but sputtered to its death when it moved northwestward over colder water and high upper level winds and fell apart.

Sandra is a powerhouse. It had 85 mph sustained winds this morning and is expected to rapidly strengthen in to perhaps the strongest hurricane for so late in the season in that region of the world.

Sandra will encounter strong upper level winds and start to fall apart tomorrow and especially into the weekend, but still might make landfall in Mexico as a tropical storm -- the latest on record.

Blame El Nino for all this. It made the water off Mexico unusually warm, which helps feed tropical storms.

Since Sandra IS supposed to weaken into just a big patch of moisture in a couple or few days, why am I making such a big deal out of it?

Again, glad you asked.

This big plume of moisture from Sandra will feed into that storm I talked about that's moving into the Plains.

Combined with the wetness the storm will already be drawing off the Gulf of Mexico, we'll get a very big area of torrential rains Thursday into Sunday, especially in northeastern Texas, eastern Oklahoma and in most of Arkansas.

I'm talking four to eight inches of rain, possibly locally more, so you can why flash flood watches are already flying for that region. It's been wet there to begin with, so it wouldn't take all that much rain to cause flooding.

By the way,  Dallas, Texas needs only about three inches of rain the rest of this year to make 2015 its wettest year. Dallas will probably get more than three inches of rain in just this one storm.

North of this potential flood zone, a broad stripe of freezing rain, sleet and snow will break out from the Texas panhandle all the way northeastward to Wisconsin and upper Michigan. That's a HUGE area to have ugly driving conditions on a holiday weekend.

Most areas won't have enough freezing rain to bring down an enormous number of trees and powerlines,  but the roads sure will be icy.

An exception to the few trees and powerline down prediction might be the Texas panhandle and parts of Oklahoma, where ice could accumulate up to a half inch thick. That's more than enough to break branches and wires

So yeah, yuck.

By the way,  up here where I live in the Northeast, we stay out of this. We'll get some rain Friday night, changing to some scattered snow Saturday, but as I said, there won't be nearly enough rain to cause flooding, and snow, in the few places it does accumulate, won't amount to much at all.

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