Monday, 11:30 AM

One of the bigger growth areas in the combined meteorological and climatological fields in recent years has been the study and use of teleconnection indices. It seems that with each passing year there's a new one out there, or a change in one of the old ones. As more study is done to try and make connections in the atmosphere at various points in time and space to our weather present and past in an effort to come up with better forecasts, we're beginning to understand how much of a global puzzle this really is. We're making progress, but as well known, we have a long, long way to go to get excellent one- to five-day forecasts, let alone forecasts that go out two weeks, two months, two years, or more.

The North Atlantic Oscillation, or NAO for short, is one such teleconnection index. It's a measure of blocking in the atmosphere in a nutshell. On average, when it's negative, there's blocking in and around the North Atlantic, and that tends to lead to a cold weather pattern over the Eastern half of the country, all other things being equal. When the NAO is positive, we tend to see a more zonal flow across the North Atlantic, which favors pulling cold air away from the Eastern part of the country, again, all other things being equal.

Here's a link to the NAO:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/verf/new.nao.shtml

Notice how it was negative during the first half of the month or more, and it correlated very nicely to a period of phenomenal cold across much of the country from the Rockies to the Appalachians, not to mention some early season snows. Here's the ensemble forecast of the NAO:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/new.nao_index_ensm.html

It suggests that going forward the rest of the month into the early days of November, we shouldn't expect to see much in the way of cold weather in the East. And when we do, it probably won't be as cold as areas farther west, or last for any long periods of time.

Another index we closely monitor is the Pacific-North America pattern, or PNA. When it's positive, we often see above-normal heights along the West Coast, especially California, which leads to a dry and relatively warm pattern there into the Southwest, at the very least. On the other hand, when the PNA is negative, we tend to see some sort of troughiness over the very same region, which leads to cooler-than-average weather, and quite often rain and mountain snow.

Here's a link to the PNA:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.sprd2.gif

Note how the PNA is forecast to transition from largely positive in the next few days to largely negative thereafter.

While not always the case, quite often these two 'line up' together in a predictable way. When I say predictable, it's from the standpoint that a negative PNA (upstream trough over the West/Southwest) is usually linked up nicely with a positive NAO (downstream ridging).

Looking at the week ahead, some of these pieces look as if they're going to come together in a fairly predictable way. We currently have an upper-level ridge well off the Southeast coast, and another well off the Southern California coast. Between the two is a fairly sharp trough extending from the eastern Dakotas and Minnesota to West Texas, with another potent upper-level disturbance riding over the top of the Eastern Pacific ridge and driving a cold front into the Northwest.

The lead trough is actually two in one, with a northern piece breaking away through the Great Lakes into Quebec. There will be a little wet weather with it this afternoon into tonight around the Lakes, but not much, and little if any snow. The southern part of this is helping drive a cold front to the lower Mississippi Valley and the Texas coast, and that's leading to a lot of rain and strong thunderstorms. Considering the fact this area is already wet from above-normal rains over the past two months, the biggest issues these areas are facing is flooding.

There's a little disturbance off the Southeast coast, somewhat tropical in nature. It will bring clouds and some rain up through the Carolinas and eastern Virginia into Maryland, Delaware, southeastern Pennsylvania, New Jersey, and extreme southeastern New York into southernmost New England later tonight into tomorrow. The east winds it will generate will at the very least produce a lot of clouds that will keep temperatures in check throughout the mid-Atlantic states into southern New England tomorrow.

As the southern part of our trough in the Plains rolls across the lower Mississippi Valley tomorrow, it will combine with a surge of tropical moisture coming out of the Gulf to produce generous rains and thunderstorms throughout the South and up into the Tennessee Valley late tonight into tomorrow. Again, while there can be some severe weather, the bigger concern would be of flash flooding in these areas. This feature will rapidly spread rain across the Ohio Valley and over to the mid-Atlantic states tomorrow night, and across the Northeast Wednesday. The farther north you go into New York and central and northern New England, the less rain there will be with this eventual storm.

Wednesday night, the storm will dart away from the mid-Atlantic coast and pass to the south and east of Cape Cod, allowing high pressure to the north to direct drier air back down into the East from Quebec. In the absence of blocking, we shouldn't expect much cool air, and there won't be. In fact, if there's enough sunshine, temperatures could be quite a bit above normal, especially if a south to southwest breeze can kick in. That becomes more and more likely Friday and Saturday ahead of the next cold front.

That front will be tied to the trough sliding through the Northwest in the next 24 hours. It will buckle the jet stream over the West tomorrow and Wednesday, lowering snow levels considerably throughout the West. It will turn much, much cooler in the process.

With so much buckling, it gives time for the ridge off the Southeast coast to strengthen and actually migrate westward to over Florida. To translate that, basically think of this disturbance coming through the South tomorrow as a glorified warm front! This warm air then will head into the Ohio Valley Thursday and Friday and push temperatures way above normal, and that warmth will likely prevail in the East Friday and Saturday before that front roars to the East later this weekend.

I do not see this front bringing with it anywhere near as much rain to the Appalachians and Northeast as the one this past weekend. And for the Southeast coast, more of the same--much ado about nothing! You're simply going to be too close to the source of the warmth to get much rain. Even with the lead disturbance tomorrow and tomorrow night, a much greater percentage of rain will be in the mountains and foothills, rather than anywhere close to the coastal plain.

After that, we may see a period where the jet stream straightens out next week to again limit the cool air across the country on average, and allow for a fairly quiet entry into the month of November.

Posted by Joe Lundberg on Monday, October 26, 2009 10:29 AM
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