Tuesday, September 4, 2012

Is the Wintertime El Nino Pattern Now Emerging?

It has come to my attention that model guidance and observed atmospheric conditions are both hinting at the possibility of the El Nino winter pattern emerging right now, in early September. Let's take a look at what the typical El Nino is comprised of.

During the winter months, an El Nino typically brings about an enhanced subtropical jet stream, which resides in the lower part of the atmosphere. This subtropical jet stream produces wet conditions across the southern US, as well as cooler than normal temperatures in the Southeast. Even though the graphic does not show this, the jet stream can then recurve north and send storms up the East Coast as Nor'easters, a slang term for monster winter storms that slam the Northeast with as much as 2 feet or more of snow, thanks to the Atlantic Ocean's influence. Additionally, a low pressure system will be spotted in the northern Pacific, in the midst of a warmer than normal western Canada and northern US.

It's time to bring to attention the most crucial part of forecasting- the observed conditions prior to now.

We'll start off with the 15 day 500 millibar height charts. This shows a mean of 500mb conditions observed over the past 15 days. What we discover when analyzing this chart is the presence of a stormy pattern across the Southeast, we well as a prevalent ridge in the West. Also shown but not marked is a dip in the lines south of Alaska.
I have marked the first two anomalies with their respective indicators. In situations like this, we can determine where the jet stream lies. Here, I believe that the jet stream itself resides in southern Canada, with a buckle into the Northeast, thanks to a recently negative NAO. Additionally, a strong high pressure ridge has made its place in the West.

You can see the arcing formation on the West, made by the ridge. However, the lack of that same arcing immediately south of where I marked the 'H' tells me that ridging is not strong in that area. This could provide a base for the presence of the subtropical jet stream that is seen in the wintertime months of an El Nino.

Now, we move into the 30 day observed 500mb field, however, this time, the chart is showing anomalies of the 500mb heights and not the height field itself. That said, let's take a closer look.

I did mark out two areas of interest- one in the Eastern US, and one on the extreme Northwest. I marked off the Northwest, because that little red circle means a monumental piece of information. As is traditional with an El Nino, a ridge is placed in the Northwest, keeping away precipitation and general cooler weather. This coincides nicely with the observed indicator shown above. I do not personally believe this is a coincidence, purely because we are already in an El Nino, and this is not the only signal I am seeing.

Also marked is a purple circle. This circle covers much of the Eastern US- in fact, it encompasses half of the nation. This purple circle is a slightly stormier than normal anomaly. It is placed in a position that is home to the stormiest weather in the nation come El Nino in the wintertime. This anomaly indicates that the areas favored for stormy weather in a winter El Nino are also shown to be stormy in this fall situation. Again, I do not believe coincidence is playing a role here.

Not marked but still of interest is another big red blob over eastern Canada into Greenland. As is common with an El Nino, a negative NAO can be observed. This negative NAO causes the buckling of the jet stream in the winter, leading to frigid temperatures in the Eastern US, and an increased likelihood for winter precipitation in the East Coast. The Negative NAO can be seen by a ridge of high pressure staying over Greenland. As we see above, that is also present. I wil say it once more: Coincidence? I think not.

Here's a little more recent data- the newest European ECMWF model forecast as of September 4, 2012. This forecast shows 500mb heights for September 9. What we see is a dramatic failure of the jet stream that sends a giant disturbance down into the Eastern US. This is quite typical of something we would see in the winter- a system diving into the East followed by cooler than normal air in the region.
Note the high pressure in the West and the arcing of the 500mb height field across the Northern Plains. Also take note of the lack of said arcing across the Southwest. This tells me two things. One, the jet stream is located in the US/Canada border, and Two, there is a lack of ridging in the Southeast. This may or may not be an indicator of a more stormy pattern in that area (possibly the subtropical jet stream)- the ECMWF offers a limited array of products for free.

This is indeed quite significant should it verify. It is unusual to see such a strong system and associated cold air dipping into the nation in the Fall. In the winter, such a phenomenon is much more likely to occur, but in the fall, the chances are drastically lowered.

Finally, we have the 8-10 day 500mb forecasts from the ECMWF and GFS. These models are forecasting the same time period side by side, and are showing 500mb height anomalies for the mentioned timeframe. On the left is the 12z ECMWF, and on the right is the 12z GFS.

While both models do have some differences, one can determine that both are showing a general high pressure regime in place across the West, with a stormy pattern in the East. Again, an El Nino leaves footprints similar to those shown above. The GFS even goes as far as to put a below normal height anomaly in the Northeast- a symbol usually observed in a wintertime El Nino.

I have showed you the observed conditions over the past month, and the forecast conditions over the past couple of weeks. If you have been following along, I believe you may be seeing a similarity in many of these images.
If we look over all of the images, we end up with a ridge in the West and a stormy pattern in the East. A low pressure system in the north Pacific is also observed, and the jet stream displaced north into the US/Canada border is also found. If you compare this to the typical winter El Nino conditions shown at the top of the post, you may find that the listed conditions match up unusually well with what forecasts and observed conditions are showing.

That said, it does appear possible that the El Nino now in place may be trying to skip fall and go into winter patterns. Based on the ECMWF forecast image shown above (not the side-by-side), an unusually strong storm system is usually seen in the winter, and the observed conditions and forecast conditions match up with the typical El Nino.


DISCLAIMER: Long range forecasts in a transition season (Fall or Spring) can be unusually difficult, so use with caution.