Thursday, September 19, 2013

Analog-Based Forecasts Favorable for Cold, Stormy Winter

Two analog-based forecasts are favorable for a cold and stormy winter in the Central and East US.

The first forecast we look at is from the TropicalTidbits site. These images show the November (left) and December (right) Z500 height anomalies. Don't look at just the colors; look closer at the contour lines. We will look at the December forecast for now.

In the December forecast, we see a few favorable things:

I. Troughing off the Baja California Coast
On the right-side image, contour lines are shown dipping down to the east of Hawaii and west of Baja California. The depression of contour lines indicates increased storminess, and this is a feature we have seen on multiple forecast models for this winter. In this case, storminess off the coast of Baja California would most likely mean storm systems would move northeast into the Plains and southern Rockies. From there, they will most likely either shift northeast to hit the Midwest, or keep south and eventually run up the Appalachians due to the feature I will discuss next.

II. Ridging in the Southeast
In the Southeast in the December forecast, you can see an arching feature. This is commonly a signal of high pressure, and this is called the Southeast Ridge in the winter. The Southeast Ridge is able to push storms north and create the Panhandle Hook, Great Lakes Cutter and Colorado Low storms. If the ridge is weak enough, we could see storms become Nor'easters down the road, but based on this forecast, I would favor a stormier Plains, Midwest and Great Lakes.

III. Ridging in the Pacific Northwest
In another common feature we have seen in model forecasts for this winter, contour lines arch up across the Pacific Northwest and western Canada. This feature is actually a specific atmospheric index called the Pacific North American Index (PNA). This analog forecast shows a positive PNA, which is shown by ridging along the western coasts of the US and Canada. The positive PNA then enables a favorable storm track for the Midwest and Great Lakes by helping to create that Southeast Ridge, and also let cold air flow south into the Plains.

IV. Negative North Atlantic Oscillation
We see ridging along the waters close to Greenland, and that is a telltate sign of a negative North Atlantic Oscillation (NAO). The negative NAO enables the jet stream to buckle south and allow cold air to flow into the Great Lakes and Northeast, while also suppressing the Southeast Ridge. With ridging down south being suppressed, storm systems can shoot up the coast and become Nor'easters. This negative NAO is a reason why I am still eyeing the Northeast for a stormy winter, even though many signs point to the Midwest and Great Lakes getting the stormy part of winter.

The other analog-based forecast we can use is the Constructed Analog, or CA model from the Climate Prediction Center. In this forecast, we see the 500 millibar anomalies on top, with temperature anomalies on the bottom. I can't really decipher the 500mb forecast because there are no contour lines, but the temperature forecast pretty much echoes the temperature forecast from the TropicalTidbits site. We see a cold Midwest, Plains and East Coast, likely a result of ridging over Greenland (negative NAO) and maybe some slight ridging along the West Coast, but the lack of contour lines does not allow me to elaborate on that further.

These two analog-based forecasts are turning out favorable forecasts for the Central and East US in terms of a wintry winter season. Whether they actually verify is to be determined, but things are looking up.