The "Cool Down"... October 6th 2012.

As I am browsing Twitter and Facebook, I have been noticing a trend in statuses talking about a major cool down this next weekend. A lot of these statues updates have been talking about snow and "winter" like conditions occurring (The weekend of Oct. 6th.). In this blog, I hope to convey an aggressive defense against these arguments and too provide what I think is correct.

So let me ask you a question... What happens when nothing more than rain actually happens during this predicted event?  All the hype that went into your forecast essentially convinces the general public that we do get paid to be right half of the time.

Here is a progression of the North American Cyclone that is going to traverse through the mid and eastern sections of the United States this coming weekend.

So what is significant about these maps? Why are people calling for snowstorms? It's because we are transitioning into a new season and a lot of forecasters do not know how to interpret these changes effectively. Whenever we get close to November and December, we notice the 5400m contour line moving ever southward. What is the 5400m contour line, you say? It is derived from the 1000-500mb thickness chart located in the upper right hand map on the chart provided above. The 1000-500mb chart's usage is for estimating where changes in surface air temperature occurs. These differences in surface air temperature can also be interpreted as air masses (a homogeneous mixture of air with like characteristics). It also locates warm and cold fronts. Essentially, the height is just a measure of how cold (compressed) or warm (expanded) the surface air temperatures are. So with that in mind, a general rule of thumb is that the 5400m line is associated as the cross over between rain and snow due to the associated temperature at which the height occurs.

Where a lot of forecasters get into trouble is when to take that rule of thumb into consideration. For an example, at this time in the year, in the Ohio Valley region, surface temperatures are not cold enough to produce the required results for snowy conditions. I have graphical explanations for my reasoning.

Here is a temperature reading for the same time period that the above chart is predicting snow for our forecast.

And

These graphs can be a little convoluted. So let me try to help you decipher the truth for yourself. If you can locate Indiana on the map, you can tell the the temperatures basically fit the average between 35-40F. While this is a cooler temperature (suggested by the lower height contour stated above) it is NOT cold enough to produce "winter" like conditions. The only areas cold enough to support snow would be Northern Wisconsin and Minnesota. The surface just hasn't had enough time to cool down to allow for snow.


So what do I think is going to happen?
Here is an example of a forecast I would give.


A deep trough is going to progressively build and push through the central United States, giving way to cold temperatures and rain. The low center over Texas will push North East, Intensify, then allow for the trough to really dig and extend southward.

                                          12z- Sunday.

As you can tell, the low pressure by 12z (8am) Sunday has significantly strengthened and the trough has dug deep into Southern Indiana and Northern Kentucky. On the out fringes of the trough, PVA can be noted on the 700mb map (PVA - Positive Vorticity Advection - Dynamic lifting) around the Ohio Valley and on into the Appalachian Mountains. This area of dynamic lifting will more than likely trigger precipitation. How much exactly is a little hard to tell. However, the point is that this area of precipitation falls well into the 40-50F, so likely form of precipitation will be liquid. Temperatures will probably top off at around 65+/- a few. As the boundary moves through, residence could very well experience stronger winds in excess of 15-20mph. Areas that could receive snow would be Northern Wisconsin as a small area of precipitation appears to form in that area.


I think the main point to take away here is to exclude wish casting from actual forecasting. To use more than one model to make accurate forecasts and to not base "snow" forecasts solely on the  5400m line or what one model says is going to happen(as stated above, I examined the 5400m line but also examined the temperatures associated with that area and came to the conclusion that there is no possible way that snow could occur in 40-50F weather).

-Thanks for reading!