Friday, February 15, 2013

February 21-23 High Impact Potential Significant Winter Storm

Have downgraded system from Blizzard to Significant winter storm on account of recent model trends. Will maintain rest of storm title.

Infrared satellite imagery of the Pacific Ocean shows the storm of interest near the Aleutian Islands. We can see some convective activity associated with the front end of the storm system, characterized by a nearly vertical line of elevated infrared values. The storm itself remains far out to sea, away from any nearby weather stations in the Mainland that would otherwise help figure out model forecasts. We will need to wait until Tuesday or Wednesday for this system to crash ashore and the National Weather Service's weather balloons to get inside the storm and help out model forecasts.

Previous model agreement has been wiped away as we enter the dreaded timeframe where the models go every possible solution before they convene on one solution. This process typically takes a few days, but can last more than 5 days. I expect we will see model agreement come Wednesday evening when the system gets onshore. Model disagreement appears to be stemming from the forecasted strength in high pressure that will be over Canada and into the East US when the storm digs into the Southwest. When this storm system hits the West Coast and shifts south, it is typical to see high pressure arise in the East. The big question is, how strong will this high pressure be? A stronger high pressure would result in a more northerly track, while a weaker ridge would indicate a more cross-country, west-to-east storm track. This cross country idea is what earlier model solutions were showing. This dilemma is shown on the 500 millibar forecast from the American ensemble prediction system above. Our storm system is shown in the West US, with high pressure in the East. In this situation, the base of the high pressure system is in Canada, so the storm takes a more cross-country track. The actual American model has the base of the high pressure system in the Great Lakes, which is how we get that north track, as shown below:

My thinking is that this storm is going to hit areas that the models were in good agreement yesterday. The reason? Well, let's go in order here. First, we'll start in the West US. We have low pressure in the area, and that can instigate high pressure response in the Southeast US, which would go hand in hand with the American model solution above. However, looking at Greenland in the upper part of both images, we see high pressure to the east of Greenland. This is what's known as an east-based negative NAO. You can probably figure out why it's named east-based, but the negative NAO infers high pressure is in place in Greenland. This high pressure can then force the subtropical jet stream (located along the South US) to act up, and this brings the storm track further south. Also, it can oppress formation of the Southeast Ridge that the West Coast low pressure is trying to do. These two indices that are battling for the Southeast may just cancel out and we could see that cross-country solution as I suggested. The third (and probably most important) reason is that storms don't head right into high pressure systems. Don't get me wrong here, they can, but in a river, does water try to go to the place where it will get stuck? No, it goes towards the most open part of the river to keep on moving. This means that the low pressure system would theoretically not want to go north, right into the Canadian high pressure system. Rather, it would want to travel west-to-east (cross-country track), where it would encounter less high pressure anomalies. Adding onto that, if we see the base of this high pressure will be in Canada, the weaker high pressure anomalies in the Southeast would not be able to properly force the storm system north, like recent model solutions have been showing.

It's times like these when you have to stop looking at the models and think about how the atmosphere works. In times like these, I like to use my aforementioned river analogy. If this storm was a leaf that had fallen off a tree and into a river, would it want to move towards the rock in the river or towards the open river channel? If you think it wants to move towards the open channel, you've just explained the big paragraph I typed above. And in all honesty, the models are worthless. Until the storm gets on land, we might as well be trying to throw a dart, in the dark, blindfolded, spun around several times, with the correct solution being the dartboard bullseye three millimeters wide and a mile away from you. But if you're going to look for hints, always look at the ensembles. They stand the best shot of being right from a time this far away, which is another part of the reason why I support the cross-country solution.

I maintain the high impact part of the storm title because I do believe that there will be an intense snowstorm somewhere with the storm. The system in question will tap into pure Gulf of Mexico moisture that will allow heavy snow to fall across a potentially wide swath of land. Right now, a North Platte, NE to Cedar Rapids, IA swath is looking good for the most snow, with similar amounts east until Gary, IN.

There will be more developments as we progress through the weekend and into next week. My thoughts will undoubtedly change, but that's what weather does: change. Please don't ask for your location's snow, I will not answer if you ask.