Saturday, February 16, 2013

February 21-23 High-Impact Potential Significant Winter Storm (Posted 2/16)

Storm title is kept the same despite crucial model differences. I do this because, despite said model differences, it is quite obvious that someone will get a thumping of snow out of this storm system. The question right now is: Who?

Satellite imagery indicates that our storm system is moving into the Gulf of Alaska at this hour, with elevated levels of infrared imagery suggesting the center of this storm system is located around the Aleutian Islands. Pressure observation charts suggest the storm system has a minimum pressure of 988 millibars, a pretty solid storm system. The comma shape of the infrared imagery depicts the storm as a well-functioning system that will certainly be able to make some noise on land, both on the wintry side and the severe weather side.

This continues to be an unresolved issue between the European model and several other reputed models. Shown above is the Day 5 forecast for 500 millibar height anomalies. Cold colors indicate low pressure systems, while warm colors display high pressure areas. We see our storm system in the Southern Plains at this time, with high pressure in the East US and additional high pressure near Greenland. This high pressure system in the East US is what is causing such intense model differences. Both the European suite and American model suite agree that the storm will be ejecting from the Rockies in the position shown above in the apricot color. That's where the similarities stop. The European model believes high pressure in the East will force the storm system to move north-northeast, cutting out many regions of the Midwest from snow that was previously shown by this model.

These are the American ensembles for the same timeframe and same parameters as the European image above. We still see our storm system in the Southern Plains, although there is no longer an apricot shade. This is to be expected, as the ensemble members begin to diverge at this stage and therefore the mean solution is not as strong as the single European model. In comparison, the high pressure in the East US is weaker and centered further north in Canada than the European model has it. This is a crucial difference, as the weakness of the high pressure does not force such a northward movement anymore. It does move northeast, but nearly as drastically as the European model. Also of interest is stronger high pressure in the Pacific than in the European model. This stronger high pressure would force the storm further south.

I'm having trouble understanding why the European model takes the storm directly into the high pressure system. It is apparent that we will see some northward movement in response to persistent low pressure anomalies in the West and high pressure anomalies in the Northeast Pacific. These two factors make up the negative Pacific North American index (PNA). The negative stage of the PNA responds to the low pressure anomalies in the West by provoking high pressure in the Southeast US, and this does correlate with the issue the models are having with the high pressure in the East US. If it was just the negative PNA, we would most definitely see the solution being shown by the European model. However, we now go back to the previously-mentioned high pressure over Greenland. When high pressure is established to the east of Greenland, it is called and East-based Negative NAO; 'East based' for being east of Greenland, and the negative NAO (North Atlantic Oscillation) implies high pressure over Greenland. The negative NAO provokes the subtropical jet stream to become more active and pushes the overall storm track south. It also suppresses the high pressure in the Southeast, which is why I'm not confident in the north-bound European model. Both images above show the east-based negative NAO, but both have different solutions. It all comes down to which factor has the stronger influence. Because the negative PNA is upstream of the Midwest, it will have a stronger effect than the negative NAO, which is far up in Greenland. However, by the time the storm system is able to reach the Plains, the negative NAO will hold some influence and ought to be able to keep the storm system south, definitely further south than the European model's solution.

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.

The 12z run of the American model shows very heavy snow in the Plains. I am very confident that Nebraska and Iowa will certainly cash in on heavy snowfall. Even South Dakota could receive some extreme snowfall. Amounts could very well exceed 18 inches based on snow-to-water ratios above the usual 10:1 (10 inches of snow equals 1 inch of water). We could see ratios flirting with 20:1, meaning there would be lighter, fluffier snow, but a lot more of it. Towards the Midwest is where confidence drops. The American model believes in a sharp cutoff near Chicago. A sharp cutoff will happen, but this is the first time the American model proposes a cutoff zone so far north. The American ensemble system has been more persistent in the rain/snow line remaining in central Illinois. This is the same solution previously shown by the American model, up until now.

As of now, the European model is a far north outlier, and the American ensembles have been very consistent. I preferred the American ensemble solution, because I don't buy into an extreme far north solution. To me, there are three stages of the models. The first is the Long Range track, where the models get a consensus roughly 10 days away from the event. The second stage is what I like to call the '60 Hours of Chaos', where the storm gets within 8 to 5 days away (Hours 180-120 on the models) and the models go in every which way. The third and final stage is the final track, where the models converge on a solution within 5 days of the storm. This final track can sometimes be nearly the same as the first Long Range track. We already observed the first stage, and are now clearly in the second stage. The system will get on land in the next few days, so model solutions should clear up as the time comes closer.

I will update daily here on the blog, and much more often on our facebook page at .


February 21 Potential Severe Weather Event

The Storm Prediction Center has outlined an area of severe weather for Day 6, or February 21st.

Prognosis... Strong storm system will be ejecting from the Rockies on the 20th-21st. As the storm system moves east, the subtropical jet stream and Pacific jet stream will merge into a single, powerful jet stream that will be centered over the outlined region. Current thinking is that the abundance of moisture and strength (albeit weakening) of the storm system will provide a base for severe weather on the 21st. Tornadoes and damaging winds would certainly be possible, but model uncertainty forbids me from making any definitive conclusions on threats for this day.

I will have more updates in coming days.