Monday, March 10, 2014

Impending Snowstorm to Drop 12"+ of Snow Tuesday and Wednesday

Note: This post will concern the March 11-14 Significant Snowstorm, but the title has been changed. If you find this sort of title better than the date and storm classification, or if you prefer the date + classification, leave your input in the comments below.

I'm carefully watching the next few days for a snowstorm that looks to drop a foot of snow or more on the United States.

We'll begin with an analyzation of the big picture of this storm. The model I will be using here will be the NAM model, as its low-resolution GFS counterpart appears to be suffering from some errors we will address later on in this post. The image above shows mean sea level pressure (MSLP) values, as well as precipitation values and high and low-pressure demarcations for Tuesday afternoon, tomorrow. We see a strong storm system in the far northeast corner of Kansas, bottoming out at 994 millibars and creating a heck of a pressure gradient over Nebraska. In Nebraska, we also see a precipitation shield breaking out, which could be accompanied by some harsh winds if that pressure gradient happens to meet up with the precipitation. Luckily, this forecast keeps the strongest winds just south and east of the inclement weather in northwest Nebraska.

In the late evening hours of Tuesday, we see that our storm system has pushed off to the east, maintaining both its tight pressure gradient and precipitation shield as it progresses east. Our storm is now at about 995 millibars, roughly the same strength as what we saw Tuesday afternoon in the image above. The 1 millibar difference really isn't anything to write home about in this situation. Typically, we might see a severe weather situation arising in the Plains and Midwest due to that tight pressure gradient, but that swath of precipitation down in Mississippi, Alabama and the panhandle of Florida indicate there's a storm system there (a closed low, to be exact) that will prohibit such a severe weather event from forming. At this point in time, accumulating snow should begin to fall over the Midwest.

By the time we hit the morning of Wednesday, March 12, we find that phasing has occurred with our storm system. What this means is that our original system has phased, or merged, with a system coming south from Canada to create a single, stronger storm. This is clearly displayed by the abundant precipitation breaking out over Michigan, Indiana and Ohio, and also shown by the tighter pressure gradient expanded across much of the Central and East US. It is also possible the western flank of the gradient is a bit amplified thanks to the strong high pressure in Canada, but that's not a big piece of the puzzle right now. At this point in time, potentially significant accumulating snow looks to be impacting the Ohio Valley.

Lastly, by the evening hours of Wednesday, we see the system has now transferred offshore, and is now putting down significant accumulating snow across the interior Northeast, leaving coastal areas (and even some inland regions) to experience rain or a mixing event.  The extreme pressure gradient remains in place, maximized in this forecast graphic over Pennsylvania and New York, as well as the rest of the New England area. After this, the storm exits the region, leaving quite a snowstorm in its wake. Check out the latest snowfall forecasts for this event:

To add some model variety, we'll also show the GFS snowfall forecast.

If you look at these maps and think something's not the same, you're right. There are some differences here. The primary difference is that the GFS is more south with the storm system than the NAM model. This is displayed well when looking at the beginning of the accumulating snow swath all the way back out in Illinois. The NAM model gives a generous 6" or more to Chicagoland, but the GFS only lays down a handful of inches across Springfield, IL.

After looking at all the data, I think it might be best to lean towards a NAM solution with the placement of heavy snow further north in the Great Lakes, but not too far north, if you know what I mean. Let's go over why.

The first red flag is the NCEP Weather Prediction Center's preferred track superimposed on an ensemble suite for this storm system. We see the NCEP WPC track for this storm in black, but it's obvious that their track is on the southern envelope of guidance. The heavy majority of ensemble members prefer to take this storm further north, into central or northern Missouri rather than southwestern MO. This would result in snow totals displaced further to the north, like the NAM says. But we can dig deeper than this. Let's keep going in our search for why these northern amounts are more preferred over the GFS.

Let's take a close look at the image above. Here, we see the NAM model projecting 300mb wind speeds in color shades and wind barbs, valid early Wednesday morning. If you look closely, you can spot two distinct jet streaks, or two areas of stronger winds than the winds around them. These are shown best by the yellows in Oklahoma and Kansas, as well as in far northern New England. These two jet streams form the classic coupled jet stream, as defined by Uccellini and Kocin in their studies. This coupled jet stream idealizes that there is a space between two jet streaks, where heightened divergence is found. In other words, in the space between two jet streaks, the atmosphere wants to create more precipitation. So, if we look above for the space between these two jet streaks, we can draw a line roughly from Missouri to Lake Michigan of where this increased divergence, or increased precipitation would be expected to appear. This would seem to put the NAM's snowfall placement 'in the right' when compared to placement of the GFS snow totals, at least in the Great Lakes region.

Also, take a look at what the 18z GFS does with the storm as it moves along the Ohio Valley.

If you watch the low pressure denotation as it moves from Indiana, it goes to the southeast rather than to the east. You might not think this is so odd, but when you compare it to the agreed notion that this storm will be moving west-to-east, a southward movement is rather odd.

Additionally, a convective feedback issue was observed as the system crossed east through Indiana, likely hampering the forecast for this storm. In this case, you may want to defer to the 12z GFS, as it is more logical in comparison to this run.

To conclude, a significant winter storm is expected for a good chunk of the US. Model guidance differs on placement of the snow, mainly in the Great Lakes, but it is agreed on that the Northeast will see a substantial snowstorm.