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.

Andrew

11 comments:

Kevin Kratka said...
This comment has been removed by the author.
Kevin Kratka said...

I've recently discovered this blog and I love it. I'm sharing it with every weather nut I know. Thanks for all the hard work.

Lynn Radabaugh said...

Andrew,
First of all thank you for all that you do. Secondly, do you have any advice as to how I can best use and differentiate the snowfall color models? I'm having difficulty matching the color strip to my area.
Thanks Andrew

Anonymous said...

thank you for all the wonderful and very thoughtful work that you do. I have learned so much about the weather and especially the el nino cycle. Thankyou

Jim Mol said...

The new headline is a better attention getter. May cause more people to read more of the blog. Being newer to the blog, a brief explanation of your misses would also be nice, it is all about probability in the end and why another model played out would be interesting. Thanks for bringing weather to life.

M in Maryland said...

I think I like the new titles better, but, of course, if you talk about next Tuesday and Wednesday in a long term forecast, the dates would be necessary.

Love your blog and read it daily.

Anonymous said...

what about the march 16-20 storm???

Anonymous said...

When do you think winter will finally slow down and snow and cold and when will warm return. Forecasts show it being warm after the storms and late next week it being warm . Your thoughs?

Anonymous said...

New title, I like it. What is your guess on Chicago, IL? Seems like the models are showing either a good hit on us or a light 2 inch at best.

Mainemanstu said...

I like the new format myself.Seems to get your attention sooner. Thanks for what you do on this blog,

Justin said...

Wonderful assessment. I live in NW Ohio and the NWS and other mainstream weather forecasters have absoukutly butchered our forecasts this year. Calling each time for the apocalyptic snowstorm....only to end up with an inch or two. This storm has been no different, I paid attention to your assessments and saved myself the average 3 hours of preperation that I take for each forecasted big storm. Thank you for the great work, it more and more seems like mainstream meteorology has turned from accuracy to primetime drama show!!