A(nother) wicked big storm is on the way! As revenge for the warmth and nastiness you warm lovers brought over the past few weeks, us winter lovers have prepared a finale nothing short of spectacular to finish out what’s been a pretty wild winter season. That finale is beginning right now over the Southeast US.
Everything is setting up almost perfectly for a very large snowstorm here in Maine. Deep tropical moisture is flooding east as the subtropical jet begins to crank up. This moisture can be seen at the surface reflected in lots of rain across Florida and the Carolinas. The surface reflection of the strong STJ is that low in the gulf. The second element is the strong polar energy dropping in from the NW. At the surface, this can be seen in the form of the two lows over the Ohio valley. Aloft, very strong energy is present which will drive explosive intensification of the storm as it travels NE tomorrow. Another thing to note on the surface map is our storm itself forming off the Carolina coast.
In every major snowstorm we have, there’s one defining feature. It might be raw totals, crazy winds, coastal flooding, or, in this case, snowfall rates. The upward motion associated with very intense atmospheric dynamics will produce a band of snow almost as heavy as is physically possible. In this band, we’ll see some pretty awesome (and dangerous) conditions. Look for snowfall rates to spike to the 3-6″/hour range with whiteout conditions and wind gusts to 45mph. Thundersnow is a distinct possibility as well. How long the band lasts in any one spot is up for debate but it shouldn’t linger over any one spot for more than an hour or two. However, it doesn’t take long under a band of 3-6″/hr to pick up some serious totals.
Here’s a representation of these heavy “crusher” bands moving through the area tomorrow afternoon. If you click on the map and zoom it in, you’ll probably note that the numbers portrayed on the map don’t quite match up with my 3-6″ per hour numbers. However, the model assumes that the snow:liquid ratio will be 10:1. While this may true along the immediate coast (within 5 miles along the SW coast and within 30 miles along the midcoast), most of the area will see ratios of 12-15:1 and folks NW of the blue line will see ratios approaching or exceeding 20:1. So for this map, take it as it is right along the immediate coast, add half for inland ares, and double for the mountains and you’ll get an idea of how hard it will be snowing tomorrow afternoon. Again, this is a result of all that upward motion (which is a result of a bunch of really awesome atmospheric processes that would take a really long time to explain. If you’re interested in stuff like that, check out my twitter feed @JackSillin or head on over to the New England part of the 33andrain.com forums where I post more technical stuff).
While I won’t get into everything that’s causing the upward motion, it is important to analyze at least one map with a zillion funny colors to determine snowfall ratios (remember how important those were in determining hourly snowfall rates above?). This is a plot of all the different dynamics important in determining snow to liquid ratios. First off is the location and depth of the layer in which snow is formed. For Portland, shown on the map, the layer is only marginally deep and fairly far removed from the ground (meaning there is lots of warmer air for flakes to fall through. In the mountains (not shown), the layer is deep and closer to the ground. However, even in Portland, other important criteria is met for efficient snow production. Rising motion through the snow producing layer (aka Dendritic Growth Zone or DGZ) is extremely strong and plenty of moisture is available in that layer. This means that we’re likely to avoid the 6-8:1 sludge present in some other storms but a general 10-12:1 would be a good bet. Farther north, my thinking is above. Much fluffier in the cold air.
Believe it or not, sleet could very well mix in especially along the midcoast and possibly along the immediate southern coast as well. Why is this? While the low pressure system at the surface will track to our SE (hence little to no threat of rain), the low pressure system in the mid levels will likely track right overhead (slightly to the east of what the NAM model shows above). This means that eastern parts of the area will see warmer temps work in (remember that warm temps are found to the east of low pressure systems). While most of the coastal plain won’t see temps warm enough for sleet, they will be warm enough to lower ratios resulting in heavier snow. Along the midcoast, they will be warm enough for sleet resulting in even lower totals.
Also note in the above image the darker colors showing the dry slot. This is what will shut precip off but as it punches in, a difference in density between dry air and saturated air will create the potential for thundersnow. The arrival of this dry slot so soon into the storm is why we won’t be seeing widespread 2-4 foot amounts. It’s just simply too fast a mover.
Another really big story will be the winds. Winds will ramp up late tomorrow afternoon with the worst arriving in the evening hours before tapering off overnight. At their peak, winds could gust to 45 or 50mph along the coast. Winds of over a hundred mph lurk only 2500 feet above the surface and while thankfully all the winds won’t mix to the surface, they’ll definitely be gusty. Along with the gusty winds comes the threat for extremely reduced visibility. Visibility could approach zero for a time in the heaviest bands with the highest winds tomorrow evening and as a result, blizzard warnings are up for southern sections where the combo is expected to last for the full 3 hours or more required for an official blizzard. Elsewhere, slightly lesser winds will result in blizzard criteria not being met and thus most of the area is under a winter storm warning. Winds will die down Wednesday.
Here’s my snowfall forecast map. The mountains will likely get the jackpot as that’s where the best intersection between moisture and cold air will occur. Closer to the coast, slightly warmer air will intrude resulting in heavier, wetter snow that doesn’t accumulate nearly as well. Along the midcoast, a changeover to sleet is likely to occur which will further cut down on totals.
In terms of timing, expect snow to overspread the area from SW to NE tomorrow morning. By noon, expect everyone to be snowing and for SW NH to have a couple inches on the ground. Snow really cranks up in the early afternoon with the heaviest snow being from 3 PM until 8 PM. After that, the best forcing continues its drive NE and we’re left with residual moderate bands. Those will taper to snow showers by Wednesday morning and roads should be all set to go by Wednesday mid day.
I’ll have any needed adjustments tomorrow morning.