A very large portion of the country is at risk for seeing a major winter storm starting tomorrow and lasting through Wednesday, with winter storm warnings up for up to a foot of snow in places from Kansas to New York.
Winter storm watches (dark blue/purple) and winter storm warnings (hot pink) are in effect in anticipation of this snowstorm. Quite a bit of real estate is covered by these watches and warnings. Those in the storm's path should be prepared for a pretty good snowfall. Nothing atypical, but it'll be enough to cause some problems.
Those of you who are going to get snow are probably wondering why I singled out New York City in particular when the storm is set to affect over a hundred million people outside of the NYC area.
Not only could the NYC Metro see a good amount of snow like the rest of you, but it could also see 1/4" to 1/2" of ice from freezing rain on top of that snow.
Follow me across the orange squiggle for an explanation as to why this is could happen...
Here's the National Weather Service's forecast for the NYC metro area:
...WINTER STORM WATCH REMAINS IN EFFECT FROM LATE TUESDAY NIGHT THROUGH WEDNESDAY AFTERNOON...
* LOCATIONS...NEW YORK CITY...NASSAU COUNTY...NORTHEAST NEW JERSEY AND THE LOWER HUDSON VALLEY.
* HAZARD TYPES...SNOW...SLEET AND FREEZING RAIN.
* ACCUMULATIONS...SNOW ACCUMULATION OF 4 TO 8 INCHES FOR METRO NEW YORK CITY AND NORTHEAST NEW JERSEY...6 TO 10 INCHES FOR THE LOWER HUDSON VALLEY AND ...ALONG WITH UP TO 1/2 INCH OF ICE.
* VISIBILITIES...ONE QUARTER TO ONE HALF MILE.
* TIMING...SNOW LATE TUESDAY NIGHT THROUGH WEDNESDAY MORNING... MIXING WITH AND CHANGING TO SLEET AND FREEZING RAIN NEW YORK CITY METRO AND NORTHEAST NEW JERSEY.
* IMPACTS...SNOW AND ICE ON ROADS WILL CREATE HAZARDOUS TRAVEL.
Note that the storm will begin as snow on Tuesday night and continue through Wednesday, changing over to sleet and freezing rain during the day on Wednesday.
Why is this happening?
Here is a surface chart for Wednesday morning, with my very poor drawing showing warm air sitting to the south and east of a deepening low pressure center just southeast of Long Island:
The low is expected to trek just close enough to the coast that areas from Philadelphia to Boston will experience a period of at- or above-freezing temperatures within a few thousand feet of the surface. This is a good setup for a snowstorm that transitions over to an ice storm, which is worse than just an ice storm because you've got ice falling on top of the weight of a heavy, wet snowpack.
Take a look at the NAM's (North American Model) surface temperatures for Wednesday morning (in Celsius). Note that temps are mostly around freezing at the surface in the NYC/Long Island area:
Now compare that to the NAM's forecast temperatures at 850 millibars (roughly 4500-5000 feet up) from the same time frame:
Granted, these are just a model's forecast temperatures and they will likely be a little bit colder, but it gives you an idea as to why this could be such a bad deal for the Big Apple.
Here's a look at some BUFKIT soundings (model simulated weather balloon observations) to help give a look at a slice of the atmosphere above the cities at risk. These images are simplified SKEW-T charts -- the diagonal blue lines are temperatures in Celsius, and the horizontal gray lines are heights in thousands of feet.
The green line is the dew point with height, and the red line is temperature with height. These lines measure the model's forecast temperature at each height. For example, to find the temperature at 10,000 feet, find the red line where it intersects with the 10 line, and follow it down diagonally along one of the blue dotted lines.
The following is an example from NAM's afternoon run for Hartford, CT, showing an all-snow sounding. From the snow growth region (area of the atmosphere where temperatures are ideal for dendritic snowflakes to form) between 16,000 and 20,000 feet all the way down to the surface, the air temperature never climbs above freezing. The flakes never have an opportunity to melt as they fall so they reach the ground as all snow.
When temperatures at the surface remain at- or below-freezing, and temperatures a few thousand feet up are considerably warmer, it sets the scene for a dangerous ice storm. Here's an excellent example I found from Philadelphia (which is NOT expecting a major ice storm, by the way):
As snowflakes fall into that red-shaded region of above-freezing temperatures, the snowflake has a chance to completely melt into a raindrop. As the raindrop falls out of the warmer air into the sub-freezing air near the surface, it becomes supercooled -- the water's temperature reaches or falls below freezing but the water doesn't freeze into ice, as it has no nucleus around which to refreeze.
The supercooled raindrop continues to fall until it lands and freezes on contact with anything exposed to the elements, such as a tree branch or car or sidewalk.
Here's another example of a freezing rain sounding from the upcoming storm, this one at LaGuardia Airport on Wednesday morning:
The forecast can and likely will change over the next day or so as the storm gathers organization and forecasters are able to more accurately predict what will happen, but as it stands, it looks like the NYC Metro area in particular is in for a nasty couple of days.
Ice itself is dangerous. I just went through the ice storm in Mobile AL and it was hell. We got 0.20" of freezing rain followed by over an inch of sleet, which all froze into solid ice overnight. It took three days to melt off the roads. You couldn't walk anywhere let alone drive. It was pretty bad even by northern standards.
This storm, if it pans out the way it's currently forecast to do, would be a significant snowfall (featuring a heavy wet snow) which would then be covered by 1/4" to 1/2" of ice. The freezing rain will melt some of the snow that falls, further adding to the weight and leaving behind a very thick solid layer of ice on everything. Snow is bearable on its own, but ice is a whole different ballgame.
This could be very dangerous for such a heavily populated metro area. Make sure you're prepared to be stuck in your home for an extended period of time (at most a day or two) without being able to go anywhere because of the ice.
As always, stay tuned to your local National Weather Service office for the latest updates, and you can always follow me on Facebook and Twitter for more frequent updates than the ones I post here.
5:55 PM PT: UPDATE: The northern and western suburbs of Washington DC could also see an ice storm resulting in >1/4" of ice, but it won't come with the snow beforehand.