Stu's A-B-C's of a Winter Storm
Posted by JAC on 12/21/2009, 10:22 am
Stu Ostro, Senior Meteorologist
MOISTURE + COLD = SNOW

When you boil it down to the most fundamental, basic level, you only need a couple of things for snow: atmospheric moisture (with a mechanism to squeeze it out), and temperatures at or below 32F continuously (or nearly so) from aloft down to the Earth's surface.

The air mass doesn't necessarily have to be the most frigid Arctic one ever seen; rather, the air just needs to be cold enough.

Then, the more moisture that falls in the form of precipitation, the more snow which falls and accumulates. (And the colder the air is, the higher the ratio from inches of snow to inches of "liquid equivalent", and also the colder the surfaces onto which the snow can more easily stick.)

Sounds easy, right? Indeed! Of course, in real life, getting a big snowstorm at any given location isn't always so easy. Maybe so in, say, the highest elevations of the western U.S. mountains. In lower elevations in the eastern U.S. south of the Mason-Dixon Line, not so much.

But the atmosphere is making it look easy with the current situation!

This map of "precipitable water" basically shows that this storm has a whole lotta moisture to work with from the atmosphere over the Gulf of Mexico and subtropical Atlantic. That even led to extreme rainfall of more than a foot last night and today in South Florida in places such as North Miami Beach and Hollywood. This was well-removed from the main precipitation area and difficult to forecast very far ahead of time, leading to the NWS expressing "dissapointment" that it wasn't more accurately predicted.





The map below of departure from average temperatures last evening in the lower part of the atmosphere shows that there's much warmer-than-average air (orange/red) over and near Greenland but to the south of that is colder-than-average air (blue/purple) over the northeast U.S. and Canadian Maritimes. Not exceptionally cold for this time of year, but cold enough.





Where the moisture and cold air meet, in the Mid-Atlantic into parts of the Northeast, is where the heavy snow will fall.

While for any particular storm there are many factors which come together, in this case there are two climate/weather drivers that stand out as influences.




EL NINO + NEGATIVE NAO = SNOW




El Nino, a periodic warming of the equatorial Pacific Ocean, is in full swing right now as shown by the orange/red shades on the map above. After having been weak earlier in the autumn, the current El Nino is now on the high end of moderate intensity, close to what would be considered strong.

Although the precipitation pattern so far this month across the U.S. isn't exactly the same as the long-term El Nino correlations for climatological winter (Dec-Jan-Feb), it's quite similar, including from southeast Texas across the immediate Gulf Coast to Florida.






Below-average amounts are denoted by brown and above-average by green.

The precipitation so far in December is reflective of an active and wet Southeast storm pattern, which is helping to energize the current system.

Meanwhile, the signal known as the "North Atlantic Oscillation," or NAO, is currently strongly negative (circled portion of the graph below).






That measure is a reflection of this atmospheric configuration aloft (map below), with a strong ridge of high pressure over and near Greenland and a trough to the south of it.

It's the setup which led to the temperature pattern I referred to earlier (and also, BTW, to a major snowstorm in parts of Europe associated with the trough represented by the dark blue in the upper right corner of the map).





And, last but not least, to go along with all that southern moisture is an intensifying low pressure system -- a classic Gulf low -- which on this "visible" satellite image from midday today even had what looks like an eye! It's not a true eye like in a hurricane because those are just mainly shallow low clouds rather than deep eyewall "convection," but neverleless the appearance is indicative of this being a feisty system. (And it's reminiscent of the image to which I linked from a February 12, 2006 blog entry about New York City's record snowfall.)

Today's cyclone will further strengthen as it heads up the East Coast and bring the blockbuster snowstorm and strong winds. As of this hour (4:30 pm EST Friday) the snowy part of the storm has cranked up in western North Carolina and southwest Virginia.





All is not as easy as meets the, uh, eye, though. The computer forecast models have been all over the dial this week with how far north the heavy snow will get, the latest runs having Philadelphia, New York City and southeast New England back in the game albeit still close to the edge. Also exactly where, to the southeast of the heaviest snow, the rain/snow line will be at any given moment is a bit tricky.

You can get TWC's latest updated local forecast at any time by entering your town/city or zip code on top of this weather.com page, and this map showing the predicted accumulations from the current time forward will be updated throughout the storm.

Already this season in the U.S. there are reports of well over 100 weather-related fatalities from vehicular accidents due to icy/snowy roads. Deaths such as those are generally not included in official weather fatality lists because they're indirect rather than directly-caused, but if the 137 figure as of this writing is accurate, then it's more than the 10-year or 30-year annual averages of hurricanes, tornadoes, lightning, or floods.

So if you can, relax at home and enjoy the snow, but if you have to travel, expect flight delays and cancellations, and please drive safely!
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A MAJOR HOLIDAY STORM BREWING - JAC, 12/21/2009, 10:00 am
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