Thursday, January 24, 2013

Jan 24, 2013: Calvert County, MD: The Snow Jackpot

Well, it's not much of a winter when I get excited about blogging over a few inches of light powder.

The Arctic Clipper came through our region last night, and was well-forecast in terms of snow timing and amount.   And yes, the morning rush is a bit of a mess.

Here are the snow amounts tallied by NWS Sterling:

Thursday AM snow totals.  NWS
Note the north-south gradient, from a dusting around Baltimore, to over an inch in NOVA...and the Jackpot, over St. Marys and Calvert Counties (MD):  A small pocket of 5"!

The storm did not have much moisture (water vapor) to work with, but it did manage to freeze out the equivalent of 0.25" of liquid over these counties...if this were rain, it would be enough to wet the ground, but probably not form puddles.   Yesterday, the NWS predicted the region south of DC would pick up the highest snow amounts.  Keep in mind that it is very, very hard to forecast exact snow amounts, to the inch, when the snow:liquid ratio is 20:1. 

Nevertheless, I did some "forensic meteorology" this morning and determined that this pocket of snow was caused by a small area of enhanced uplift north of the low pressure center.  Anytime you observe a snowband, you look for something called "frontogenetic forcing" which means a region in the lower-middle atmosphere where a weather front is developing.  In this case, a mid-level front formed where chilly air to the north was sliding under slightly milder air to the south.  The cold air, acting like a narrow wedge, lifted up a narrow strip of moist air, forming the snowband that dumped on St. Marys and Calvert Counties.   You can see how this unfolded in the graphic below:

10,000 foot weather chart showing snowband (green) and pocket of frontogenesis (purple lines).  NWS.
This weather chart is at the 10,000 foot level, in the wee morning hours.  The black lines are isobars.  The blue dashed lines are isotherms (the air temperature over Baltimore is -24 C or about -11 F - plenty, plenty cold enough for snow).   The blue and green solid colors portray the radar's detection of snow.  Note the narrow, green band of moderate snowfall located south of D.C., over St. Marys and Calvert Counties.  Now, the solid purple lines indicate regions of frontogenesis.  Note the purple bulls eye, located over the snowband, marking the location of the mid-level front.   Subtle, yes.  But these are the features a meteorologist must look for, to determine where the heaviest pockets of snow will develop.

And all this while you and I were sound asleep!

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