Posted by: splillo | February 27, 2012

Long duration snowstorm

Flakes start flying on a Sunday afternoon; harmless, a gentle winter scene. The storm ramps up overnight, and it rages through Monday with heavy snow piling up against doors and windows. The snow might lighten up once, and everyone thinks the storm is over, only for blizzard conditions to return in full force. It doesn't stop Monday night either, and on Tuesday everyone wakes up to see big fat flakes still tumbling to the ground. There's three feet of snow on the ground, and the storm doesn't wrap up until later that afternoon.

That's my dream storm.

Well we may be looking at a long duration storm threat by the middle of this week. It's not exactly on the magnitude of that in my dream, but a few forecast models suggest we might be looking at continuous snow for a good 24 hours from Wednesday evening through the day Thursday.

A major trough digging into the western U.S. tomorrow will track across the Rockies and support the development of a strong low pressure system in the Central Plains on Tuesday. A large precipitation shield will develop with this storm, stretching from the Ohio Valley back into the Northern Plains by Wednesday morning.

There are two distinct components to this storm.

The first is what is often termed a "southwest flow event" which, as the name suggests, occurs with southwest winds aloft. The forcing for precipitation in this situation is from warm moist (low density) air flowing in from the southwest and forced to rise over cold (high density) air. The warm moist air condenses as it rises, forming clouds and precipitation within the cold airmass.

The southwest flow ahead of the trough will produce this effect, supporting the rain and snow moving from the Ohio Valley into the Northeast ... well ahead of the low pressure system. However, most of the precipitation from this round will probably miss us to the south during the day Wednesday.

The second component of the storm is with the upper level low. The cyclonic flow around the upper low (remember that term "vorticity"?) supports bands of snow (what is often referred to as the "comma head" in a storm). Depending on the organization of the system, these bands can consist of light intermittent snow showers or heavy snow that produces significant accumulations. There are two areas where banding is favored ... in the northeast quadrant and the northwest quadrant of the storm system. So in general, you want the upper low (or "vorticity maximum") to track just south of you for the heaviest snowfall. So the next time you're hoping for a snowday, you can say "boy, I sure hope the vorticity maximum tracks south of us!"

The upper low will roll into the Great Lakes Wednesday night introducing these bands of snow into New England Wednesday night. As the low sticks around on Thursday over New England, so will the bands of snow. The big question is how organized this system will be ... will we just get light snow showers all day amounting to a few inches, or will we get bands of heavy snow that produce as much as a foot by Thursday afternoon? Too early to tell right now, but I can tell you the weather will certainly be maintaining a wintry appeal in New England this week.


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