Sunday, February 7, 2016

Eastern U.S. trough is bringin' winter back (yeah)

A SUPER day in the weather department today as temperatures push towards the 60° mark thanks to a warm southwest breeze and a good deal of sunshine. A big shift in the upper level pattern takes place this week though, transitioning overnight tonight as an Arctic cold front moves through the Mid-South. Yep, it's bringin' sexy winter back!

On Monday morning, you'll likely notice the difference immediately, even though temperatures won't be any colder than the past few mornings (mid 30s). A strong northwest wind will be in place and keep temperatures from climbing much at all Monday as clouds circulate around a large trough of low pressure that builds south at the upper levels of the atmosphere.

A large upper level trough is depicted in blues in the map above valid at 6pm Monday. Air over the Mid-South will have origins in Canada, meaning a very cold pattern. Under the trough, upper level disturbances will rotate around the low bringing scattered snow showers. The Mid-South will be on the edge of these systems with borderline temperatures to support light snow. Graphic courtesy Pivotal Weather.
Under the trough, scattered snow showers are expected for much of the Midwest, Ohio and Tennessee Valleys, and northeast U.S. The Mid-South will be on the periphery of the coldest air, which in this case is both good and bad. Areas under the deeper cold air will have the best chances of light, but accumulating, snow, but they'll also be bitterly cold, while we'll only be "coat-and-gloves" cold.

Simulated radar from the NAM model valid at 6pm Monday shows the potential for very light precipitation. With temperatures falling to the mid 30s by this time, sprinkles or flurries could result. The blue narrow line is the forecast freezing line. Graphic courtesy Pivotal Weather.

Upper level flow around the main low over the Great Lakes will drive pieces of energy around it like spokes on a wheel. For us, northwest flow will mean fairly dry, cold air, but also low precip chances late Monday into early Tuesday. Most snowfall will occur to our northeast closer to the main low. Graphic courtesy Pivotal Weather.
A forecast sounding (conditions aloft over Memphis) at 3pm Monday shows the temperature (red line) falling below the freezing level (blue line) at about 1200' above the ground with a surface temperatures in the lower 40s. This means that precip would fall from the clouds as snow but would likely melt in the lowest 1000' to rain. Once the surface temperature drops into the mid 30s by sunset, there is a better chance the snow survives the lowest several hundred feet without completely melting.
The trough will hang around for the first half of the week, keeping colder than normal conditions around through that period.

Average surface temperatures from Monday night through Saturday night will be well below normal across the eastern half of the country and well above normal out west, reflecting the upper level pattern of a large trough of low pressure in the east and ridge of high pressure in the west. Graphic courtesy WeatherBell.
Once the trough moves east, upper level ridging will move in, bringing temperatures back to near or above normal by the end of the week into next weekend. Other than the Monday to early Tuesday threat of very light precipitation, the pattern supports a dry forecast until late next weekend.

Trends indicate a warming trend back to near or slightly above normal temperatures by next weekend, according to the American Ensemble model system. Graphic courtesy WeatherBell.
Erik Proseus
MWN Meteorologist

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