DISCUSSION: As very cold air began to make its way into parts of central and eastern Europe over the past several days, there was a major uptick in the threat for accumulating snowfall across many different sections of central and southern Europe. Once such region which received some accumulating snowfall was in and around the city of Rome, Italy. As a particularly cold air mass made its way across central and southern Europe, this facilitated a noticeable clash of warmer and colder air masses. Thus, this helped to set the stage for more cyclonic flow in the vicinity of the boundary created between the colder air mass to the north and the warmer (Mediterranean-based) air mass to the south.
This aforementioned cyclonic flow is due to the fact that in the Northern Hemisphere, winds associated with a extra-tropical low-pressure systems rotate counter-clockwise as a consequence of the impacts of the Coriolis effect. Thus, with a low-pressure system delivering accumulating snowfall across an area such as central/southern Italy, this would mean that there would a reasonably strong influx of warmer air into the colder air mass located to the north courtesy of warm-air advection. Warm-air advection is best defined as a surge of warmer air which heads in a poleward direction most often in association with a Winter-time extra-tropical low-pressure system which helps to generate the precipitation shields on the eastern, northeastern, northern, and northwestern flanks of extra-tropical cyclones. Hence, warm-air advection acts as a key in-storm mechanism to continue pumping critical warm, moist air into the precipitation shield which fuels winter storms such as the one which recently impacted central and southern Italy as reflected by the graphic above with a great shot of a snow-covered Rome, Italy.
It is also worth noting that the particularly (and unusually) cold air which made it this far south across portions of central eastern Europe had far-reaching impacts which reached far beyond just Italy. For example attached below is an excerpt from the ABC News which details some other impacts which were inflicted on the region.
"Elsewhere in Europe, the storm set dangerously low temperatures: Lithuanian officials said temperatures that plunged to as low as minus 24 degrees Celsius (minus 11 Fahrenheit) in some places were to blame for the deaths of at least three people over the weekend. Hospitals in Lithuania and Latvia have reported an uptick in people being treated for hypothermia and frostbite.
Scientists say the big chill in Europe is partly caused by the fact that strong winds which normally keep cold air 'locked' over the Arctic have weakened, releasing icy blasts across the northern hemisphere. Similar sudden drops in temperature have occurred over North America in recent years and climate researchers say they could become more frequent as global warming further saps strength from the air currents around the pole."
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© 2018 Meteorologist Jordan Rabinowitz
DISCUSSION: Along most coastlines, the ocean has a very large influence for much of the atmospheric phenomena and temperatures that occur in these locations. With everything else being equal, continental air masses will be more extreme than maritime ones (meaning the land heats up and cools down quicker creating larger temperature departures whereas maritime air masses modify the temperature). We see this come into play across the entire west coast of the U.S., where the coldest air during the winter comes straight from the North American continent as opposed to the milder Pacific Ocean air.
This concept is very much related to portions of western Europe. Typically, a westerly wind advects mild Atlantic air keeping portions of the countries such as Portugal and Spain relatively mild. Over the next week or so, high-pressure systems migrating north and east from the Azores will produce a more north to northeasterly wind over these regions shown above (notice 850mb winds from the north and northeast). This allows for cooler continental air masses to advect chiller temperatures as shown below (2-meter temperature anomaly chart with blues and purples meaning well below average).
While this colder air is in place, these regions, especially in the northern mountains of Spain, have a chance to see snow based on the latest GFS model guidance below (colors and numbers correspond to accumulations in centimeters). The GFS even hints at some light snow making its way all the way down to the northern regions of Portugal. This is uncommon for this region, but not unheard of. In any event, an interesting pattern ahead for southwestern portions of Europe is on the way!
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©2018 Weather Forecaster Joseph DeLizio