Year in and year out there is always an interest across the United States and the world for matter regarding whether or not there is a decent chance of there being a White Christmas. Being as though it is Christmas Day and millions of people living in more than a dozen cities spread across the United States have already experienced a White Christmas here in 2017, it is always interesting to understand more about why the percentages of there actually being a White Christmas varies so much in going from north to south across the nation.
It is important to acknowledge the fact that the verification of there being a White Christmas is contingent upon there being snowfall cover on the ground during the period between Christmas Eve and Christmas Day and/or there being a verification of new snowfall accumulating during that same period of time. More often that not, the reason why the odds of there being a White Christmas is due to the predominant storm track in place across a good portion of the lower 48 states. From year-to-year, this average national storm track position obviously varies based on the influences from various global climatic teleconnections such as the El Nino Southern Oscillation. Having said that, based on the fact that we are currently under the influence of a Central Pacific Ocean-based La Nina event. Thus, the typical storm track position during a typical La Nina Winter is roughly positioned along a track from the Pacific Northwest and southeastward towards the Southeastern United States.
Nonetheless, as with any national climatic tendencies based on trends tied to seasonal climatic variability, there is always subs-seasonal variability associated with where low-pressure systems enter various parts of the contiguous U.S. For instance, over the past 48 to 72 hours, a relatively weak "Alberta Clipper" low-pressure system entered the U.S. and progressed east-southeastward towards the North-Central U.S. before heading towards the interior Northeastern U.S. Upon reaching the greater Mid-Atlantic coastline, the system transferred its energy towards the area just offshore from the Delmarva Peninsula before moving off to the northeast with time. As a result, this system delivered mostly rainfall to the greater NYC metro area. However, more inland locations across New York and up towards New England all witnessed a White Christmas. Hence, areas which typically have less than a 40% chance of witnessing a White Christmas got that and more today in some cases.
It just goes to show that dreams can come true when the atmospheric storm track and timing decides to play along with the hopes and dreams of younger generations who anticipate this time of year all year long.
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©2017 Meteorologist Jordan Rabinowitz
Understanding What Forces Different Types of Wintry Precipitation (credit: National Weather Service)
DISCUSSION: As we are heading deeper into the 2017-2018 Winter season, it is always important to bear in mind what factors come into play when it comes to forecasting for winter weather events. One of the first questions which should always be asked when it comes to anticipating any variety of winter weather event is to what extent (i.e., both with respect to vertical and horizontal difference) there is cold air in place. In addition, the other major issue pertains to if there happens to be warmer air (i.e., above-freezing air) at the surface and/or above the surface. If there is warmer air above the surface, this can often lead to either icing events, mixed precipitation events, or even sleet events. The main difference is determined by the exact presence as well as depth of the relatively warmer air.
If there is warmer air present through the depth of the precipitation all the way down to the surface, this leads to the occurrence of an all-rain event. However, when there ends up being a shallow layer of colder air very close to the surface, this quite often leads to the occurrence of a freezing rain event due to the fact that rain falling through a somewhat warmer layer above the surface reaches a below-freezing layer very close to and/or right above the surface of the Earth which leads to the rainfall freezing upon contact with the ground. Thus, this leads to what are most commonly recognized as destructive freezing rain events. The other relatively common possibility is for the "frozen layer" of the lower part of the atmosphere to be substantially deeper than that observed with the freezing rain scenarios which leads to sleet events where rain freezes well above the surface. This leads to ice pellets which can often have quite a high impact since upon reaching a frozen surface, they can adhere to the surface and create a dangerous situation as well. The message to be taken from all of this is that all-out snowstorms are not the only issues to be concerned about when it comes to winter weather events.
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©2017 Meteorologist Jordan Rabinowitz
DISCUSSION: Over the last 24 to 48 hours, the first legitimate winter storm impacted several parts of the northern tier of the I-95 corridor (i.e., from about Baltimore, Maryland to Bangor, Maine). However, as this winter storm made its way out of the coastal and semi-interior sections of the Northeastern United States, the return flow behind the departing low-pressure system made for a particularly favorable set-up for the current lake-effect snowfall event which is occurring to the east of both Lake Erie and Lake Ontario. With lake-effect snow warnings in place across these respective areas, the threat for adverse conditions to intense lake-effect snow squalls will remain in place for at least the next 12 to 18 hours. In addition, there are also many reports of thundersnow coming out of both lake-effect snow bands which indicates the presence of very intense snow bands as well. Therefore, if you or anyone you know is in the path of these respective lake-effect snow bands should take extra precautions as you head out today and/or tomorrow. To learn more about this latest lake-effect snowfall event, watch the brief video briefing attached above.
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©2017 Meteorologist Jordan Rabinowitz
DISCUSSION: The above figure shows snowfall totals over the past 24 hours from the National Operational Hydrologic Remote Sensing Center (NOHRSC) showing measureable snowfall in areas that don't regularly see snow (e.g., New Orleans, Brownsville, TX, southern Mississippi). Because these areas don't see snow often, they generally are not as prepared for snow as more northern locations. For example, many cities in the south don't have snow plows or salt trucks. Thus, even a little snow can make travel treacherous. But, some of these areas received more than 8 inches of snow over the past 24 hours (i.e., southern Mississippi), according to the above graphic. It is best to avoid driving in these conditions if you can help it. But, if you must go out, be sure to drive slowly and avoid making any sudden changes in speed or direction. In other words, drive extremely carefully. The storm system responsible for this snowfall is also causing issue with air travel. For example, Delta has canceled 600 flights in Atlanta as described here. This winter storm is expected to move up the eastern seaboard. Hence, people in the northeast U.S. who are better prepared to handle winter weather should nevertheless prepare for the upcoming impacts of this storm.
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© 2017 Meteorologist Dr. Ken Leppert II