This page will explain some terms and weather phenomenons that we commonly use or describe in our client weather updates.  We hope that this page will help you understand our content better and learn more about weather as well.  Be sure to check back as this page will update as deemed necessary with new weather information.  If you would like us to add new weather terms or information to this page, please Contact Us to let us know.



cold_air_damming_explained CADRegion CADThis is short for Cold Air Damming and Cold Air Damming occurs when a low-level cold air mass is trapped on the east side of a mountain range. An area of high pressure at the surface will be positioned to the north of the region and the clockwise flow around the high means the low-level winds coming into the region will have an easterly component. These easterly winds will push up against the east side of the mountain range and the cold air becomes trapped at the surface.  If the CAD is strong enough it can produce sleet/snow and ice in the region shown on the map.  The strength of the CAD is dependant on the sterngth of the high to the north.  Typically, if you have a high between 1028mb-1030mb then you will get a snow/sleet and ice changing to cold rain event.  If the high is 1032mb-1036mb, it normally will feature more in the way of sleet/snow and ice, but depending on position could still change to a cold rain thereafter.  When the high is 1038mb and greater, you will normally get a sleet/snow and ice event in the areas shown in the CAD, but it depends on the position of the high.  In less common circumstances, you can get a mostly snow event for most areas in NC/SC and North GA, which are along and north of I-85 (Ex. December 8-9, 2018).  However, it takes a perfectly placed high over Pennsylvania with a strength of at least 1038mb or stronger.



The overrunning process involves a stationary front that is draped across the USA.  Then, you get little weak low pressure impulses that ride along the front and sometimes repeat several times over the course of a few days.  These fronts can be located further north or south and different circumstances will produce different end results.  If the cold air is deep enough and it is winter, you can get a wintry mix scenario for some areas of the Southeast and Middle Atlantic regions.  In rare cases, the cold air can be some deep that a mostly snow event will take place.



ao_neg new_positive_pna1 epo_neg

You may see us mention a +PNA, -EPO or -AO and these are part of the large-scale teleconnections that are responsible for determining who gets what type of precipitation.  The graphics shown will explain how each of these work to help bring winter storms into the Southern US.



nao_neg BlocksExplained

Blocking patterns come in several different kinds of setups.  The classic -NAO that we speak of is the precursor 9 times out of 10 for the Eastern US of a winter storm.  Other blocks may not be as easily recognized but they each play an important role in the overall setup of out atmosphere and where large-scale systems go.  The graphic will help to explain these different blocks in more detail.