Pacific air makes for cool rather than cold fronts

Published 7:00 am Tuesday, December 15, 2015

Sunday night’s cool front was followed by relatively mild air for December. A stronger cold front on Wednesday night will finally bring temperatures for Thursday through Sunday to near or even a little below average.
Today’s air mass owes its moderate temperatures to the fact that it came to us from the Pacific Ocean rather than the interior of northern Canada.
Meteorologists use the term air mass to refer to a large horizontal expanse of air with relatively uniform temperature and humidity. The air masses behind most of our cold fronts this fall and early winter entered the USA as “maritime polar” air masses. They took on their basic moisture and heat characteristics over the northern Pacific Ocean.
Cold fronts moving from west to east across our area separate these Pacific maritime polar air masses from warm, moist maritime tropical air streaming northward from the Gulf of Mexico.
By the time Pacific maritime polar air masses reach us, they have been substantially modified by their transit across the American landmass to our west. Not only does interaction with the land modify the air mass, but they often mix with polar air of more continental origins.
One noticeable change in the Pacific air masses is that they become much drier compared to when they first surge ashore on the West Coast of the United States. They also can become cooler.
Wintertime maritime polar air masses tend to be significantly warmer than their continental polar cousins. Water heats and cools much more slowly than land. This results in the oceans staying warmer than the continents in the winter and cooler during the summer.
Really cold air masses that affect the Gulf Coast are either continental polar air masses coming to us via Canada, or continental arctic air masses originating even farther north in Canada, Alaska, and over the ice-covered Arctic.
So far this season, there has been only one significant southward surge of continental polar air. That surge brought us a light freeze early in Thanksgiving week.
The dominance of Pacific maritime polar air behind cool fronts is very typical for the Gulf Coast during winters when there is a strong El Nino underway.
The 2015 El Nino is a real doozy, to use a not-quite-precise non-scientific term. Sea surface temperatures in the main El Nino region of the tropical Pacific are some of the warmest observed since scientists began collecting those data in the 1950s.
That doesn’t mean we won’t have bone-chilling continental polar or arctic outbreaks this year. It just means that they are less likely than in other winters and there probably won’t be many of them.
Friday through Sunday, enjoy a cool, dry air mass, which looks to be a mixture of air from the Pacific Ocean and Canada regions, having both maritime polar and continental polar influences.
Next week expect a return of downright warm maritime tropical air from the Gulf of Mexico during the lead up to Christmas. It’s still too early to know whether we’ll get a cool break for Christmas day. But, all the computer models indicate that it’s a safe bet that you won’t need an Arctic parka.

By Skip Rigney

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