Doug Simonian

Colder weekend could lead to a wintry mess on Sunday night

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As temperatures have moderated along the east coast, an abnormally strong cold airmass has entered the Western half of the United States. This cold airmass is thanks to a strong -EPO ridge over Alaska, which helps to dislodge Arctic cold into the United States. However, because of the +NAO, +AO, and -PNA, this cold is forced into the West, and will initially have a lot of resistance as it tries to head eastward. This pattern is illustrated very well by today’s 12z European model, valid for Thursday night, shown below.

Today's 12z European model valid for Thursday night shows what we call a "gradient" pattern with very cold air to the north and warm air to the south and east. This battle zone between the two airmasses will create the chances for precipitation. Image credit goes to the WSI Model Lab.

Today’s 12z European model valid for Thursday night shows what we call a “gradient” pattern with very cold air to the north and warm air to the south and east. This battle zone between the two airmasses will create the chances for precipitation. Image credit goes to the WSI Model Lab.

The strong -EPO ridge is evident as there are high 500mb heights into Alaska, which is forcing an Arctic airmass into the Northern Plains. This airmass has the chance to break records, and will try to spill eastward with time; however, it has a lot of resistance thanks to the strong ridge in the Southeast. This very warm air in the Southeast and Arctic air in the Northern Plains will create an impressive temperature gradient in the northern Mississippi Valley and Ohio Valley region, and this gradient is where some storms will form. As these storms form, the ridge in the Southeast will force them to track to our west, leading to warm air and rain for Thursday night and Friday. However, the passage of each low pressure will try to gradually shift this gradient eastward due to counterclockwise flow behind them — it is a step-down process. The image below illustrates how strong this gradient really is, and it’s pretty mind-boggling.

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John Homenuk

PM Update: Milder this week, more wintry by weekend

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The trend towards milder temperatures continues this week, as high temperatures rose into the upper 40’s and the 50’s throughout the area on Tuesday. The modifying airmass and stagnant high pressure are initially to blame this week, with no reinforced source of cold arctic air. But by Wednesday into Thursday, the warm air will be forced northward by a surging warm front and associated storm system moving through the Great Lakes.

The warm front moving northward on Thursday will bring an unseasonably warm airmass with it. Southerly winds will pump in very mild air, with temperatures rising into the 60’s by Thursday afternoon. But the warmth will be short lived, as a cold front moving through the area on Friday will mark the beginning of another infiltration of arctic air.

NAM model showing high temperatures in the lower to middle 60's in parts of the area on Thursday afternoon.

NAM model showing high temperatures in the lower to middle 60’s in parts of the area on Thursday afternoon.

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Tom Stavola

The forecast headache that is next week

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Arctic air will be pouring into the Rockies, Pacific NW, and Northern Plains over the next several days as the mid level ridge centered near Alaska energizes. Whenever we see the Alaskan ridge, it suggests that cross polar flow has initiated from Siberia to North America due to the clockwise flow around that mid level high. This pattern is polar opposite of what we saw the past two winters over Alaska. Remember the stories of yards of snow and endless bitter cold in AK? That has not been the case as of late due to the strong positive height anomaly at the 500mb (18,000ft) level of the atmosphere.

The composite for November 2013 illustrates the ridging that has been taking place in the northeastern Pacific and AK, exactly where a trough pattern dominated in 2012.

 

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John Homenuk

PM Update: Increasing clouds, becoming milder

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Low clouds and fog were the main story early on Monday morning, with plenty of areas reporting low visibility. The cloudiness also kept temperatures a bit cooler to start. But by Monday afternoon, the low clouds had lifted and most areas were reporting sunshine. Temperatures warmed into the upper 40’s to near 50, which felt especially refreshing after several days of cold weather. The warmth will continue to slowly build this week.

However, with the building warmth will come a variable increase in cloudiness. With a weak storm system offshore, low clouds will increase again early from late Monday night into early Tuesday morning, with the potential for fog in the morning once again. Temperatures are expected to top out in the 50’s — but not until the late afternoon when the clouds burn off a bit.

NAM model showing high temperatures reaching near 50 degrees on Tuesday afternoon, with a storm system offshore.

NAM model showing high temperatures reaching near 50 degrees on Tuesday afternoon, with a storm system offshore.

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