Winter Making A Huge National April Comeback. Here's Why

Ice from unseasonable freezing rain covers trees in St. Albans,
Vermont Tuesday afternoon.

For two days in a row, Monday and Tuesday, freezing rain fell on different parts of Vermont - in April!  Then many of us Vermonters woke up to a thin but real new covering of snow this morning.

In Quebec, things were even worse. The ice accumulated so thickly on trees and power lines that branches broke and wires snapped. More than 250,000 people, mostly a little north of Montreal and northeast of Quebec City, lost power due to the unseasonable ice.

There were reports of carbon monoxide poisonings from people using generators and house fires caused by candles lit to ward off the darkness.

Meanwhile, in northern Maine, it just keeps snowing and snowing, with more snow yesterday and last night, leaving more than two feet of snow on the ground in some areas.

And of course, there's that gigantic blizzard in the central and northern Plains, which might end up bringing record snows to some areas of Colorado, Nebraska, South Dakota and Minnesota.

Geez! What happened to April?

Blame it mostly on something called the North Atlantic Oscillation,  or NAO. It's a weather pattern well known to meteorologists in the northern Atlantic Ocean and around Greenland that often has a big influence on the weather here in the United States and often in western Europe.

A schematic showing a NAO pattern. Note the high
pressure in red near Greenland. It's suppressing the jet
stream south, so that storms are centered
in California and New England, not further north.

The NAO flip flops between two phases. If it's in it's positive phase, there's generally low pressure over in the far North Atlantic and higher pressure further south in the ocean, in the eastern United States and often western Europe.

The opposite is called the negative phase of the NAO. In this case air pressure rises in the North Atlantic, suppresing the jet stream southward. It also tends to lower air pressure in the mid-latitudes of the United States and Europe.

That means we get who live in those mid-latitudes get cold and stormy weather. The more negative the NAO is, in general the worse it is for people in much of the United States or Europe. Or sometimes both.

The NAO is in a relatively intense negative phase at the moment. In the spring, the jet stream usually begins retreating to the north, and that's one reason it gets increasingly warm during this season.

The current negative NAO has stopped or even slightly reversed that northward seasonal trend in the jet stream. That means colder air can sneak further south into our neck of the woods. Storms are following the jet stream that's more or less over us. Parts of those storms can take advantage of that southward moving cold air and give us a winter redux, like many of us are currently experiencing.

In some ways, this current negative NAO is making things worse it might in the winter, when we'd expect it to bring us nasty cold snaps and snowstorms. Psychologically, it's always tough for most of us to endure freezing rain or snow when we're supposed to be outdoors in the sunshine, enjoying blooming daffodils.

Plus, it is spring, and you can't violate the laws of physics. The increased sun angle is warming the Northern Hemisphere. That warm air is fighting those cold conditions brought south in part by the negative NAO.

This state of affairs increases temperature contrasts over short distances.  A sharp temperature contrast like this can be one of several factors that create storms more intense than they otherwise would be. Or this state of affairs can create weather systems that are weirder than they otherwises might be.

A perennial bed in my St. Albans, Vermont gardens is yet to show
much activity this chilly April. Freezing rain yesterday and this
coating of snow this morning is not helping.

Part of the reason why the Midwest storm is so strong and that blizzard is because of a sharp temperature contrast.

 There's a difference of around 50 degrees over a 100 to 200 mile wide area near the storm. That is  one factor that will deepen the storm to possibly become one of the most intense on record for this time of year.

Thunderstorms in the very nearby warmer air in the southern part of the storm are feeding northward into the cold air, intensifying the snowfall in the blizzard zone.

This, and lots of moisture feeding north from the Gulf of Mexico into the storm, are leading to the intense rates of precipitation. Rain and melted snow in the already flooded central and northern Plains will probably reach two to four inches.

Here in the Northeast, the storms this week haven't been especially strong, but they have been weird. On Monday, temperatures ranged from the 20s in Maine, to the 50s in Massachusetts. At around 10 p.m. Monday night it was 56 degrees in New York's Central Park but 72 degrees in Newark, New Jersey, only 17 miles away.

This tight temperature gradient has been noticeable in Vermont the past few days, too. Late Tuesday morning, it was just below 32 degrees with freezing rain in the northern Champlain Valley but close to 60 degrees in Bennington.

By the way, we've now had freezing rain in two Aprils in a row. Although freezing rain has happened in other Aprils, I've personally never seen before this year and last. Especially not like Tuesday, when there was freezing rain in the early afternoon. With the strong sun angle, some radiation would have come through the clouds to melt it. But the cold air was too tough on Tuesday.

There are signs of hope, though, because spring is inevitable. The track of the storm causing the blizzard out west will mean some relatively warm air will briefly make its way into Vermont Friday and Saturday, bringing temperatures to near 60 degrees. That's not unusually warm for this time of year, but I'll take it.

Another storm on Monday that I feared would bring us a snowstorm might go a little further west than initial forecasts. If that happens, we get mostly or all rain, not snow.

Any particular phase, either positive or negative, can last for days, weeks or even months.  A negative NAO lasting for an entire season has given us some memorably harsh winters.

This particular negative phase of the NAO won't last months. It is already showing signs of reversing itself, or at least becoming more neutral later this month. That would also help open the door to more springlike weather.

Or at least we hope

Mid-May Weather Pattern Created Extremes Around Northern Half Of World

Wildfires in Greece this week amid record early season heat
Photo by Aris Messinis, AFP/Getty Images

An especially wavy jet stream in the Northern Hemisphere is causing some pretty extreme weather in some areas of the world this week.

The jet stream - that upper atmospheric river of fast moving air that controls weather patterns, is experiencing wild bulges to the north in some areas, and deep plunges southward in others.

Depending on where these ridges and dips are and move the weather can change quite a bit.

As noted yesterday, this is affecting us here in Vermont: It snowed in some high elevations Sunday. By Thursday, a few towns in the warmer valleys of the Green Mountain State will be around 90 degrees.

As noted in Weather Undergrounds Category 6 blog, the weather elsewhere has taken some strange turns as well. Greece and parts of northern Africa are having some of their hottest May weather on record. It was as high as 105 degrees in one town in Greece, and Athens reached 92 degrees, its hottest day for so early in the season.

Brush fires broke out in Greece, killing one person.

Meanwhile, in parts of northern Europe, winter hangs on. Norway had its snowiest May since the 1960s. Parts of eastern Finland received six to 10 inches of snow on Mother's Day. In  Russia, bursts of heavy snow hit Moscow on May 11.

A snowy scene in Moscow on May 11

Meanwhile, back in the United States, parts of the Rocky Mountains, especially in areas of Idaho and Montana, a winter storm watch is in effect through Thursday because of expected heavy snow.

Part of the reason for the wild weather extremes and the wavy jet stream is something called a "negative NAO."  

NAO stands for "North Atlantic Oscillation."  Sometimes it's positive, sometimes it's negative, and there's a complicated index that tells you how positive and how negative it is. If it's really positive or really negative, weather can go off the rails.

A negative NAO can mean less than normal low atmospheric pressure near Iceland and less than normal high pressure in the subtropics. The weak pressure gradient helps give the jet stream more room to meander more to the north or south.

In the first part of May it the "NAO" was really  negative. That meant weather patterns that favored colder weather in the Northeastern United States and in northern Europe and hot weather in soutern Europe.

The NAO is trending toward zero, neither negative nor positive, so the weather is calming down somewhat. (By the way, when the NAO is positive, it tends to get stormy in western Europe, warm in the United States and frigid up in northern Canada.)