It’s all about the jetstream baby.

So by now I am pretty sure almost everybody is sick of the cold, windy, snowy days here in the New River Valley. In fact I was walking on campus the other day and some goofy guy was screaming ” I HATE THE COLD! I HATE THE COLD!”. I’m sure most of us have felt the same over the past few weeks. It should be getting sunny, warm, and we should be laying out in the sun down by the New River by now right??? In fact last March this was the case. On the first day of spring last year the high was 77. The high on the first day of spring this year, was in the mid 30’s. Why was there such a major difference in temperatures from one year to the next?? Why are we continuing to see temperatures barely reach 40, strong winds, and borderline winter precipitation events?? We’ll take a look at each of these questions as well as explain some dynamics of the polar jet stream and mid-latitude cyclones. Let’s dive in baby.

So people keep on asking me, “When will it warm up?”. My answer is always, “Eventually.”. In reality it is somewhat impossible to tell when it will warm up exactly. The model runs only provide data for a few weeks out, and the farther out you look, the less accurate the models are. There are just way too many variables that come into play to  correctly project output temperatures that far out. However, the Climate Prediction Center does produce maps projecting percent probabilities of temperatures being either above or below normal for the near future. The following map shows the most recent percent probability output from the Climate Prediction Center:

Temperatureprobability814

So what does that map tell us?? Essentially the Climate Prediction Center is forecasting that for the next 8 to 14 days  the New River Valley has a 40% chance of experiencing temperatures below the average for this time of year. So this means that we can most likely expect below average temperatures to continue through the first 10 days of April. Now before everyone blows up over this, rest assured the warmth is on its way. I’ll show you another output map from the Climate Prediction Center later in this post which will have many of you breathing a sigh of relief.

The next question many of you are probably asking is, what is causing this cold pattern to continue this late into spring? To make things simple, we can blame the polar jet stream. The jetstream level in the atmosphere is around 300 mb(in meteorology we measure heights in the atmosphere at different pressure levels. The higher the pressure, the closer to the surface. So 300 mb is extremely high in the atmosphere). In the winter time, the jetstream is our main weather maker. Due to higher temperature contrasts,  this massive stream of air migrates further and further south as we get deeper into the winter.

Jetstream Pic

Credit: National Weather Service
You can see the massive wave patterns created by the jetstream. Look as one giant wave exits the North East, another dips deep down into Texas behind it. We refer to these dips as “troughs”. The other part of the wave pattern is known as “ridges”. You can see a high amplitude ridge entering California.

The jetstream creates massive waves in the atmosphere. Large dips in the jetstream are known as “upper level troughs”, while other end of the wave is known as “upper level ridges. Without diving too deep into the physics, on the eastern side of an upper level trough is where upward motion is enhanced at the surface. In meteorology, upward motion (convection) is associated with precipitation and storms. On the other hand, when an upper level ridge is located over your area, we associate these areas with descending air, clearing skies, and calm weather. The jetstream evacuates massive amounts of air in the upper atmosphere  On the eastern side of a trough, wind speeds pick up greatly and the air rounding the bottom of the trough fails to keep up with the wind in front of it. This creates an area of divergence high in the atmosphere.  This is where rising air from the surface comes in. To fill the void of evacuated air high in the atmosphere, air rises from the surface. What does rising air lead too?? Cloud formation and precipitation. On the other hand, air slows down on the eastern side of a ridge. Fast moving air rounding out the top of a ridge will converge with this slower moving air. When air above the surface converges, it has nowhere to go but down. What do we associate descending air with? Clear skies with no precipitation in the forecast.

Upper level ridge and trough.


Credit: Dave Carrol
Upper level ridge and trough.

Divergence aloft, leads to upward motion from the surface.

Credit: Dave Caroll
Divergence aloft leads to upward motion from the surface, cloud formation, and precipitation.

convergencealoft

Credit: Dave Carrol
Convergent air aloft leads to descending air, clear skies, and no precipitation.

So the further you can drive these jetstream troughs south, the more powerful wind dynamics you can create. These stronger wind dynamics are what we need in order for mid-latitude cyclones to develop. If you can keep producing rising air at the surface eventually a low pressure system can develop at the surface. Air will begin to circulate counter-clockwise into a developed area of low pressure. If you can keep evacuating air aloft at the jet stream level(see divergence diagram above), you can continue to strengthen this surface low pressure system, and continue to launch air vertically. Winds at the surface will eventually begin to circulate in towards the low creating fronts at the surface(think cold and warm fronts). The diagram below best depicts this phenomenon:

As air aloft diverges an area of low pressure may develop at the surface. Winds will begin circulating inwards towards the low creating a warm front and cold front around the low.

As air aloft diverges an area of low pressure may develop at the surface. Winds will begin circulating inwards towards the low creating a warm front and cold front around the low.

Our location in reference to these low pressure fronts dictates the type of winter precipitation we see. If the low tracks along the coast(say through Georgia), we are in the prime snow area. Areas northwest of the low receive the cold air which is wrapped around the back side of the Low. If the low tracks through Virginia or say Tennessee we will be influenced by the associated warm front, creating a wintry mixed event. This past weekend we saw a few inches of snow here in Blacksburg. First, a low pressure system blew up off the South Carolina coast. We received remnants of that system which was the initial snowfall we experienced throughout the day on Sunday. If you remember though it stopped snowing completely around 4 pm. Although we didn’t receive much more snow overnight, we were being foretasted to pick up another couple inches Sunday night into Monday morning. This was due to the fact that a separate low pressure system was being forecasted to move through West Virginia. Well this location of the low put us in the southeast sector. Sometimes this sector can receive “wrap around” snow as the low wraps around cold winds. Unfortunately for us topography completely destroyed any precipitation that tried to make it over the Blue Ridge Mountains(a phenomenon known as the rain shadow effect. Something I will blog about in a later post).

Back to the jetstream. Along with vertical motion development, the jetstream plays a large role in advecting warm and cold air. Large troughs drive cold Canadian or arctic  air into the lower 48 states. Large ridges on the other hand drive warm air from the tropics northward. This phenomenon is pictured below:

Warm and cold air advection due to jetstream dynamics.


Credit: Unisys
Warm and cold air advection due to jetstream dynamics.

So as long as the jetstream continues to dive deep into the lower 48 states we will continue to see cold air advecting into our area, strong wind dynamics, and borderline wintry precipitation events due to mid-latitude cyclogenisis.

For example, a cold front and associated low pressure system is being forecasted to move through the region this Sunday into Monday.  Let’s take a look what’s happening.

First you can see that a jetstream trough is driving southward by 7 am on Sunday:

Jetstream trough diving southward on Sunday at 7 am.


Credit: National Weather Service
Jetstream trough diving southward on Sunday at 7 am.

As you can see they are expecting an area of low pressure to develop under the eastern side of that upper level trough as well as an associated cold front:

12zsurface


Credit: National Weather Service
Low pressure system develops in Ohio. An associated cold front develops west of our area.

Now let’s look at 24 hours later. The jet stream trough has migrated eastward:

The upper level trough has migrated eastward at 7 am on Monday.


Credit: National Weather Service
The upper level trough has migrated eastward at 7 am on Monday.

The low pressure system continue to follow the upper level pattern and meanwhile the cold front sweeps through our area, bringing northwest winds and cooler temperatures early next week:

Low pressure migrates eastward keeping up with the jetstream. Meanwhile the associated cold front clears our area.

Low pressure migrates eastward keeping up with the jetstream. Meanwhile the associated cold front clears our area.

At this point in the year the jetstream begins to migrate northward and we usually don’t have these deep penetrating troughs develop. So why is this not the case this year? One of the main reasons is because of a phenomenon known as atmospheric blocking.  When unusually large areas of high or low pressure settle in the upper atmosphere, they can create blocking patterns in the jet stream. In our case unusually high pressure over Greenland can create a blocking pattern which creates increased deep level toughing in the eastern United states. And as we know, deeper troughing leads to cold air intrusion and cyclone development. This is a phenomenon that is not extremely well understood, however it is something that has been occurring this late winter and early spring, creating deeper troughing in the east coast.

High pressure sits over Greenland creating an Omega pattern in the jetstream. This leads to cold air intrusion into the eastern US through troughing.

Credit: Accuweather
High pressure sits over Greenland creating an Omega pattern in the jetstream. This leads to cold air intrusion into the eastern US through troughing.

 This upcoming weekend the block will diminish slightly and we’ll see pretty nice weather with high’s in the upper 50’s. But I wouldn’t be surprised to see it re-intensify as we enter April.

So my answer to the question, “When will it warm up?”, is still “Eventually”. I think we’ll see cooler temperatures in early April, but it will gradually get warmer and warmer by the end of April.

In fact the Climate Prediction Center also produced three month temperature outlooks. This was their latest map:

Climate Prediction Center three month temperature probability outlook.

Climate Prediction Center three month temperature probability outlook.

They are predicting that we have a 40 % chance of temperatures being above normal over the next three months.

Warm weather is on its way, this much is for certain. For now, enjoy wearing your thermals for these last few weeks, because we all know how much we’ll miss them come summer time.

-Nick Luchetti

Meteorology student at Virginia Tech.

I leave you with some phenomenal music from Railroad Earth.

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