Today we are taking a look at an atypically strong (for mid-May) mid-latitude cyclone.
In the northern hemisphere, cyclones (low pressure) have counter-clockwise winds, and anticyclones (high pressure) have clockwise winds.
Mid-latitude cyclones (MLCs) typically feature the low pressure center, a warm front, and a cold front, and the cold front usually has a faster forward speed than the warm front. When the cold front overtakes the warm front, an occlusion forms.
South of a warm front and east of a cold front we have the "warm sector" of a storm system. Winds are typically out of the south or southeast, and bring copious moisture from either the Gulf of Mexico or the mid-Atlantic ocean. This is what our area was in Thursday evening. Abundant moisture-laden, warm, unstable air was pushed into the Carolinas on the coattails of this southerly to southeasterly flow. As well, there was a lot of SHEAR, which is a change of wind speed and/or direction with height. These two combined to bring the at-times tornadic thunderstorms to the region. The winds just off the surface (within the first mile or so) were very strong and coming from a different direction when compared to the surface winds.
A warm front indicates where warm air from the south is rising up and over cooler air (relatively speaking) at the surface. This typically brings lots of clouds, along with periods of rain but generally not severe weather (although, again, there are exceptions).
A cold front is usually the focus for severe weather in the springtime across our region. (Drylines are prevalent in the Plains but that's a different story.) A cold front provides a source of "forcing," or "lift." It literally lifts up a parcel of air (think 3D), which causes the air to cool faster than it surrounding environment, and causing thunderstorms to develop. When you see a "squall line" (a narrow-ish but discernible LINE of thunderstorms) you can infer the presence of a cold front. As the front pushes eastward, the squall line pushes eastward.
So what makes this system different? Several times this spring we have had MLCs that are "vertically stacked." What? Most of the time, low pressure is only at the surface and perhaps to 5,000 or maybe 10,000 feet. This year we have had systems that have low pressure extending upwards of 18,000 to 30,000 feet. The implication is that there are many layers of low pressure from the surface to that level... they are "stacked up," like when you stack dinner plates in the sink for someone else to wash. Hence, our MLC is vertically stacked.
(Click or tap to enlarge.)
The visible satellite images above show the comma-shaped MLC quite nicely. The annotated version of the visible satellite shows the position of the isobars (lines of equal barometric pressure), the fronts, lows, and highs. A 993-millibar low pressure system is quite deep for this time of year. The term "993 millibar" is air pressure. Defining that in terms that we might hear on the nightly news weathercast, 993 millibars equals a barometric pressure of 29.32 inches of Mercury. When the pressure is low like that, we say that the MLC is very "deep." The 1013-millibar high is equivalent to a TV meteorologist saying the barometric pressure is 29.91 inches of Mercury. The "normal" barometric pressure on the Earth is 1013.25 millibars, or 29.921 inches of Mercury.
The first Earth Wind Map demonstrates the surface winds from about 4:00 PM Friday May 5. This one is not annotated, other than the major cities for reference points.
Looking at the annotated version (map #2), we see that the location of the low pressure center, and the frontal boundaries, aren't as "nice and neat" as they are on the visible satellite (or what you'd see on TV). As I noted on the slide, science and nature are perfect, where man is not, and there is no doubt I made errors in my annotations of boundaries.
Anyway, a frontal boundary is merely a shift in wind direction. In the northern hemisphere, where winds go from blowing out of the south, to suddenly blowing out of the west, one can imply a cold front lies at that junction. Winds blowing out of the south, turning to blowing from the east or southeast, indicates the presence of a warm front. Also remember I said winds around low pressure flow counter-clockwise.
If you look carefully you can see a boundary right along the spine of the Appalachian mountains. This is merely a surface trough; winds are blowing northward into the cyclone. Since the winds are parallel to the mountains, the air is rising on both sides of the mountains and funneling northward.
You'll also notice the winds over the ocean, as well as over the great lakes, are stronger, whereas the winds over the land surface are considerably slower. This is due to friction from the land surface. Over the ocean, that friction doesn't exist as strong so the winds can be much stronger. Over land, however, things get a bit more "sticky" so to speak... especially when the land features include various valleys and mountains. Look carefully north of New York City (on map #1 above) and you can make out the outline of the Hudson River Valley.
Surface weather features are "driven" by processes higher up in the atmosphere. Typically a forecaster will examine charts at 5000 feet, 10,000 feet, 18,000 feet, 30,000 feet, etc. I posted the wind map from 18,000 feet (about 3.4 miles off the ground). One of the first things we note is that friction doesn't matter here. There is a southwest wind blowing at about 92 mph over central North Carolina at 18,000 feet. We also notice our vertically stacked low center and the "upper level trough." The upper trough indicates the strength of our area of low pressure, as the lower air pressure extends the length of that trough. When you have a strong trough such as this, air movement picks up speed as it rounds the base of the trough (the southern end). In this case, the strongest winds are directly west and east of the low 'center', but we have increasing wind speeds from the base of the trough east-northeast all the way into New England.
Well what in the world does that mean? Simply put, the strengthening winds strengthen the weather features below that. As well, thunderstorms routinely reach the 18,000 foot level (sometimes much, much higher) and can "mix down" some of these very strong winds to the surface, thus making the thunderstorm classified as "severe." Furthermore, if the winds are blowing in different directions at the 18k foot level, versus the 5k foot level, versus the surface, we have that directional shear I mentioned waaaaay up above in this lengthy blog. Shear is needed for thunderstorms to become severe; shear helps keep the storm moving forward (as opposed to stationary); shear is also a factor in tornado development.
In the environment the evening of the 4th and overnight into the morning of the 5th, there wasn't much in the way of directional shear (change of direction with height), but there was a good level of speed shear (change of speed with height). However, there was just enough directional change to put a "spin" on even the relatively "shallow" thunderstorms (storms that weren't all that tall), that they generated areas of rotation seen on radar.
The last map,#4, is the jet stream level. Take a second to acclimate yourself; I realize there are a lot of colors and a lot of text on the image. I have highlighted Miami, Wilmington, and New York City in yellow font. Troughs and ridges were apparent at the 18k foot level, but are REALLY apparent at the jet stream level (34,000 feet). A ridge is a broad area of high pressure, of sinking air, and usually you have dry weather underneath a high pressure ridge. Alternatively, a trough is a broad area of low pressure, of rising air, clouds, precipitation... an unsettled weather pattern.
Our vertically stacked low is spinning its wheels over Appalachia with a very deep, broad trough extending from Canada all the way down to the Gulf of Mexico. Similar to the 18k foot chart, we see the strongest winds where they round the base of the trough and head northeastward. In this case, the southern jet stream (highlighted, over Mexico) "phases" with the northern, or "polar" jet. This is also somewhat unusual for mid-May and added additional strength to our MLC. In the middle of the Atlantic ocean you see the well-established Bermuda High pressure ridge that will eventually be responsible for our weather in June, July, August... you know... mid 90s... high humidity... etc.
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Our atmosphere is a fluid. Air is a fluid... it is water in the gaseous state. The atmosphere is a dynamic, always-changing, always-moving entity (for lack of a better word). When you see blue skies and scattered clouds, there are many different processes at work, and all are connected with each other to bring the weather you and I on the surface experience. I hope to have shown you just a sampling of this web as it relates to our classic MLC.
Thanks for reading!