Climates and microclimates – Part 2

The Central Plains

With tall mountains to the west to cut the power of storms from the pacific, you would think that the weather would be gentle as it reaches the central plains, but just the opposite is true. The Central Plains – extending from the Dakotas almost to the Gulf of Mexico are subject to some of the worst weather conditions in the world.

The Plains are subject to frontal activity from the west, north and south. Due to the planetary rotation, Coreolis forces in the US latitudes generally cause our winds aloft to move from west to east.

As weather systems rotating around the north pole tend to spill southwards, they bounce off of the mountains and flow down the Plains.

In the equatorial regions of the planet, the winds flow from east to west. Moisture from the Caribbean therefore can be picked up by strong low pressure systems and pushed across the gulf.

When all three of these forces are acting in concert, they create dramatic weather changes. Try placing a house plant (one you don’t like) between three fans all on high.

Sometimes none of these forces is strong enough to do anything, remember the plains are fairly far from the origin of the forces so they can peter out before they reach the area, so there can be long periods of dry weather or droughts, especially when a large strong high pressure clamps down over the area, steering weaker systems around it.

In the summer, the western forces cause a dry windflow that intensifies with afternoon heating. The big storms occur when the southern and northern forces feed moisture into the winds and heat. Since the moisture is spread out, the storms usually occur in wide areas. The really severe storms have two or more of these lifting mechanisms.

Lifting mechanisms include troughs, fronts, upper air disturbances and heat. In order for storms to form, the hot moist air from the surface must be lifted up into the cooler air above, creating cumulus clouds. With only a little lift and moisture you then have a delightful afternoon with fluffy white clouds that kids imagine shapes in.

Add a stronger lift and more moisture and the clouds develop upwards into Towering Cumulus. Add more, and the Cumulo Nimbus, or storm cloud is created.

When you are reading a weather chart, a trough of low pressure is depicted with dashed line, usually colored brown. There is a perpetual north-south one located between New Mexico and Texas that moves east or west about a hundred fifty miles, but never really goes away. It is also usually a dryline – meaning that air to the east of it is moister than air to the west. This trough draws moisture up out of the gulf into the area, then afternoon heating intensifies the already present lifting mechanism forming storm activity that usually extends northwards from Midland to Amarillo. These storms generally move east-northeast. They lessen in intensity as temperatures cool in the evening. but if there is a secondary disturbance aloft, they can stay alive and regenerate the next day.

If another front is dropping down the plains it can also generate the lift for afternoon storms. The superstorms happen when you mix all these together, add a pinch of jetstream and hot summer temperatures.

In the winter the plains are affected most by the Polar fronts as they blast their way southwards. Again, the Rocky mountains act as a barrier on the west, so the frustrated front is contained on the plains. The Ozarks mountains to the east also help funnel the majority of the storm’s forces southward before the prevailing westerly upper winds drag them off to the east.

In general, the night before you are planning to fly in the Plains , look at the temperature/dewpoints across your route. At 8pm, if the spread between temperature (T) and Dewpoint (D) is 5 to 6 degrees Celcius or less, you may have morning mist or fog. If there are no other clouds aloft, then the fog will be fairly shallow and should burn off by about 9am. If the T/D spread is lower than 3 degrees the fog will probably be thicker and last until noon or later.

If it is summer, low T/D’s herald afternoon thunderstorms. However, there will almost always be a period of time between morning fog and afternoon thunderstorms where you can go VFR. The same lifting mechanism that starts the upward development of storms also dissipates the morning fog.

If the T/D spread is between 8 degrees and 15 degrees you may wake to clear skies with fluffy clouds in the afternoon. If it is more than 15 degrees you can plan on it being clear and hot with lots of afternoon turbulence.

What will compromise this generality is if there is an airmass that moves in overnight with a front-which changes the whole picture. If you are looking at a surface analysis, cold fronts are blue lines with spiky teeth. Warm fronts are red lines with rounded bumps. Strong ones move fast and weak ones move slow and will tend to become stationary and eventually dissipate. Airmasses are piled up behind the fronts moving them forward. What determines the difference between a cold front and a warm front is the temperature of the airmass behind it relative to the one it is moving into.

Manmade conditions due to farming on the plains and the building of cities contribute to the microclimates in the area. Fields that have been freshly tilled release moisture and heat. Large parking lots, streets and buildings radiate heat which causes lift…microclimates that glider pilots take advantage of. Conversely, the air over lakes and streams is cooler and more dense than the surrounding areas.

When you watch the weather channel or your local weather, the charts they use will show the High pressure systems as an H, and the Low pressure systems as an L. In our hemisphere, the winds around a High circulate clockwise and push air down to the ground, while the winds around a Low circulate counterclockwise and push air upwards. Look at the weather maps and imagine how the winds are flowing in those areas. Remember that winds flowing over water will pick up moisture and carry it aloft onto the land areas.

The southern plains, from Texas through Kansas will get most of their moisture from the south, so look for a strong High (H) pressure system in the gulf of Mexico to push that moisture northwards.

Though Hurricanes are the most colossal form of a Low Pressure system on the planet, one that hits the gulf coast moving northwest can also throw moisture as far west as Arizona before the prevailing winds push it back to the east. Did you know that a tornado is the strongest form of a low pressure system in terms of wind strength?

The Northern plains, from Nebraska through the Dakotas, receive their moisture from either the pacific or the arctic/Hudson bay area. The jet stream changes its pathway as the planet travels around the sun. As the planet’s tilted axis causes winter to set in, it also causes the northern jet stream to begin waving north to south, catching the moisture trapped in layers below and feeding them southwards as well. This cold moist air brings blizzard conditions to plains from the north.

This website from the NOAA’s weather service center is designed for aviation and has weather advisories available - http://adds.aviationweather.gov/metars/index.php

Rose Marie Kern is a Flight Service Specialist at ABQ AFSS. You can ask questions by emailing her at author@rosemariekern.com.