Weather Discussions

Welcome to the Weather Discussion page! The SDWC will have meetings where YOU will analyze our atmosphere and talk about the weather. Here is a helpful page to help you get started. Learners of all levels are welcome to particpate.

Weather Discussion Format:

At the start of the meeting, we start with a geography game and talk about our week!

We then will divide everyone into breakout groups, and then each group will take a few minutes to discuss one or more set of products (satellite imagery, upper air charts, surface analysis etc.). This should give everyone a chance to ask plenty of questions and be ready to discuss the weather as a group! If you don't know anything about weather, no problem! Our club leadership will guide you through how to read maps and other weather data.

Step 1: Look at this Satellite Imagery (Band 2, 13, 8) ------> https://www.star.nesdis.noaa.gov/goes/conus.php?sat=G16 (CONUS)

https://www.star.nesdis.noaa.gov/goes/sector.php?sat=G16&sector=sr (AZ)

Step 2: Look at these Mandatory layers (850mb, 700mb, 500mb, 300mb, 250mb @ 12Z ) ------> https://www.spc.noaa.gov/obswx/maps/

4 Ingredients to Create a Storm

Water Vapor (Band 8):

So you've clicked through some of the links in the previous section and you're probably wondering what these products are telling you. In this section, we explain meteorolgical terms and how to analyze the surface and upper air charts.

4 Ingredients to Create a Storm

In order to create weather you need to have 4 things happening in the atmosphere:

Moisture- energy needed to get storms started

3 maps to analyze: Surface, 850 mb, 700 mb
Look for Dew Point Depressions (Tdd) 5 or below on surface and 850 mb
Look for Tdd above 20 on 700 mb map

Vorticity - counterclockwise spin
- 1 Map to analyze: 500mb
- Look for area at the bottom of the trough
Primary Uplift
Secondary Uplift

Ideally, each of these mechanisms would be present and occurring at a high intensity for more intense precipitation. The goal of our weather discussions is to determine whether or not each of these ingredients are present across the different levels of the atmosphere.

Surface Analysis

The surface analysis is a logical place to start analyzing atmospheric conditions. Start at the bottom and work your way up. Surface station observations of temperature, dew point, pressure, wind speed/direction, and present weather are plotted, and isobars (lines of equal pressure) are added and adjusted. If you have seen any sort of topographic map, then the idea of what contours are telling you should be somewhat familiar. At any point along each contour, the elevation is the same. Contours will typically enclose areas of high or low elevation. When you look at a surface analysis then, the contours represent barometric pressure, given in millibars. For reference, the pressure under normal conditions at sea level is about 1013 mb. The contours, or isobars, are always done in four millibar increments. The center of high-pressure areas are marked with an H, and low-pressure with an L. Distinct fronts are associated with mature Mid-Latitude Cyclones, which are strong low-pressure systems with cyclonic (counter-clockwise) rotation and typically one or more fronts associated with it.

Cold Front

This of course is the leading edge of an advancing cold airmass. The cold air, being much more dense than the warmer air ahead will rapidly lift air, potentially leading to strong thunderstorms right at the front. A pressure tongue (an elongated extension of the isobars away from the center of the low) is the first clue to placing a cold front. Obviously, cold temperatures and drier air will be behind the front. Winds are also typically from the west or northwest. Ahead of the front will be the warm sector, with warmer temperatures and moisture. Winds will typically be from the south or southwest. The changes in these variables are typically much more abrupt than for warm fronts.

Warm Front

Warm air will advance over denser cold air, lifting gradually. Most of the precipitation will occur as steady but generally light rail ahead of the front, where the warm air has lifted sufficiently ahead of the boundary at the surface. There is actually very little uplift at the boundary itself, with few or no clouds aloft and perhaps some fog near the surface (given enough instability though, severe weather is occasionally possible around a warm front). Because of this pattern, warm fronts can be more difficult to place. Pressure tongues are the primary clue, though they may not be nearly as distinct as for a cold front. The pressure tongue is typically found east or northeast of the central low. Behind the warm front, there will be warm temperatures and moisture in the warm sector as expected.

Stationary Front

This type of front should be fairly self-explanatory. This type of front often occurs when a jet-max is located overhead and parallel to the boundary. Since it is parallel, the upper-level winds will not move the front. Despite the lack of movement, this is still a boundary between two different airmasses and can act as a lifting mechanism.

Occluded Front

This type of front begins to form usually as the mid-latitude cyclone begins to weaken. The cold front moves much faster than the warm front, and eventually will catch up with the warm front starting near the central low. At this point, the cold air has uplifted all of the warm air, leaving only cool air at the surface. With the loss of strong uplift, light rainfall is typical. Occluded fronts are common especially over the Pacific Northwest and occasionally the East Coast. If you think about it, Seattle is known for receiving lots of rain, but usually not intense rain.

Dry line

This is a boundary between two air masses with substantially different amounts of moisture. They are especially common during the spring and summer in western Texas and the Great Plains. Moisture will flow from the Gulf of Mexico north, where it meets dry air from the west.

Upper Air

The Mandatory Levels

As we start looking at the weather above the surface, we look at a series of somewhat different charts. Upper air charts depict what are called ‘constant-pressure levels’, where at any given point on the map, the pressure is the same. What changes (what the contours represent) is the height of t pressure level is observed. For example, the 500mb chart displays the height of the 500mb level. This is the height that the pressure was 500mb. The pressure at any point on the 500mb chart is 500mb, so again what is varying is the height of that pressure-level.

They are, from lowest altitude to highest: the 850mb level, 700mb, 500mb, 300 and 250mb. Each of these levels is represented on a map as a constant pressure level, meaning that at any point on, say, the 850mb map, the pressure is 850mb. What the contours are telling us then is the height (in meters) at which the pressure in the atmosphere is 850mb. The height of this constant-pressure surface will vary based on differences in temperatures and pressure throughout the atmosphere. The contours, wind and other variables will tell different parts of the story at each level.

850mb:

As we said above, this is the lowest (closest to the ground) of the mandatory levels. Though the contours correspond to height, they actually tend to match the features on the surface analysis. This layer is generally analzyed for moisture which is the energy in weather. This can be done by looking for dew point depression less than 5 degrees farenheight or below.

Another item fo interest is convergence. Convergence is when the wind barbs come together. This forces the air to lift, which is an important ingredient to a storm.

700mb:

This layer of the atmosphere magnifies areas that provide uplift. These areas are known as "shortwaves" and they appear as dents in the isobars. To show you we have attached an image:

This layer is also used to analzye dryness aloft, or dewpoint depressions greater than 20 degrees celsius. Dry air at this level allows moist air below it to be lifted.

500mb:

The 500 mb chart shows the general flow of the atmosphere. It is useful in tracking storms to forecast where they will move to in the future.

300mb:

This layer is used to evaluate the upper air flow, usually in the winter. Most times meteorologists look at this level for jet maxes which are the areas that have the fastest air flow. These areas are usually shaded in blue and have wind speeds greater than 70 knots.

Divergence, or areas where the wind barbs spread apart, is also an important item to analzye. This process forces the air to rise in order to replace the air that is spreading apart.

250mb:

Similar to the 300 mb chart, this layer is utilzed to analyze the upper air flow in the summer. Most times meteorologists look at this level for jet maxes which are the areas that have the fastest air flow. These areas are usually shaded in blue and have wind speeds greater than 70 knots.

Divergence, or areas where the wind barbs spread apart, is also an important item to analzye. This process forces the air to rise in order to replace the air that is spreading apart.

Soundings

Satellite Imagery

Visible (Band 2):

Visible Imagery is utilized alongside IR imagery to determine cloud heights. Clouds that appear on Visible but not IR imagery tend to be lower and warmer, while clouds that appear equally well on both are likely colder and indicate areas of strong uplift. Areas of snow cover are also easily located.

Infrared (Band 13):

Infrared imagery shows the temperature of the Earth and atmosphere, and together with Visible imagery can be used to locate areas of significant uplift and cloud development. Temperature decreases with height, so naturally the highest cloud tops should be much colder than low clouds. Consequently, clouds that show up extremely well on IR imagery and that are highlighted in green, yellow, and especially red correspond to high cloud tops and strong uplift. Clouds that are difficult to distinguish or are undetectable on IR imagery but show up clearly on Visible imagery are likely very low-level and warmer clouds.

Water Vapor (Band 8):

Water Vapor imagery shows where moisture is present. It is important to note that it is the water vapor throughout the whole air column and not just a single layer. Areas of high levels of water vapor are designated by green, moderate levels are white, and low levels are either blue or yellow.

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