Ridges and Troughs - The Daily Weather Makers

The middle latitudes are home to a strong west to east flow around both the Northern and Southern Hemispheres. The map below shows the flow over most of North America this morning (5-25-2023). The winds aloft are shown by the green arrows.

As you can see the winds curve around troughs and ridges in the atmosphere. On the east sides of troughs is where precipitation is mostly likely to occur while the east side of ridges are mainly dry. That is highly simplified but for our purposes today it is mostly valid. The location of the troughs and ridges is illustrated with the up and down V symbols. NOTE: The Trof along the East Coast is mislabeled. The blue ridge marks should be flipped to point down like the Trof in the west.

Upper Air Flow at 500 millibars (approximately 18,000 feet) on May 25, 2023. The 500 millibar map used on this chart was plotted using Digital Atmosphere which is available at www.weathergraphics.com.

GOES 16 Visible Satellite Image courtesy of NOAA/NESDIS/STAR. Text added by Weather Briefing LC. 5-25-2023, 15:01 CDT

5-25-2023

Here is the USA Surface Map Plot this afternoon at 1600 CDT. Compare the satellite and radar images above with this map.

Surface high pressure over the Great Lakes with its clockwise rotation has spread much cooler air from eastern Canada across the Great Lakes and New England westward across the lower Great Lakes and the Ohio Valley to the Upper Midwest. The southern edge of the cooler air extends from the Carolinas to Tennessee and then northwest up through eastern Kansas, Nebraska, to the eastern Dakotas into Canada. Temperatures are in the 70s over Iowa but in the 80s in eastern Nebraska and the eastern Dakotas. Brandon, Manitoba reports 86 degrees on this map. The clouds, showers, and thunderstorms from Nebraska to Oklahoma are near and west of the boundary in eastern Nebraska southward. Dew points are in the 50s west of that boundary and in the 30s and 40s to the east. The air pressure here (northeast Iowa) peaked this morning and has been falling today as the high pressure begins to inch eastward away from Iowa. Our dew point is 32 degrees while Omaha, Nebraska’s dew point is 57 degrees.

The cumulus cloud fields on the satellite image are mainly along and west of the boundary that extends from southern Canada south through the eastern Dakotas, eastern Nebraska and Kansas and then to the East Coast through Tennessee and the Carolinas. The satellite image and radar show the showers and thunderstorms that are near and west of that boundary in the unstable air mass. We are stable here with sinking air motion overhead due to the high pressure ridge aloft and the high pressure centered at the surface over the Great Lakes.

This discussion is meant to show the interrelationships between weather systems that are affecting the eastern and central United States. Wind, temperature, dew points, air pressure, relative humidity, and precipitation are all determined by the location and movement of very large global and smaller regional weather systems - but they are all tied together.

This is our sky today in northeastern iowa - A clear sky under the ridge of high pressure aloft over the Great Plains and a surface high pressure centered over the Great Lakes.

Scrolling down to the next map we see where precipitation is expect to occur sometime during the next 7 days. Notice that the primary areas of precipitation are east of the troughs (west of ridges) shown on the map above. The primary dry areas are east of ridges (west of troughs). Why? Areas east of troughs (west of ridges) are where rising motion is often found. Rising air cools and expands as it rises and the cooling may cause the air to become saturated with moisture which leads to precipitation.

NOTICE: The forecast on the chart below is only valid on Thursday, May 25, 2023. The discussion in this post is using data and forecasts that are updated daily. It is used here to illustrate the explanation about how the position of troughs and ridges influences the daily weather.

Because the overall pattern is not expect to change much during the next week most of the precipitation will be found in the favored areas east of the troughs, which will not move much during the next week. The third chart below shows the expected amounts of precipitation for the next 7 days. Notice how the most precipitation is placed in the favored areas east of the troughs. Since the pattern is not expected to move much the heaviest rain is expected east of the troughs where little movement will concentrate the rain totals. You will notice that precipitation is expected in a few areas that are not within the favored regions. The explanation for that is beyond the scope of this post.

When the weather does not change much over long periods it usually means the overall pattern is not changing much. It stays wet where it is already wet and dry where it is already dry. When droughts occur the same thing happens, only for a longer time than a week. These types of events are a normal occurrence that is caused by a stalled weather pattern that will eventually change.

NOTICE: The forecast on the chart below is only valid on Thursday, May 25, 2023. It is updated daily.
The discussion in this post is using data and forecasts that are routinely updated. The forecasts will be out of date after 5-25-2023.