Northwest flow - we have all heard of it. In the Winter, it can mean round after round of snowfall thanks to clipper systems coming in from Canada and across the Midwest, but it also plays a significant role in our summer pattern! It can lead to widespread rainfall and rounds of damaging severe thunderstorms. However, in the right scenarios, it can also lead to cool conditions and sunny skies. So how do meteorologists determine what kind of weather the northwest flow pattern will bring? First, let’s look at what a northwest flow is.
A northwest flow describes the mid to upper-level wind pattern, specifically when winds across the northern and eastern portions of the country (Midwest included) flow from generally northwest to southeast. This is created by two prominent upper-level features. The first is an upper-level ridge of high pressure that develops or moves into the west-central, southwest, or western portions of United States, or at times Canada. These upper-level ridges, often referred to as “heat domes” bring very hot air to the areas under them with temperatures often reaching and exceeding 100° F. It also allows for humid air to be pulled north from the Gulf of Mexico and transported into the nation’s heartland.
The second feature is a trough of low pressure that sets up across the northeast or eastern portions of the U.S. Where the troughing sets up is very important to note for meteorologists and has bigger implications on the forecast. The upper-level troughing allows for cooler, drier air to move in from the north. As winds flow around the north side of the ridge of high pressure (anti-cyclonic flow) then south of the trough of low pressure (cyclonic flow), it creates a northwest flow.
While it is common to see precipitation in association with a northwest flow across the Midwest, there are certain situations where it can make for a dry pattern as well. Let’s start by examining the first of the two. It is very common to see rounds of rain, thunderstorms, and severe weather rumble across the Midwest in the Summertime on a northwesterly flow. This kind of pattern occurs when the upper-level troughing is focused more into the northeastern portions of the country, with the ridging back into southwest or central U.S.
This pattern creates a more favorable environment for mid to upper-level waves of energy to track east-southeast across the Midwest region. However, just as importantly, it allows more of a southerly flow to transport in very moist air out of the Gulf of Mexico, providing the needed atmospheric moisture and instability for storms to develop. These waves create lines or clusters of rain and storms that move across the region and can produce severe weather. These rounds of storms are sometimes associated with more impactful and significant damaging wind events, like Derechos, but can also lead to large hail and tornadoes in the right environmental conditions.
However, not all northwest flows are the same and, in some situations, can lead to drier conditions across the Midwest and be a pain for those in the nation’s AG belt. This is where having a credible and experienced meteorologist can truly come in handy! While it is common to see upper-level troughs build into the northeastern United States, we also have times where the troughing tends to reside further south over the Mid-Atlantic or into portions of the TN/OH Valley. This placement can create drier conditions for many of those who would normally see more precipitation chances with this pattern…but why?
It all comes down to the placement of the troughing/ridging and the wind flow around them as previously discussed. When the troughiness settles further south into the eastern U.S., it essentially pinches off the moisture flow into a larger portion of the Midwest region. The southerly flow that transports moisture is pushed further west, resulting in more moisture into the Great Plains and Missouri/Mississippi Valley regions. All of this occurs while cooler, drier air is transported into the Great Lakes and Ohio valley, allowing for sunny, calmer conditions there leading to more dryness.
Through the next 2-4 weeks, the upper-level pattern continues to suggest we will stay locked into a northwest flow across the Midwest region. It also suggests periods of dry weather and periods of more active weather can be expected! If you or your company rely on accurate forecasts to make short or long-range decisions, make sure to reach out to us for custom forecasting solutions!