High and Low Pressures


Could you please explain why the HIGH is staying in this part of the country so long?  Also, what stirs highs and lows?


This is a good--but very complex--question. I'll try to answer.

Truth is, we often see centers of mid-upper-level HIGH and LOW pressure centers "lock" into position and remain there for days, even weeks, at a time.

For example ... remember all the excessive rains in Texas just a few weeks ago?  That was largely due to a semi-permanent upper LOW over the Lone Star State.

Obviously (and thankfully) these patterns eventually fade, so the question becomes, "How long will it last?"

And THAT is a VERY difficult question because to answer it, we need to know something about the HIGH or LOW of interest (like our current heat-wave generating HIGH over the SouthEast/South Central U.S.). PLUS, we need to know how the upper-levels of the atmosphere are behaving "ahead of" and "behind" that  pressure system.

Think of the upper-air flow in the mid-latitudes as basically a west-to-east flowing river.  Whenever the river takes a slight shift to the south, it must be followed downstream by a shift to the north.

These meanders in the flow produce regions of RIDGES (Highs, indicated by northward shifts) and TROUGHS (Lows, indicated by southward shifts).

Our current SE/SC U.S. upper-air ridge (which I would define as a "variety" of the summertime Bermuda High) -- is currently locked into place because of troughs on either side of it: one centered near the U.S. Pacific Coast ("upstream") and the other along the U.S. East Coast and western Atlantic ("downstream").

So, to change our current weather pattern, all three features must be modified to move, displace or dissipate the heat-wave HIGH.

We typically use computer weather models to forecast moves or changes in the upper-air flow.  Assume that a few days ago, it looked like a HIGH would push to the west (in other words, drift "upstream") by the weekend, but on Thursday evening the models indicate that any movement (or weakening) would be minimal until early to middle of the following week. Result: we begin thinking that the heat is with us well into the following week.

However, we must keep watching, as the models are always re-working the forecast, with updates coming every 12 hours in most cases.

Hope this helps!

Jay Grymes
Chief Meteorologist
WAFB Storm Team