Today, just for fun, I am going to present my personal explanation of what caused Wednesday’s extraordinary warm anomaly in the central part of North America. The weather maps are telling me that two separate and distinct atmospheric rivers (ARs) are flowing over much of this area at the same time, each of which is adding a whole new layer of concentrated precipitable water (PW) to the overhead atmosphere of the surface below. The combined total of all this PW, which includes a relatively small amount of ambient humidity, generates a greenhouse energy effect that is several times greater than the total effect that would normally be produced by a much lesser amount of overhead PW on this day of the year—which is the principal reason why there was such a large spread of exceptionally warm anomaly. We’ll start this with the anomaly map, where you can see how the largest spot of all (in Iowa and Missouri) has broken into the +18-21C bracket (about 35F) on the scale. Overall, a truly vast portion of the continent has a fever greater than +10C, requiring nothing less than a full double of overhead PW content above the historical average from three decades back.
As for PW content, a quick look at this next map will tell you that something significant is going on above the entire region, which extends into the far reaches of northern Canada. The historical averages for total PW over this entire area, registered in kg per square meter, would actually run the gamut of many different numbers over so much geography. I have no means of displaying those numbers, because there is no such map available, but have high confidence that they would be much lower than what you see on the current map, even as low as one quarter in the case of the Iowa/Missouri hot spot.
This same map includes a fair amount of evidence about the origin of all the PW we are seeing. It obviously cannot be rising straight up from water sources on land, which leaves outside sources in the form of one or more ARs, initiated by warm ocean waters, as the only reasonable alternative. As I’ve already said, I think we see two of these coming in at the same time—from different directions, with once crossing over the top of the other. This is not a common situation, nor is it highly unusual. What is unusual is that both are heavily loaded with PW concentrations that have not had much in the way of previous reduction by raining out—as seen on a later map. I think one of the ARs, the higher up of the two, is being carried toward and across the northern Mexico coastline of North America on the underbelly of a jetstream wind, which is collecting vapor from Pacific Ocean waters well to the south of Hawaii. The next map will show you the course of this jet’s pathway over the continent, first heading northward over the midsection, then abruptly reversing to the south as it continues eastward. (For some reason reason the PW it is carrying does not show up well on the PW map while it is crossing the mountains in Mexico, nor is there any sign of dispersal, an enigma that is commonly seen and always hard to explain.)
The second AR, probably the larger of the two, collects most of its PW from the Gulf of Mexico, which still has an abundance of warm water ready for evaporation in spite of the late season. This PW, instead of being lofted all the way to the level of jet streams, gets picked up by strong surface winds that remain effective in the mid to higher parts of the lower atmosphere. These winds are very common, heading generally to the north across the plains and well into Canada. On this day, as we see on the next map, they are maintaining a great deal of power as they proceed into the far north of Canada, then swing around toward the west and continue southward for a considerable distance:
It is interesting to note that this lower level wind action does not seem to be directly affected by the passage of the South Pacific AR passing over it, or by the passage of a very strong jet stream that we can observe on the previous map looping down from the north and then swinging to the east. This jet, which has no sign of any AR (or PW) content, is traveling to the east while the upper surface wind directly below it is moving in the exact opposite direction. These upper and lower wind systems are indeed well-separated!
Finally, I need to show the map of cloud cover and precipitation for this 24-hour period. Clouds, yes, but almost no sign of precipitation over the entire area of anomaly in spite of the remarkable volume of PW in the air. There is some snowfall in northern Canada where the AR from the Gulf swings around, but that is about it. The AR out of the Pacific did all of its raining long before it reached land. Over land, these two ARs have both chosen to hang on to their PW content, which has kept their concentrations of PW largely intact over their respective courses of movement. This factor certainly has helped to keep the anomaly as high as we see it over the entire extent of the large territory overrun by these ARs.
Carl