(Image source: NOAA)
On about July 11th, I began to report on a strange weather phenomena involving an upper level low moving backwards against the Jet Stream. At that time, there were a few reports among meteorologists that such an event was possible and could affect the weather in the central and western US, causing more rainfall and storms for a longer period of time. There was some discussion that the low might end up in Mexico before moving back east.
All this prediction was strange enough. Retrograde systems do happen now and then, but their motion is normally truncated, only progressing a few hundred miles west at most then returning to the typical pattern of west to east movement. But this retrograde acted differently, traveling almost directly against the Jet Stream flow for hundreds and then, ultimately, for about 3,000 miles. As of last week, the motion was so abnormal that climate scientists began to comment on its likelihood. In general, there’s about a 1 in 300 chance of such an event happening in July, making this retrograde system a 1 in 300 year event.
So the low did move backwards into Arizona, New Mexico and eventually California where it dumped rain day after day, alleviating drought conditions there while also setting off flash floods. Today, flooding in Phoenix was so intense that numerous motorists had to be rescued as their vehicles were inundated under flash floods no current infrastructure could handle. Up to two inches of rain fell within about a half an hour in numerous locations. Large storms also doused fires even as they set off localized flooding events in California. Some areas of New Mexico experienced their first rainfall in over 100 days.
The flood event in Phoenix today is described in the following video:
Over the next few days, the low is expected to slowly move northward, drifting into northern California then Oregon before being picked up by the Jet and lazily pushed eastward by its feeble flow. Over the next 7-10 days it will transition once more to the place it started — the Ohio river valley. Models are too uncertain, at this time, to determine if the low will again get sucked into the eddy that drew it all the way to the west coast. But with the Jet Stream so weak odd weather events such as this one are now becoming the norm.