(Front and associated trough sweep from east to west over Central US. Image source: NOAA)
An extreme US summer that has featured floods and heavy rains in the east and drought and devastating fires in the west boasts yet one more bizarre weather pattern: a backward moving storm system.
As of last week, a strong frontal boundary had swept into the southeastern US bringing with it another dose of heavy rains and storms. Then the system stalled. Over Friday, Saturday and Sunday, the front and associated low pressure systems have backed up, moving from east to west, passing over the Appalachians, then the Tennessee River Valley, then the Mississippi, until today it reached a central region stretching from Texas all the way north to the Dakotas.
This retrograde weather is a very uncommon, but not unheard of, event. In the context of an already strange summer, it adds yet one more anomalous weather pattern to the list. In the AQUA/NASA image below, we can see the position of this retrograde frontal boundary and low pressure system as of yesterday. Note how the eastern front essentially collapsed as it pushed westward forming a bow from Texas to the Dakotas in the image at the top of this post.
(Frontal System begins to move in reverse on Saturday, July 13. Image source: NASA)
The cause for this retrograde storm motion is an extraordinarily weak Jet Stream. Over the past two decades, the Northern Hemisphere Jet has continued to weaken as both sea ice and summer time snow cover rapidly eroded. This helped reduce the strong north-south temperature differentials that drove the Jet to move weather systems rapidly from west to east. Now, temperature differences between high and low lattitudes are greatly reduced and, as a result, the Jet proceeds more slowly.
One result is that weather tends to persist much, much longer over a given region. Droughts, heat, and dry weather persist in areas where the Jet is pushed into a south-north configuration. Rain, storms, and cooler weather persist in regions where the Jet is pushed into a north-south configuration.
In the case of this weekend, the Jet became so weak that prevailing local influences overcame a hemisphere wide system to drive weather patterns against its ebbing flow. In this case, though, the result may be a somewhat positive influx of rainfall to drought and fire stricken western regions. Let’s hope this doesn’t turn into too much of a good thing.
UDPATE:
The weather system I reported on earlier today is still moving in its retrograde fashion, east to west, across the United States. It now stretches almost to Utah and appears to be dumping rain in a bow shaped arc from Texas into New Mexico and then re-curving northward back through the Dakotas and all the way through to Quebec. A moist upper level flow issuing from the Atlantic Ocean is still pushing the arching front westward.
You can see the progress of this retrograde storm in the most recent satellite image here:
(Image source: NOAA)
This retrograde action has now spanned 2/3 of the Continental US and is still proceeding westward. May well be something for the record books…
Links:
robertscribbler 
Illuminate by Tanya
/ July 15, 2013That’s so weird!! What do you think will happen if this retrograde system stays on longer?
LikeLike
robertscribbler
/ July 15, 2013Looks like the center of the upper low is now stalled over Texas. Expect a lot of moisture to get pulled in from the Gulf of Mexico and the possibility for some heavy rains, especially in Texas. The US east coast could see the weird appearance of weather systems moving in from the Atlantic, if this kind of situation lasts for an extended period. Eventually, a Pacific system should collide with the retrograde and push it back east. If the Pacific system is strong enough, this could result in yet more severe weather.
LikeLike
Illuminate by Tanya
/ July 15, 2013When was the last time this happened? And for how long?
LikeLike
robertscribbler
/ July 15, 2013Now and then, we get an upper air low that stalls and moves slowly backward. They usually don’t move so far, though. For my part, I’ve never seen one transition from the East Coast all the way back to the Desert Southwest.
LikeLike
coyoteyogi
/ July 15, 2013New Mexico desperately needs rain, from any direction it is welcome. As a farmer there is no way to plan for this. As you say, let’s hope it dumps and moves on so floods are avoided.
LikeLike
bugwump
/ July 15, 2013I’ve read on a few pages that this will make it all the way to the west coast, then dive south into Mexico and church up moisture and send it up to Southern CA and Nevada. Is this the pattern you see as well? What kind of forecast can we expect in Southern CA given this weather anomaly? It seems like most weather apps and new stations aren’t addressing this – maybe because they just don’t know how to forecast this model.
LikeLike
robertscribbler
/ July 15, 2013If it becomes embedded in the monsoonal moisture flow and is able to access both Gulf of Mexico and Pacific moisture, the result would probably a high risk precipitation even for the southwest.
I’ll take another look at the models and provide an additional update for you guys.
LikeLike
GrandDaddy
/ July 15, 2013Good- it will blow the Fukushima crap back to Fukushima.
LikeLike
Doug
/ July 16, 2013If you look at the GOES10 satellite loop: http://www.intelliweather.net/imagery/intelliweather/sat_goes10fd_580x580_img.htm
You will notice another Low pressure system going Retrograde in the northern pacific, north of Hawaii. So, it is not just over the Continental US. This, in my opinion, is very alarming & concerning…It’s a drastic departure from the norm, it seems…
LikeLike
robertscribbler
/ July 16, 2013The deep meanders in the Jet provide more openings for this kind of retrograde action. Usually, Northern Hemisphere summers primarily see retrograde only in the Tropical Convergence Zones. Cyclonic formation over oceans is, in part, encouraged by these actions. One wonders if formation of new retrograde zones over the oceans, when combined with warming waters, will result in secondary, further northward, formation zones for tropical systems. May be worth looking into if these new weather patterns continue to proliferate.
LikeLike
robertscribbler
/ July 16, 2013Just looked at your link. That is rather chilling.
LikeLike
Bob
/ July 19, 2013Ok so the weekend is here, I am on the west coast and it’s cloudy here and about 7 degrees below the projected high. I looked at the latest intelliweather animation and it looks like the clouds we are getting are being pulled up from Mexico….is this residual from the backwards system or is this something else? It looks like the clouds are spinning counter-clockwise which I think means backwards, but I am no weather bug…just interested in the phenomenon.
LikeLike
robertscribbler
/ July 19, 2013Yep. The upper level low has moved into your region and is the cause of cool, cloudy and rainy conditions there.
The upper level low is now moving south-west and is centered near Baja and the Gulf of California. It will probably soon be over open water.
http://www.goes.noaa.gov/GSSLOOPS/wcwv.html
LikeLike
Bob
/ July 19, 2013So why aren’t my weather apps like Yahoo Weather and The Weather Channel app forecasting a “wetter” day? We’ve seen no rain, but definitely cloud cover that appears to be coming up from the south (I am about 45 minutes from the border). Is it because their computer models aren’t built for a system moving like this?
LikeLike
robertscribbler
/ July 19, 2013The latest national weather service radar shows spotty light drizzle running through your region. A more dense area of rainfall is to your south over the Bay of California. Firing of thunderstorms and showers to the north of the low as the afternoon progresses is possible. But for you to see more consistent rain would require the low to move more in a northwestward fashion or to interact with another system, which hasn’t happened yet.
To this point, the jet does seem, now to be shoving this system to the south, which would lessen your chances for consistent rainfall. That said, I think your forecasters should have noted a chance of rain for today and tomorrow, especially in the afternoon.
http://radar.weather.gov/ridge/Conus/pacsouthwest_loop.php
I know you guys are starved for rain in that area, so let’s hope you get some. But one thing to point out is that the dry soil itself sets the stage for less rain as it tends to suck moisture out of the air.
LikeLike
jai mitchell
/ July 23, 2013This morning the weather channel dubbed it the “wrong way rerun” because after dipping into Baja and then moving up into California and Nevada, the reformed upper level (cutoff) low is projected to move back east to Baltimore by Saturday the 27th. This means that this cutoff low was formed on the 13th and sustained monsoonal moisture, over land, for over 2 weeks.
This is unprecedented in my understanding of this kind of weather pattern, they usually dissipate in 3 or 4 days.
Similarly there has been a double formation of North Pacific cut off lows for the entire month of July.
http://www.ssd.noaa.gov/goes/west/nepac/flash-wv.html
LikeLike
robertscribbler
/ July 23, 2013Ah, they’re predicting it to reach Baltimore by the 27th now. What a mess.
I agree, there’s probably no precedent for this kind of pattern. I’ve never seen it and haven’t yet seen a meteorologist or climatologist who says they’ve seen it.
Saw one Pacific retrograde last week, so this is yet one more. Since the start of summer, I’ve seen numerous cut off upper level lows. Most meandered or were stationary. These retrogrades, though, were extraordinary.
LikeLike
mieprowan
/ July 24, 2013I measured 5.5″ at my place in Carlsbad, NM over about 60 hours mid-July. To put this in perspective, our average yearly rainfall is about 13″ and average July is 1.8″. We can get twice as much during El Ninos, but we are ENSO neutral now and the drought was predicted to continue.
We do occasionally get weather in from the east but it’s usually violent. This was highly atypical, with no thunder at all, more like west coast rains, long soaks. We won the jet stream lottery…for now…
LikeLike