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I asked this in the chat and it was suggested I ask on the main site.

There's been a seemingly perpetual severe weather outbreak over the central and eastern U.S. over the past two weeks. Killer EF3s in Jefferson City, MO, Dayton, OH, and El Reno; flash flooding in Oklahoma; severe storms in Illinois/the Ohio Valley/Mid Atlantic; the tornadic supercells in northern NJ/Staten Island, the list goes on, and on, and on.

On top of this, the southeastern U.S. is seeing record high temperatures (earliest-ever triple digit temps for Alabama) and a heat wave is currently (as of today, Wednesday 5/29) smothering the east coast. What has been the synoptic and mesoscale (fronts, upper-air) setup that can explain this unusual pattern?

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  • $\begingroup$ The midwestern US (with rain & tornados) isn't the entire US. Parts of the west (where I live) have been unusually cool & wet, though I wouldn't call that "severe". More like a blessing, given a near-record snowpack after several drought years :-) $\endgroup$ – jamesqf May 30 at 17:41
  • $\begingroup$ @gansub: Current western US weather is not typical of El Nino years. $\endgroup$ – jamesqf May 31 at 5:29
  • $\begingroup$ @gansub: That report is from Midland, Texas, which is in the middle of the country, not all that far from the Missouri, Oklahoma, &c places that're having floods & tornados. Not much connection to the California/Sierra Nevada West, AFAIK. Anyway, I never said there wasn't an El Nino, just that from what I've read it's a fairly weak one, and this spring's weather hereabouts is not typical of such. $\endgroup$ – jamesqf May 31 at 17:55
  • $\begingroup$ @gansub: It depends on where you're looking from, of course :-) But Colorado, and particularly the parts east of the Front Range, are roughly a thousand miles from the Sierra Nevada, so I would expect different weather patterns - e.g. influence from the Gulf of Mexico & Canadian prairies. $\endgroup$ – jamesqf Jun 1 at 17:45
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Summarizing the comments I made above in this question and in this one - Does this weather pattern have a name ? I believe significant parts of the US are experiencing a Stationary Front for the past one month. There has been plenty of media coverage of this event and if you google the term "stationary front" under news one will receive a lot of links to the TV news media with meteorologists giving detailed analysis of this event.

A few summarized below -

1) Week's bleak weather not an unusual spring occurrence

2)A stationary front is stuck across the middle parts of Illinois and Indiana

I will attempt to provide a synoptic scale explanation of the event from reanalysis data.

1) From JMA's site - wave activity flux derived from stream function from April 29th through May 28th 2019

an upper level look at the 200 hPa surface reveals that an equatorward trajectory of the wave activity flux vector(seen as small arrows) is seen from the Upper North Pacific. Climatologically Rossby Wave Breaking can occur from boreal fall to boreal spring. The signal is seen at 30 day , 10 day period and 7 day period and 5 day period. The signal is present in the raw data as well as in the anomaly. Most importantly this wave activity flux is directed towards the continental USA all the way from New Mexico to the Great Lakes region.

A popular science version of the above information has been discussed over at this link - What's up with all this wet weird weather ?

2) At the surface level this "stationary front" presents itself as air masses of two different temperatures as seen from this plot and one can see the frontal boundary more or less coincides with the rising contour of the stream function plot in (1). Here the south easterlies are bringing in warm moist air from the Gulf coast and the Atlantic and the colder air masses are coming from Canada and North East Pacific. So the key question is whether the upper level influence is directing the surface level weather and that question can be answered by calculating PV anomalies when ERA 5 reanalysis data does become available.

Surface Air Temperature And 10 m wind anomaly

Source - JMA reanalysis data

3) The moisture flux being pumped into the south eastern part of the USA can be seen from this plot over a seven day period - Moisture Flux for North America 7 day mean. Again the signal is present over 30 day period, 10 day period and 3 day period and over a 1 day period. I would like to point out the moisture flux is seen at different levels of the atmosphere from surface level to the 300 hPa surface.

Source - NCAR NOAA reanalysis derived variables

4) Last year (2018) apparently a wavenumber - 7 was found to be causing weather stalls as shown in this link - Weather stalls becoming longer. This year in April it is a wavenumber 5 pattern as seen in this site - Current Weather reports and summarized here

From a global point of view, the weather pattern in the beginning parts of the 6-10 day (around day 7) will enter into a wavenumber 5 pattern. That's where you have 5 distinct upper-level troughs (areas where the weather is cool and stormy) around the globe. It's like a major traffic jam in the atmosphere and could lead to weather stagnation with little to no variation for days and/or weeks. This could also lead to extreme weather events such as droughts and severe weather/flooding across some parts of the world.

and I am able to pick up the wavenumber 5 or 6 as pointed by user Deditos (as well in this twitter feed - Wave number 6 planetary wave) from NCAR NOAA reanalysis data for the 500 hPa geopotential height anomaly for the month of May. enter image description here

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    $\begingroup$ Yes, wavenumber 6 according to some Euro peoples: twitter.com/rahmstorf/status/1133834691826638848 $\endgroup$ – Deditos May 31 at 13:27
  • $\begingroup$ Whenever I try to get a flux plot from NCAR, it throws an error. Strangely enough, geopotential heights get plotted just fine. What am I doing wrong? Error message here: hastebin.com/xazuyeceme.http $\endgroup$ – spillthrill May 31 at 20:22
  • $\begingroup$ @spillthrill Confirmed. You can send them an email - esrl.psd.data@noaa.gov. I will do so right away. It was working till couple of days back. $\endgroup$ – gansub Jun 1 at 0:30
  • $\begingroup$ @spillthrill moisture flux seems to be working now. Can you try it out? $\endgroup$ – gansub Jun 29 at 6:57

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